Fifteenth Supplemental Indenture

Exhibit 4.1

SPECTRA ENERGY CAPITAL, LLC,

AS ISSUER,

SPECTRA ENERGY CORP,

AS GUARANTOR,

AND

THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A.

AS TRUSTEE

Fifteenth Supplemental Indenture

Dated as of August 28, 2009

$300,000,000 5.65% Senior Notes due 2020

TABLE OF CONTENTS ¹

Exhibit A – Form of Note

Exhibit B – Certificate of Authentication

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THIS FIFTEENTH SUPPLEMENTAL INDENTURE is made as of the 28th day of August, 2009, by and among Spectra Energy Capital, LLC, a Delaware limited liability company (formerly known as Duke Capital LLC and successor to Duke Capital Corporation), as issuer (“Spectra Capital”), Spectra Energy Corp, a Delaware corporation, as guarantor (“Spectra Energy”), and The Bank of New York Mellon Trust Company, N.A. (formerly known as The Bank of New York Trust Company, N.A.) (successor to JPMorgan Chase Bank, N.A. (formerly known as The Chase Manhattan Bank)), a national banking association, as trustee (the “Trustee”).

W I T N E S S E T H:

WHEREAS, Spectra Capital has heretofore entered into a Senior Indenture, dated as of April 1, 1998, with the Trustee, as amended and supplemented to the date hereof with applicability to the Notes (as defined below), including by the Twelfth Supplemental Indenture dated December 14, 2007 (the “Twelfth Supplemental Indenture”), whereby Spectra Energy entered into a Guarantee for the benefit of the Holders and the Trustee (as so amended and supplemented, the “Original Indenture”);

WHEREAS, the Original Indenture is incorporated herein by this reference and the Original Indenture, as amended and supplemented to the date hereof with applicability to the Notes, including by this Fifteenth Supplemental Indenture, is herein called the “Indenture”;

WHEREAS, under the Indenture, a new series of Securities may at any time be established in accordance with the provisions of the Indenture and the terms of such series may be described by a supplemental indenture executed by the parties hereto;

WHEREAS, Spectra Capital hereby proposes to create under the Indenture a new series of Securities, and Spectra Energy hereby proposes to Guarantee such Securities in accordance with the terms of the Indenture;

WHEREAS, additional Securities of other series hereafter established, except as may be limited in the Indenture as at the time supplemented and modified, may be issued from time to time pursuant to the Indenture as at the time supplemented and modified; and

WHEREAS, all conditions necessary to authorize the execution and delivery of this Fifteenth Supplemental Indenture and to make it a valid and binding obligation of Spectra Capital and Spectra Energy have been done or performed.

NOW, THEREFORE, in consideration of the agreements and obligations set forth herein and for other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, the parties hereto hereby agree as follows:

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ARTICLE 1

NOTES

SECTION 1.01 Establishment.

There is hereby established a new series of Securities to be issued under the Indenture, to be entitled the “5.65% Senior Notes due 2020” (the “Notes”). There is to be authenticated and delivered $300,000,000 aggregate principal amount of Notes, and no further Notes shall be authenticated and delivered except as provided by Section 304, 305, 306, 906 or 1106 or the last paragraph of Section 301 of the Original Indenture; provided that any additional Notes authenticated and delivered as provided in the last paragraph of Section 301 of the Original Indenture shall be fungible with the original Notes for United States federal income tax purposes. The Notes shall be issued in fully registered form without coupons.

The Notes, together with the endorsement of the Guarantee thereon, shall be in substantially the form set forth in Exhibit A hereto, and the form of the Trustee’s Certificate of Authentication for the Notes shall be in substantially the form set forth in Exhibit B hereto.

The Notes shall be dated the date of authentication thereof.

SECTION 1.02 Definitions.

The following defined terms used herein with respect to the Notes shall, unless the context otherwise requires, have the meanings specified below. Capitalized terms used herein for which no definition is provided herein shall have the meanings set forth in the Original Indenture.

“Interest Payment Dates” means each March 1 and September 1, commencing March 1, 2010.

“Original Issue Date” means August 28, 2009.

“Regular Record Date” means, with respect to each Interest Payment Date, the close of business on the February 15 and August 15, as applicable, immediately preceding such Interest Payment Date (whether or not a Business Day).

