Multifamily Demand Factor Question

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PHA

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I am designing a 208 3-phase electrical service for a multifamily apartment project. Meter bank #1 consists of (32) apartment units and meter bank #2 consist of (21) apartment units. Meterbanks #1 and #2 are served by a MDP which is fed from the utility transformer. I have sized meterbank #1 based on the demand factor of 0.31 found in NEC table 220.84 and 0.37 for meterbank #2. This gives me 974 demand load amps (3142 total connected amps) on meterbank #1 and 759 demand load amps (2051 total connected amps) on meterbank #2. Since I have already applied the apartunit unit demand factors, I simply add the loads together to size my MDP (974 + 759 amps). I would need a 2000A MDP.

The electrical contractor is coming back and saying that we need to size the meter banks as I mentioned above, but the MDP needs to be sized based on a demand factor of 0.25 (because there are 53 apartment units) of the total connected load (3142 + 2051 amps). After applying the 0.25 demand factor, this equates to a 1600A MDP.

The contractor's method does not seem code permitted to me. It is my understanding that I can not apply a demand factor at the meter banks and then apply an overall demand factor to size the MDP. Is my understanding correct? Can anyone please provide me with an NEC code section to confirm one way or the other? NEC section 220.84 does not seem to clearly adress this issue to my understanding. Thank you in advance for your help.
 
You size any component based on the load that that component will serve. But you can't "double dip" on the demand factors. As I understand your design, if you take the total connected load from the two meter banks, and then use the demand factor that applies to the total number of units, you are following what the code allows.

Welcome to the forum.
 
Are you saying that if you had just one set of meters you would have less of a load on the MDP then if you have two sets of meters? Somehow that does not make any sense to me.
 
charlie b said:
You size any component based on the load that that component will serve. But you can't "double dip" on the demand factors. As I understand your design, if you take the total connected load from the two meter banks, and then use the demand factor that applies to the total number of units, you are following what the code allows.

Welcome to the forum.
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Thank you Charles. That helps clarify things for me.
 
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