Sizing wire to 400 amp service

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frofro19

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Location
VA.
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Master Electrician
I have (2) 400 amp 3 phase panels that will supply 7 apartments each with 100 amp sub-panels. When doing the load calculations of 25,000 va each before demand factors and after using demand factors of 40 percent on total load I came up with around 388 amps. Splitting that into (2) 3 phase panels 400 amps each. Do those 400 amp panels have to have conductors rated at 400 amps if the total load after demand factors are around 194 amps each?
 
Do you have a 1-line from the customer end of the service to these panels? Without that and in general, if the calculated load is less than the ampacity of the panel(s) then the conductors and OCPD can be sized to the calculated load. You might consider sizing the conductors or at least the conduits to 400 amps in case of future expansion.
 
frofro19 said:
I have (2) 400 amp 3 phase panels that will supply 7 apartments each with 100 amp sub-panels. When doing the load calculations of 25,000 va each before demand factors and after using demand factors of 40 percent on total load I came up with around 388 amps. Splitting that into (2) 3 phase panels 400 amps each. Do those 400 amp panels have to have conductors rated at 400 amps if the total load after demand factors are around 194 amps each?
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Your discussion seems to be using the optional calc, assume your apartments qualify based on the loading but the 40% does not apply to ever item in the list of loads.
I believe you should be using 220.84 multipliers.
If you are spilitting into two meter stacks I think you then would use a multiplier for each stack based on the specific units connected to it.
Why are you splitting into TWO ? Isn't this more expensive on component costs ?
 
MyCleveland said:
Your discussion seems to be using the optional calc, assume your apartments qualify based on the loading but the 40% does not apply to ever item in the list of loads.
I believe you should be using 220.84 multipliers.
If you are spilitting into two meter stacks I think you then would use a multiplier for each stack based on the specific units connected to it.
Why are you splitting into TWO ? Isn't this more expensive on component costs ?
Click to expand...
 
The conductors from the switchgear to the 400 amp panel have to be rated for the calculated load and protected accordingly.
 
After using the demand factor of 44 percent (7 apartments) I came up with 214 amps per panel. 25,000 x 7=175,000 x 44 % =77,000 ÷208÷1.732= 213 amps. So I installed (2) panels 400 amp 3 phase to distribute the load. 14 apartments total and 7 on each 400 amp panel. I'm assuming I could have installed (2) 300 amp panel to take care of the total load?
 
Beaches EE said:
The conductors from the switchgear to the 400 amp panel have to be rated for the calculated load and protected accordingly.
Click to expand...
The calculated load is around 213 amps using the demand factor for 7 apartments. Would conductor size be based to calculated load of 213 amps even though it has a 400 amp breaker supplying the panel?
 
Beaches EE said:
The conductors from the switchgear to the 400 amp panel have to be rated for the calculated load and protected accordingly.
Click to expand...
After using the demand factor of 44 percent (7 apartments) I came up with 214 amps per panel. 25,000 x 7=175,000 x 44 % =77,000 ÷208÷1.732= 213 amps. So I installed (2) panels 400 amp 3 phase to distribute the load. 14 apartments total and 7 on each 400 amp panel. I'm assuming I could have installed (2) 300 amp panel to take care of the total load?
 
frofro19 said:
The calculated load is around 213 amps using the demand factor for 7 apartments. Would conductor size be based to calculated load of 213 amps even though it has a 400 amp breaker supplying the panel?
Click to expand...
The conductors have to meet BOTH of the following:

1. Meet the calculated load
2. Have ampacity greater than or equal to the OCPD(unless next size up rule applies)

About the only exception is common service conductors that serve multiple disconnects can be sized to the load calc alone, but I'm pretty sure the conductors you are talking about are not service conductors.
 
frofro19 said:
The calculated load is around 213 amps using the demand factor for 7 apartments. Would conductor size be based to calculated load of 213 amps even though it has a 400 amp breaker supplying the panel?
Click to expand...
The breaker must be sized to protect the wire.
If the feeder breaker is 400 amp for example, then you need at least 500 MCM.
 
Beaches EE said:
Do you have a 1-line from the customer end of the service to these panels? Without that and in general, if the calculated load is less than the ampacity of the panel(s) then the conductors and OCPD can be sized to the calculated load. You might consider sizing the conductors or at least the conduits to 400 amps in case of future expansion.
Click to expand...
I already have the panels and they have a 400 amp breaker in them.
 
frofro19 said:
Or a 500 kcmil CU conductor at 380 amps?
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Yes, it's actually good for 400 amps due to an exception in the code. (2) 500's will also be ok at 800 amps per code exception.
Three wires per phase and above you must use 380A per code. So for 1200 amps, I always recommend 4 sets of 350's. Lot less headaches.
 
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