Pepco is working to restore service to customers left powerless by damage to underground cables.
Crews have been working non-stop since the equipment was damaged on Tuesday. Outages caused the Federal Energy Regulatory Commission and the U.S. Equal Employment Opportunity Commission to close.
Pepco said in a statement Wednesday afternoon that they hoped to have the power back on to the affected areas by midnight. And the heat may be at least partially to blame for the outage, NBC Washington's Chris Gordon reported.
Also among the closings, the D.C. Department of Health, the Department of the Environment, the D.C. Housing Authority and apartment buildings housing 1,500 residents.
Pepco's Andre Francis says the problem started with a power feeder.
Most of the customers without power are north of Union Station near North Capitol Street.
A few customers in Montgomery and Prince George's counties in Maryland are also affected.
D.C. Fire and EMS performed checks on apartments and treated and transported people for heat exhaustion Wednesday, Gordon reported. They powered fans in lobbies with their fire trucks.
A cooling station will remain open at Walker Jones Education Campus on New Jersey Avenue near New York Avenue until power is restored.
Metro sent coolinges bus so people affected by the outage could get out of the heat and sit or sleep in air conditioning, Gordon reported.