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?
Sizing wire to 400 amp service
- Thread starter frofro19
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- Licensed Electrician
No, but the conductors feeding them have to be protected at their rating.
Beaches EE
Senior Member
- Location
- NE Florida
- Occupation
- Electrical Engineer / Facilities Manager
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.
MyCleveland
Senior Member
- Location
- Cleveland, Ohio
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 ?
So the conductors from the switchgear to the 400 amp panel would have to be rated for 400 amps?
Beaches EE
Senior Member
- Location
- NE Florida
- Occupation
- Electrical Engineer / Facilities Manager
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?
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?
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?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
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.
powerpete69
Senior Member
- Location
- Northeast, Ohio
- Occupation
- Professional Electrical Engineer
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.
I already have the panels and they have a 400 amp breaker in them.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
You can use next standard size up so you need a minimum of 351 amps of wire.
Is that based on 90 degree wire and cu?
So a 700 AL would satisfy this requirement?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
No that's based on the 75° C ampacity and the next standard size up rule.
Yes a 700 kcmil Al conductor at 375 amps would work.
Or a 500 kcmil CU conductor at 380 amps?
powerpete69
Senior Member
- Location
- Northeast, Ohio
- Occupation
- Professional Electrical Engineer
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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