Parallel conductors and derating

[lectrician7]

If I have a compressor that draws 467 amps and I feed it with 2 sets of 300s in parallel, do I derate due to 6 current carrying conductors and now I need 400s, or do I not have to derate because each pair is a considered a single conductor making a total of 3 conductors in the conduit? The 300mcm conductor size already takes into account the increase to 125% for continuous duty.

Thanks in advance

 

[infinity]

If the two sets are within the same raceway then derating applies to the 6 CCC's. In separate raceways it does not apply.

Welcome to the Forum.

 

[wwhitney]

If I have a compressor that draws 467 amps and I feed it with 2 sets of 300s in parallel

I assume you mean 300 kcmil copper, as 300 kcmil aluminum has a 75C ampacity of 230A, so two sets would be 460A, less than 467A.

do I derate due to 6 current carrying conductors and now I need 400s

As infinity said, you derate if you run all six in one conduit, and you don't if the two sets are in separate conduits.

But with 300 kcmil copper, the 90C ampacity is 320A, with 6 CCCs in one conduit, that becomes 256A, and with two sets you have 512A. Still more than 467A.

Cheers, Wayne

 

[lectrician7]

I assume you mean 300 kcmil copper, as 300 kcmil aluminum has a 75C ampacity of 230A, so two sets would be 460A, less than 467A.


As infinity said, you derate if you run all six in one conduit, and you don't if the two sets are in separate conduits.

But with 300 kcmil copper, the 90C ampacity is 320A, with 6 CCCs in one conduit, that becomes 256A, and with two sets you have 512A. Still more than 467A.

Cheers, Wayne

Thanks for the reply. The reason I’d still have to derate the 300 copper is upsized due to 125% for a continuous load.

 

[augie47]

Most compressors would not meet the definition of a continuous load.
Continuous Load. A load where the maximum current is expected to continue for 3 hours or more

 

[wwhitney]

Thanks for the reply. The reason I’d still have to derate the 300 copper is upsized due to 125% for a continuous load.

If it's a motor load, that already has a 125% factor in it, I don't believe you ever need an additional 125% for continuous loads. Plus what Augie said.

But say you have a 410A continuous load, not a motor. The checks you'd need to satisfy are:

(1) The 75C conductor ampacity is at least 125% * 410A =512A. No derating is applied.
(2) The 90C conductor ampacity, with adjustment and correction, is at least 410A.
(3) If you are using a 450A 100% rated OCPD (unlikely), you are done. If you are not using 100% rated OCPD, the OCPD must be at least 125% * 410A = 512A, or 600A if using a standard size. Now your 90C conductor ampacity, with adjustment and correction, needs to be at least 501A, per 240.4(B).

The point is that in the above, you never use 125% for continuous use simultaneously with derating. The termination check (1) gets the 125% continuous factor with no derating, and the wire ampacity check (2) gets derating with no continuous factor.

The other point is that if not using a 100% rated OCPD, the OCPD check (3) above is basically always stricter than the wire ampacity check (2).

Cheers, Wayne