Sizing wire to 400 amp service

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powerpete69 said:
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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Great, do you have a code reference for that exception? Also I'm going to size the neutral full size. What size ground do I need to go with if I use the 500 cu? Was originally going to go with AL but want to keep my conduit size down also.
 
frofro19 said:
Great, do you have a code reference for that exception? Also I'm going to size the neutral full size. What size ground do I need to go with if I use the 500 cu? Was originally going to go with AL but want to keep my conduit size down also.
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I don't have the code reference handy, but I have seen it in the past. I'm sure someone else will know it. Inspectors allow the 500 mcm to be good for 400 amps a piece all the way up to a 800 amp breaker. Above the 800 amp breaker, the ratings hold at the 380 rating.
Definitely use a neutral that matches the current carrying wires, 500 mcm in this case.

I believe this is a service entrance, table 250.66 in NEC shows a 1/0 copper ground for your application assuming you are using a 400 amp breaker and 500 MCM to feed downstream. A 1/0 copper ground will be required in each conduit.

Again, breakers are sized to protect wires in overload situations and short circuit situations.
 
Back to sizing wire for 1200 amp breaker -
Back in 2017, I had a 1200 amp breaker and sized (3 sets) of 600 MCM for the drawings.
The inspector still called me out on the wire size. It was a newer Square D breaker.
I had to literally prove to him that the breaker was a 80% breaker instead of a 100% breaker. (long time setting)
I had to pull out the serial number of the breaker and prove by one of the "letters" that it was 80%!
Sort of an odd thing to call out since almost all breakers are 80% breakers.....but yes, you can order 100% breakers, just make sure your wire size jumps up 1.25 the size compared to the 100% breaker.

Nonetheless, I still prefer (4 sets) of 350 MCM for this 1200 amp application for standard 80% breaker.
 
powerpete69 said:
Back to sizing wire for 1200 amp breaker -
Back in 2017, I had a 1200 amp breaker and sized (3 sets) of 600 MCM for the drawings.
The inspector still called me out on the wire size. It was a newer Square D breaker.
I had to literally prove to him that the breaker was a 80% breaker instead of a 100% breaker. (long time setting)
I had to pull out the serial number of the breaker and prove by one of the "letters" that it was 80%!
Sort of an odd thing to call out since almost all breakers are 80% breakers.....but yes, you can order 100% breakers, just make sure your wire size jumps up 1.25 the size compared to the 100% breaker.

Nonetheless, I still prefer (4 sets) of 350 MCM for this 1200 amp application for standard 80% breaker.
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Hmm, unless I am misunderstanding, you do not need to size conductors at 125% for a 100% breaker. I believe you need to use 90 degree rated conductors but sized off the 75 degree collum.
 
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 ?
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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...
I used the 40% demand factor to get the service size for the total load of the building and if I have 7 apartments on one 400 amp panel I'm using the demand factor of 44%. 25,000x7x44%=77,000÷208÷1.732=213 amps per panel. Each panel will also have 2 dryers and 2 washers. There is only one meter as the owner is paying for all utilities
 
I can't tell you if you came up with load calculation correctly, but assuming you did and it was 214 amps - then you need a minimum of 214 amp of supply conductor no matter what. Next standard overcurrent device up is 225 so you could have 214 amp conductor and a 225 OCPD.

Since you already are planning to use a 400 amp breaker then it wouldn't matter if you only had 10 amps of calculated load, you still need 400 amp conductors (or at least 351 because of next size up rule) because you have a 400 amps OCPD.
 
kwired said:
I can't tell you if you came up with load calculation correctly, but assuming you did and it was 214 amps - then you need a minimum of 214 amp of supply conductor no matter what. Next standard overcurrent device up is 225 so you could have 214 amp conductor and a 225 OCPD.

Since you already are planning to use a 400 amp breaker then it wouldn't matter if you only had 10 amps of calculated load, you still need 400 amp conductors (or at least 351 because of next size up rule) because you have a 400 amps OCPD.
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That's what I used. Installed the 400 amp panels with 400 amp main breakers and ran (3) #500 cu for the ungrounded conductors, #500 cu for the neutral and a #1/0 cu for the ground in each panel.
 
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