Calc load of ac

[Krusscher]

If I had to prove to an inspector that removing the heat package from the Bard unit would be enough so that I could tap off the 60A circuit feeding it to also feed the back LG mini split unit how would you do that on paper? I don't think I am going to have an issue but I want to be prepared incase he ask to see it.

 

[Krusscher]

How exactly do I calculate a feeder for 2 hvac units? Is it as easy as adding the 2 MOP together and its under 60A so its good? or do i do 125% of the largest motor/compressor and add the rest?

 

[ActionDave]

125% of the largest unit and 100% of the other one is how I would do it.

 

[don_resqcapt19]

You use 430.24 for AC equipment feeders. 440.3(A) says that the rules in 430 apply to AC installations unless modified by a rule in 440. There is no feeder rule in 440, so you have to use the one in 430.
The minimum circuit ampacity on the nameplate already includes the 125% required by 430.24(1). I would use the minimum circuit ampacity from the largest unit and the actual running current for any additional units.

 

[wwhitney]

Often the largest motor in such a unit is the compressor, and the nameplate gives its RLA. In that case MCA = 125% * RLA + all the other equipment and motors.

So when computing the MCA for a feeder for two such units, add the two MCAs and subtract 25% of the smaller RLA. Because with multiple motors, only the largest motor needs the 125% factor, while just adding the MCAs applies the 125% factor to two different motors.

Cheers, Wayne