And we expect the call to last about 60 minutes.
At this time last year, during our 2020 third quarter call, we laid out a long-term goal of our pathway to achieving annual revenues of $10 billion plus.
It's important to remember, at that time, MasTec was on a path to generate just over $6 billion of revenue in 2020.
Fast forward 12 months, this year, we expect to generate $8 billion in revenue.
And our long-term goal of reaching annual revenues exceeding $10 billion is now within reach in what we hope will be a much shorter time frame.
Since becoming CEO in 2007, we've been able to grow MasTec from $900 million in revenue to $8 billion today.
Revenue for the quarter was $2.404 billion.
Adjusted EBITDA was $278 million.
Adjusted earnings per share was $1.81.
And backlog at quarter end was $8.5 billion, a year-over-year increase of $821 million.
Our Communications revenue for the quarter was $670 million.
The second quarter of this year represented the largest quarterly sequential segment backlog increase in the company's history, and in the third quarter, we were again able to sequentially grow segment backlog by over $200 million.
Margins for the segment were 10.7% in the third quarter; and were impacted by both lower wireless revenues than expected, along with project closeouts related to a large fiber build that is nearing completion.
Over the last few quarters, we've talked about the opportunities related to the Rural Digital Opportunity Fund or RDOF, which will provide $20 billion of funding over the next 10 years to build and connect gigabit broadband speeds in underserved rural areas; and the 5G Fund for Rural America, which will provide up to $9 billion in funding over the next decade to bring 5G wireless broadband connectivity to rural America.
In addition to these programs, the current pending infrastructure bill has another $65 billion allocated for broadband infrastructure.
Revenue was $365 million versus $129 million in last year's second (sic) third quarter.
The increase was driven by organic growth of nearly 50% in the quarter on a year-over-year basis; and the first full-quarter contribution of INTREN, which we acquired during the second quarter.
Margins for the segment were 9.5%, which exceeded our expectations.
Revenue was $858 million and margins remained strong.
As a reminder: Last year, we forecasted a longer-term recurring revenue target of $1.5 billion to $2 billion a year, assuming a continued depressed oil and gas market.
Pipe materials often account for nearly 50% of project costs.
Revenue was $518 million for the third quarter.
Backlog at quarter end in Clean Energy was $1.570 billion versus $891 million at the end of last year's third quarter, a year-over-year increase of nearly $700 million and a slight sequential reduction of over $100 million from the second quarter.
Since quarter end, we've either signed or been verbally awarded another roughly $800 million in projects.
In summary, we had strong third quarter results with revenue of approximately $2.4 billion, a 42% increase over last year; adjusted EBITDA of approximately $278 million; and adjusted EBITDA margin rate at 11.6% of revenue.
Yesterday, we announced a new and increased credit facility of $2 billion, which adds to our ample liquidity, improves pricing and eliminates security requirements.
Our strong cash earnings profile, coupled with our focus on working capital management during 2021, has allowed us to easily fund organic working capital needs associated with approximately $1.5 billion in year-to-date revenue growth while investing approximately $600 million in strategic acquisitions.
At the end of our third quarter, we maintained a strong balance sheet and capital structure with liquidity approximating $1.3 billion, comfortable leverage metrics and with net debt at only 1.3 times adjusted EBITDA at quarter end.
Third quarter Communications revenue was $670 million, approximately 4% growth compared to last year.
Our third quarter Communications segment adjusted EBITDA margin rate was 10.7% of revenue, an 80 basis point sequential decline primarily related to the overhead impacts of lower-than-expected third quarter revenue levels.
Our annual 2021 Communications segment expectation is that revenue will range somewhere between $2.5 billion to $2.6 billion, with annual 2021 adjusted EBITDA margin rate approximating 11%.
Third quarter Clean Energy and Infrastructure segment or clean energy revenue was $518 million.
And adjusted EBITDA was approximately $14 million or 2.7% of revenue, below our expectation.
At one point during this project, approximately 1/3 of the project field crew and 50% of critical path electricians were either infected with COVID or in quarantine, effectively stopping project production.
We believe the issues that have negatively impacted our clean energy segment year-to-date adjusted EBITDA margin performance are largely behind us and, based on project timing, expect that fourth quarter segment revenue will be the largest revenue quarter of the year with over 60% year-over-year revenue growth and strong fourth quarter adjusted EBITDA margin rate improvement to a high single-digit level.
As we have previously indicated, our clean energy segment has grown from $300 million of revenue in 2017 and will approach $2 billion in revenue during 2021.
Third quarter Oil and Gas segment revenue was $858 million, and adjusted EBITDA was $171 million.
