AC circuit requirements.

[KasseemF]

I am working on a building with old piping. I need to install two Air conditioning units one rated a t 40A and the next at 30A.

There is an existing circuit that I am going to reassign but I am unable to pull the wires again. Can I install a 70A circuit to deal with that will service both of these A/C units?

 

[Smart $]

I am working on a building with old piping. I need to install two Air conditioning units one rated a t 40A and the next at 30A.

There is an existing circuit that I am going to reassign but I am unable to pull the wires again. Can I install a 70A circuit to deal with that will service both of these A/C units?

You can use the 70A circuit as a feeder, but you will have to provide ocpd and disconnecting means for each unit somewhere along the way and meet tap rule requirements.

 

[kwired]

A 70 amp feeder is likely to be fine. There is a good chance that you can use even smaller feeder. Breaker size is not really what you need to be looking at but the rated load is. The 40 amp unit is likely the breaker size but actually may only draw maybe 20-26 amps fully loaded, the same with 30 amp unit but a full load of 15 - 20 amps.

lets assume MCA of 23 and 16.

23x1.25+16= 44.75

Minimum ampacity of the feeder would only need to be 45 amps. A 45 amp breaker will likely not allow for starting, a 50 may work but I'm guessing a 60 will work in most cases.

 

[curt swartz]

A 70 amp feeder is likely to be fine. There is a good chance that you can use even smaller feeder. Breaker size is not really what you need to be looking at but the rated load is. The 40 amp unit is likely the breaker size but actually may only draw maybe 20-26 amps fully loaded, the same with 30 amp unit but a full load of 15 - 20 amps.

lets assume MCA of 23 and 16.

23x1.25+16= 44.75

Minimum ampacity of the feeder would only need to be 45 amps. A 45 amp breaker will likely not allow for starting, a 50 may work but I'm guessing a 60 will work in most cases.

Actually the MCA already has the 125% included. You would use the MCA of the largest unit and add the individual motor loads of the other unit. Adding just the MCA's together works but will give you a larger ampacity that really required.

 

[kwired]

Actually the MCA already has the 125% included. You would use the MCA of the largest unit and add the individual motor loads of the other unit. Adding just the MCA's together works but will give you a larger ampacity that really required.

When I was writing that I was thinking about saying just the largest compressor times 1.25 and add the rest of the loads, and probably should have said that, for some reason it slipped my mind that 1.25 is already included in the MCA even though I did know that.

OP could easily have a possibility of 18 amp compressor with 2 amps condenser fan on one unit and maybe 12 amp compressor with 1.5 amps fan on the other.

In that case he could possibly only need a 38 amp feeder. I still think the feeder breakers I previously mentioned would still hold true. Probably not going to start both without tripping anything under 50 amps and 50 is likely questionable with some brands of breakers. Available current / voltage drop values during starting will make some difference here also.

 

[Ponchik]

How about if you use the 70A as a feeder and install a subpanel near the condensors and then install two separate branch circuits for each A/C unit.

Or do as smart$ suggested