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    <link:label id="lab_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA_terseLabel_en-US" xlink:label="lab_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">EPC Electric Transmission Project</link:label>
    <link:label id="lab_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA_label_en-US" xlink:label="lab_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">EPC Electric Transmission Project [Member]</link:label>
    <link:label id="lab_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA_documentation_en-US" xlink:label="lab_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">EPC Electric Transmission Project [Member]</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_EPCElectricTransmissionProjectMember" xlink:label="loc_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA" xlink:to="lab_pwr_EPCElectricTransmissionProjectMember_C633CA08982F9083898EEEE9133E25FA" xlink:type="arc" />
    <link:label id="lab_pwr_ProcessingFacilityConstructionProjectTexasMember_9BFA299C75E98EA8809CEEE9133F1246_terseLabel_en-US" xlink:label="lab_pwr_ProcessingFacilityConstructionProjectTexasMember_9BFA299C75E98EA8809CEEE9133F1246" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">$150 Million Lump-Sum Processing Facility Construction Project</link:label>
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    <link:label id="lab_pwr_ProcessingFacilityConstructionProjectTexasMember_9BFA299C75E98EA8809CEEE9133F1246_documentation_en-US" xlink:label="lab_pwr_ProcessingFacilityConstructionProjectTexasMember_9BFA299C75E98EA8809CEEE9133F1246" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Processing Facility Construction Project - Texas [Member]</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_ProcessingFacilityConstructionProjectTexasMember" xlink:label="loc_pwr_ProcessingFacilityConstructionProjectTexasMember_9BFA299C75E98EA8809CEEE9133F1246" xlink:type="locator" />
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    <link:label id="lab_pwr_NaturalGasPipelineConstructionProjectNortheastUnitedStates1Member_A63B6A27F4FF9F6F7E25EEE9133F4C19_terseLabel_en-US" xlink:label="lab_pwr_NaturalGasPipelineConstructionProjectNortheastUnitedStates1Member_A63B6A27F4FF9F6F7E25EEE9133F4C19" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">$145 Million Lump Sum Natural Gas Pipeline Construction Project</link:label>
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    <link:label id="lab_us-gaap_ContractWithCustomerAssetNet_80EB5784C01296530642EEE97DCB4C62_terseLabel_en-US" xlink:label="lab_us-gaap_ContractWithCustomerAssetNet_80EB5784C01296530642EEE97DCB4C62" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Contract assets</link:label>
    <link:label id="lab_us-gaap_ContractWithCustomerAssetNet_80EB5784C01296530642EEE97DCB4C62_label_en-US" xlink:label="lab_us-gaap_ContractWithCustomerAssetNet_80EB5784C01296530642EEE97DCB4C62" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Contract with Customer, Asset, Net</link:label>
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    <link:label id="lab_us-gaap_ContractWithCustomerLiability_A21E4C67B823E5DE5FF2EEE97DCB3238_terseLabel_en-US" xlink:label="lab_us-gaap_ContractWithCustomerLiability_A21E4C67B823E5DE5FF2EEE97DCB3238" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Contract liabilities</link:label>
    <link:label id="lab_us-gaap_ContractWithCustomerLiability_A21E4C67B823E5DE5FF2EEE97DCB3238_label_en-US" xlink:label="lab_us-gaap_ContractWithCustomerLiability_A21E4C67B823E5DE5FF2EEE97DCB3238" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Contract with Customer, Liability</link:label>
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    <link:label id="lab_us-gaap_DebtDisclosureAbstract_3101E7A841AC73169A67EEE97C17C15B_label_en-US" xlink:label="lab_us-gaap_DebtDisclosureAbstract_3101E7A841AC73169A67EEE97C17C15B" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Disclosure [Abstract]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtDisclosureAbstract" xlink:label="loc_us-gaap_DebtDisclosureAbstract_3101E7A841AC73169A67EEE97C17C15B" xlink:type="locator" />
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    <link:label id="lab_us-gaap_LineOfCreditFacilityTable_5F7A9E9AB9FF0632C652EEE97C1788F5_terseLabel_en-US" xlink:label="lab_us-gaap_LineOfCreditFacilityTable_5F7A9E9AB9FF0632C652EEE97C1788F5" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Line of Credit Facility [Table]</link:label>
