Power mains question: wire gauge

I would, since the starting current is much higher.

Cant hurt. It would definitely help, in fact. multiple #14s to make each #10 would work better, keeping each segment electrically separated except at the nodes, giving a sort of Litz effect. At that gauge, you can get SPC too (Silver Plated Copper), reducing ohms per foot without increasing gauge above #10.

And you want a #12 return run for ground fault carriage too. A single piece would pop a breaker.

"DaveC"

** Giant HUH ??

Such a motor normally draws around 3 amps.

... Phil

#12 is 1.6 mohms/foot. That's 0.32 ohms total. The short-circuit current from 220 volts is almost 700 amps. A half horse is only around

400 watts, about 2 amps. I'd use #14.

ance

make

considered what the skin deepth at 60Hz is?....

-Lasse

Up to 4.9 Amps, according to the 2011 NEC (Table 430.248). And that's all you have to provide in terms of feeder/branch circuit ampacity.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com 
------------------------------------------------------------------ 
Just an armadillo on the shoulder of the information superhighway.

Maybe the decimal placement error is on the amps. 1.5 instead of 15. That math works too, btw. 220 * 1.5 = 330 or ~ .5 HP

There is s thing called "voltage drop". Voltage will drop in a wire running a long distance, sometimes so much, the device at the end will not operate!

The amount of voltage drop depends on the voltage, the type of metal used for the wire (copper / aluminum), the wire size, and the load in amperage at the end of that wire.

The internet has made this easy for you. Just search google.com for the words...

Voltage Drop Calculator

200' of 14AWG solid copper wire has a resistance of about 0.5 ohms, so if your locked rotor current is 15A, then the drop across the cable will be 7.5V when the motor first starts up, leaving 212.5V for the motor. No problem.

If your motor is about 80% efficient, then at full load it'll be taking about 466 watts from the mains, which is about 2.1A.

2.1A through 200' of 14AWG ohm will drop about 1.1V across the run, leaving about 219 for the motor. Absolutely no problem with 14AWG, so don't waste your money on what you don't need.

--
JF

Until the motor or compressor is replaced with a larger unit. Then all bets are off. It's cheaper to do it right the first time. You could even put a small breaker box by the compressor & run AWG 6 to it, to power other tools, as needed without starting from scratch.

That's a good idea. I'll talk it over with the owner.

But for now I want to settle on what size conductor to use if it's just to supply this one compressor.

Thanks.

nce

The NEC motor tables for 1/2 HP single phase at 230 specify FLA of 4.9A and LRA of about 30A. NEC also recommends sizing branch line for no more than

5% voltage drop. So if you want to power through the start-up with less tha n 5% line drop, that would be less than 0.05 x 220= 11V at 30 Amps for a maximum line resistance of 11V/30A= 0.37R. For a total line length (suppl y and return) of 200 feet this comes to 0.37R/200=1.8 milliOhms/ft wire. Looking at the AWG table , 12 gauge comes in at 1.588 milliOhm/ft. Therefore 12 gauge is within the g uidelines required for trouble free service. Turning the problem around and asking for maximum acceptable length with 14 gauge, 2.525 milliOhms/ft, yo u would then solve 2 x L x 2.525E-3=0.37R for L= 73 ft one-way length, so you might want to look at the exact distance. Circuit protection should be for 1/2HP motor load in any case.

Thanks for your reply.

Sorry you did all that calculation on the wrong HP rating. (See my post earlier today.) Motor is actually rated 3.1 HP.

Thanks.

If it's 1/2HP, now, it's unlikely to grow to >5HP.

Run several #12s, instead, when needed. It's cheaper.

Pull additional #12s in the future? I've had bad experience pulling additional conductors in a conduit with existing conductors.

If I misunderstand your statement, please try again. ;-)

Thanks.

No. A bundle of a few 12s will make a nice oversized #10. The finished 'wire' will be stronger and more flexible too.

Of course, the return has to match. So you pull all 7 at once.

Two sets of 3 #12s and one #12 fault return. Seven wires.

I don't see earlier post. At any rate, the FLA for 3HP is 17A and LRA is 10

0A. NEC allows for 15% line drop at locked rotor, so 0.15 x 220V/100A=0.3 3 Ohm, or 0.33/200=1.65 milliOhm/ft, and 12 gauge does it again. The moto r will have no problem because NEMA takes the NEC guidlelines into account in their specification. Motor will easily start but any other loads on the branch will brown out during the start-up, just so you know. If you don't l ike that then you need 6 gauge, much more $$$.

Strictly against code!

Conduit?