On Thursday, the company said that fire lines were holding around the transmission lines and that it was more hopeful. Soot and ash also posed a danger to electrical equipment.

In Las Cruces, N.M., which gets all of its power from El Paso Electric, officials were bracing for the possibility of blackouts. With temperatures reaching the 90s, city officials were preparing to set up cooling stations complete with generators, fans and portable air-conditioners.

Despite the work of more than 3,000 firefighters and the expenditure of an estimated $15 million, the blaze, known as the Wallow Fire, was zero percent contained on Thursday morning, meaning that it continued to have the potential to extend in any direction. Across the New Mexico line, residents eyed the fire bearing down on them warily.

Nadine Handy, who owns two assisted living centers and six child-care centers in Las Cruces, said that with the possibility of rolling blackouts, staff members at the facilities have been told to ready generators and emergency lights.

“I feel like we’re doing everything we can,” she said. “We’re even doing a little hoping. Hoping that this fire will go out.”

Jeanne Lambert, who lives in the tiny community of Quemado, about 40 miles from the border, has been getting up at 2 or 3 in the morning to serve breakfast to firefighters at the Largo Cafe. Ms. Lambert, 71, said she had seen a lot of fires come through in nearly half a century of living there, but nothing like this one.