    <link:label id="lab_us-gaap_LineOfCreditFacilityTable_5F7A9E9AB9FF0632C652EEE97C1788F5_label_en-US" xlink:label="lab_us-gaap_LineOfCreditFacilityTable_5F7A9E9AB9FF0632C652EEE97C1788F5" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Line of Credit Facility [Table]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_5F7A9E9AB9FF0632C652EEE97C1788F5" xlink:type="locator" />
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    <link:label id="lab_us-gaap_CreditFacilityAxis_864DDD97BE989B5642C0EEE97C186037_terseLabel_en-US" xlink:label="lab_us-gaap_CreditFacilityAxis_864DDD97BE989B5642C0EEE97C186037" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Credit Facility [Axis]</link:label>
    <link:label id="lab_us-gaap_CreditFacilityAxis_864DDD97BE989B5642C0EEE97C186037_label_en-US" xlink:label="lab_us-gaap_CreditFacilityAxis_864DDD97BE989B5642C0EEE97C186037" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Credit Facility [Axis]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_864DDD97BE989B5642C0EEE97C186037" xlink:type="locator" />
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    <link:label id="lab_us-gaap_CreditFacilityDomain_8968D17D4D71B2F2CEB2EEE97C18C4BC_terseLabel_en-US" xlink:label="lab_us-gaap_CreditFacilityDomain_8968D17D4D71B2F2CEB2EEE97C18C4BC" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Credit Facility [Domain]</link:label>
    <link:label id="lab_us-gaap_CreditFacilityDomain_8968D17D4D71B2F2CEB2EEE97C18C4BC_label_en-US" xlink:label="lab_us-gaap_CreditFacilityDomain_8968D17D4D71B2F2CEB2EEE97C18C4BC" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Credit Facility [Domain]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_8968D17D4D71B2F2CEB2EEE97C18C4BC" xlink:type="locator" />
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    <link:label id="lab_pwr_LettersOfCreditAndBankGuaranteesMember_E7182A83DC90FE23ACE9EEE97C189006_documentation_en-US" xlink:label="lab_pwr_LettersOfCreditAndBankGuaranteesMember_E7182A83DC90FE23ACE9EEE97C189006" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">The Letters Of Credit and Bank Guarantees.</link:label>
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    <link:label id="lab_pwr_BorrowingsUnderCreditFacilityMember_44B794943275125E2328EEE97C1945B4_documentation_en-US" xlink:label="lab_pwr_BorrowingsUnderCreditFacilityMember_44B794943275125E2328EEE97C1945B4" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">The borrowings under the credit facility.</link:label>
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    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_CAD" xlink:label="loc_currency_CAD_C9608E62F010076CB5D5EEE97C19E50F" xlink:type="locator" />
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    <link:label id="lab_pwr_CanadianAndAustralianDollarsMember_C9ECB8835CFC14F09A19EEE97C1A4D6A_terseLabel_en-US" xlink:label="lab_pwr_CanadianAndAustralianDollarsMember_C9ECB8835CFC14F09A19EEE97C1A4D6A" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Primarily Canadian and Australian Dollars</link:label>
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    <link:label id="lab_pwr_UnitPriceContractsMember_FDB12EE4871C58ADB6CAEEE97DE1DE7A_terseLabel_en-US" xlink:label="lab_pwr_UnitPriceContractsMember_FDB12EE4871C58ADB6CAEEE97DE1DE7A" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Unit-price contracts</link:label>
    <link:label id="lab_pwr_UnitPriceContractsMember_FDB12EE4871C58ADB6CAEEE97DE1DE7A_label_en-US" xlink:label="lab_pwr_UnitPriceContractsMember_FDB12EE4871C58ADB6CAEEE97DE1DE7A" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Unit-Price Contracts [Member]</link:label>
    <link:label id="lab_pwr_UnitPriceContractsMember_FDB12EE4871C58ADB6CAEEE97DE1DE7A_documentation_en-US" xlink:label="lab_pwr_UnitPriceContractsMember_FDB12EE4871C58ADB6CAEEE97DE1DE7A" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Unit-price Contracts [Member]</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_UnitPriceContractsMember" xlink:label="loc_pwr_UnitPriceContractsMember_FDB12EE4871C58ADB6CAEEE97DE1DE7A" xlink:type="locator" />
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    <link:label id="lab_us-gaap_FixedPriceContractMember_CCF83D4E7F5B8421FCBDEEE97DE23842_terseLabel_en-US" xlink:label="lab_us-gaap_FixedPriceContractMember_CCF83D4E7F5B8421FCBDEEE97DE23842" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Fixed price contracts</link:label>
