Sub-panel calculations

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frofro19

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VA.
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Master Electrician
Can any demand factors be applied for a sub-panel in the same building as the main panel? Can't seem to find anything in the code for calculating this. I have a garage addition that has a small sub-panel I'm upgrading and trying to decide if I can get by with #2AL ser on a 90 amp breaker vs #1 AL on a 100 amp breaker.
They are adding a 10 kw back-up heat, 2 ton AC, 4.5 kw water heater, and moving the well pump I believe is a 1/2 hp pump. Also has a sewer pump that pulls about 7 amps. Also one garage door opener. Area is around 960 square ft. That's over 100 amps if no demand factors and be used. If so, I'll go with a mlo 125 amp panel with #1CU or 2/0 AL.
 
Demand factors are for load calculations. If using a subpanel the demand factors would be included in the main service calculation as the service would be providing power for everything.
In your case, demand factors would only effect the total load calculation for the complete service. They might keep you from upsizing the existing service, but not for a subpanel. The sub would be sized for loads from it.
 
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Uisng the Art 220 demand factors isn't going to help you much with those loads. The key may be in the HVAC sysstem and whether the the compressor can operate at the same time as the 10kw heat (some heat pumps allow that). If so, a quick run on the info given shows you might push a 100 amp feeder.
 
augie47 said:
Uisng the Art 220 demand factors isn't going to help you much with those loads. The key may be in the HVAC sysstem and whether the the compressor can operate at the same time as the 10kw heat (some heat pumps allow that). If so, a quick run on the info given shows you might push a 100 amp feeder.
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I was afraid of that. I'll check with the HVAC Contractor this week when I go back. If the compressor doesn't operate at the same time I should be good with a 90 amp breaker and #2 AL, correct?
 
Little Bill said:
Demand factors are for load calculations. If using a subpanel the demand factors would be included in the main service calculation as the service would be providing power for everything.
In your case, demand factors would only effect the total load calculation for the complete service. They might keep you from upsizing the existing service, but not for a subpanel. The sub would be sized for loads from it.
Click to expand...
I will do a load calculation for the total service but was just making sure I was sizing the feeder and OCPD correctly to the sub- panel. They added a lot that I wasn't aware of at first.
 
Little Bill said:
Demand factors are for load calculations. If using a subpanel the demand factors would be included in the main service calculation as the service would be providing power for everything.
In your case, demand factors would only effect the total load calculation for the complete service. They might keep you from upsizing the existing service, but not for a subpanel. The sub would be sized for loads from it.
Click to expand...
So everything in the sub calculated at 100% as far as the OCPD and feeder size is concerned?
 
augie47 said:
Uisng the Art 220 demand factors isn't going to help you much with those loads. The key may be in the HVAC sysstem and whether the the compressor can operate at the same time as the 10kw heat (some heat pumps allow that). If so, a quick run on the info given shows you might push a 100 amp feeder.
Click to expand...
The compressor does run at the same time as the 10 kw heat. I talked to the HVAC contractor the other day and he said he could drop it down to 5 KW of heat. So I should be good with #2 AL on a 90 amp breaker. In all reality the 200 amp meter should have been switched out to a 400 amp meter with a 200 amp disconnect feeding to the garage addition before it was build but unfortunately I wasn't there during that part of the construction.
 
Sorry, I'm not following the answers. To my understanding if you just look at the loads on the subpanel, any demand factors available in 220 Part III can use used in calculating that load. You just can't use 220 Part IV, as that applies only to the whole dwelling unit service or a feeder carrying all the loads.

So are the above answers just "yes, but 220 doesn't really have any factors in it that would reduce the calculated load on the feeder?"

In which case let me ask if it is plausible to consider the water heater, the well pump, the sewer pump, and the garage door opener to be appliances, so that the 220.53 demand factor of 75% for 4 or more appliances in a dwelling unit (other than ranges, clothes dryers, and HVAC) would apply to those loads in the subpanel? I've not parsed through 220 Part III for other possibly applicable demand factors, that's just one I remember.

Cheers, Wayne
 
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wwhitney said:
Sorry, I'm not following the answers. To my understanding if you just look at the loads on the subpanel, any demand factors available in 220 Part III can use used in calculating that load. You just can't use 220 Part IV, as that applies only to the whole dwelling unit service or a feeder carrying all the loads.

So are the above answers just "yes, but 220 doesn't really have any factors in it that would reduce the calculated load on the feeder?"

In which case let me ask if it is plausible to consider the water heater, the well pump, the sewer pump, and the garage door opener to be appliances, so that the 220.53 demand factor of 75% for 4 or more appliances in a dwelling unit (other than ranges, clothes dryers, and HVAC) would apply to those loads in the subpanel? I've not parsed through 220 Part III for other possibly applicable demand factors, that's just one I remember.

Cheers, Wayne
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Interesting as to what others think about that. Why would we consider everything in a sub-panel at 100 % but not anywhere else? The chances of everything on at the same time often would be rare IMHO.
 
IMO, 220.40 allows you to apply the Part II calculations. In your situation, the demand factors will not make much difference as I mentioned in Post #3 since the majority of your load is HVAC and you addressed that.
 
why is everyone always trying to cheapen things up like this. just put in a 200 A panel and be done with it. eventually someone is going to want to add something anyway.
 
petersonra said:
why is everyone always trying to cheapen things up like this. just put in a 200 A panel and be done with it. eventually someone is going to want to add something anyway.
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That is what should have been done from the beginning but as stated before, I wasn't there during the construction process. And not a option at this point to upgrade the existing 200 amp service to a 400 amp service and supply the additon with a 200 amp panel. That's why we opted to downsize the 10 kw heat to 5 kw heat because we don't have the needed service there. The owner stated he may install a small propane heater which would offset the 5kw of heat. Not trying to "cheapen things up" was just looking for solutions to the existing situation.
 
frofro19 said:
Interesting as to what others think about that. Why would we consider everything in a sub-panel at 100 % but not anywhere else? The chances of everything on at the same time often would be rare IMHO.
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Question as to everyones opinion on the "Largest Motor = 125%". Ive always been told the largest motor demand gets added to what ever is the largest load carried on that panel.

Say panel A (400A) feeds panel B (200A). Say there is a large 100A/3Ø motor connected to panel A. Panel A would give 125A/3 demand for that circuit. Technically this largest motor demand would carry on to panel B as well if no other larger motor loads are connected. Is this statement incorrect?
 
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