“Stated Maturity” means March 1, 2020.

SECTION 1.03 Payment of Principal and Interest.

The principal of the Notes shall be due at the Stated Maturity (unless earlier redeemed). The unpaid principal amount of the Notes shall bear interest at the rate of 5.65% per annum until paid or duly provided for, and such interest will accrue from the Original Issue Date or from the most recent Interest Payment Date to which interest has been paid or duly provided for. Interest shall be paid semi-annually in arrears on each Interest Payment Date to the Person or Persons in whose name the Notes are registered on the Regular Record Date for such Interest Payment Date, even if such Notes are canceled after such Regular Record Date and on or before such Interest Payment Date. Any such interest that is not so punctually paid or duly provided for shall

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forthwith cease to be payable to the Holders on such Regular Record Date and may either be paid to the Person or Persons in whose name the Notes are registered at the close of business on a Special Record Date for the payment of such defaulted interest to be fixed by the Trustee (“Special Record Date”), notice whereof shall be given to Holders of the Notes not less than ten days prior to such Special Record Date, or be paid at any time in any other lawful manner not inconsistent with the requirements of any securities exchange on which the Notes may be listed, and upon such notice as may be required by any such exchange, all as more fully provided in the Original Indenture.

Payments of interest on the Notes shall include interest accrued to but excluding the respective Interest Payment Dates. Interest payments for the Notes shall be computed and paid on the basis of a 360-day year of twelve 30-day months. In the event that any date on which interest is payable on the Notes is not a Business Day, then payment of the interest payable on such date shall be made on the next succeeding day that is a Business Day (and without any interest or payment in respect of any such delay) with the same force and effect as if made on the date the payment was originally payable. “Business Day” means a day other than (i) a Saturday or a Sunday, (ii) a day on which banking institutions in The City of New York are authorized or obligated by law or executive order to remain closed or (iii) a day on which the Corporate Trust Office is closed for business.

Payment of principal of, premium, if any, and interest on the Notes shall be made in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts. Payments of principal of, premium, if any, and interest on Notes represented by a Global Security (as defined below) shall be made by wire transfer of immediately available funds to the Holder of such Global Security; provided that, in the case of payments of principal and premium, if any, such Global Security is first surrendered to the Paying Agent. If any of the Notes are no longer represented by a Global Security, payments of principal, premium, if any, and interest on such Notes shall be made at the office of the Paying Agent maintained for such purpose; provided that (i) in the case of payments of principal and premium, if any, such Notes are first surrendered to the Paying Agent; and (ii) payments of interest may be made, at the option of Spectra Capital, (A) by check mailed to the address of the Person entitled thereto as such address shall appear in the Security Register or (B) by wire transfer at such place and to such account at a banking institution in the United States as may be designated in writing to the Trustee at least 16 days prior to the date for payment by the Person entitled thereto.

SECTION 1.04 Denominations.

The Notes shall be issued in denominations of $2,000 or any integral multiples of $1,000 above such amount.

SECTION 1.05 Global Securities.

The Notes shall initially be issued in the form of one or more Global Securities registered in the name of the Depositary (which initially shall be The Depository Trust Company) or its nominee (the “Global Securities”). Except under the limited circumstances described below, Notes represented by such Global Security or Global Securities shall not be exchangeable for,

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and shall not otherwise be issuable as, Notes in definitive form. The Global Securities described above may not be transferred except by the Depositary to a nominee of the Depositary or by a nominee of the Depositary to the Depositary or another nominee of the Depositary or by the Depository or any such nominee to a successor Depositary or its nominee.

A Global Security shall be exchangeable for Notes registered in the names of persons other than the Depositary or its nominee only if (i) the Depositary notifies Spectra Capital that it is unwilling or unable to continue as a Depositary for such Global Security and no successor Depositary shall have been appointed by Spectra Capital within 90 days of receipt by Spectra Capital of such notification, or if at any time the Depositary ceases to be a clearing agency registered under the Exchange Act at a time when the Depositary is required to be so registered to act as such Depositary and no successor Depositary shall have been appointed by Spectra Capital within 90 days after it becomes aware of such cessation or (ii) subject to the Depositary’s procedures, Spectra Capital in its sole discretion determines that such Global Security shall be so exchangeable. Any Global Security that is exchangeable pursuant to the preceding sentence shall be exchangeable for Notes registered in such names as the Depositary shall direct.

