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Actually .... regardless of what's on the 20 amp circuit..... you should never load it past 16 amps.
Don't forget the requirement for dedicated ground as well.
The question was about "detatched pole signs"...... not sure I would have called it this but I'm preety confident I know what CStone is refering too.
Depending on the municipality you may also need a diconnect within so many feet or on the sign.
Check Your Local Code People. NEVER rely on a sign forum for the final answer.
You should have a 20 amp circuit for a sign, but for your load requirement you will need to ad up all of your ballast / transformer primary load amps to see how many circuits you will need for your sign.