400a Main breaker fed with 350mcm

[infinity]

I get 350s to the left of the equipment.

The 350's are spliced to the 500's in the wireway and feed a 400 amp non-fused disco then a 400 amp main breaker panel.

 

[ptonsparky]

Post 11, OP says they are 500 up to the gutter.

Okay, I'm cross eyed again.

 

[Npstewart]

id prefer to use 500mcm, it always escapes me when can I


legally use 500mcm for a 400a service when 500mcm is only rated at 380 75degree?

Because 240.4(b) allows you to round up to the next standard size circuit breaker. They don't make a 380A breaker so you're allowed to round up to the next standard size breaker. However, there are limitations to this.

 

[raberding]

Don't forget that if the load exceeds 380 amps, 500mcm won't get it. Parallel #3/0 or 600mcm will be needed.

 

[Npstewart]

Don't forget that if the load exceeds 380 amps, 500mcm won't get it. Parallel #3/0 or 600mcm will be needed.

Agreed, those are some of the limitations. Also, can’t be used on secondary side of a transformer off memory, or anything 800A or greater, etc.

I’m not an electrician but I do my own electrical work, and I’d much prefer to work with 2 sets of 3/0 wire vs. something like #500 or 600s.


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[kwired]

Agreed, those are some of the limitations. Also, can’t be used on secondary side of a transformer off memory, or anything 800A or greater, etc.

I’m not an electrician but I do my own electrical work, and I’d much prefer to work with 2 sets of 3/0 wire vs. something like #500 or 600s.


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Next size up rule ends once you go over 800 amps. People sometimes get into trouble with this thinking 500 copper is good for 400 amps and they run say three sets for a 1200 amp circuit. That gives you 1140 amps of conductor but you are no longer allowed to use next size up rule.

 

[Npstewart]

Next size up rule ends once you go over 800 amps. People sometimes get into trouble with this thinking 500 copper is good for 400 amps and they run say three sets for a 1200 amp circuit. That gives you 1140 amps of conductor but you are no longer allowed to use next size up rule.

Agreed. Personally, I only use 500s for large branch circuit wiring provided the load is less than 380A. Otherwise, it’s parallel 3/0 (or) #600s.


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[Jraef]

I’m wondering if the thing they got wrong was the NUMBER of cables, ie it should have been put down as 2x350 per phase… (and they wanted some future growth capacity).

 

[Elect117]

There are couple of red flags to me:

1) the transformer secondary written as 500KCM (Typical). Those conductors exist and should be verifiable at the main disconnect. He also calls them feeders, which is odd.

2) the new disconnect & panel PM are feed from a tap, and should follow the rules for 240.21(B), and so there is no rounding up. They also called panel PM main breaker.

3) I don't know what a double tub is, but it looks like PM feeds PM1? if so, then PM1 will need to be rated 400A or it needs protection based on 408.36.

Yes, i would say it needs 600kcmil.