    <link:label id="lab_us-gaap_FixedPriceContractMember_CCF83D4E7F5B8421FCBDEEE97DE23842_label_en-US" xlink:label="lab_us-gaap_FixedPriceContractMember_CCF83D4E7F5B8421FCBDEEE97DE23842" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Fixed-price Contract [Member]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FixedPriceContractMember" xlink:label="loc_us-gaap_FixedPriceContractMember_CCF83D4E7F5B8421FCBDEEE97DE23842" xlink:type="locator" />
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    <link:label id="lab_us-gaap_TimeAndMaterialsContractMember_BBD04721E2F6E9DFE2D6EEE97DE2B273_terseLabel_en-US" xlink:label="lab_us-gaap_TimeAndMaterialsContractMember_BBD04721E2F6E9DFE2D6EEE97DE2B273" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Cost-plus contracts</link:label>
    <link:label id="lab_us-gaap_TimeAndMaterialsContractMember_BBD04721E2F6E9DFE2D6EEE97DE2B273_label_en-US" xlink:label="lab_us-gaap_TimeAndMaterialsContractMember_BBD04721E2F6E9DFE2D6EEE97DE2B273" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Time-and-materials Contract [Member]</link:label>
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    <link:label id="lab_us-gaap_DisaggregationOfRevenueLineItems_D81A8697341A8FFB1815EEE97DE2DBA5_label_en-US" xlink:label="lab_us-gaap_DisaggregationOfRevenueLineItems_D81A8697341A8FFB1815EEE97DE2DBA5" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Disaggregation of Revenue [Line Items]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_D81A8697341A8FFB1815EEE97DE2DBA5" xlink:type="locator" />
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    <link:label id="lab_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_09C78DE07ACC40D5FB36EEE97DE227CF_terseLabel_en-US" xlink:label="lab_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_09C78DE07ACC40D5FB36EEE97DE227CF" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Revenues</link:label>
    <link:label id="lab_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_09C78DE07ACC40D5FB36EEE97DE227CF_label_en-US" xlink:label="lab_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_09C78DE07ACC40D5FB36EEE97DE227CF" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Revenue from Contract with Customer, Excluding Assessed Tax</link:label>
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    <link:label id="lab_us-gaap_UnrecordedUnconditionalPurchaseObligationTable_E4A2112D135DDEAA0117EEE97B55BF24_label_en-US" xlink:label="lab_us-gaap_UnrecordedUnconditionalPurchaseObligationTable_E4A2112D135DDEAA0117EEE97B55BF24" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Unrecorded Unconditional Purchase Obligation [Table]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationTable" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationTable_E4A2112D135DDEAA0117EEE97B55BF24" xlink:type="locator" />
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    <link:label id="lab_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_3FA40C3BF65C46A55C7FEEE97B55DF25_terseLabel_en-US" xlink:label="lab_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_3FA40C3BF65C46A55C7FEEE97B55DF25" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Category of Item Purchased [Axis]</link:label>
    <link:label id="lab_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_3FA40C3BF65C46A55C7FEEE97B55DF25_label_en-US" xlink:label="lab_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_3FA40C3BF65C46A55C7FEEE97B55DF25" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Category of Item Purchased [Axis]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_3FA40C3BF65C46A55C7FEEE97B55DF25" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_3FA40C3BF65C46A55C7FEEE97B55DF25" xlink:to="lab_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_3FA40C3BF65C46A55C7FEEE97B55DF25" xlink:type="arc" />
    <link:label id="lab_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_B93F33E13F5B50B80E2BEEE97B56349E_terseLabel_en-US" xlink:label="lab_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_B93F33E13F5B50B80E2BEEE97B56349E" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Long-term Purchase Commitment, Category of Item Purchased [Domain]</link:label>
    <link:label id="lab_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_B93F33E13F5B50B80E2BEEE97B56349E_label_en-US" xlink:label="lab_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_B93F33E13F5B50B80E2BEEE97B56349E" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Long-term Purchase Commitment, Category of Item Purchased [Domain]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_B93F33E13F5B50B80E2BEEE97B56349E" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_B93F33E13F5B50B80E2BEEE97B56349E" xlink:to="lab_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_B93F33E13F5B50B80E2BEEE97B56349E" xlink:type="arc" />