SECTION 1.06 Guarantee of the Notes.

The Guarantor hereby agrees that the Guarantee set forth in Section 102 of the Twelfth Supplemental Indenture shall apply with respect to the Notes as if the Notes were listed on Schedule A to the Twelfth Supplemental Indenture and constituted “Notes” thereunder. The Guarantor agrees that Sections 103 and 106 of the Twelfth Supplemental Indenture shall be for the benefit of the Notes.

SECTION 1.07 Redemption at the Option of Spectra Capital.

The Notes shall be redeemable, in whole at any time or in part from time to time, at the option of Spectra Capital on any date (a “Redemption Date”), at a Redemption Price equal to the greater of (i) 100% of the principal amount of the Notes to be redeemed and (ii) the sum of the present values of the Remaining Scheduled Payments thereon, discounted to such Redemption Date on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate plus 35 basis points, plus accrued and unpaid interest on the principal amount being redeemed to such Redemption Date.

“Treasury Rate” means, with respect to any Redemption Date, the rate per annum equal to the semi-annual equivalent yield to maturity of the Comparable Treasury Issue, calculated using a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such Redemption Date. The Treasury Rate shall be calculated on the third Business Day preceding such Redemption Date.

“Comparable Treasury Issue” means the United States Treasury security selected by an Independent Investment Banker as having a maturity comparable to the remaining term of the Notes to be redeemed that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of comparable maturity to the remaining term of such Notes.

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“Comparable Treasury Price” means, with respect to any Redemption Date, (i) the average of the Reference Treasury Dealer Quotations for such Redemption Date, after excluding the highest and lowest of such Reference Treasury Dealer Quotations, or (ii) if an Independent Investment Banker obtains fewer than four such Reference Treasury Dealer Quotations, the average of all such quotations.

“Independent Investment Banker” means one of the Reference Treasury Dealers appointed by Spectra Capital.

“Reference Treasury Dealer” means each of RBS Securities Inc. (or its affiliates and its successors), Credit Suisse Securities (USA) LLC (or its affiliates and its successors), UBS Securities LLC (or its affiliates and its successors) and one other nationally recognized investment banking firm that is a primary U.S. government securities dealer (a “Primary Treasury Dealer”) specified from time to time by Spectra Capital; provided, however , that if any of the foregoing shall cease to be a nationally recognized investment banking firm that is a Primary Treasury Dealer, Spectra Capital shall substitute therefor another nationally recognized investment banking firm that is such a dealer.

“Reference Treasury Dealer Quotations” means, with respect to each Reference Treasury Dealer and any Redemption Date, the average, as determined by an Independent Investment Banker, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to an Independent Investment Banker by such Reference Treasury Dealer as of 3:30 p.m., New York City time, on the third Business Day preceding such Redemption Date.

“Remaining Scheduled Payments” means, with respect to each Note to be redeemed, the remaining scheduled payments of the principal thereof and interest thereon that would be due after the related Redemption Date but for such redemption; provided, however , that, if such Redemption Date is not an Interest Payment Date with respect to such Note, the amount of the next succeeding scheduled interest payment thereon will be reduced (solely for the purpose of the definition of “Remaining Scheduled Payments”) by the amount of interest accrued thereon to such Redemption Date.

Notwithstanding Section 1104 of the Original Indenture, the notice of redemption with respect to the foregoing redemption need not set forth the Redemption Price but only the manner of ascertainment thereof.

In the event that any Redemption Date is not a Business Day, then payment of the Redemption Price shall be made on the next succeeding day that is a Business Day (and without any interest or payment in respect of any such delay) with the same force and effect as if made on such Redemption Date.

Spectra Capital shall notify the Trustee of the Redemption Price with respect to the foregoing redemption promptly after the calculation thereof. The Trustee shall not be responsible for calculating said Redemption Price.

If less than all of the Notes are to be redeemed, the Trustee shall select the Notes or portions of Notes to be redeemed by such method as the Trustee shall deem fair and appropriate. The Trustee may select for redemption Notes and portions of Notes in amounts of whole multiples of $1,000.

