Posted by David Sherman on Monday, September 17, 2007 at 17:24:28 :
In Reply to: OT: ingersoll-rand 242-5n compressor posted by Joel Wilkins on Monday, September 17, 2007 at 16:06:17 :
Yes, it's single-phase. Home voltage in most areas now is 240 volts, but that'll be okay. The "208" is because in an industrial 3-phase system there are two ways to hook up a single-phase load to a 3-phase source, one of which gives you 240 and the other gives you 208. The nameplate says this motor will work either way.
A 20 amp breaker with #12 wire should be adequate. A 30 amp circuit would be better. I use 20 amp circuits for both of my consumer-model 5 hp compressors with no problem, but that's because the "5 HP" rating on those units is, shall we way, "optimistic". Theoretically, 5 hp at 240 V and 80% efficiency works out to 19.5 amps. Code says the steady-state design load on any circuit must not exceed 80% of the breaker's rating, which is 16 amps on a 20 amp circuit. So, you can see that if the motor really is doing 5 HP of work, it'll be on the hairy edge of tripping the breaker and will violate the code as well.
An air compressor is not an easy load to start, since if the receiver is nearly full, the motor has to work against air pressure right from the get-go, when it doesn't have much torque. That means compressors draw a pretty hefty surge when starting. If your compressor is any distance from the electrical panel, or if it's a real industrial model that really will be drawing an honest 5 hp (rather than a Chinese 5 hp) worth of electricity while running, you're better off going with a 30 amp circuit and #10 wire. This is no big deal, since electric water heaters are usually wired this way. You can use 10/2 WG wire (no need for 10/3) since the machine is wired directly across the two hot phases (again, like a water heater). put black tape around the white wire at each end so that future electricians will know that you're using the white as a "hot". The code allows this.