    <link:label id="lab_pwr_VehicleFleetCommittedCapitalMember_6E287E9C0219A603EAE1EEE97B560DBF_terseLabel_en-US" xlink:label="lab_pwr_VehicleFleetCommittedCapitalMember_6E287E9C0219A603EAE1EEE97B560DBF" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Vehicle Fleet Committed Capital</link:label>
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    <link:label id="lab_pwr_VehicleFleetCommittedCapitalMember_6E287E9C0219A603EAE1EEE97B560DBF_documentation_en-US" xlink:label="lab_pwr_VehicleFleetCommittedCapitalMember_6E287E9C0219A603EAE1EEE97B560DBF" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Vehicle Fleet Committed Capital [Member]</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_VehicleFleetCommittedCapitalMember" xlink:label="loc_pwr_VehicleFleetCommittedCapitalMember_6E287E9C0219A603EAE1EEE97B560DBF" xlink:type="locator" />
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    <link:label id="lab_us-gaap_UnrecordedUnconditionalPurchaseObligationLineItems_19CE3CDB12190C0F8599EEE97B569C30_terseLabel_en-US" xlink:label="lab_us-gaap_UnrecordedUnconditionalPurchaseObligationLineItems_19CE3CDB12190C0F8599EEE97B569C30" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Unrecorded Unconditional Purchase Obligation [Line Items]</link:label>
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    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationLineItems" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationLineItems_19CE3CDB12190C0F8599EEE97B569C30" xlink:type="locator" />
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    <link:label id="lab_us-gaap_UnrecordedUnconditionalPurchaseObligationDueInRemainderOfFiscalYear_FD3383CB6673EE094CA3EEE97B56FB63_terseLabel_en-US" xlink:label="lab_us-gaap_UnrecordedUnconditionalPurchaseObligationDueInRemainderOfFiscalYear_FD3383CB6673EE094CA3EEE97B56FB63" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Estimated committed capital in current year</link:label>
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    <link:label id="lab_pwr_OtherCreditFacilitiesMember_2D82CF47D5D3E900C2BAEEE97BCF5417_terseLabel_en-US" xlink:label="lab_pwr_OtherCreditFacilitiesMember_2D82CF47D5D3E900C2BAEEE97BCF5417" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Other Credit Facilities</link:label>
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    <link:label id="lab_pwr_RestrictedStockUnitsToBeSettledInCashMember_1C04F63DE33C739EDEF1EEE97B8CC15C_terseLabel_en-US" xlink:label="lab_pwr_RestrictedStockUnitsToBeSettledInCashMember_1C04F63DE33C739EDEF1EEE97B8CC15C" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Restricted Stock Units to be Settled in Cash</link:label>
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    <link:label id="lab_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_EC486F73FCE8BDC5A081EEE97B8E5CA4_verboseLabel_en-US" xlink:label="lab_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_EC486F73FCE8BDC5A081EEE97B8E5CA4" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:type="resource" xml:lang="en-US">Vesting period for RSUs to be settled in cash</link:label>
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    <link:label id="lab_pwr_AmountInCashReceivedForEachRSUIsEqualToFairValueOfThisNumberOfQuantaCommonStockShares_92470C28E833C6202009EEE97B8E0E3A_terseLabel_en-US" xlink:label="lab_pwr_AmountInCashReceivedForEachRSUIsEqualToFairValueOfThisNumberOfQuantaCommonStockShares_92470C28E833C6202009EEE97B8E0E3A" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Amount in cash received for each RSU is equal to the fair value of this number of Quanta common stock shares (in shares)</link:label>
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    <link:label id="lab_us-gaap_AllocatedShareBasedCompensationExpense_BB9A2C325A850CEF3FBDEEE97B8EED64_terseLabel_en-US" xlink:label="lab_us-gaap_AllocatedShareBasedCompensationExpense_BB9A2C325A850CEF3FBDEEE97B8EED64" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Compensation expense related to RSUs to be settled in cash</link:label>
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    <link:label id="lab_us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards_6F0123E66336D1FFE0F8EEE97B8F6C18_terseLabel_en-US" xlink:label="lab_us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards_6F0123E66336D1FFE0F8EEE97B8F6C18" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Payments to settle liabilities under compensation plan</link:label>
