I dig the trench at least 18" down and set the pole in cement mix. A 40lb bag should do. Be sure not to forget to encase the wire (if it's UF) in RNC to comply with 340.12 (8). I also like to use a 4' level just to make it look really good. I would also set the light fixture in place before setting the pole so you won't disturb the cement mix. And thats from experience!
I just did one a couple of days ago. The owner bought the pole and fixture from Cosco for $120, great price for the quality. It came with 3 anchor bolts and a cast template. The instructions said 12" dia concrete 15" deep. We put a 12" sonotube 24"+ deep, fed with 1/2" HW and pulled a ground.
Jim, I've had to do a few like that. Here's what I suggest:
First, select the fixture. Most are sized to fit on 2" pipe, but some are made for 2 1/2".
Next, dig your hole, and your trench. Set the stuff in to see how it lines up. Use a 90 degree sweep, to see where you need to cut into the pole for the wires to enter. I know, code soed not require this- but, if you ever want to re-pace the wires, you'll be thankful for that sweep! Even if you are running UF, there should be pipe run well away from the pole. If you're using UF, make the pipe large enough to actually be able to pull the stuff through!
Having set it all in place, I very strongly reccoment pouring concrete around the bottom. Remember, someone will be setting a ladder against that pole.
I set poles by placing a coupling on the top. The coupling has been drilled to accept guy ropes, to hold it in place while the concrete sets. You need this support for at least two hours.
CAMPMOR sells rope / guy lines that are reflective. A nice touch, as folks tend to trip over them at night. I don't care how inaccessible the site... some fool will trip over the guy line. Otherwise, make a fence with EMT and warning tape.
If you need a box - and I reccomend one- place a 10" diameter plastic handhole at the point where the sweep ends. Other arrangements - such as bell boxes set in the concrete- are not very practical.
To seal the connections, I dip my wire nuts in Skotch-Kote.
Hrm.. ya I install alot of these. I dig em down to the height I want it. Come in with the UF. pour in the dry cement... backfill, level the fixture. Then I'll use a hose or pour some water over the hole. Done deal.
This is Michigan and going 18 or so " deep I haven't seen any come up due to frost.
I did one out front of here some years ago.. Overkill I know but we dug down 4 feet and "tapered" the hole (wider at the top). Brought the RNC in the side of the pole, ( 10' of painted 2 1/2" EMT) set it in the hole and filled with concrete. Done.. Finished the pipework and pulled in conductors the next day and that was it.. It hasnt moved in 5 years so I doubt it ever will. Trick was the "taper" of the post hole, when the ground freezes and pushes in on the concrete its going to try and push the base down into the earth, not up and out.