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.

The quick answer is: Yes all conductors may be sized based on their insulation rating. De-rating factors need to be considered, and the terminations MUST be rated for the final temperature.

For example - There are no overcurrent protective devices that are rated for termination of conductors sized per the 90 deg C column. The lug is not the limiting factor, changing to a higher lug will not increase the rating of the complete device.

I can only think of one example where a wire could actually be loaded to its 90 C rating, but I doubt very much if it would pay to do it except maybe on extremely long runs.

You could size the wire between two junction boxes to the 90 degree rating if the slices (kerneys, crimps etc.) are rated for 90C, and then size the wire at each end of this run using the 75 degree column.

Matt

Matt,
If everything in the circuit including the equipment is listed for use at 75°C, then yes we could use the 75°C ampacity forom the table.

In the long term, it would never pay to run a 90°C conductor at its full ampacity between junction boxes, do to the increase in energy costs to heat that wire to 90°C.
Don(resqcapt19)

Don,

I agree, that it would seldom if ever pay, but it would be legal. However, I don't quite understand what you're getting at when you make the statement "do to the increase in energy costs to heat that wire to 90°C." Heat the wire? LOL

Matt

Matt,
It takes energy to heat the wire. This costs money. If you have any circuit that runs 4,000 or more hours in a year and is loaded to near the conductors ampacity, it often makes economic sense to install a conductor one or two sizes larger. The smaller wire size has a higher resistance and creates more heat and wastes more energy then the larger wire does at the same load.
Don(resqcapt19)

>Heat the wire?
Yes, seriously. The heating of the wire comes about because the cmil cross-section is heavily utilized. Think of the effort it takes to breathe through a soda straw. The larger your conductive pathway, the lower the resistance, and the less loss you have to heating. 90°C means just that. The conductor can heat to that temperature carrying that many amps.

Try running 50 amps through a #14 sometime (Be sure to use heavier wire at both ends before connecting to anything you can't throw away when your test is over). It will get quite hot, might even make water sizzle.

OK guys, I understand all that. It just sounded funny worded that way.

Matt

This is one reason that I rarely use 14 AWG wire - especially to power anything other than solid state electronics.

Derating probably is less costly than many folks think.

Here's a page on wire size and Energy efficiency from the Copper Development assn.

Wire Size Up\"


Bill

Bill;
Note all calc's are for industrial distances and constant loads.

Is it purely for economical reasons?

oh yes...but who's is my Q

[This message has been edited by sparky (edited 08-03-2001).]