I am wiring a A/C condenser on the roof of a building. The unit pulls 18 amps max,at 220v. The unit is second hand and the wiring that is coming out of the conduit of the unit is 10 gage( I think) 2 hots and 1 ground.
I have 220 running to it now . I have 2 hots 1 neutral and a ground. The wire that I have running to the unit is 12-3 ( I think my breaker is 30 or 40 amps).
My questions are
1. how do I go from 2 wire to 4 wire ( There is a service disconnect between the two different types of wire)
2. is 12-3 big enough for the ampacity and heat rating of 18 amps?
Thanks

Jadenat,
Please check the "nameplate" on the condensing unit...
The FLA (full load amps) or MCA (minimum circuit ampacity) along
With MOCP (maximum overcurrent protection) may be listed.
Please write back and let us know what this says.

If the running amps are 18 at 220V, single phase, then #10 AWG protected by a 30 ampere circuit breaker or #8 AWG if the circuit breaker is 40 ampere. For a typical condensing unit only two "hot" wires and a ground a required without a neutral.

Is the condensing unit at least 10 feet from the roof edge?
How are you running this cable? Romex is not listed for exposure to the weather. Usually PVC conduit with the final connection made with sealtite flexible conduit most common.

Can you tell us more?