Subpanel head room

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TechSTL

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I've gotten a few bids from some electrical contractors to install a subpanel for my garage. I have 200 amp service at my house, and different contractors have quoted different sizing for the subpanel.

I realize the actual capacity of my panel will vary based upon the actual load of the equipment using it. I don't have all of that information, and may not have it all until after the electrical is complete. So, I thought it would be a good idea to calculate the theoretical headroom assuming the devices use the maximum VA supported by the proposed circuits.

Here are the circuits going into the subpanel:

Air compressor (240v * 25 amp = 6000 VA)
Ductless mini split (240v * 30 amp = 7200 VA)
2-post car lift (240v * 30 amp = 7200 VA)
Tesla charger (240v * 60 amp = 14400 VA)
Misc outlets (500 sqft * 3VA = 1500 VA)

Total VA = 36,300 VA

Adjustment per NEC 220.82
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100% of the first 10kW = 10,000 VA
40% of the remainder = (36,300-10,000) *4 = 10,520VA

Total Adjusted VA = 20,520 VA
Total Adjusted Amps = 20,520/240 = 85.5 amps

If all of that is correct, then I basically need to choose between 100 amp or 125 amp subpanels. Of the 3 quotes I got, only one quoted a 100 amp panel. They also happen to be _half_ the cost of the other two bids. If the above calcs are correct, I'll have at least 8700 VA left of headroom. I believe that means I have room for another 240v 30 amp circuit or a couple of 120v circuits. Are my calculations above correct?

Thanks!
 
I allowed this post as the OP is not asking "how to" but is verifying information.
 
I would ignore the two post car lift, since it will probably run less than 10 minutes per year and something else is sure to be off while you are doing that.
 
I believe you need to account for 100% of the AC unit load outside of the 10 kVA + 40% method per 220.82(C), which puts you at 103.5 A.
 
Those amp ratings look like breaker sizes, not actual loads. ?

That looks a lot like my (800 sq-ft) garage, except I don’t have a mini split, but do have a 3HP table saw, 1 HP dust collector, and a NEMA 6-50 for my welder. I have a 100A subpanel.
 
retirede said:
Those amp ratings look like breaker sizes, not actual loads. ?

That looks a lot like my garage, except I don’t have a mini split, but do have a 3HP table saw, 1 HP dust collector, and a NEMA 6-50 for my welder. I have a 100A subpanel.
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You are correct in that they are not actual loads. I attempted to address that in my original post. I won't have all of the equipment on-hand before electrical is complete. The compressor is 24.4 SLA (21.5 FLA) and the documentation of the Tesla charger says 48A.
 
TechSTL said:
You are correct in that they are not actual loads. I attempted to address that in my original post. I won't have all of the equipment on-hand before electrical is complete. The compressor is 24.4 SLA (21.5 FLA) and the documentation of the Tesla charger says 48A.
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If the electrician uses those actual values for his load calculations, his choice of a 100A panel is probably sound.

You should be able to get the technical data from the manufacturers’ websites.
 
Looks like they're using the Optional calculation. That calculation needs to come to 100A or more in order to be valid. The Standard calculation will probably come in a bit higher. Either way, I think a 100A panel is probably sufficient, as the HVAC should use the MCA value and not the breaker value so it is probably less than what you posted. Are there other fastened-in-place item you've missed, such as garage door openers?
 
suemarkp said:
Looks like they're using the Optional calculation. That calculation needs to come to 100A or more in order to be valid. The Standard calculation will probably come in a bit higher. Either way, I think a 100A panel is probably sufficient, as the HVAC should use the MCA value and not the breaker value so it is probably less than what you posted. Are there other fastened-in-place item you've missed, such as garage door openers?
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You have stated in other threads 220.82 requires the the load needs to be 100 amps or greater. It says the service or feeder conductors must have an ampacity of 100 amps or greater not the load must be 100 amps or greater.

I'm not ever sure why 220.82 is being discussed in this thread. A garage is not a dwelling unit.
 
Looks like you are correct. The beginning of 220.82(A) says "having a total connected load..." which made me think the resultant calculation had to be 100A or more. But at the end it does say "service or feeder conductors with an ampacity of 100A or greater" :oops:. So putting in a 100A service or feeder would work if the Optional Calculation is allowed. And as you pointed out, a garage by itself is not a dwelling unit, so the Standard calculation should be used.
 
suemarkp said:
Looks like they're using the Optional calculation. That calculation needs to come to 100A or more in order to be valid. The Standard calculation will probably come in a bit higher. Either way, I think a 100A panel is probably sufficient, as the HVAC should use the MCA value and not the breaker value so it is probably less than what you posted. Are there other fastened-in-place item you've missed, such as garage door openers?
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Yes but anything else is fed from the main panel.
 
OP said the air compressor load is based on the motor nameplate, the rest of them are breaker sizes. The air compressor is rated 24.4 SLA and 21.5 FLA. SLA probably means Stall Load Amperes. I would base the breaker size on 125% of the FLA, which is 26.875 amps. Go to the next larger size, that is 30 amps. But, for the purposes of the load calculation, use the 21.5 amps.
The car lift, mini-split, and car charger are listed with breaker sizes. Load calculation amps will be not greater than 80% of those. So 24 amps, 24 amps, and 48 amps. The mini-split would probably be even smaller if you find and use its MCA rating.

The car charger won't run at its maximum for very long, so we have some margin built in by counting it at 48 amps.

Add the motors, including an extra 25% for the largest one. That brings us back up to around 30 amps for either the air compressor or the lift. 48+30+24+24, now we're already over 125 amps, and haven't even added the lighting and receptacles. Since this is not a dwelling unit, the 3VA/sqft is not applicable. Commercial garages can be counted at 1.5 VA/sqft for the actual lighting only (not the receptacle load). Receptacles should be counted at a minimum of 180 VA per device yoke (so, 180 for a duplex or 360 for a quad), but that is a minimum. If actual appliance loads are known and they are greater than that, the actual appliance loads should be used. I'm going to guess a total of 20 amps for lighting and receptacles. So, I get a total of 146 amps.

Unfortunately, the demand factor of 220.82 doesn't apply since this is not a dwelling unit. The real question is, can we use a demand factor for any of the motor loads? We know the air compressor and the car lift will be off more often than they're on.
 
This garage is attached to a dwelling unit? And, the garage door openers, lights, and receptacles are already powered from the main panel of the house? 125 amps might not be too small, then.
 
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