How deep should I embed a 30-foot light post?
5” cover around the steel pole is not enough. 

Most electrical contractors have augurs in the 12”, 18”, 24” diameter. I’d go with the 24” diameter for four reasons: 1) it will give you 9 1/2” cover (if the pole ends up EXACTLY in the middle...doubtful), 2) the 24” width will help resist overturning, if it’s windy like you say, and 3) the rebar needs to be 3” from the outside edge and this leaves you plenty of room between the rebar and pole, and 4) the weight of the concrete base will help overturning. 

For depth, I’d go 4’ in the sandy soil where you’re located. At 24” diameter and 4’ deep, that gives you about 630 lbs of concrete resisting overturning. 

A light and bar at the top of the pole will have about 8” x 12” =96 sq. In. / 144 = .66 sq. ft. X 30’ pole = 20 ft. lbs. (not counting the pole).  Wind equals about 25 psf x 20 = 500 lbs. 

Therefore, the concrete footing has more resistance than the wind on the light at the top of a 30’ pole...and the pole will break before it rotates.