The Poco Fire east of Payson has grown to more than 11,000 acres.
More than 700 firefighters from 12 states (among them California, Idaho, Utah, New Mexico, Wyoming and Montana), including 11 hotshot crews, are trying to get a handle on the fire before the expected monsoon arrives this weekend.
The fire near Young is 15 percent contained and no homes are in immediate danger.
"It's a concern when the storms start building because you get the downdrafts (strong winds) and dry lightning," said fire information officer Punky Moore.
The fire spread under a portion of the electrical transmission lines that provide energy to Phoenix and Tucson Metro areas earlier this week. The lines were shut off and then re-energized Wednesday afternoon.