Sizing cable using 90Deg C XXHW-2 insulation, 600V. 2017 NEC

[Isaiah]

I have a 300A CB at an indoor SWGR feeding a 3ph load of 270A located outdoors. I want to use 350kCMIL cable.
I used the 90 degree C column for ambient correction of Table 310.15(B)(2)(a) at 36-40C (i.e. 350A x 91=318.5A) for the outdoor run in rigid galvanized steel conduit.
The switchgear resides in a climate controlled environment so I’m assuming the 310A doesn’t require ambient correction since the CB/lugs are in a cool environment. At the lugs, you’d have 310A per the 75 degree C column in 310.15(B)(16) of the 2017 NEC.
Is this correct?


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[infinity]

At the lugs, you’d have 310A per the 75 degree C column in 310.15(B)(16) of the 2017 NEC.
Is this correct?

That is correct. You adjusted ampacity is 310 amps and the OCPD is 300 amps so you're good. How was the 270 amp load calculated? Is it continuous?

 

[Isaiah]

That is correct. You adjusted ampacity is 310 amps and the OCPD is 300 amps so you're good. How was the 270 amp load calculated? Is it continuous?

Yes it would be considered continuous. If were not continuous would it change the temperature ratings?


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[infinity]

Yes it would be considered continuous. If were not continuous would it change the temperature ratings?

No but your conductors would need to add in a 25% factor for a continuous load. Is that already part of the 270 amps?

 

[Dennis Alwon]

That would mean the conductor and overcurrent protective device must be good for 338 amps with 500kcm conductors and a 350 ocpd

I didn't do the calc for de-rating so this is probably not correct

 

[Isaiah]

That would mean the conductor and overcurrent protective device must be good for 338 amps with 500kcm conductors and a 350 ocpd

I didn't do the calc for de-rating so this is probably not correct

Thanks Dennis and Infinity


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[ramsy]

Yes it would be considered continuous. If were not continuous would it change the temperature ratings?

NEC tables solve for ampacity, which may rise as Power Factor drops.

Load Pf <=0.92 may exceed capacity of #350cu with balanced 224kW load @ 40c