Feeder cable size

[hhsting]

I have feeder breaker 400A that has feeder cable 500kcmil CU feeding 400A MLO panel A.

Panel A demand load comes out to 390A both continuous and non continuous load.

Would the feeder cable size of 500kcmil ok or does it need to be upsized?

Assume 75C

 

[infinity]

Needs to be at least 390 amps so 500 kcmil is too small.

 

[hhsting]

I dont get it whats the point of 240.4(B) breaker rating less than 800A.

I mean 240.4(B) allows the 500kcmil on 400A it does not refer you back to calculating demand load 215.2 or nothing stated in 240.4(B) exception or Table to refer back to 215.2. Its confusing. Can someone please explain whats the point of 240.4(B) and when can someone use it?

 

[DrSparks]

I dont get it whats the point of 240.4(B) breaker rating less than 800A.

I mean 240.4(B) allows the 500kcmil on 400A it does not refer you back to calculating demand load 215.2 or nothing stated in 240.4(B) exception or Table to refer back to 215.2. Its confusing. Can someone please explain whats the point of 240.4(B) and when can someone use it?

Here are the conditions where you can:

(1) The conductors being protected are not part of a multioutlet branch ckt
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).

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[DrSparks]

That code says nothing about the wires, only the OCPD.

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[infinity]

Can someone please explain whats the point of 240.4(B) and when can someone use it?

You can use the next size up rule if your calculated load were 380 amps or less.

 

[hhsting]

Here are the conditions where you can:

(1) The conductors being protected are not part of a multioutlet branch ckt
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).

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I think both of those condition satisfy post#1. Condition 1 this is not branch circuit at all and #2 500kcmil has 380A ampacity and next standard size is 400A.

So how come the conductor size of 380A are smaller with 400A breaker and demand load of 390A? Its allowed by 240.4(B)

 

[DrSparks]

It doesn't matter what size breaker you use. It's the ampacity of the wires we're talking about here. We need 390 amps, so you need wire that is allowed to carry 390 amps. If you could find wire that had exactly 390 ampacity, you would be allowed to use a 400 Amp breaker, which exceeds the rating of the wire because it's the next higher standard breaker. 390 Amp fuses and breakers are not easy to come by.

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[hhsting]

It doesn't matter what size breaker you use. It's the ampacity of the wires we're talking about here. We need 390 amps, so you need wire that is allowed to carry 390 amps. If you could find wire that had exactly 390 ampacity, you would be allowed to use a 400 Amp breaker, which exceeds the rating of the wire because it's the next higher standard breaker. 390 Amp fuses and breakers are not easy to come by.

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But what does ampacity of wire matter at all? 240.4(B) says 400A breaker would protect cable with 380A ampacity. Note 240.4(B) does Not say if demand load exceeds 380A then 400A breaker will not protect 380A breaker.

240.4(B) still say demand load of 390a with cable ampacity of 380A then cable would be protected by 400A breaker Then why upsize in cable size is needed?

 

[infinity]

How is a 380 amp conductor going to safely carry a 390 amp load?

 

[DrSparks]

But what does ampacity of wire matter at all? 240.4(B) says 400A breaker would protect cable with 380A ampacity. Note 240.4(B) does Not say if demand load exceeds 380A then 400A breaker will not protect 380A breaker.

240.4(B) still say demand load of 390a with cable ampacity of 380A then cable would be protected by 400A breaker Then why upsize in cable size is needed?

I know. This one took me a while to grasp when I had to come to terms with it. 240.4(B) does not alleviate you from having to use conductors large enough to carry the calculated load it is to serve as in 310.16.

You have a calculated load to 390 amps. 500 MCM is good for 380, so you are violating 310.16. Let's say your load was 379 amps. 500 would be fine and a 400 Amp breaker would be fine too because of 240.4(B). Yes it's over fused a bit but the NEC allows that little fudge factor because they can't make breakers for every possible amperage readily available.

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[DrSparks]

Could you run that circuit at 395 amps continuously? Yes. Would you be overloading this circuit according to the NEC? Yes. Would you be overloading this circuit in reality? Probably not. This is why the allowable ampacity tables are purposely conservative. Ask an engineer

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[wwhitney]

I mean 240.4(B) allows the 500kcmil on 400A it does not refer you back to calculating demand load 215.2 or nothing stated in 240.4(B) exception or Table to refer back to 215.2. Its confusing. Can someone please explain whats the point of 240.4(B) and when can someone use it?

215.2 always applies; nothing in 240.4(B) relieves you from complying with 215.2

Say you had a calculated non-continuous load of 370A. 215.2 could be satisfied with 500 kcmil Cu, with a 75C ampacity of 380A. But 215.3 requires a breaker of at least 370A, which is not a standard size. So you go to a 400A breaker.

Without 240.4(B), you couldn't use a 400A breaker on a circuit with a 380A ampacity. You'd have to upsize the wires to 600 kcmil Cu so the 400A breaker can protect them. You end up with 420A ampacity for a 370A load, a bit wasteful.

With 240.4(B), you can stick with the 500 kcmil Cu conductors, even though 380A < 400A.

Cheers, Wayne

 

[electrofelon]

215.2 always applies; nothing in 240.4(B) relieves you from complying with 215.2



Cheers, Wayne

Right. It always helps to remind yourself what section a given code is in. The conductors in question is a feeder, so you go to 215 and it tells you how to size the conductor. Next you need to select an OCPD to protect that conductor. That is what article 240 is, overcurrent protection. Article 240 is where the next size up rule is. If you are in 240 and have not selected a conductor size yet, please return to article 215 and choose the appropriate conductor. Sorry, you will lose your place in line, next please.....

 

[infinity]

As Wayne stated to utilize the next up rules in Article 240 you have to wrap your head around other code articles like 215.

 

[hhsting]

All that is a problem when plan reviewer and lets say 400A feeder breaker is existing, feeder cable of 500kcmil is existing and Panel A MLO 400A is existing but then you are adding new loads to Panel A and new demand is 390A due to adding load. Would have to send plans back to provide 600kciml cable?

 

[infinity]

All that is a problem when plan reviewer and lets say 400A feeder breaker is existing, feeder cable of 500kcmil is existing and Panel A MLO 400A is existing but then you are adding new loads to Panel A and new demand is 390A due to adding load. Would have to send plans back to provide 600kciml cable?

Either 600's or some other size that will be large enough for 390 amps. Could be parallel #3/0's.

 

[DrSparks]

All that is a problem when plan reviewer and lets say 400A feeder breaker is existing, feeder cable of 500kcmil is existing and Panel A MLO 400A is existing but then you are adding new loads to Panel A and new demand is 390A due to adding load. Would have to send plans back to provide 600kciml cable?

If you added new loads to a panel and feeder that has a tight design load with no room for growth, you bet you would have to start over from scratch and up everything.

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[DrSparks]

Either 600's or some other size that will be large enough for 390 amps. Could be parallel #3/0's.

Yeah I'd definitely opt for parallel. So much easier to work with.

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[electrofelon]

All that is a problem when plan reviewer and lets say 400A feeder breaker is existing, feeder cable of 500kcmil is existing and Panel A MLO 400A is existing but then you are adding new loads to Panel A and new demand is 390A due to adding load. Would have to send plans back to provide 600kciml cable?

It's not compliant, send the plans back. What is the problem? It's not your role to come up with a solution