2 Family Service Calculation

DannyBoiii

Member
Location
Rhode Island
Occupation
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
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
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.
 

letgomywago

Senior Member
Location
Washington state
Occupation
residential electrician
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.
 

HEYDOG

Senior Member
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
You would size it based on part III of article 220. How did you come up with 125 amps?
 

DannyBoiii

Member
Location
Rhode Island
Occupation
Master Electrician
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.
whats the optional triplex? I'm unfamiliar with this
 

DannyBoiii

Member
Location
Rhode Island
Occupation
Master Electrician
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
 

letgomywago

Senior Member
Location
Washington state
Occupation
residential electrician
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
Plan on atleast a 30 amp from the house panel atleast.
 

kwired

Electron manager
Location
NE Nebraska
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
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.
 

DannyBoiii

Member
Location
Rhode Island
Occupation
Master Electrician
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.
Where would you get these demand factor calculations from?
 

HEYDOG

Senior Member
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
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!
 

HEYDOG

Senior Member
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!
 

kwired

Electron manager
Location
NE Nebraska
Where would you get these demand factor calculations from?
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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