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]