Now that Peco has restored power to thousands of customers knocked out by Tuesday's North Philadelphia substation fire, the hard work begins.

The utility must quickly rebuild a large substation at 2634 W. Westmoreland Ave. in Tioga before summertime demand adds more stress to a system patched together after the fire.

Until the substation is rebuilt, 36,000 customers in North Philadelphia are getting electricity rerouted from neighboring areas, supplemented by power produced by 12 semi-trailer-size diesel generators Peco has brought in to support the distribution grid, said Ben Armstrong, the utility's spokesman.

"We want to have this system back to normal sooner rather than later," said Armstrong.  Some brief service interruptions will occur during the restoration. He said repairs would require "at least a few weeks, and maybe a few months."

Armstrong said the Tuesday afternoon fire caused "substantial damage" to the substation, which converts power from transmission lines to 13,000-volt current that is distributed in the area. It's one of 62 such substations on the Peco system.

No cause was determined for the fire, and damage estimates are still being assessed. The substation was unoccupied at the time the blaze broke out.

The fire was confined to the distribution side of the facility, Armstrong said, so none of the custom-built transformers that convert high-voltage electricity were damaged.

Engineers are still assessing what equipment can be salvaged, but much will need to be removed and replaced, including up to 60,000 feet of heavy electrical cable, he said.

An immediate concern in the next few days is cold weather, which Peco anticipates will drive up demand from customers with electric heating. The utility is swapping some of the portable 2-megawatt power generators, which were brought in from as far as Ohio, to try to match supply with expected demand.

Peco said it is trying to notify customers when the generators are disconnected, which will cause outages of about 10 minutes.