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The Notes shall not have a sinking fund.

SECTION 1.08 Defeasance.

In addition to the conditions set forth in Section 1304 of the Original Indenture, the application of Section 1302 or 1303 of the Original Indenture with respect to the Notes shall be subject to the condition that (a) in the event of Covenant Defeasance of the Notes pursuant to Section 1303 of the Original Indenture, Spectra Capital shall have delivered to the Trustee an Opinion of Counsel from a nationally recognized counsel acceptable to the Trustee or a private letter ruling issued by the United States Internal Revenue Service to the effect that the Holders of the Notes will not recognize income, gain or loss for United States Federal income tax purposes as a result of Spectra Capital’s exercise of its option under Section 1303 of the Original Indenture and will be subject to United States Federal income tax on the same amount and in the same manner and at the same times as would have been the case if such option had not been exercised; and (b) in the event of Defeasance of the Notes pursuant to Section 1302 of the Original Indenture, Spectra Capital shall have delivered to the Trustee an Opinion of Counsel from a nationally recognized counsel acceptable to the Trustee to the effect referred to in clause (a) with respect to such Defeasance, which opinion is based on (i) a private letter ruling issued by the United States Internal Revenue Service addressed to Spectra Capital, (ii) a published ruling of the United States Internal Revenue Service pertaining to a comparable form of transaction or (iii) a change in the applicable United States Federal income tax law (including regulations) after the date hereof.

SECTION 1.09 Modification of Guarantee.

Clause (1) of Section 902 of the Original Indenture is hereby amended and restated in relation solely to the Notes to read as follows:

“(1) change the Stated Maturity of the principal of, or any installment of principal of or interest on, any Security, or reduce the principal amount thereof or the rate of interest thereon or any premium payable upon the redemption thereof, or reduce the amount of the principal of an Original Issue Discount Security or any other Security which would be due and payable upon a declaration of acceleration of the Maturity thereof pursuant to Section 502 or change the coin or currency in which any Security or any premium or interest thereon is payable, or impair the right to institute suit for the enforcement of any such payment on or after the Stated Maturity thereof (or, in the case of redemption, on or after the Redemption Date), or change the obligations of Spectra Energy under the Guarantee in any manner adverse to the Holders, or”.

SECTION 1.10 Amendment to Section 101 of the Original Indenture.

The definition of “Subsidiary” in Section 101 of the Original Indenture is hereby amended and restated in relation solely to the Notes to read as follows:

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““Subsidiary” means, as to any Person, a corporation or other entity of which more than 50% of the outstanding shares of capital stock or other ownership interests having ordinary voting power (other than capital stock or other ownership interests having such power only by reason of contingency) is at the time owned, directly or indirectly through one or more intermediaries, or both, by such Person.”

SECTION 1.11 Paying Agent.

The Trustee shall initially serve as Paying Agent with respect to the Notes, with the Place of Payment initially being the office of The Bank of New York Mellon, 101 Barclay Street, New York, New York.

ARTICLE 2

MISCELLANEOUS PROVISIONS

SECTION 2.01 Recitals.

The recitals in this Fifteenth Supplemental Indenture are made by Spectra Capital and Spectra Energy only and not by the Trustee, and the Trustee assumes no responsibility for their correctness. The Trustee makes no representations as to the validity or sufficiency of this Fifteenth Supplemental Indenture. All of the provisions contained in the Original Indenture in respect of the rights, privileges, immunities, powers and duties of the Trustee shall be applicable in respect of the Notes and of this Fifteenth Supplemental Indenture as fully and with like effect as if set forth herein in full.

SECTION 2.02 Ratification and Incorporation of Original Indenture.

As supplemented hereby, the Original Indenture is in all respects ratified and confirmed, and the Original Indenture and this Fifteenth Supplemental Indenture shall be read, taken and construed as one and the same instrument.

SECTION 2.03 Executed in Counterparts.

This Fifteenth Supplemental Indenture may be executed in several counterparts, each of which shall be deemed to be an original, and such counterparts shall together constitute but one and the same instrument.

SECTION 2.04 Governing Law.

This Fifteenth Supplemental Indenture, the Notes, the Guarantee