NEC 2020 Table 310.15(B)(1)

Zyb

Zyb

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For derating conductors base on Table 310.15(B)(1). if I use 90 degree rated conductor but will terminate on 75 degree breaker. do I use the 90degree column or the 75 degree column?
 
Zyb said:
For derating conductors base on Table 310.15(B)(1). if I use 90 degree rated conductor but will terminate on 75 degree breaker. do I use the 90degree column or the 75 degree column?
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It's the primary value in having 90C wiring, despite being limited to 75C terminations most of the time. Gives you headroom for these calculations.
 
@Dennis Alwon

Just want to make sure my calcs is right. Example the current is 78A, conduit fill and temp. derate is 0.80 & 0.96 respcectively.

78A x 1.25 = 97.5A

Using #2 ; 130A @ 90Ddeg, 115A @ 75deg

130A x 0.80 x 0.96 = 99.8A

99.8A < 115A OK
97.5A < 99.8A OK
 
Zyb said:
Just want to make sure my calcs is right. Example the current is 78A, conduit fill and temp. derate is 0.80 & 0.96 respcectively.
Click to expand...
If that's 78A non-continuous, 75C termination rating, 90C insulation rating, and an 80A breaker, then you need a conductor for which:

(1A) The 75C column value is at least 78A.
(2A) The 90C column value is at least 78/0.8/0.96 = 102A.

If instead that's 78A continuous, 100A breaker, then you need a conductor for which:

(1B) The 75C column value is at least 78*125% = 98A
(2B) The 90C column value is at least 91/0.8/0.96 =118A.

Why 91A in (2B)? The rules in 210/215 only impose the requirement that is (2A), which is unchanged for a continuous load. But 210/215 do impose a minimum 98A breaker, so 100A breaker in practice. And 240.4 imposes a minimum ampacity of 91A for use with a 100A breaker (as 90A is the next lower size). So that gives us (2B), which is strictly stronger than (2A).

Cheers, Wayne
 
Hi, I need to follow-up on this thread for clarification. I was looking at a third party design then upon checking how they compute their wire size, it is more aggressive resulting to smaller wire size. My question is, is there formula correct? and my formula is just on a conservative side.

The third party, my interpretation and @wwhitney have the same formula on 1B column, we differ in 2B.

The current 210.5A is per set, there are two sets, hence conduit fill derate is 0.80.

Third Party Computation on 2B
derate the 90degree column and compare to the current
210.5 < 0.91 x 0.80 x 320 (300kcmil 90deg)
210.5 < 233

My interpretation
similar to the third party computation but it is compared to the design current 210.5A x 1.25 = 263.1A, thus resulting to a bigger wire 400kcmil

Can someone explain, why the third party did not multiply the current by 1.25 on 2B. Is this also correct? Thank you





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Zyb said:
My question is, is there formula correct?
Click to expand...
Grasshopper is not considering the most critical information.

What is the load, how many phases, Wye, or Delta transformer?

Most loads wont need 1.25% factor, unless 210.19, 215.2, or 220.18 apply.

Configurations shown in 310.15(E) may count the neutral as CCC.
 
Zyb said:
Can someone explain, why the third party did not multiply the current by 1.25 on 2B. Is this also correct?
Click to expand...
The third party is correct on the 1.25 factor. The 1.25 factor for continuous loads is only for terminations, not for the wire itself. The ampacity of a wire is already a continuous rating, per the definition.

This is reflected in the NEC in the various sections discussing this. For feeders, 215.2(A)(1)(a) is the termination check and includes the 125% factor for continuous loads (and does not include derating, although that is only expressed by the wording contrast with 215.2(A)(1)(b)). While 215.2(A)(1)(b) is the wire check and does not include a 125% factor for continuous loads.

Note that the third party selection is still wrong if the continuous load is on a 300A non-100% rated breaker, as reflected by your "this forum" line. This is due to the interaction of 240.4 and 215.3 (or comparable section if this is not a feeder).

To summarize:

Termination check:, no derating, use 125% factor for continuous portion of load, use ampacity column based on termination temperature rating

Wire check: derate, use ampacity column based on insulation temperature, check ampacity against both the load (no 125%) and the minimum for the breaker size selected. If a major portion of the load is continuous, the breaker size check is likely to control this check.

Cheers, Wayne
 
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