400 amp multifamily service

[Chevys10zr2]

Hi,
First time posting. Long time lurker.
I usually find my answer just searching through old threads, but I can't find the answer to this one.
So I was asked to do a 4 unit multifamily and I have never done one before. I have always done single family dwellings.
I did a multifamily service load calculation for this multifamily and it comes out to 380 amps.
Would you just divide the 380 amps by 4 and that should be good for each panel in each unit?
380÷4= 95
For shits and giggles, I did a standard and optional single family load calculation for 1 unit and my calculation comes out to about 90 amps for optional calculation and 123 amps for standard calculation.
At first I was thinking installing 100 amp subpanels in each unit, but that standard single family calculation is telling me maybe I should put a 125 amp panel in each unit. I would rather be safe than sorry so my main question would be can you install 4 - 125 amp panels on a 400 amp service? I think the pedestal says it's rated at 125 per position. The meter center is a Square D MPH44125.

 

[augie47]

No problem. If your calculated load for the 4 units is 380, build a 400 amp service and feed four 125 amp panels.

 

[tortuga]

Hi,
First time posting. Long time lurker.
I usually find my answer just searching through old threads, but I can't find the answer to this one.
So I was asked to do a 4 unit multifamily and I have never done one before. I have always done single family dwellings.
I did a multifamily service load calculation for this multifamily and it comes out to 380 amps.
Would you just divide the 380 amps by 4 and that should be good for each panel in each unit?
380÷4= 95

No, and welcome to the forum.

For shits and giggles, I did a standard and optional single family load calculation for 1 unit and my calculation comes out to about 90 amps for optional calculation and 123 amps for standard calculation.

That is actually the correct way to calculate the feeders,
If the optional (220.80) comes out under 100 Amps you cant use it so you have to go with the general 123 amps per unit.

At first I was thinking installing 100 amp subpanels in each unit, but that standard single family calculation is telling me maybe I should put a 125 amp panel in each unit. I would rather be safe than sorry so my main question would be can you install 4 - 125 amp panels on a 400 amp service?

Yes, the calculation for the entire service 220.84 has more of a demand factor (45%) than the feeder to each unit.

 

[wwhitney]

If the optional (220.80) comes out under 100 Amps you cant use it so you have to go with the general 123 amps per unit.

Not quite. The restriction in 220.82 just says that the service or main feeder conductors must have an ampacity of at least 100A. So as long as you install a 100A service or main feeder to the dwelling unit, if the 220.80 calculation comes out to 100A or less, there's no need to do a Part III calculation.

Cheers, Wayne

 

[curt swartz]

OP, will you share your load calcs so we can see what you did?

The service load should be significantly less than adding the 4 individual dwelling calcs together.

Also, if you area is using the 2020 or 2023 NEC check with the AHJ to see if they will accept the SquareD meter center you are proposing. It does not comply with 230.71(B).

 

[Chevys10zr2]

Not quite. The restriction in 220.82 just says that the service or main feeder conductors must have an ampacity of at least 100A. So as long as you install a 100A service or main feeder to the dwelling unit, if the 220.80 calculation comes out to 100A or less, there's no need to do a Part III calculation.

Cheers, Wayne

So it's common to do the Optional Calculation first? And if the Calculation comes out to be 100 amps or less you don't have to use the standard calculation? If the calculation comes out to over 100 amps you are supposed to use the standard?
I always thought you could use either one no matter the circumstances.

 

[Chevys10zr2]

OP, will you share your load calcs so we can see what you did?

The service load should be significantly less than adding the 4 individual dwelling calcs together.

Also, if you area is using the 2020 or 2023 NEC check with the AHJ to see if they will accept the SquareD meter center you are proposing. It does not comply with 230.71(B).

I'm not quite sure how to post pictures on here... to show you my calculations. If you could direct me on how to do it I can share them.
I'm in Wisconsin. We are on 2017 nec code. I believe I should be good with that meter socket. I think we are allowed to have up to 6 disconnects. That meter setup has 4 disconnects total... 1 for each unit.

 

[wwhitney]

So it's common to do the Optional Calculation first? And if the Calculation comes out to be 100 amps or less you don't have to use the standard calculation? If the calculation comes out to over 100 amps you are supposed to use the standard?

No.

Tortuga seemed to say that if that if the optional calculation came out under 100A, you couldn't use it. That's not what 220.82 says; it says that you can only use the optional calculation if you provide at least 100A ampacity of conductors. So when the optional calculation comes out under 100A, just round the service size up to 100A and that's fine. Which is in accordance with the requirements of 230.79(C) anyway.

Cheers, Wayne