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    <link:label id="lab_us-gaap_DeferredCompensationShareBasedArrangementsLiabilityCurrent_ECE329C37E391A7F22F7EEE97B8F14A0_terseLabel_en-US" xlink:label="lab_us-gaap_DeferredCompensationShareBasedArrangementsLiabilityCurrent_ECE329C37E391A7F22F7EEE97B8F14A0" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Accrued liabilities under compensation plan</link:label>
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    <link:label id="lab_pwr_EmployersLiabilityInsuranceClaimsDeductible_DC7CAEDC97E896521CCAEEE910A60532_terseLabel_en-US" xlink:label="lab_pwr_EmployersLiabilityInsuranceClaimsDeductible_DC7CAEDC97E896521CCAEEE910A60532" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Employer's liability claims subject to deductible per occurrence</link:label>
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    <link:label id="lab_pwr_WorkersCompensationInsuranceClaimsDeductible_FA82975E08B68BFDC580EEE910A6D51C_terseLabel_en-US" xlink:label="lab_pwr_WorkersCompensationInsuranceClaimsDeductible_FA82975E08B68BFDC580EEE910A6D51C" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Worker's compensation claims per occurrence</link:label>
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    <link:label id="lab_pwr_AutoLiabilityInsuranceClaimsDeductible_A39BBFB1E04D31981D61EEE910A677F2_terseLabel_en-US" xlink:label="lab_pwr_AutoLiabilityInsuranceClaimsDeductible_A39BBFB1E04D31981D61EEE910A677F2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Auto liability insurance claims deductible</link:label>
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    <link:label id="lab_pwr_GeneralLiabilityInsuranceClaimsDeductible_21493EE823E3A24FF6E6EEE910A6116D_terseLabel_en-US" xlink:label="lab_pwr_GeneralLiabilityInsuranceClaimsDeductible_21493EE823E3A24FF6E6EEE910A6116D" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">General liability insurance claims deductible</link:label>
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    <link:loc xlink:href="pwr-20180930.xsd#pwr_DebtInstrumentIncreaseDecreaseInMaximumAdditionalInterestAndFeeRates" xlink:label="loc_pwr_DebtInstrumentIncreaseDecreaseInMaximumAdditionalInterestAndFeeRates_71A3AE1143FA309966E5EEE97BF131F4" xlink:type="locator" />
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    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_426183C4D66DDB93651DEEE97BF17C9D" xlink:type="locator" />
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    <link:label id="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_53CDDA896B8F3B2F2376EEE97BF23749_terseLabel_en-US" xlink:label="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_53CDDA896B8F3B2F2376EEE97BF23749" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Reduction in Quanta's funded indebtedness reduced by cash and cash equivalents in excess of this amount</link:label>
    <link:label id="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_53CDDA896B8F3B2F2376EEE97BF23749_label_en-US" xlink:label="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_53CDDA896B8F3B2F2376EEE97BF23749" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Reduction In Company Maximum Funded Debt And Maximum Senior Debt By All Cash And Cash Equivalents In Excess Of Amount</link:label>
    <link:label id="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_53CDDA896B8F3B2F2376EEE97BF23749_documentation_en-US" xlink:label="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_53CDDA896B8F3B2F2376EEE97BF23749" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Reduction in Quanta's maximum funded debt and maximum senior debt by all cash and cash equivalents in excess of amount.</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount" xlink:label="loc_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_53CDDA896B8F3B2F2376EEE97BF23749" xlink:type="locator" />
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    <link:label id="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_42C38ACF0D0166BB1460EEE97BF2D35F_terseLabel_en-US" xlink:label="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_42C38ACF0D0166BB1460EEE97BF2D35F" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Percentage of capital stock of direct foreign subsidiaries of wholly owned U.S. subsidiaries to secure credit agreement</link:label>
    <link:label id="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_42C38ACF0D0166BB1460EEE97BF2D35F_label_en-US" xlink:label="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_42C38ACF0D0166BB1460EEE97BF2D35F" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Percentage Of Capital Stock Of Direct Foreign Subsidiaries Required To Secure Credit Agreement</link:label>
