2 Family Service Calculation

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DannyBoiii

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Rhode Island
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Master Electrician
I am doing my first multiunit job and was wondering if someone could direct me on how to size the service wires. There will be a three gang meter stack with (2) meters for the 2 units and 1 meter for house panel. I was planning on running 125a to each panel. I cant find much on how to size the service wire providing power for the property. Also I guess this wouldn't fall under a multifamily calculation because the code would define a multi family as "3 or more" dwellings.
Thanks for any help
 
230.23 tells you how to size the service conductors.

but I think if you have already decided on 125 Amp panels the conductors going to those panels would need to have at least that much ampacity.
 
Remember to see if using the optional triplex way gives a lower number. Good measure would be to size it fully though since 125 amps is small that way if someone wants to add to the house panel a small pay car charger or something there's room to expand.
 
DannyBoiii said:
I am doing my first multiunit job and was wondering if someone could direct me on how to size the service wires. There will be a three gang meter stack with (2) meters for the 2 units and 1 meter for house panel. I was planning on running 125a to each panel. I cant find much on how to size the service wire providing power for the property. Also I guess this wouldn't fall under a multifamily calculation because the code would define a multi family as "3 or more" dwellings.
Thanks for any help
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You would size it based on part III of article 220. How did you come up with 125 amps?
 
letgomywago said:
Remember to see if using the optional triplex way gives a lower number. Good measure would be to size it fully though since 125 amps is small that way if someone wants to add to the house panel a small pay car charger or something there's room to expand.
Click to expand...
whats the optional triplex? I'm unfamiliar with this
 
So to also throw a twist into this scenario they are planning on putting an elevator in the building. Right now theres not enough information on the elevator so its being priced as if there isnt an elevator but that would need to be factored in the future if they decide to go through with it
 
DannyBoiii said:
So to also throw a twist into this scenario they are planning on putting an elevator in the building. Right now theres not enough information on the elevator so its being priced as if there isnt an elevator but that would need to be factored in the future if they decide to go through with it
Click to expand...
Plan on atleast a 30 amp from the house panel atleast.
 
DannyBoiii said:
I am doing my first multiunit job and was wondering if someone could direct me on how to size the service wires. There will be a three gang meter stack with (2) meters for the 2 units and 1 meter for house panel. I was planning on running 125a to each panel. I cant find much on how to size the service wire providing power for the property. Also I guess this wouldn't fall under a multifamily calculation because the code would define a multi family as "3 or more" dwellings.
Thanks for any help
Click to expand...
There is single and two family then there is multifamily - in the NEC.

Your house panel is a non dwelling load plus you also serving a two family load.

As mentioned if you have 125 amp individual OCPD's then each must have 125 amp conductor, or the 83% that is specifically allowed for individual dwellings can apply to the two dwelling feeds. But a common supply conductor can be sized to the calculated load that is served, and this can take advantage of demand factors of combined loads of the whole installation. Depending on how such demand factors work out all three 125 amp supplies could possibly be supplied by only a ~200 amp conductor or maybe even less.
 
kwired said:
There is single and two family then there is multifamily - in the NEC.

Your house panel is a non dwelling load plus you also serving a two family load.

As mentioned if you have 125 amp individual OCPD's then each must have 125 amp conductor, or the 83% that is specifically allowed for individual dwellings can apply to the two dwelling feeds. But a common supply conductor can be sized to the calculated load that is served, and this can take advantage of demand factors of combined loads of the whole installation. Depending on how such demand factors work out all three 125 amp supplies could possibly be supplied by only a ~200 amp conductor or maybe even less.
Click to expand...
Where would you get these demand factor calculations from?
 
HEYDOG said:
Read 310.12 starts at bottom of right hand column on page 156!
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My understanding of this is that they are talking about the feeders for individual units being allowed to be sized that way and not the service wire itself that would be supplying all three panels at the meter stack
 
DannyBoiii said:
My understanding of this is that they are talking about the feeders for individual units being allowed to be sized that way and not the service wire itself that would be supplying all three panels at the meter stack
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I see what you are saying that you have a house panel also. I will have to think so more about that. Maybe someone else will come up with the answer. If I find something I will get back with you!
 
If you know the square footage of each dwelling and the appliances going into each one, dishwasher, disposal, dryer, A.C. , heat, stove , etc. plus square footage of common hallways and any other loads …you can calculate it using the standard method!
 
DannyBoiii said:
Where would you get these demand factor calculations from?
Click to expand...
If you've calculated load using art 220 you likely have used them. There are standard calculations and in some cases there are optional calculation methods that can be used. Anything that allows you to reduce calculated load from the actual connected load had some demand factor applied to attain the final result.
 
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