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The utility said late Tuesday that the storm's rampage through the New York City area shoved trees and branches onto power lines, bringing down equipment that left 257,000 customers without power.
"The destruction surpassed Hurricane Irene, which caused 204,000 customer outages in August 2011," the company said in a news release.
According to ConEd, the record for storm-related outages is 1.1 million caused by Hurricane Sandy in October 2012 and a Nor'easter that following the storm.
The company said that service has already been restored to more than 48,000 customers, but "it is clear" that restoration for all will take "multiple days."
In the greater New York City area, Westchester County has about 97,000 customers out of service.
In city limits, the borough of Queens has about 45,000 customers out of service and in Brooklyn, 16,000. In Staten Island, 29,000 customers are without service, and in the Bronx, 23,000.
ConEd said the company has brought in 220 additional line workers to restore service and has another 100 scheduled to begin work Wednesday.
"Including tree trimmers and other workers, the company will have brought in more than 500 additional personnel to help with the arduous task of replacing poles, wires, transformers and other equipment," the company said in a statement.
Crews are working with local public works crews to clear hundreds of roads blocked by fallen trees and branches from the storm.
Downed wires that are entangled in trees must be de-energized by crews before wires can be cut and removal can take place.
"Crews will give priority to making repairs that will provide power to the most customers quickly, then restore smaller groups and individual customers," the company said.
As of Wednesday morning, power-outage tracking website poweroutage.us said over 2.4 million customers remain without power, with the greatest number in New Jersey, Connecticut and New York.
At the height of the storm, over 3.6 million customers were without power.
"Over 3 million people without power because of this tropical storm, another indication it doesn't take a major hurricane to cause incredible damage," Fox News senior meteorologist Janice Dean said Wednesday.
At least six people were killed as Isaias spawned tornadoes and dumped rain Tuesday along the East Coast after making landfall as a hurricane in North Carolina, where it caused floods and fires that displaced dozens of people.
"One of the most damaging storms the Northeast has ever experienced," Dean said.
Two people died when Isaias spun off a tornado that struck a North Carolina mobile home park. Another person died in Pennsylvania when their vehicle was overtaken by water and swept downstream. Two others were killed by falling trees toppled by the storm in Maryland and New York City, and a sixth person died in Delaware when a tree branch fell on them, authorities said.
The Associated Press contributed to this report.