ECN Forum Forums General Topics of the Trade General Discussion Area Conductor Ampacity

In general, a no. 14 awg. with thhn insulation can safely carry 25 amps according to table 310.16. Switch and receptacle terminals as well as circuit breaker terminals are typically rated 75 deg. C. max. which limits circuit current capability to 15 amps. My question is why don't manufacturers make 90 deg. C terminals so table 310.16 can be used to it's fullest? Is it purely for economical reasons?

Good question Frank,

I think that one problem would be that the higher terminal temperatures would be affecting the operation of thermal breakers.

Bill

And who is going to accept an electrical system where the parts are operating at 194 degrees F? Also this heat production costs money. It is wasted energy.
Don(resqcapt19)

I agree with Don on all points.
It's nice that wires can carry that much juice. But heat is a warning sign that something is wrong. So for safety's sake, wires need to remain cool. No one wants to touch a 150° F wallplate. Such a temperature should be a clue that a hazard exists.

If we start running 25 A over 14 AWG, where is the safety margin when the OCPD allows 200% for a minute or two? 50 A on 14 AWG would turn a junction box into a toaster even with 200° C insulation.

The insulation doesn't keep the wire from getting hot. It just resists degradation and melting at that temperature.

The hotter (actual temperature) the wires are, the higher resistance is.

Frank,

Some breakers may be rated 60C which would need further derating (conductor ampacity) for them to operate properly and be in compliance.

Wasn't there some older ones at 40C too?

Bill

It seems like I still see 40°C a lot. Perhaps I need to pay more attention to that.

Gentlemen,

Are we not technically bound by section 110-14c to using the 60 degree rating of the conductors for circuits rated 100 amps or less, or marked for #s 14 through #1 conductors, unless that we can prove that every last termination involved is rated for the higher temperature?

I was told that the large 40 C stamp that you see on the side of some breakers is not the actual rating of the termination. If you notice, most of these breakers also state that they are suitable for use with 60 C or 75 C rated conductors. It sure would be nice to get this clarified.

Matt

The 40°C marking on breakers is usually the maximum ambient temperature that the breaker is rated for.
110-14(c) only requies that the current on the circuit does not exceed the 60°C ampacity. We can use the 75° and 90°C ampacities for derating or adjustment purposes.
Don(resqcapt19)

Don,

I agree with everything you said. But just to clarify further, if every termination on the circuit as described in 210-14c is rated for 75 C or more, we can use the ampacity of the wires listed in the 75 C column, even if derating is not called for on a particular circuit. Do you agree?

Matt

Don,

Take Matt's question one level higher. If every termination on the circuit as described in 210-14c is rated for 90 C or more, can we use the ampacity of the wires listed in the 90 deg. C column, even if derating is not called for on a particular circuit? If for no other reason, in theory only? That was the intent of my original posted question.