Not sure where you are located but SMECO gives away used telephone poles. U-Haul. (however, back in the day, some of the local crews working nearby would drop a few off for a case of beer
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If you are in maryland, and the poles arent going to be used for anything other than a light pole 3-4 feet is plenty! MOST telephone poles are set 3' in the ground, depending on how much tension is on them. The local line crews who have to occasionally set them buy hand, told me what the depth requirement was. Most augers dig about 3-4 ft. I have a 9 inch auger that is PTO driven, and we set our high tensile fence posts (that had tension on them) at 4 feet, and we had to actually dig the last little bit. I have 1000lbs of tension on my poles.
NOTE: Augers are dangerous and unless you are very familiar with farm implements and PTO, I recommend you get someone to do it for you.
Augers have the potential to kill you in a matter of seconds!!!