Arizona Public Service, the area's main utility, said hot weather was expected to push power demand to record levels at the same time repairs continued on a substation that was seriously damaged by fire on July 4.
Maricopa County residents were asked to set air conditioners no lower than 82 degrees and put off the use of washers and other major appliances during the afternoon peak demand period.
Forecasters called for temperatures approaching 110 degrees Monday afternoon with rising humidity levels.
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