3 phase service conductor sizing?

[Electron_Sam78]

Hey all, I came across a 600a, 3 phase, 480/277V service that has 2 sets of 4/0 copper service conductors. This seems under rated with a 460 circuit ampacity. The conductors come directly from the utility transformer into an ATS then to a 600amp CB in the MDP. As a caveat, I've never had to install a service higher than 240V, am I missing something?

:?

 

[Gregg Harris]

Hey all, I came across a 600a, 3 phase, 480/277V service that has 2 sets of 4/0 copper service conductors. This seems under rated with a 460 circuit ampacity. The conductors come directly from the utility transformer into an ATS then to a 600amp CB in the MDP. As a caveat, I've never had to install a service higher than 240V, am I missing something?

:?

480/277
600 Amp
20 feet
4/0
Vd 0.7
Vd % 0.3
voltage 276.3

 

[erickench]

NEC 240.4(B) would allow you to size the OCPD to the next higher standard rating which in this case would be 500 amps. Look's like a violation. What equipment is upstream from the ATS?

 

[kwired]

Next size up allows for 500 amp overcurrent device, but the calculated load must be 460 or less because conductor ampacity is 460.

Someone possibly replaced 450 or 500 amp fuses with 600's, or used 90 deg C ampacity values instead of 75 deg C. to come up with 520 ampacity.

 

[Smart $]

... The conductors come directly from the utility transformer into an ATS then to a 600amp CB in the MDP. ...

The ATS should be service-rated, i.e. containing disconnect with OCPD. What's its OCPD rating?

 

[Electron_Sam78]

NEC 240.4(B) would allow you to size the OCPD to the next higher standard rating which in this case would be 500 amps. Look's like a violation. What equipment is upstream from the ATS?

That's what I get too when looking at the NEC. I've been looking at the code book so think about this, if they're using 90 degree rated wire and the 90 degree column of T310.15(B)(16) then 4/0 would be 260 x 2 = 520. That would allow a 600 a breaker. The calculated load just couldn't exceed the rating from the 75 degree column which is 460a total. The calculated phase amps, according to the drawings, are A-N: 466.6 , B-N: 421.9, C-N: 403.6. So they probably just fudged that 6.6amps over 460 to save money and go with the 4/0.

We have a 275KW standby generator and a 300KVA utility transformer upstream from the ATS.

 

[Electron_Sam78]

The ATS should be service-rated, i.e. containing disconnect with OCPD. What's its OCPD rating?

this ATS does not have an OCPD. There's a CB downstream at the MDP and upstream at the generator

 

[Smart $]

this ATS does not have an OCPD. There's a CB downstream at the MDP and upstream at the generator

Non-compliant with 230.82 and 230.94, equipment and circuits permitted on the supply side of the service disconnect and ocp.

 

[don_resqcapt19]

That's what I get too when looking at the NEC. I've been looking at the code book so think about this, if they're using 90 degree rated wire and the 90 degree column of T310.15(B)(16) then 4/0 would be 260 x 2 = 520. That would allow a 600 a breaker. ...

That is not correct, unless the equipment and terminations at both ends are rated for 90?C (very unlikely) the only thing you can use the 90?C ampacity ratings for is derating.

 

[Electron_Sam78]

That is not correct, unless the equipment and terminations at both ends are rated for 90?C (very unlikely) the only thing you can use the 90?C ampacity ratings for is derating.

I agree but that is probably how the process they used to get the 4/0

 

[kwired]

So they probably just fudged that 6.6amps over 460 to save money and go with the 4/0.

Even if you let the 6.6 amps too much slide, the next size standard overcurrent device is 500 amps.

 

[augie47]

I concur with the others, assuming your 4/0s are in separate conduits and not all in one, loads can not exceed 460 amps, fuse can not exceed 500 and ATS must have overcurrent ahead of it unless it's service rated.