Motor Starting vs. Long Wires

A recent email conversation may prove relevant to someone else …

I have a pole barn which has approximately 100′ run of 10 gauge copper supplying power to the building. I … did not care to pay … $12,000 for a new 200′ line from the road … [with] only lights and 2 door openers for demand.

I … put a 30 gallon air compressor in […]. When I first put it in, it struggled to start @<40 F. They called it a 1.6 running h.p. (whatever that means) motor. Nameplate shows 15/7.5 F.L.A. I switched it to 240v and the problem went away.

Aren’t I likely to get the same problem as I had before or do 240 volt motors start easier?

I screwed up when they buried the wire – in retrospect I would have buried 6ga to the barn to lessen the voltage drop.

After running a few numbers, here’s what I came up with …

do 240 volt motors start easier?

The trouble with motors is they draw far more current while starting than they do while running. A factor of ten more is a good rule of thumb.

So a “1.6 running HP” motor draws 1.2 kW while running at full load:
 – 10 A at 120 V
 –  5 A at 240 V

The “full load amps” will be higher than that, because the motor isn’t 100% efficient. You can plug the FLA values into the calculation for an even more depressing result.

During the fraction of a second when it’s starting, however, it will (try to!) draw 100 A or 50 A, depending on which line voltage you’ve wired it for.

100′ run of 10 gauge copper

That’s 200 feet of wire out-and-back.

Look up the resistance per foot in a wire table, finding 10 AWG wire has a (convenient!) resistance of 1 mΩ/ft, so a 200 ft length has 0.2 Ω of resistance:

 – A 10 A load drops 2 V
 – A  5 A load drops 1 V

Both of which are survivable in normal operation at their respective line voltages.

However, the motor starting currents will be completely different. A 100 A current will (try to!) drop 20 V, reducing the line voltage to 100 V and stalling the motor. Running the motor from 240 V means the 50 A starting current drops only 10 V and the remaining 230 V can get the motor up to speed.

Now, 240 V service isn’t a complete solution. The new compressor draws 15 “full load amps”, so it’ll drop 3 V while it’s running and 30 V while starting. It’ll probably start at 210 V, but it may grunt for a bit longer than you like as the speed comes up and the current goes down.

in retrospect I would have buried 6ga to the barn

There’s a Pennsylvania Dutch saying: “We grow too soon old and too late smart.” [grin]

3 thoughts on “Motor Starting vs. Long Wires

  1. When we bought our place, the previous owner did a 500′ extension cord with #6 wire. It was wired wrong, with the neutral and #10 ground switched. Ouch. Now have 200A service to the barn.

    I had a spirited discussion with the electrician when we were setting up the 330′ line for the pumphouse. 20A/220V, with the pump 1/2 HP on 110V.

    The electrician wanted to do absurdly small wires, but I insisted and paid for #4 wire. He thought I was nuts, but the voltage drops were acceptable with that figure. Those lines are now setting idle, since the solar system is doing the work (quietly hides emergency plans to have solar system power a single phase of the house if the grid goes sideways for days on end).

    Copper isn’t cheap, but new pumps are expensive.

    FWIW, I have a 3HP 220V compressor that I tried running with the 20A/220V outlet on our “5500W” generator. It started all right, but by the end of the cycle, the generator bogged down. Between age (2003 generator) and elevation, I’m not getting the watts that were promised. The solution to that was some long lengths of air line with the compressor in the barn.

    1. When we connected to the town water main out front, I had the plumber run 1 inch copper pipe instead of the usual half (maybe 3/4) inch pipe. He thought it was absurd, but I wanted to make absolutely sure all 150 feet of pipe wasn’t the flow-limiting factor.

      Yeah, probably absurd. I still feel better.

      1. 300 feet of 1-1.4″ PEX pipe from the pumphouse to where it tied to the old water system. Comes in 100′ rolls, too.

        Once the surge tank is emptied (40 gallons in reserve), the limiting factor is the pump; 7 gallons/minute. Not a whole lot of flow, but a pump that does well on 110VAC is nice to have. It’s been on solar since May of last year, with one break last winter when I wasn’t home to clean the panels in a stormy period.

        (Protip: if you have to stay at a hotel for a minor medical procedure, pick some time other than the dead of winter. Julie’s had to shovel too much snow on my medical trips, though there’s always thunderstorms for summer. Sigh.)

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