I purchased a 20 foot 8x8 PT post to hold 2 large lights for a bb court. I live in Upstate New York and was wondering if I have to have a post hole deeper then 4 feet due to the height and weight of this post? I bought a 4 ' - 12" diameter sona tube and was going to fill with cement. Any suggestions?
Your 12" x 4' Sonotube is more than enough. The small weight of the post and lights is minimal and less than the weight the concrete puls down on the soil.
The real design point is the wind or lateral load. Since you have a couple of lights and not a "sail" on the post, a 12"x4' base is enough. Frost is obviously not a question for a single post structure. Since you have a somotube there is no way for the frost in the upper levels to cause heaving upward.