Re: Voltage Drop Question for Air Compressor


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Posted by Sherman in Idaho [72.47.153.105] on Friday, January 03, 2014 at 21:34:05 :

In Reply to: Voltage Drop Question for Air Compressor posted by Dave Horvath [76.125.248.166] on Friday, January 03, 2014 at 19:49:13 :

You're thinking the right way. The wire gauge and breaker amperage tables in the electrical code are based on preventing fires. The wire will carry the rated amount of current without overheating. On long runs, however, there will be too much voltage drop.

An air compressor is a relatively tough load to start, and if the voltage droops too much when starting, the motor will never pull in. On the other hand, the "compressor duty 5 HP" motors really don't put out an honest 5 hp continually. But for worst case, guesstimating, let's say it does. Figure roughly 1 kW/hp (that allows for inefficiency in the motor), so you're looking at 5 kW/240V = 21 A.

A 30 amp breaker would be okay for this. They might suggest 40 amp due to nuisance tripping on startup. But back to what you really want to know. A fair rule of thumb is you wouldn't want more than a 5% drop under load. 3% will make lights dim noticeably. 5% of 240 is 12 volts. The resistance that will drop 12 volts at 21 amps is 12/21 = .57 ohms. You have roughly 600' of wire (out and back). Wire tables list resistance in ohms per 1000 feet, so you need wire with a resistance of less than .57 * (1000/600) = .95 ohms/1000 feet. Looking in the wire tables (link below) we see #10 wire has a resistance of .999 ohms/1000'. Therefore #10 wire would be adequate, but far from excessive.

If you already have #10 installed, I would go with it, but the breaker on the upstream end must be no more than 30 amps. It would be easy enough to try it and see if you're satisfied with the performance of the compressor and the flickering of the lights. On the other hand, if you have #8 wire already in there, a 40 amp breaker is safe, and your loop resistance would be down to .37 ohms and your voltage drop to 7.8 volts (3%), which would be my preference. Anything larger than #8 would just be a bonus.

This assumes you're using copper wire. If it's aluminum, it might be something like 4/4/2 (for buried wiring to an outbuilding). In very rough terms you can assume aluminum of a given size is equal to copper of one size smaller, so #4 al would be like #6 copper, and so also adequate.

On the other hand if by "support the breaker" you mean provide 40 amps continually at the barn with a 5% drop, with the compressor plus a bunch of other stuff on, then you're looking at #6 copper for sure. Theoretically #7 would do it, but of course that's not an available size.

Now if it was my setup, cheapskate that I am, and expensive as copper is these days, I would try to run whatever wire is already there. If all I had was #10 or #12 already in the ground, I would put a 240/480 transformer at each end and run 480V through the buried wire. But that's only because I have a bunch of such transformers on hand. Or if voltage drop under load was an issue, I would put an automatic motorized variac voltage regulator at the barn end, but again that's only because I have two of them and would like to use them for something.



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