    <link:label id="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_42C38ACF0D0166BB1460EEE97BF2D35F_documentation_en-US" xlink:label="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_42C38ACF0D0166BB1460EEE97BF2D35F" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Percentage of capital stock of direct foreign subsidiaries required to secure credit agreement.</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement" xlink:label="loc_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_42C38ACF0D0166BB1460EEE97BF2D35F" xlink:type="locator" />
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    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_BB20BF70B77BEDD78EC7EEE97BF76598_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_BB20BF70B77BEDD78EC7EEE97BF76598" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Maximum consolidated leverage ratio</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_BB20BF70B77BEDD78EC7EEE97BF76598_label_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_BB20BF70B77BEDD78EC7EEE97BF76598" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Leverage Ratio, Maximum</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_BB20BF70B77BEDD78EC7EEE97BF76598_documentation_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_BB20BF70B77BEDD78EC7EEE97BF76598" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Leverage Ratio, Maximum</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_DebtInstrumentCovenantLeverageRatioMaximum" xlink:label="loc_pwr_DebtInstrumentCovenantLeverageRatioMaximum_BB20BF70B77BEDD78EC7EEE97BF76598" xlink:type="locator" />
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    <link:label id="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_2618607797E7C56A1CA7EEE97BF7D890_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_2618607797E7C56A1CA7EEE97BF7D890" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Acquisition threshold for leverage ratio</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_2618607797E7C56A1CA7EEE97BF7D890_label_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_2618607797E7C56A1CA7EEE97BF7D890" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Acquisition Threshold For Leverage Ratio</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_2618607797E7C56A1CA7EEE97BF7D890_documentation_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_2618607797E7C56A1CA7EEE97BF7D890" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Acquisition Threshold For Leverage Ratio</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio" xlink:label="loc_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_2618607797E7C56A1CA7EEE97BF7D890" xlink:type="locator" />
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    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_B786AA54830250A5AA5CEEE97BF76495_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_B786AA54830250A5AA5CEEE97BF76495" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Maximum leverage ratio acquisition completed in current and two subsequent quarters</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_B786AA54830250A5AA5CEEE97BF76495_label_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_B786AA54830250A5AA5CEEE97BF76495" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Leverage Ratio Acquisition Completed In Current And Two Subsequent Quarters, Maximum</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_B786AA54830250A5AA5CEEE97BF76495_documentation_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_B786AA54830250A5AA5CEEE97BF76495" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Leverage Ratio Acquisition Completed In Current And Two Subsequent Quarters, Maximum</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum" xlink:label="loc_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_B786AA54830250A5AA5CEEE97BF76495" xlink:type="locator" />
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    <link:label id="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_B774325CA1FF706CB087EEE97BF7D3D9_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_B774325CA1FF706CB087EEE97BF7D3D9" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Minimum consolidated interest coverage ratio</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_B774325CA1FF706CB087EEE97BF7D3D9_label_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_B774325CA1FF706CB087EEE97BF7D3D9" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Interest Coverage Ratio, Minimum</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_B774325CA1FF706CB087EEE97BF7D3D9_documentation_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_B774325CA1FF706CB087EEE97BF7D3D9" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Interest Coverage Ratio, Minimum</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum" xlink:label="loc_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_B774325CA1FF706CB087EEE97BF7D3D9" xlink:type="locator" />
