Conductor Sizing Question
Conductor Sizing Question
(OP)
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
Thanks,
Doug






RE: Conductor Sizing Question
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
RE: Conductor Sizing Question
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.
RE: Conductor Sizing Question
Thanks for your answers,
Doug
RE: Conductor Sizing Question
Also, unless your upstream breaker is 100%-rated, it can carry only 80% of 800 A on a continuous basis.
RE: Conductor Sizing Question
RE: Conductor Sizing Question
RE: Conductor Sizing Question
RE: Conductor Sizing Question
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!
RE: Conductor Sizing Question
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.
RE: Conductor Sizing Question
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
RE: Conductor Sizing Question
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.