THHN/THWN 90C Damage Curve 3

[JJHorak]

I am doing some short circuit and TCC studies for a customer using SKM products. The customer asked me to put cable damage curves on the TCCs. They have 90C THHN/THWN PVC 600V cable in general. SKM has buried in thier default databases a damage temperature for cable of 150C, yet I can find no backup for it. Further, at this web site, the mfr is selling 90C THHN/THWN PVC cable with a 250C damage temperature.

http://www.houwire.com/products/products.asp?catid=45&parentcatid=40

I am at a loss on what damage temperature I should use. Any ideas?

Is time a loom, weaving all together, or a broom, sweeping all away?

 

[jghrist]

The IEEE Buff Book (Protection and Coordination), IEEE Std 242-1986 references the following short-circuit temperature ratings from ICEA P-32-382-1969:

Rubber (75°C continuous) - 200°C
Rubber (90°C continuous) - 250°C
Silicone rubber - 250°C
Thermoplastic (60, 75, 90°C continuous) - 150°C
Paper - 200°C
Varnished Cloth - 200°C

 

[JJHorak]

Thanks; that gives me a good resource. I dug up the 2001 buff book and it says the same thing. I take this to say PVC(thermoplastic) has the same damage temperature, no matter what the continuous rating is. An engineer from Soutwire emailed me some pages out of the Southwire cable manual that says the all PVC.

This indicates a downside to using 90C cable. If you really plan on running the cable to 90C, your damage temperature is closer to your running temperature, and you are more likely to damage the cable during a fault.

 

[dpc]

It is rare (in NEC land) that you can legally load 600 V conductors up to their 90 deg C ampacity, for a variety of reasons. Normally the terminations are rated for a maximum of 75 deg C ratings.

If you're planning to load 600 V cables up to their 90 deg C ratings, better take another look.

 

[alehman]

I think the curves are from ICEA.

http://www.skm.com/Equipment Damage Curves Cables.shtml

See page 74:

http://www.bussmann.com/library/docs/spd02/SPDSection10.pdf

Alan
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"It’s always fun to do the impossible." - Walt Disney

 

[jghrist]

This indicates a downside to using 90C cable. If you really plan on running the cable to 90C, your damage temperature is closer to your running temperature, and you are more likely to damage the cable during a fault.

This only applies to thermoplastic insulation. You can use 250°C for thermosetting insulation like XHHW. If you are limited by fault current capacity, XHHW might be a good choice. As dpc notes, you are often limited by terminations for continuous loading capacity.

 

[JJHorak]

Thanks to everyone; I consider my question answered. Thanks for pointing out the breaker termination as a limiting factor, and I have to check that I do not have some higher temp cable in the mix, such as XHHW. I suppose using 90C cont. cable but using it at its 75C rating helps hide some sins associated improper cable derating, but it is bad form to even suggest that is acceptable practice. If cable is reaching 90C in an over-filled cable tray somewhere, my cable damage curves using 75C-150C will be a bit in error.

Everything thus far points to 150C as the damage point for typical PVC insulation. I think there was an error on the Houston Wire and Cable page, and their PVC insulated cable is not really rated to 250C, or at least it is not the norm. I called them on the matter yesterday, but no response yet.

I half-finished a sentence in my last post. I should have written, "An engineer from Southwire emailed me some pages out of the Southwire cable manual that says PVC insulation is rated only to 150C."

 

[jghrist]

If you have to derate cable because of conduit or tray fill, you can derate starting at the 90°C rating as long as the derated ampacity is less than the 75°C rating if you are using 75°C terminations and 90°C cable.

 

[dpc]

There is absolutely no problem in using 90 deg C insulation at 75 deg C ampacity. It is done all the time.

Also, if the cable is being de-rated for high ambient or conduit/tray fill, it is permissible to de-rate from the 90 deg C ampacity, as long as the 75 deg C ampacity is not exceeded.

Lastly, if you have any 100% rated breakers, these require 90 deg C conductors, but applied at 75 deg C ampacity.

 

[JJHorak]

Both dpc and jghrist flag a problem I have on how to derate cable:
Suppose I have cable rated at 260A base per NEC Table 310.16, 90C, (i.e.,4/0 CU THHW 3 conductors in raceway) and due to conduit fills I derate it 88% to 230A (which happens to be the 75C rating). If I run 230A into it, it seems I can assume that the cable is running at 90C down in the conduit. Due to heat conduction in the copper, I may have a cable running at 90C all the way into the breaker, depending on whether I can assume the cable cools off in the distance distance between the exit from the conduit and the point where it reaches the breaker.
--Or should I derate based on a 75C rating of the cable because I cannot take the assumption of any cable cooling between the exit from the conduit and the breaker. If I derate the entire cable down from 230A down to 203A, I can assume the cable runs at 75C down in the conduit, and hence the breaker is not at risk of seeing hot conductors.
--i.e., do I have 203A cable or 230A cable?

 

[alehman]

I don't think I've seen a minimum length specified anywhere. Maybe it's assumed to be sufficient if adequate wire bending space is provided per code.

Alan
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"It’s always fun to do the impossible." - Walt Disney