Subpanel size for 5 HVAC units

[kwired]

I think we need to clarify if/when we are talking about feeder ampacity vs feeder OCPD. But if there is a motor panelboard supplied by a motor feeder, wouldnt 408.36 prohibit us from using a larger than ampacity feeder OCPD?

Edit: 430.62(B). apparently there is no allowance for a larger than ampacity OCPD for a motor feeder like there is for motor branch circuits.

AFAIK yes, you must still protect the panelboard in accordance with 408.36.

You could have a feeder with the permitted higher OCPD and then make up feeder taps to each motor circuit

 

[john100]

You could, but you don't have to. Five units at 33A each plus 25% of the largest motor = 171.1A. You could run a 2/0 feeder from a 200A feeder c/b to a 200A panel.

I'm not saying I would do it that way...makes more sense to just make the feeder conductors 3/0...but Code allows it.

Don't have to match the conductor to the panel size? 200amp panel would need a 3/0 at 75c. Am I not correct?

 

[david luchini]

Don't have to match the conductor to the panel size? 200amp panel would need a 3/0 at 75c. Am I not correct?

No, I don't believe there is any such requirement in the Code.

 

[kwired]

Don't have to match the conductor to the panel size? 200amp panel would need a 3/0 at 75c. Am I not correct?

I don't believe so, you must protect 200 amp panelboard with 200 amp or less device but nothing prohibits less than 200 amp conductor or lesser overcurrent protection. Other codes sections will require minimum ampacity and OCPD in accordance with the load served.

 

[petersonra]

Hello, i want to make sure i got this correct. We are bidding job for hvac and electrical. The hvac will need a subpanel installed. The name plate says min circuit ampacity is 40 amps and the max breaker size is 60 amps. I am thinking we need a 300 amp sub panel 60*5=300 or a 320 amp sub panel 60*1.25=75 plus the 4 other units is 315 amps? can i get some input if this is correct

MCA has to do with conductor sizing. Not breakers.

 

[Dennis Alwon]

That would be 60+1.7+6.9+33+33+33+33 = 200.6, or a 200A breaker with an mca of 171.1A.



Not sure how you got the 90, but the 225% is already figured in the 60A MOCP of the unit.

If the 40 amp is the min then I don't believe the 60 is 225%. I got 90 by multiplying 40 x 2.25

 

[Dennis Alwon]

I didn't read the posts so I see the motor is 34 amps not 40. Not sure where the 33amps is coming from nor the 1.7 and 6.9

 

[david luchini]

If the 40 amp is the min then I don't believe the 60 is 225%. I got 90 by multiplying 40 x 2.25

The largest motor is 24.4A, not 40A.

 

[Dennis Alwon]

If the largest motor is 24 then why is the minimum cir ampacity 40 amps

 

[david luchini]

If the largest motor is 24 then why is the minimum cir ampacity 40 amps

24.4A (compressor) *1.25 + 1.7A (outdoor fan) + 6.9A (indoor fan) = 39.1A, the listed MCA of the unit.

 

[Dennis Alwon]

Maybe I should read the entire thread. There are so many numbers thrown around I am lost.

The indoor unit should not have anything to do with the outside a/c units MCA

 

[david luchini]

Maybe I should read the entire thread. There are so many numbers thrown around I am lost.

The indoor unit should not have anything to do with the outside a/c units MCA

Yes, too many numbers...OP mentioned 40 mca, but cut sheet shows 39.1. See cut sheet in post #4.

There is no "indoor unit". The unit is a rooftop heating/cooling unit..gas heat, electric A/C.