JOINDER AND THIRD AMENDMENT TO UNDERWRITING, CONTINUING INDEMNITY ANDSECURITY AGREEMENT

Exhibit 10.30

JOINDER AND THIRD AMENDMENT TO UNDERWRITING, CONTINUING

INDEMNITY AND SECURITY AGREEMENT

THIS JOINDER AND THIRD AMENDMENT TO UNDERWRITING, CONTINUING INDEMNITY AND SECURITY AGREEMENT (this “Third Amendment”) made as of the 19th day of December, 2008, by and among QUANTA SERVICES, INC., a Delaware corporation, and certain of its Affiliates and Subsidiaries identified on Exhibit A to this Third Amendment (individually and collectively, in their capacity as a named principal under any Bond, “Principal” and individually and collectively “Indemnitors”); FEDERAL INSURANCE COMPANY, an Indiana corporation, AMERICAN HOME ASSURANCE COMPANY, NATIONAL UNION FIRE INSURANCE COMPANY OF PITTSBURGH, PA., and THE INSURANCE COMPANY OF THE STATE OF PENNSYLVANIA (individually and collectively “Surety”).

W I T N E S S E T H:

WHEREAS, Surety and certain Indemnitors entered into that certain Underwriting, Continuing Indemnity and Security Agreement dated March 14, 2005 (the “Original Agreement”), as amended and modified by that certain Joinder Agreement and Amendment to Underwriting, Continuing Indemnity and Security Agreement dated November 28, 2006 (the “First Amendment”) and that certain Second Amendment to Underwriting, Continuing Indemnity and Security Agreement dated January 9, 2008 (the “Second Amendment”); and

WHEREAS, the terms of the Original Agreement, as amended and modified by the First Amendment and the Second Amendment (as so amended and modified, the “Agreement”), are further amended and modified as set forth in this Third Amendment; and

WHEREAS, the parties desire to add certain additional Domestic Subsidiaries of Quanta Services, Inc. as Principals and Indemnitors under the Agreement, as amended by this Third Amendment;

NOW, THEREFORE, in consideration of the foregoing premises, and other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto agree as follows:

1. Definitions. All capitalized terms used in this Third Amendment (including the recitals hereto) will have the respective meanings assigned thereto in the Agreement, unless otherwise specifically defined in this Third Amendment.

2. Amendments.

(a) The definition of “Bonded Contract” in the Agreement is hereby modified to delete “and/or Sunesys, LLC” immediately after “Island Mechanical, Hawaii” and the proviso to Section 1(b) of the Second Amendment is hereby deleted in its entirety.

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(b) The definition of “Bonds” in the Agreement is hereby modified to delete “and/or Sunesys, LLC” immediately after “Island Mechanical, Hawaii” and the proviso to Section 1(c) of the Second Amendment is hereby deleted in its entirety.

(c) Clauses (c) and (f) of the definition of “Event of Default” in the Agreement are hereby modified to delete “and/or Sunesys, LLC” immediately after “Island Mechanical, Hawaii” wherever such term appears in each such clause.

(d) The definition of “Retainage” in the Agreement is hereby modified to delete “and/or Sunesys, LLC” immediately after “Island Mechanical, Hawaii” wherever such term appears in such definition and the proviso to Section 1(e) of the Second Amendment is hereby deleted in its entirety.

(e) The definition of “Surety Loss” in the Agreement is hereby modified to delete “and/or Sunesys, LLC” immediately after “Island Mechanical, Hawaii” wherever such term appears in such definition.

(f) The definition of “Work” in the Agreement is hereby modified to delete (i) “and/or Sunesys, LLC” immediately after “Island Mechanical, Hawaii” wherever such term appears in such definition and (ii) “and/or Sunesys, LLC’s” immediately after “Island Mechanical, Hawaii’s” where such term appears in such definition.

3. Exhibit A. Exhibit A to the Agreement is hereby deleted in its entirety and replaced with Exhibit A to this Third Amendment.

