Dave: Suggestions & comments:
MH start current is higher than 'run'
Is the CB a HID rated/labeled breaker?
How many fixtures on the circuit?
What is the voltage?
Does the breaker 'trip' instantly?
or do the lights 'start-up' partially?
IF they do, then the ones that 'started' may require a 'cool-down' before re-striking; lowering the start-up current!
I would replace the breaker with a known 'good' (or new) breaker and clamp the circuit to get more info.
You may have a bad breaker; or you may have a ballast that is going bad.
Another reason I try to spec fuses at each hi-bay, and at each pole or head. Better to have one light out then the whole circuit!
John
