A fire at a power substation in east Mesa set off a cascade of electrical failures Thursday that left about 150,000 homes and businesses without power on a sweltering summer afternoon.

Power has been restored for all customers, SRP spokesman Scott Harelson said Friday morning. The last customers in Mesa without power had their service restored at about 11:30 p.m. Thursday, he said.

SRP does not expect asking any of its customers to conserve electricity Friday, Harelson said.

Thursday's problems began with a fire in an electrical transformer. Though the fire was doused, the extra load placed on other stations led to failures throughout the afternoon. There is still no answer as to what caused the fire, Harelson said. There will be an investigation, but it could still be weeks until a cause is declared, he said.

Blackouts hit nearly every area of Mesa and Apache Junction at various times, forcing hospitals to use emergency generators and prompting companies to send thousands of workers home early.

Business took a hit as stores and theaters closed, and staffers at nursing homes worried about their most vulnerable residents as the Valley's official high reached 105 degrees.

SRP officials estimated that 150,000 homes and businesses lost power at one time or another throughout the day. As of 10 p.m., there were still an estimated 22,500 customers without power.

City officials opened the Mesa Convention Center as an emergency shelter Thursday evening for anyone who needed to escape the heat.

Doug Noble, 84, a retired Maricopa County sheriff's deputy, has heart trouble and grew concerned as the temperature reached 100 inside his east Mesa mobile home. He drove to the shelter for respite.

"I'm glad this place was here, and I'm glad they have cold water," he said.

The fire

The fire started about 11:45 a.m. at the Thunderstone substation near Power Road and University Drive in Mesa, SRP's Harelson said.

Spokesman Jeff Lane said that the transformers use a type of mineral oil for cooling and that the oil was likely what burned. SRP officials said they don't know what caused the fire, but Mesa Fire Chief Harry Beck said substation fires usually are caused by a surge somewhere on the power grid that damages the transformer.

Fire crews could not put out the fire for three hours or more because the power lines first had to be shut down, then grounded to remove static electricity, which can pose a threat to firefighters, Beck said.

"The fire itself was rather small and contained in an area where there were not a lot of other things to burn," Beck said. "Once we had the electrical current under control, we put out the fire rather quickly."

Of the station's four transformers, only No. 3 caught fire. But although the transformers are designed so that a fire on one does not affect the others, all four were shut down, Harelson said.

Two of the transformers have been restored, Harelson said Friday morning. One was destroyed, and another is being inspected and tested Friday. SRP hopes to have it restored by noon, Harelson said.

As the blaze sent billowing plumes of smoke across east Mesa, customers continued to get electricity from SRP as it was rerouted through other substations.

But about three hours after the fire started, at 2:11, another SRP substation tripped offline.

The Rogers substation northeast of Stapley Drive and University apparently could not handle the demand from all the power being rerouted around the burning Thunderstone station, Harelson said.

"It proved to be more load than those transformers could handle," he said.

An additional 15,000 customers get power from Mesa's city-owned electric utility. Their power is delivered through the same substation, and they also briefly lost service.

SRP began to restore power to the substation, but workers had to manually close electrical switches in several neighborhoods to restore power, which left many people without power into the night.

Outages across town

Falcon Field Municipal Airport in Mesa was forced to suspend operations for several hours and divert traffic to other airports after the control tower lost power, and nearby Boeing was forced to cease operations for the day and send 4,000 workers home.

Mesa City Hall lost power for about 45 minutes before it was restored about 3:30 p.m.

At Caffe Boa Bistro near Power and McDowell roads in east Mesa, owners worried that pricey hand-cured meats could be in danger of spoiling.

"We're losing business. It's killing us because Thursdays are pretty good for us," Jay Wisniewski said.

Larger businesses were hit hard, too. Movie theaters were forced to refund tickets, and the 160,000-square-foot Costco at U.S. 60 and Sossaman, the largest in Arizona, closed for the day and sent its employees home.

The failure knocked out traffic lights, but Mesa police said only two minor accidents were reported.

A key concern was with area hospitals, many of which were forced to go to generator power for most of the afternoon. No surgeries or emergency-room services were affected at Banner's Mesa hospitals, however several surgeries at Mountain View Medical Center were postponed.

Power went out about 2 p.m. Thursday at Banner Baywood Medical Center and Banner Heart Hospital. The two hospitals were running on generators.

Several elder-care facilities were doing their best to cope with the heat.

The Citadel Care Center, a nursing home on East Broadway Road, was still relying on a single generator and waiting for a second to arrive just before 6 p.m. to run fans and swamp coolers. Employees hung sheets over the windows to block the sun.

Residents of the Orchard Active Senior Living Complex, near Greenfield Road and Main Street in east Mesa, congregated outside during the failure to avoid their steamy apartments.

"I'm worried about the people who are on oxygen," said 64-year-old Marilyn Suiter.

Republic reporters John Genovese, Matt Haldane, Angelique Soenarie, Brittany Smith, Art Thomason, Caitlin McGlade, Gary Nelson and Laurie Merrill contributed to this article.