This increased our third quarter revenue by approximately $100 million, accelerating revenue previously expected to occur in the fourth quarter.
We currently expect annual 2021 Oil and Gas segment revenue will range between $2.5 billion to $2.6 billion, with annual 2021 adjusted EBITDA margin rate for this segment expected in the high-teens to low-20% range.
Third quarter Electrical Transmission segment revenue was $365 million, and adjusted EBITDA margin rate of 9.5% of revenue.
Third quarter results reflected a full quarter of electrical distribution and storm services from INTREN, which contributed revenue of approximately $175 million to the quarter.
Excluding INTREN, organic segment revenue during the third quarter grew $64 million and adjusted EBITDA margin performance was strong.
We expect annual 2021 revenue for the Electrical Transmission segment to approximate $1 billion and annual 2021 adjusted EBITDA margin rate to range somewhere between 6.5% to 7% of revenue.
This expectation includes the assumption that second half of 2021 segment adjusted EBITDA margin rate will approximate a low-8% range, a significant improvement when compared to first half 2021 performance.
Now I will discuss a summary of our top 10 largest customers for the third quarter period as a percentage of revenue.
Enbridge was 21% of revenue, reflecting the previously mentioned pipeline project acceleration.
Newly defined AT&T services totaled 7% of revenue.
NextEra Energy was 6% of revenue, comprising services across multiple segments including clean energy, Communications and Electrical Transmission.
Equitrans Midstream was 5%.
Entergy and Comcast were each 4% of revenue.
Duke Energy, DIRECTV and Exelon reached 3%; and Enel Green Power was 2%.
Individual construction projects comprised 63% of our third quarter revenue, with master service agreements comprising 37%.
As of September 30, 2021, we had total backlog of approximately $8.5 billion, up approximately $821 million when compared to last year.
And we continue with the expectation that 2022 segment revenue will range somewhere between $1.5 billion to $2 billion, with potential sizable growth opportunities in 2023 and beyond.
As I mentioned earlier in these remarks, yesterday, we announced closing of a new unsecured $2 billion credit facility, which reflects a $250 million increase from our prior facility with improved pricing and extended term.
During the third quarter, we managed to reduce our net debt levels by approximately $80 million despite the working capital associated with approximately $450 million in sequential revenue growth.
We ended the quarter with $1.3 billion in liquidity; and net debt, define as total debt less cash and cash equivalents, at $1.26 billion, which equates to a very comfortable 1.3 times leverage metric.
2021 year-to-date cash provided by operating activities was approximately $500 million.
We ended the third quarter with DSOs at 72 days compared to 85 days in Q3 last year.
Assuming no Q4 acquisition activity, net debt at year-end is expected to approximate $1.2 billion, leaving us with ample liquidity and an expected book leverage ratio slightly over one times adjusted EBITDA.
We predict an annual 2021 revenue of $8 billion, with adjusted EBITDA of $930 million or 11.6% of revenue; and adjusted diluted earnings of $5.55 per adjusted diluted share, which is a $0.10 per share increase over our prior expectation of $5.45 per adjusted diluted share.
This translates into a fourth quarter revenue expectation of $1.85 billion, with adjusted EBITDA of $218 million or 11.7% of revenue; and earnings guidance of $1.33 per adjusted diluted share.
As previously mentioned, our fourth quarter revenue view includes approximately $100 million in lower revenue expectations for the Oil and Gas segment due to the acceleration of project revenue during the third quarter.
We anticipate net cash capex spending in 2021 at approximately $120 million, with an additional $160 million to $180 million to be incurred under finance leases.
We expect annual 2021 interest expense levels to approximate $54 million, with this level including approximately $600 million in year-to-date acquisition funding activity.
For modeling purposes: Our estimate for 2021 share count continues at 74 million shares.
We expect annual 2021 depreciation expense to approximate 4.3% of revenue, inclusive of year-to-date 2021 acquisition activity.
We expect annual 2021 corporate segment adjusted EBITDA to be a net cost of slightly under 1% of overall revenue.
And lastly, we expect that annual 2021 adjusted income tax rate will range approximately 22%, with our third and fourth quarter adjusted income tax rates ranging in the 19% to 20% range primarily due to the benefits of income mix and tax true-up adjustments.

Summary:
In summary, we had strong third quarter results with revenue of approximately $2.4 billion, a 42% increase over last year; adjusted EBITDA of approximately $278 million; and adjusted EBITDA margin rate at 11.6% of revenue.
This translates into a fourth quarter revenue expectation of $1.85 billion, with adjusted EBITDA of $218 million or 11.7% of revenue; and earnings guidance of $1.33 per adjusted diluted share.