i saw mention of this on another message board and its giving me pause.

Can someone please explain, in simple terms, the theory of derating of the wires when there are more conductors in a conduit or raceway than there should be?

I understand higher temps mean lower amp carrying ability of the conductor.

I understand that the sheathing of the wire (conductors)
is rated for a certain temp. I understsnd the danger of crimping the wire.

Is this something we should be delving into as Home Inspectors???
I mean, beyond noting the obvious “super crowded” conduit filled with wire, where do we draw the line?

This is a way to adjust for the amp and voltage drop? yes ?

Seems to be the domain of a experienced licensed elecrician imo

This is a way to adjust for the amp and voltage drop? yes ?

Seems to be the domain of a experienced licensed elecrician imo

It’s a matter of heat dissipation or the lack thereof.

More wires in a pipe = more heat buildup

More heat buildup = insulation of the conductors gets hotter

conductor insulation exposed to heat for long periods of time = brittle conductor insulation

Brittle conductor insulation = insulation failure

Insulation failure = possible fire, catastrophic equipment damage, etc.

Same thing goes for bundles of cables that ara bundled for great lengths.

I see Marc, and Michael, thank you.

Now- how can this be determined for the whole house?
this is just for conduit in hot areas like an attic, correct?

No, it applies to all conductors in conduit, regardless of where they’re installed. As the number of conductors in the bundle increases, they “self heat”. There is additional “math” to account for when the conduit is also installed in a hot ambient, like in an attic or running accross a flat roof.

The actual determination of whether a conduit is overloaded or not, in accordance with the gauge of conductor installed, is way beyond the scope of a home inspector’s purview. If you see some crazy amount of conductors coming out of a pipe into a panel, or lots of cables tied together in a bundle running across a basement, the best thing you can do is call it out as suspicious enough to warrant a look-see by an electrician.

Thank you MARC, I think i understand now.

In the past i have commented on " too many wires in conduit"

before, i imagined that the action of pulling and forcing it in there would damage the outer coating, leading to arcs and sparks. i guess thats possible too, especially if the conduit is bent wrong.

Where derating is probably ignored the most is large conductors in the attic.
Usually 240.4(D) covers you on small conductors (14/12) since the breaker size is artificially lower than the 60c column and you can derate from the 90c column. Not so much on #10.
When you might have a problem is if you used the label to size a conductor for an A/C compressor @ 75c and then ran it through a 130 degree attic. We usually get away with it in Florida by having the air handler and condenser on the same circuit and you size the circuit for the strip heaters. When the strip heat is on the attic is cool so the derating table is actually a plus number.
The condenser is usually considerably smaller than strip heaters. In my case the air handler is on 6ga which extends to the 40a disconnect for the condenser. I could have used 12ga for the whip (23a on the label) but I went nuts and used #10. Again with an ambient of close to 100f that is probably closer to right. If you had the condenser in the sun (dumb idea) you might want to figure an ambient more like 130.

Intersting stuff.

this can be very serious.
It will be way better when the entire distribution panel is ARC FAULT protected.

ok, I found some charts in my pocket ref book that help

I see some wire ( sheathing ) is rated at 140 degrees F, ( TF, UF)
some are rated at 167 F (THW,THWN,ZW,RHW, THHW, XHHW )
And there are a lot rated at 194 F

ampacity of a 140 F rated sheathing 12 awg wire is 30 at 86 degrees, BUT FOR AMBIENT TEMPS ABOVE THAT, there needs to be adjustments made.

So if that wire is in an attic that is 115 degrees F, You multiply ampacity by .58 and find that the wire now has an ampacity rating of 17.4.

has anyone seen the newest charts or info from NEC on this and is it available online?

i can try to post it here, but i dont know if what i have is still the standard

““has anyone seen the newest charts or info from NEC on this and is it available online?”” – me

AH ! wait a minute , THANK YOU Mr Joe Tedesco you already did that - That link is great!

The NEC recently addressed rooftops ref derating, I bet attics are next

Free Access(read only) to NEC 2008

Gerald Newton has an interesting article about table 310.15 and the Neher-McGrath formula in general. I imagine it is on, his web site. I can’t seem to find a copy on my hard drive. It is something like “Where did 310.16 come from”

Thanks Joe. I lost that somewhere in 300 gig of stuff on this machine.