"A bomb destroyed a high tension pylon on the grid which connects Baiji and Kirkuk and because of this attack, 80% of the city is without electricity," the head of the city's power department Yalcine Muhammad said.
Friday's blast served yet another reminder as to how insecure the occupied country remains.
Ferocious attacks on US-led occupation troops continue across Iraq, though their numbers have marginally come down.
It was the eighteenth attack on the city's power supply infrastructure since US President George Bush declared major combat in Iraq over on 1 May.
In the town of Taza, 25 km away to the south, police shot dead an assailant who had attacked officers manning a checkpoint, Kirkuk police chief General Turhan Yussef said.
"Jamal Hamed Abbad was wounded in the chest and legs and taken to hospital in a critical condition," he said.