Derating 90 degree column

[mstrlucky74]

My ups vendor and shop engineer seem to disagree. Have a 4-20a ckts in a 3/4" conduit with each ckt having 1h&1n. We all agree there are 8 ccc but ups engineer is using 75 degree column to derate which is why he shows #10 but my shop engineer is using 90 degree column which give you 21a and thus he's showing #12's. Which column should be used to derate in this situation ? Thanks

 

[oldsparky52]

I would side with your in house guy, 90c column for derating.

 

[kwired]

Terminations are likely 75C, so conductor still needs to be at least whatever 75C calls for as minimum.

You need to find minimum conductor size required for the terminations @ 75C and determine minimum size needed in the raceway per adjustments @ 90C and select whichever results in the larger conductor.

In general you usually don't need to increase conductor size for number of conductors in raceway until you get past 9 current carrying conductors - as long as ambient temp isn't a factor in adjustments. It always works out this way for small conductors like 6 AWG and smaller anyway.

 

[luckylerado]

My ups vendor and shop engineer seem to disagree. Have a 4-20a ckts in a 3/4" conduit with each ckt having 1h&1n. We all agree there are 8 ccc but ups engineer is using 75 degree column to derate which is why he shows #10 but my shop engineer is using 90 degree column which give you 21a and thus he's showing #12's. Which column should be used to derate in this situation ? Thanks

What type of insulation and is it run underground? We probably can assume THHN but...

 

[Sierrasparky]

Just remember if these are non linear loads in a 3phase system you may get some additional heating in the conduit.

 

[electrofelon]

In general you usually don't need to increase conductor size for number of conductors in raceway until you get past 9 current carrying conductors - as long as ambient temp isn't a factor in adjustments. It always works out this way for small conductors like 6 AWG and smaller anyway.

ID have to double check, but I thought 9 only works for 12 and 14 awg, max 10's is 6 no?

 

[kwired]

ID have to double check, but I thought 9 only works for 12 and 14 awg, max 10's is 6 no?

10 and 8 get you into that borderline area where next standard size OCPD works as long as your load isn't over the ampacity of the conductor - so yes you need to watch those a little more carefully.

 

[mstrlucky74]

Thhn

 

[Carultch]

Just remember if these are non linear loads in a 3phase system you may get some additional heating in the conduit.

How exactly does one objectively know if the non-linear loads are significant enough to matter?

Is there something I can look for in an oscilloscope to know? Is there a metric such as total harmonic distortion?

 

[kwired]

How exactly does one objectively know if the non-linear loads are significant enough to matter?

Is there something I can look for in an oscilloscope to know? Is there a metric such as total harmonic distortion?

I don't think they will matter much on the ungrounded conductors . It is the neutral of multiwire circuits that you can have issues with as some of the harmonic currents are additive instead of canceling one another in the neutral.

 

[luckylerado]

Thhn

Underground?? THHN/THWN is derated from the 75 deg column in wet locations.

 

[kwired]

Underground?? THHN/THWN is derated from the 75 deg column in wet locations.

But THWN-2 is 90C wet or dry, most of what is available today is rated THWN-2, but one may want to double check just in case.

 

[charlie b]

Since nobody has yet cited a code reference, I will mention that the last sentence of 110.14(C) is the basis of this whole discussion. :happyyes:

 

[charlie b]

ID have to double check, but I thought 9 only works for 12 and 14 awg, max 10's is 6 no?

True. The 90C rating of #10 THHN is 40 amps. With 7 or more CCCs, you take 70% of 40 and the result is lower than 30 amps. Thus, you can't use a 30 amp breaker to protect this circuit.

 

[Carultch]

Since nobody has yet cited a code reference, I will mention that the last sentence of 110.14(C) is the basis of this whole discussion. :happyyes:

In my explanation, it states that derate factors of both kinds only apply to the wire ampacity rating, and not to the terminations. Terminations get the unadjusted ampacity, usually from the 75C column, except for 100A and less that are not listed and marked otherwise, which get the 60C column by default (a more academic rule, than practical). The wire itself can start with an ampacity at a temperature higher than the termination rating, and it is this ampacity that gets adjusted for ambient temperature (possibly increased due to direct sunlight) and bundling.

 

[Smart $]

In my explanation, it states that derate factors of both kinds only apply to the wire ampacity rating, and not to the terminations. Terminations get the unadjusted ampacity, usually from the 75C column, except for 100A and less that are not listed and marked otherwise, which get the 60C column by default (a more academic rule, than practical). The wire itself can start with an ampacity at a temperature higher than the termination rating, and it is this ampacity that gets adjusted for ambient temperature (possibly increased due to direct sunlight) and bundling.

This is one of the downsides of current 110.14(C). All sizing-for-termination determinations are based on 30°C ambient temperature though the actual ambient at terminations may be different.

 

[Carultch]

This is one of the downsides of current 110.14(C). All sizing-for-termination determinations are based on 30°C ambient temperature though the actual ambient at terminations may be different.

So in your opinion, you feel as if it would make more sense if you'd derate terminations at least for ambient temperature? I agree too. As long as bundling derates wouldn't apply, because it isn't bundled at the terminations.

There may have been a reason why it isn't required, or maybe it is an oversight.

 

[Smart $]

So in your opinion, you feel as if it would make more sense if you'd derate terminations at least for ambient temperature? I agree too. As long as bundling derates wouldn't apply, because it isn't bundled at the terminations.

There may have been a reason why it isn't required, or maybe it is an oversight.

I think it is an oversight. However, with no problems over the years that can be attributed directly to this oversight, and backed up with empirical evidence, makes it hard to get the CMP to include ambient correction for terminations.

 

[infinity]

My ups vendor and shop engineer seem to disagree. Have a 4-20a ckts in a 3/4" conduit with each ckt having 1h&1n. We all agree there are 8 ccc but ups engineer is using 75 degree column to derate which is why he shows #10 but my shop engineer is using 90 degree column which give you 21a and thus he's showing #12's. Which column should be used to derate in this situation ? Thanks

You use the column that corresponds to the conductor you're using. THNN would use the 90° C column for derating purposes.