THHN/THWN 90C Damage Curve
THHN/THWN 90C Damage Curve
(OP)
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.
ht tp://www.h ouwire.com /products/ products.a sp?catid=4 5&pare ntcatid=40
I am at a loss on what damage temperature I should use. Any ideas?
ht
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?






RE: THHN/THWN 90C Damage Curve
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
RE: THHN/THWN 90C Damage Curve
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.
RE: THHN/THWN 90C Damage Curve
If you're planning to load 600 V cables up to their 90 deg C ratings, better take another look.
RE: THHN/THWN 90C Damage Curve
h
See page 74:
http://ww
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: THHN/THWN 90C Damage Curve
RE: THHN/THWN 90C Damage Curve
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."
RE: THHN/THWN 90C Damage Curve
RE: THHN/THWN 90C Damage Curve
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.
RE: THHN/THWN 90C Damage Curve
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?
RE: THHN/THWN 90C Damage Curve
Alan
----
"It's always fun to do the impossible." - Walt Disney