Wiring temp rise

icarus

This is a bit off topic, but since I trust the experience and guidance of most everyone here, I though I would pose the question.

I just instqlled a large ceramic kiln for a friend (who has the grid needles to say!). It is wi to 240 single phase, is protected with a 90 amp two pole breaker, perhaps 60' from the main panel. It is wired with #2al triplex cable with proper connectors and anti ox paste. The Kiln draws nearly 75 amps when going full bore, and it runs for up to 16 hours at a a shot.

While the wire is rated for the current, I am concerned that it is a bit under wired. When he was running it full bore the other day, I went over to check to make sure the connectors weren't getting warm. Using an IR thermometer, the connectors were pretty close to ambient (which was warm, just near the kiln) but the wire was warm throughout it's length. With the IR it was about 85-90F witha 70F ambient.

So the question is, should I get him to spring for a chunk of #2copper, or something else?

The breaker itself was also a bit warm, Perhaps 90F

Any thoughts?

Tony

Note edit to reflect the breaker Rating and wider size!

niel

Re: Wiring temp rise

i, for one, don't like al wire as it becomes brittle easily and that leads to higher resistances down the road. i think it also flexes more than copper. now i haven't put my vdrop calculator in this pc yet and that's something i need to do, but did you try the results entered to a vdrop calculator to show what the losses look like? if warm it is a bad sign to me and given the extra heat from the kiln is present it will certainly make things worse as far as vdrops go. i would change it out, but i can't say with certainty to what gauge copper wire without my running the numbers.

icarus

Re: Wiring temp rise

THe voltge drop is minimal, 2 volts over the length, under load.

T

waynefromnscanada

Re: Wiring temp rise

75 amps @ 2 volts, 150 watts spread out over 60 feet. 2.5 watts per foot would over time warm the wire somewhat. Is it excessive? My opinion - - borderline. If it were mine I'd like to reduce the loss, but in practical terms where does one stop? Is it worthwhile and sensible to make changes to the system as it is? Probably not.
Circuit breakers carrying heavy currents do normally get warm, unless they're way oversized, and even then I'd expect to see some warmth over time @ 75 amps.

niel

Re: Wiring temp rise

as it is 2v is not bad enough in my opinion to warrant the wire change unless the wire itself is not rated for high temps as it is being compounded with the extra heat given off by the kiln. something to keep a good eye on in the future anyway you shake it.

vtmaps

Re: Wiring temp rise

icarus wrote: »

So the question is, should I get him to spring for a chunk of #2copper, or something else?

Something I've wondered about... is it possible to develop a rule-of-thumb for when Aluminum is cheaper than copper? For example, if your cable was only 5 ft, the aluminum cable would be cheaper than copper, but the connectors at each end would make the aluminum setup more expensive.

The question of aluminum vs copper comes up often on this forum when folks are installing their panels 100 ft or more from their charge controller. In most of these situations there is little danger of overheating the cable, because the cable is sized large to reduce voltage drop and is nowhere near its ampacity ratings.

Of course, there are always other considerations... for example aluminum needs to be sized larger than copper and may be difficult to pull through 100 ft of conduit, or may even require larger conduit.

Has anyone used aluminum so save money? How long was your run? How much did you save? Would you do it again?

--vtMaps

icarus

Re: Wiring temp rise

First, thanks for the input. My primary concern is safety. His studio is in a classic old barn that would be irreplaceable today, not to mention the human risk!

I don't care much about the voltge drop per se, I only care about getting the wire too warm. It is rated 75C, so we are no whe near there.

As for Vts comment. I have used AL wire for years for service drops, Range and drier wire because it was cheaper. I have never used AL wire for normal branch circuits because of the known issues with connectors etc.

Tony

RCinFLA

Re: Wiring temp rise

If you look up the resistance per foot for various gauge wires you will find the NEMA rating for max current carrying capability is based on ballpark of 2 watts per foot. Larger gauge has more surface area to dissipate heat. There is some variance to the rule based on insulation layers the wire is wrapped in and conduit air flow that reduces ability of wire to dissipate heat. One the best example of necessary derating is SOOW cable which is wrapped with many layers that inhibit heat dissipation.

The primary concern is insulator degradation (brittleness) due to continual heating. New material hold up much better then older plastics.

Slappy

Re: Wiring temp rise

Updated to nec2011. http://www.electriciancalculators.com/calculators/wireocpd_ver_1.html