Thanks for the replies and confirmation.
1) A T8 ballast only requires 1 hot and 1 neutral connection. The T8 ballast also requires a ground connection.
2) Possibly, the 2nd black wire was installed as a switched loop conductor to control other downstream light fixtures? Another possibility is that the circuit was configured as a shared neutral circuit where a 2nd hot conductor is required if configured as such. Another possibility is that the 2nd black wire is an abandoned wire and is not in use.
3) Most likely, the circuit breaker will be a 20 amp. If a 20 amp breaker, the switched loop conductors will be sized as 12 AWG Copper unless the conduit run is lengthy and there were voltage drop issues that required a larger conductor. The switched loop wire that will splice to the new T8 ballast will be of the same wire size as that landing on the controlling circuit breaker.
4) Since you have a conduit system in place, gently tug on the 2nd black wire to determine where it extends to.
5) If the 2nd black wire just dead-ended and was not spliced to any other wires, then put a plastic wire nut on it as it won't be needed.
If you have any additional questions, just let me know and I’ll be glad to answer them for you.
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