400A Service with #400MCM

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DST83

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Hannibal, Missouri
We are working on a job where the prints call out for us to install a 480/277V 400A breaker to be fed by (4) #400MCM THHN/THWN conductors and (1) #4 ground. The feed from the breaker goes to a 480/277V 130kVA UPS with an input voltage of 151A. The UPS then feeds a 480/277V 300A panel which feeds a 480-120/208V 45kVA transformer which then feeds another 208/120V 150A panel.

The question here is whether or not the (4) #400MCM conductors is sufficient as the feeder from the 400A breaker to the UPS. By the code table it does not appear to be but (and not to be caddy) this was engineered for a reason the way it is, we just can't figure out why and if it will really meet NEC code.

Any advice?

Thanks -
 
I don't believe a 400 amp breaker can feed 400 mcm copper conductors. Those conductors are only good for 335 amps so a 350 amp overcurrent protective device could be used - not 400
 
dkidd said:
Are the 10 or 25 foot tap rules applicable? Otherwise, it isn't compliant.
Click to expand...

Nope, all the runs are longer than 25'.

We are scratching our heads in our office because we all seem to remember there being some sort of caveat related to #400MCM wire...can you think of anything? I'm not sure it applies here anyway, now we are just annoyed we can't remember.
 
Engineers make mistakes all the time. I'm sure it's a simple mistake overlooked by the engineer, however 400A OCP and 500 MCM CU conductors are such common installations that it must have been an engineers consultant and overlooked by the engineer when proofing.
Did the building department approve the drawings for installation?
 
So what options does the OP have? Use a 350A or smaller breaker? Do 400A breakers have trip adjustment to trip at <400A? Rewire with 500MCM? Fused disco with 350A or smaller breakers? Another option?

Of the listed options, which would be the least expensive in T&M for a new install? For correcting an existing install?

Btw, Welcome to the Forum!
 
Maybe what the drawing actually says is a 400 amp frame breaker not a breaker with a 400-amp trip point. I think a request for information is in order.
 
DST83 said:
We are working on a job where the prints call out for us to install a 480/277V 400A breaker to be fed by (4) #400MCM THHN/THWN conductors and (1) #4 ground. The feed from the breaker goes to a 480/277V 130kVA UPS with an input voltage of 151A. The UPS then feeds a 480/277V 300A panel which feeds a 480-120/208V 45kVA transformer which then feeds another 208/120V 150A panel.

The question here is whether or not the (4) #400MCM conductors is sufficient as the feeder from the 400A breaker to the UPS. By the code table it does not appear to be but (and not to be caddy) this was engineered for a reason the way it is, we just can't figure out why and if it will really meet NEC code.

Any advice?

Thanks -
Click to expand...

2017 NEC 215.2(A)(1)(a) allows feeder conductors to be sized @ 100% of the continuous load (not 125%) if the ends of the feeders are terminated on 90 Degree C. Lugs and not terminated in switchgear.
I'm not positive, however you may be able to terminate the 400 MCM wire (380A @ 90 Degrees), on a 400 A C/B rated at 100% ( not 80% as most are).

Take a look on Mikeholt.com and there he has many videos. Scroll down to the feeder sizing video and see what I mean.
 
DST83 said:
We are working on a job where the prints call out for us to install a 480/277V 400A breaker to be fed by (4) #400MCM THHN/THWN conductors and (1) #4 ground. The feed from the breaker goes to a 480/277V 130kVA UPS with an input voltage of 151A. The UPS then feeds a 480/277V 300A panel which feeds a 480-120/208V 45kVA transformer which then feeds another 208/120V 150A panel.

The question here is whether or not the (4) #400MCM conductors is sufficient as the feeder from the 400A breaker to the UPS. By the code table it does not appear to be but (and not to be caddy) this was engineered for a reason the way it is, we just can't figure out why and if it will really meet NEC code.

Any advice?

Thanks -
Click to expand...
NEC 2017 215.2(A)(1)(a) allows feeder conductors to be sized using the 90 degree column, as long as the terminals at each end are rated at 90 degrees. If terminated in a panelboard, the C/B must be rated at 100% (not 80%). It also permits feeder sizing at 100% of the continuous load (not 125%) and 100% of the non-continuous load.
400 MCM @ 90 Degrees equals 380A. 240.4 permits the installation of a 400A C/B (next higher)

Mike Holt.com has a video that explains this more detailed on his website. Look for his Feeder Sizing video link on his home page.

Good Luck:thumbsup:
 
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