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    <link:label id="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_8DFD36AFA6CB56939CB8EEE97BF8BAE9_terseLabel_en-US" xlink:label="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_8DFD36AFA6CB56939CB8EEE97BF8BAE9" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Amount of availability under the credit agreement and/or cash and cash equivalents on hand that must be present to allow for cash payments of dividends and stock repurchases</link:label>
    <link:label id="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_8DFD36AFA6CB56939CB8EEE97BF8BAE9_label_en-US" xlink:label="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_8DFD36AFA6CB56939CB8EEE97BF8BAE9" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Availability Amount To Allow Dividends And Stock Repurchases</link:label>
    <link:label id="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_8DFD36AFA6CB56939CB8EEE97BF8BAE9_documentation_en-US" xlink:label="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_8DFD36AFA6CB56939CB8EEE97BF8BAE9" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Amount of availability under the credit agreement and/or cash and cash equivalents on hand that must be present to allow for cash payments of dividends and stock repurchases.</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases" xlink:label="loc_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_8DFD36AFA6CB56939CB8EEE97BF8BAE9" xlink:type="locator" />
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    <link:label id="lab_pwr_CrossDefaultProvisions_717407910AF13CBFEE5EEEE97BF87248_terseLabel_en-US" xlink:label="lab_pwr_CrossDefaultProvisions_717407910AF13CBFEE5EEEE97BF87248" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Cross default provisions with debt instruments exceeding this amount</link:label>
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    <link:label id="lab_pwr_CrossDefaultProvisions_717407910AF13CBFEE5EEEE97BF87248_documentation_en-US" xlink:label="lab_pwr_CrossDefaultProvisions_717407910AF13CBFEE5EEEE97BF87248" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Cross-default provisions.</link:label>
    <link:loc xlink:href="pwr-20180930.xsd#pwr_CrossDefaultProvisions" xlink:label="loc_pwr_CrossDefaultProvisions_717407910AF13CBFEE5EEEE97BF87248" xlink:type="locator" />
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    <link:label id="lab_us-gaap_ScheduleOfDebtTableTextBlock_D0785CB10E51C2712D02EEE97C78B414_label_en-US" xlink:label="lab_us-gaap_ScheduleOfDebtTableTextBlock_D0785CB10E51C2712D02EEE97C78B414" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Schedule of Debt [Table Text Block]</link:label>
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    <link:label id="lab_us-gaap_AssetsAbstract_3659432CF5E96F19B5EAEEE97E354F12_label_en-US" xlink:label="lab_us-gaap_AssetsAbstract_3659432CF5E96F19B5EAEEE97E354F12" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Assets [Abstract]</link:label>
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    <link:label id="lab_us-gaap_AssetsCurrentAbstract_DB28F7607360C5B68194EEE97E3514C0_terseLabel_en-US" xlink:label="lab_us-gaap_AssetsCurrentAbstract_DB28F7607360C5B68194EEE97E3514C0" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Current Assets:</link:label>
    <link:label id="lab_us-gaap_AssetsCurrentAbstract_DB28F7607360C5B68194EEE97E3514C0_label_en-US" xlink:label="lab_us-gaap_AssetsCurrentAbstract_DB28F7607360C5B68194EEE97E3514C0" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Assets, Current [Abstract]</link:label>
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    <link:label id="lab_us-gaap_AccountsReceivableNetCurrent_27D7A6261AEE61E74BA4EEE97E36066C_terseLabel_en-US" xlink:label="lab_us-gaap_AccountsReceivableNetCurrent_27D7A6261AEE61E74BA4EEE97E36066C" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Accounts receivable, net of allowances of $6,662 and $4,465</link:label>
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