Breaker size

[hhsting]

I have 400A feeder breaker fed from 500kcmil conductors to Panel A.

Panel A demand load comes out to be 390A but Contractor says 390A next standard size breaker according to NEC 240.4(B) can be 400A. I dont see anything in NEC 2017 that says to upside conductor to 600kcmil? How can this be?

 

[infinity]

One stipulation of using the next size up rule is that the calculated load cannot be greater than the ampacity of the conductor. Since 380 amps is less than 390 amps 500 kcmil cannot be used.

 

[augie47]

Its not fed from a transformer is it ?

 

[hhsting]

One stipulation of using the next size up rule is that the calculated load cannot be greater than the ampacity of the conductor. Since 380 amps is less than 390 amps 500 kcmil cannot be used.

Where I dont see that stipulation NEC 2017 ?

 

[don_resqcapt19]

Where I dont see that stipulation NEC 2017 ?

The rule in 240.4(B) does not change the ampacity of the conductor, it only permits the conductor to be protected by an OCPD with a rating that exceeds the ampacity of the conductor. The ampacity of 500kcmil is 380 amps and you cannot supply a load greater than 380 amps with that conductor.

 

[hhsting]

The rule in 240.4(B) does not change the ampacity of the conductor, it only permits the conductor to be protected by an OCPD with a rating that exceeds the ampacity of the conductor. The ampacity of 500kcmil is 380 amps and you cannot supply a load greater than 380 amps with that conductor.

But I cant state that can I. As reviewer I need code reference. Where is it in NEC 2017?

 

[curt swartz]

But I cant state that can I. As reviewer I need code reference. Where is it in NEC 2017?

240.4

 

[hhsting]

240.4

240.4 does have exception one of them is 240.4(B). So still stuck with OP post #1 question in regards to NEC 2017 code reference

 

[wwhitney]

But I cant state that can I. As reviewer I need code reference. Where is it in NEC 2017?

215.2(A)(1). A feeder with ampacity of 380A may not be used to supply a load of 390A.

Cheers, Wayne

 

[don_resqcapt19]

But I cant state that can I. As reviewer I need code reference. Where is it in NEC 2017?

Table 310.15(16)

 

[hhsting]

215.2(A)(1). A feeder with ampacity of 380A may not be used to supply a load of 390A.

Cheers, Wayne

NEC 2017 Article 240.4 first paragraph has exception to that and Table 310.15 and that exception is 240.4(B)

 

[wwhitney]

NEC 2017 Article 240.4 first paragraph has exception to that and Table 310.15 and that exception is 240.4(B)

No. 240.4(B) is an exception on breaker size vs conductor ampacity. It is not an exception to ampacity vs load.

And 240.4 can't amend 215.2(A)(1) anyway.

Cheers, Wayne

 

[hhsting]

Table 310.15(16)

See NEC 2017 Article 240.4 first paragraph that says exceptions to 310.15 including 240.4(b)

 

[wwhitney]

See NEC 2017 Article 240.4 first paragraph that says exceptions to 310.15 including 240.4(b)

Look, you have 3 different numbers:

Breaker rating
Conductor ampacity
Calculated load

Load is determined by Article 220. Conductor ampacity is determined by Article 310. Breaker rating is just printed on the breaker.

Then 240.4 says "Breaker rating <= Conductor ampacity unless . . ." Fine, the install would comply with 240.4.

But 215.2(A)(1) says "Conductor ampacity >= Calculated Load". The exceptions do not apply. The proposed install of 390A of load on a conductor with 380A ampacity is violation of 215.2(A)(1).

That's it.

Cheers, Wayne

 

[curt swartz]

Article 240 is about overcurrent protection. It does not change the ampacity of the conductor.

 

[infinity]

But 215.2(A)(1) says "Conductor ampacity >= Calculated Load". The exceptions do not apply. The proposed install of 390A of load on a conductor with 380A ampacity is violation of 215.2(A)(1).

That's it.

That pretty much ends the discussion.

 

[hhsting]

Look, you have 3 different numbers:

Breaker rating
Conductor ampacity
Calculated load

Load is determined by Article 220. Conductor ampacity is determined by Article 310. Breaker rating is just printed on the breaker.

Then 240.4 says "Breaker rating
But 215.2(A)(1) says "Conductor ampacity >= Calculated Load". The exceptions do not apply. The proposed install of 390A of load on a conductor with 380A ampacity is violation of 215.2(A)(1).

That's it.

Cheers, Wayne

Thanks for that info. Helpful.

 

[hhsting]

Its not fed from a transformer is it ?

I was about to post that on another post. But since you mentioned it i have another instance where transformer has not over 25ft secondary conductors that are 2 sets of 500kcmil with 800A secondary main breaker. NEC 2017 240.21(C)(6)(2) I get cannot be less than 1153A. Transformer primary 480v, 120/208V three phase secondary , primary overcurrent protection of 1500A. However according NEC 2017 Article 240.21(C)(6)(3), should not the minimum ampacity be greater than or equal to 800A? NEC 2017 Article 240.21(C)(6)first paragraph says must comply with all 1,2 and 3. How?

 

[infinity]

transformer has not over 25ft secondary conductors that are 2 sets of 500kcmil with 800A secondary main breaker

For two sets the minimum conductor size would be 600 kcmil.

Transformer primary 480v, 120/208V three phase secondary , primary overcurrent protection of 1500A

Why is the primary protection 1500 amps on the 480 volt side?

 

[hhsting]

For two sets the minimum conductor size would be 600 kcmil.



Why is the primary protection 1500 amps on the 480 volt side?

500kva transformer primary breaker 250% of FLA.

What is 240.21(C)(6)(1) for? When I use that then it comes to be 1153A and it says amapcity shall not be less then that.