4. Warranties and Covenants of Indemnitors. Each of Pauley Construction Inc., an Arizona corporation, Sunesys, LLC, a Delaware limited liability company, and Winco, Inc., an Oregon corporation (each being a “New Indemnitor”) represents and warrants to Surety that all of the representations and warranties made by the Indemnitors (as defined in the Agreement) in the Original Agreement (whether made as an Indemnitor (as defined in the Agreement) or as a Principal (as defined in the Agreement)) are true and correct as applicable to such New Indemnitor in all material respects, as of the date hereof (except to the extent such representations and warranties specifically relate to an earlier date). Each New Indemnitor hereby acknowledges, agrees, and confirms that, by its execution of this Third Amendment, such New Indemnitor will be deemed to be a party to the Agreement, as amended by this Third Amendment, and an “Indemnitor” (as defined in the Agreement) and “Principal” (as defined in the Agreement) for all purposes of the Agreement, as amended by this Third Amendment, and shall have all the obligations of an Indemnitor (as defined in the Agreement) and Principal (as defined in the Agreement) thereunder as if it had executed the Agreement. Each New Indemnitor hereby ratifies, as of the date hereof, and agrees to be bound by, all of the terms, provisions, and conditions contained in the Agreement, as amended by this Third Amendment, applicable to such New Indemnitor (whether as an Indemnitor (as defined in the Agreement) or Principal (as defined in the Agreement)). Without limiting the generality of the foregoing terms of this Section 4, each New Indemnitor hereby grants to the Surety a security interest in any and all right, title and interest of such New Indemnitor in and to the Collateral of such New Indemnitor to secure the prompt payment and performance in full when due of any Surety Loss,

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and the payment and performance of all other obligations and undertakings now or hereafter owing to Surety with respect to the Bonds and/or under the Surety Credit Documents, as same may now or hereafter be modified, replaced, extended or renewed.

5. Due Diligence Items Required to be Delivered by New Indemnitors. Each New Indemnitor will deliver to Surety the following, in form and substance reasonably satisfactory to Surety and its counsel:

(a) Favorable opinions of both outside and in-house counsel to Principal and Indemnitors, with respect to the New Indemnitors, substantially in the form attached to the Original Agreement as Exhibit C thereto, with such modifications thereto as are requested by such counsel and acceptable to Surety and its counsel in their reasonable discretion;

(b) an officer’s certificate of such New Indemnitor certifying appropriate resolutions authorizing the execution, delivery, and performance of this Third Amendment and performance of the Agreement, as amended by this Third Amendment, certifying that such resolutions have been approved in accordance with such New Indemnitor’s governing documents together with copies of such governing documents, and certifying incumbencies and true signatures of the officers so authorized; and

(c) evidence of the good standing of such New Indemnitor in the jurisdiction in which such New Indemnitor is formed.

6. Power of Attorney. Each New Indemnitor hereby irrevocably constitutes and appoints Quanta Services, Inc. (and all officers, employees, or agents designated by Quanta Services, Inc.), with full power of substitution, as such New Indemnitor’s true and lawful attorney-in-fact with full irrevocable power and authority in the place and stead of such New Indemnitor and in the name of such New Indemnitor or in its own name, from time to time in Quanta Services, Inc.’s discretion, to take any and all appropriate action and to execute and deliver any and all documents and instruments which may be necessary or desirable to accomplish the purpose of this Third Amendment or the Agreement and to amend, modify or supplement the Agreement or other Surety Credit Documents in any manner. Each New Indemnitor hereby ratifies and agrees to be bound by, to the fullest extent permitted by law, all that Quanta Services, Inc. will lawfully do or cause to be done by virtue hereof.

7. Miscellaneous.

(a) Upon the effectiveness of this Third Amendment, each reference in the Agreement to “this Agreement,” “hereunder” or words of like import shall mean and be a reference to the Agreement, as affected and amended by this Third Amendment (except to the extent such reference specifically relates to an earlier date). The foregoing is not intended to otherwise affect the definition of “Agreement” or “this Agreement.”

(b) This Third Amendment shall be governed by and construed in accordance with the internal laws of the State of New York.

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(c) Section headings in this Third Amendment are included herein for convenience of reference only and shall not constitute a part of this Third Amendment for any other purpose.

8. Binding Effect. By executing this Third Amendment, each New Indemnitor will be deemed to be an Indemnitor (as defined in the Agreement) under the terms of the Agreement, as amended hereby, as though such New Indemnitor were an original signatory thereto and such New Indemnitor hereby confirms its grant of a security interest in the Collateral to Surety as provided in Section 5 of the Agreement.

9. Continuing Effect. Except as specifically set forth in this Third Amendment, the Agreement remains in full force and effect in accordance with its terms.

10. Counterparts. This Third Amendment may be executed by the parties independently in any number of counterparts, all of which together will constitute but one and the same instrument which is valid and effective as if all parties had executed the same counterpart. This Third Amendment may be validly executed and delivered by facsimile or other electronic transmission. Notwithstanding the foregoing, the parties are required to deliver original signature pages to each other.

IN WITNESS WHEREOF, each of the parties has caused this Third Amendment to be executed by its duly authorized officer as of the day and year first above written.

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LIST OF PRINCIPAL/INDEMNITORS

EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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EXHIBIT A

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