What size circuit for a 5 HP air compressor?

[Brian Elfert]

I have a used Ingersoll-Rand air compressor I need to supply power to. It has a true 5 HP motor that says it takes 15 amps 230 volts on the motor data plate. The compressor has a 14 gauge cord with a 15 amp 230 volt plug on the end. (I have no manual and not available online.)

It seems to me that something that takes 15 amps continuous would need a 20 amp circuit. Why the 14 gauge cord and 15 amp plug then? I assume it would be okay by electric code to put a 15 amp receptacle on a 20 amp circuit? I know that putting a larger plug on a smaller circuit would be a violation.

[Keith Hankins]

I'm going to go out on a limb here and ask does it say 5hp or does it say "develops up to 5hp or develops a maximum of 5hp". If it's the latter, then it only get's 5hp when you turn it on. Now if it is 15A at 230 volts, thats 30A at 120 so its' not a total slouch. g15A at 230 volts means 7.5A on each leg and hence 14Awg wire supporting 15A Max.

[CPeter James]

First off, if that is a true 5 hp motor, you need a lot more than 15 amps at 230V to run it. Even a Baldor high efficiency 5 hp motor needs at least 23 amps at 230V.

CPeter

[Van Huskey]

Based on the plate it isn't a true 5hp motor more like 3hp. If the cord and plug were on it from the factory then it is fine on a 15A circuit.

[Brian Elfert]

The nameplate on motor says 5 HP and also says 15 amps at 230 volts. I agree that 15 amps at 230 volts is a bit low for 5 HP. 5 HP is right about 3725 watts which is about 16.2 amps. The motor is probably closer to about 4.5 HP in reality.

15 amps at 230 volts is not 7.5 amps on each leg.

[Brian Elfert]

The motor is probably somewhere between 3 HP and 5 HP if it says it uses 15 amps. Wouldn't I need a 20 amp circuit as a 15 amp circuit can't typically be used at full 15 amps for very long?

[David Kumm]

If it is used the wiring may not be original. If you don't have an amp meter to put on it I would replace the cord and plug-less than $10 and not worry. There does seem to be some cheating done with intermittant duty motors which is what I suspect you have. Probably about 80% efficient so 3-3.5 hp. Dave

[Kevin W Johnson]

I assume it would be okay by electric code to put a 15 amp receptacle on a 20 amp circuit? I know that putting a larger plug on a smaller circuit would be a violation.

Am I reading this right? Your wondering about using a 15 amp wall outlet on a circuit with a 20 amp breaker?

If so, then no. The circuit would allow 20 amps of current on a 15 amp receptacle, which would be a fire hazard.

[Van Huskey]

The nameplate on motor says 5 HP and also says 15 amps at 230 volts. I agree that 15 amps at 230 volts is a bit low for 5 HP. 5 HP is right about 3725 watts which is about 16.2 amps. The motor is probably closer to about 4.5 HP in reality.

15 amps at 230 volts is not 7.5 amps on each leg.

That would assume 100% efficient motor. A $1,000 Baldor Premium Efficiency 5hp motor is going to be right at 20 FLA. The average single phase 3hp 240v motor is going to pull 13-18 FLA.

[Michael W. Clark]

Its not 3 phase is it? My 5Hp TS came with 14g wire, but 3 hot conductors. IR is a popular industrial brand and they make a lot of 3ph equipment.

Mike

[Kevin W Johnson]

The motor is probably somewhere between 3 HP and 5 HP if it says it uses 15 amps. Wouldn't I need a 20 amp circuit as a 15 amp circuit can't typically be used at full 15 amps for very long?

It's not so much that a 15a circuit can't handle 15a for an extended period (I wouldn't want to run a circuit like that personally, as I like "cushion"), but you have to consider the starting current of the equipment your running. So at a minimum, yes I'd use a 20a circuit.

[Scott T Smith]

Brian, air compressor manufacturers have played games with stated HP on their compressors for years. You DONT have a true 5 hp motor, you have a "compressor duty 5 hp motor", which is different. Compressor duty motors are typically longer and develop more startup torque, but less total HP. If I compare the size of a 5 hp compressor motor with the 5 hp motor on my table saw, the latter is almost twice as much larger.

According to my Ugly's Electrical Reference, full load current in Amps for a 5hp single phase AC motor on 230V is 28 amps. Full Load Current on a 3hp 230VAC motor is 17 amps, so it's most likely that you have a true 3 hp motor.

And then you have the fact that motors pull more amps on startup, so you should upsize accordingly.

If you want to be safe, put it on a 10 guage, 30 amp circuit. I sure wouldn't run it on a 12 ga / 15A circuit if it were mine.

[Van Huskey]

If you want to be safe, put it on a 10 guage, 30 amp circuit. I sure wouldn't run it on a 12 ga / 15A circuit if it were mine.

This assumes the motor is controlled by adequate overload protection on the compressor, if not you are trading one danger for another.

[Brian Elfert]

Am I reading this right? Your wondering about using a 15 amp wall outlet on a circuit with a 20 amp breaker?

If so, then no. The circuit would allow 20 amps of current on a 15 amp receptacle, which would be a fire hazard.

I guess I'll have to spend the $30 for a 20 amp locking plug and receptacle then. I was really hoping to avoid that expense. I have no idea why they would have put a 15 amp plug on something that draws a full 15 amp. It is a molded cordset so nobody replaced the plug. The darn thing is I think I have 20 amp locking receptacle somewhere, but it isn't in my box of electrical stuff.

I was looking at some Leeson motors and most of their 3 HP motors show 13 amps full load. It is really hard to know how many amps a "real" 5 HP motor should draw. All I know is this motor is not a full 5 HP if it draws only 15 amps.

[Scott T Smith]

This assumes the motor is controlled by adequate overload protection on the compressor, if not you are trading one danger for another.

Good point but I've never seen an IR compressor w/o overload protection on the motor. I'm sure that they exist though.