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

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).]