In both Mike Holt's and Stallcup's books the standard calculation for multifamily dwellings is performed using the demand factors of NEC Table 220.42 regardless of how many dwelling units are in the building. These demand factors are applied to all the dwelling units combined. The NEC is not very clear on whether NEC Table 220.42 should be applied on single or multifamily dwelling units. Except in Part IV, optional calculations, demand factors are applied to all the dwelling units combined in accordance with NEC Table 220.84. It seem's to me that NEC 220.42 should be applied in sizing the subfeeders of each dwelling unit and then the calculated load is added up to size the main feeder for the entire building. This would be the way I would do the calculation. Your thoughts on this?
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220.40 says the calculated feeder or service load should not be less than the sum of the loads of the branch circuits as determined by Part II after demand factors of Part III have been applied...
220.42 is one of the demand factor of Part III. I think the demand factor in 220.42 could be applied to the service or feeder that serves the multifamily dwelling for the total load of all the dwelling units.

When calculating the feeder to the panel inside any single unit, you make a list of all the loads in that unit, and apply whatever demand factors you can. Specifically, you take the square footage of that unit, and obtain the lighting load from Table 220.42.
When calculating the service to the building as a whole, you start from scratch, and do not simply add the results of the individual unit calculations. Make a list of all the loads in the entire building, and apply the appropriate demand factors. Specifically, when using Table 220.42, you use the square footage of the entire building.Charles E. Beck, P.E., Seattle
Comments based on 2017 NEC unless otherwise noted.
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A you know for some cases there is a standard and an optional method of calculation. We do not expect the numbers to be exactly the same but we do expect them to be reasonably close to the other. In the case of a multifamily dwelling the numbers are way off from each other.Eric Kench, P.E.
If it's not broken don't fix it
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There really is no reason to expect them to be anywhere close to one another. I have never come across a situation in which the standard method provides a lower result than the optional method. I am sure it happens. But you do both, and you accept the lower result, and then you check with the client to see if they want to get more than the minimum required.Charles E. Beck, P.E., Seattle
Comments based on 2017 NEC unless otherwise noted.
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Originally posted by erickench View PostOkay, but do you at least agree that the subfeeders to the apartments should be sized according to NEC 220.42? Then I would apply the optional calculation method.
A 1500sf apartment would have 4500va lighting per 220.12. Using 220.42 the feeder to the apartment would include 3525va lighting plus the additional loads in 220.40. Per the optional method in 220.82, 4500va of lighting plus the additional loads listed would be added together and applied with the first 10kVA at 100% and the remainder at 40%.
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Originally posted by david luchini View PostI don't quite understand what you are asking. The subfeeders to the apartments could be sized EITHER according to the method in 220.40 or the optional method in 220.82.
A 1500sf apartment would have 4500va lighting per 220.12. Using 220.42 the feeder to the apartment would include 3525va lighting plus the additional loads in 220.40. Per the optional method in 220.82, 4500va of lighting plus the additional loads listed would be added together and applied with the first 10kVA at 100% and the remainder at 40%.Last edited by erickench; 061711, 06:19 PM.Eric Kench, P.E.
If it's not broken don't fix it
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Originally posted by charlie b View PostWhen calculating the feeder to the panel inside any single unit, you make a list of all the loads in that unit, and apply whatever demand factors you can. Specifically, you take the square footage of that unit, and obtain the lighting load from Table 220.42.
When calculating the service to the building as a whole, you start from scratch, and do not simply add the results of the individual unit calculations. Make a list of all the loads in the entire building, and apply the appropriate demand factors. Specifically, when using Table 220.42, you use the square footage of the entire building.
It would be possible to use the optional method part IV just for the calculation of the feeders of each dwelling, and then use the standard method for the calculation of the service conductors that supply all the dwellings?
Thanks in advance,
Regards,
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