Conductor Sizing Question

[dougjl]

I have a question about conductor sizing. I have been reviewing an MCC drawing that shows 2 X 3/C 500 MCM Armored cables running from the switchgear to the MCC incoming section. The lugs are 75C, the voltage 480VAC, the cable run via open cable tray. The switchgear breaker is 800A. My understanding from NEC is that cable ampacity is taken from 310.16 which for a 500MCM at 75C is 380A, 2 X 380 = 760A which is much less than the 800A available. Can anyone tell me why this arrangement would be allowable?

Thanks,
Doug

 

[resqcapt19]

Doug,
As long as the calculated load is 760 amps or less, 240.4(B) permits the use of the next larger standard size OCPD as long as the OCPD is rated 800 amps or less.
Don

 

[BJC]

Keep reading your code book. When conductors are sized you can use the next standard size breaker. If your conductors are sized for 140 amps you can use a 150 amp breaker.
I don't have a code book here so I can find chapter and verse, but you can.
800 amps is the cut off point. If your breaker is greater than 800 amps the conductor have to have amapcity equal to or greater than the breaker rating.
Two 500 kCmil conductors are OK with an 800 amp breaker but three 500s are not OK with a 1200 amp breaker.

 

[dougjl]

The MCC is rated at 800A so I would have assumed and maybe incorrectly that you could put 800A of load on it safely, but not if the feed conductors are unable to carry the load. I understand the next size breaker, but I guess I would figure MCC load as the capacity of the MCC (800A). Wouldn't this seem like a reasonable assumption?

Thanks for your answers,
Doug

 

[dpc]

As already stated, this is allowed by the NEC and is very common. There is a lot of conservatism built into the NEC conductor ampacities.

Also, unless your upstream breaker is 100%-rated, it can carry only 80% of 800 A on a continuous basis.

 

[dougjl]

Upstream breaker is 100% rated

 

[dpc]

That's great - but the feeder is still large enough per NEC.

 

[jghrist]

I would figure MCC load as the capacity of the MCC (800A). Wouldn't this seem like a reasonable assumption?

No, it is not a reasonable assumption. I would expect the MCC load to be no more than the MCC capacity. The load could well be considerably less than the MCC capacity.

 

[fredpar]

Dear Doug:

I believe that if you need to pick the right conductor to handle up to 800 Amps, according to table 310.16 there is no single conductor to handle it. Then you got to use at least two single conductors per phase. In your case you want to use either 500MCM [380Amps] or 600MCM [420Amps]. BUT because you are using more than three single conductors in total you need to apply the ADJUSTEMENT FACTOR FOR MORE THAN THREE CURRENT CARRYING CONDUCTORS which in this case [Table 310.15(B)(2)(a)] is 0.8. That means now the ampacities for the above mentioned conductors are: 304Amps and 336Amps respectively. From table 310.16 you want to use either two 900MCM or two 1000MCM [545Amps].
Hope this helps!

 

[rcw retired EE]

Fredpar - Your comment on derating for multiple conductors in a raceway is correct if the cables are in conduit. But in this case, the 3/c armored cables are run in open cable tray, so the NEC derating does not apply. If the cable is spaced by at least one diameter, the ampacity rating in the tray for a 75C conductor could be as high as 416 amps. (2002 NEC Table B310.3, I can’t check the latest NEC, someone borrowed it.)

Note that derating may apply if installation is in Canada. CEC requires derating for multiple power conductors in tray if spacing is not maintained.

Just because the MCC has an 800 A main bus does not mean it needs to be fed with an 800 Amp feeder. We use 800A bus as a standard, but many MCC’s may only have 150 Amps of connected load, so we run a 200 or 250 amp feeder and protect it accordingly. Running 800A feeds to the MCC's for 150A of load is not economically justified.

In this case, the 800A Main Breaker on the MCC does imply that the load could grow to 700+ amps so the 2-500’s per phase makes sense.

 

[jraef]

I think dougil has a valid point though, even though the NEC allows it. If at some future point someone looks at that MCC and sees it rated for 800A, then they see the breaker feeding it is rated 800A continuous, they might in fact jump to the conclusion that it is OK to load that puppy up to 800A continuous. An engineer would likely think to double check the conductors, but would an electrician?

One thing though dougil,
You stated that the cable is 500kCMil, and the lugs are rated 75[°]C, but by any chance is the cable itself rated at 90[°]C? that would make a big difference! The ampacity of the lugs may be higher at 75[°]C than the cable would be at that temperature.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dougjl]

Thanks all for your input. FYI the mill that this work is being done for normally has connected loads of greater than 800A on a 800A MCC, since some of this equipment is backup and most of the load does not run at full load they don't normally have problems.

Since I know this will eventually happen to this MCC, the connected load that I am adding is over 700A then I want to size the conductors to carry the full capability of the MCC. This is why I am assuming a connected load of 800A, even though I realize that it will never actually reach that limit.

I spoke with the folks at NEC today and since armored cable in cable tray has its ampacity taken from 310.16 and the lugs are 75C this means that the ampacity of 2 X 500 per phase is 380 X 2 or 760A. This would not satisfy my needs, however the fact that the MCC and transformer rooms are cooled to a temperature of less than 77F I can correct the ampacity of the wire by a factor of 1.05. This results in an ampacity of 798A which is should be more than close enough.