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Old 08-05-2009, 11:22 PM   #16
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240 Volt Air Compressor


If I were installing this compressor at my house, or installing it at your house as an employee of an electrical contractor, I'd install a 30 amp breaker in the panel, then use a 12/2 from the panel to a disconnect of some sort, then either cord or flex from the disconnect to the compressor.

If I were advising you to install it, do the same thing except use 10/2.

The reason is because I know the code well enough to legally justify my installation. I've 'educated' (tactfully) several inspectors over the years.

Unless you know the code well, it's better to grossly overkill the installation so as to not raise eyebrows.

Rob.


Last edited by micromind; 08-06-2009 at 12:20 AM.
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Old 08-06-2009, 10:23 AM   #17
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240 Volt Air Compressor


What some are missing here is starting current. Compressors are notoriously under protected and tripping of the OCPD (circuit breaker) is the result. When sizing a breaker or fuse for a motor you must consider this in the calculation. FLA x 1.75 = 29.75. This is the value you MUST use to size the OCPD and wire*. Since they do not make a 29.75 amp breaker, you need a 30 amp breaker. While you may legally install #12 wire on this 30 amp circuit*, I would not do it. I would use #10 following NEC Art.310.16.

The OP will have no external overload to adjust, (unless the compressor comes with one, or he installs one himself) so the breaker is the only protection he will have. The 30 amp breaker will allow this compressor to start. The internal (non-adjustable) thermal overload in the motor will protect at (running current).

I agree with Rob and InPhase regarding the addition of the disconnect and the subsequent wiring after the disco. In this application we are addressing a significant motor load. You cannot just look at the name plate and size the OCPD and wire to this value. I would be willing to bet, if you put this motor (17 amp) on a 20 amp breaker and push the start button the OCPD (breaker) would trip instantly. It will not work, period.

*The NEC has exceptions for wire sized smaller than allowed in Art.310.16. For the sake of not confusing the OP we can save this for another discussion.

Note: Yes 10/3 for future expansion.

Last edited by J. V.; 08-06-2009 at 01:01 PM.
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Old 08-06-2009, 10:49 AM   #18
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240 Volt Air Compressor


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Originally Posted by J. V. View Post
What some are missing here is starting current. Compressors are notoriously under protected and tripping of the OCPD (circuit breaker) is the result. When sizing a breaker or fuse for a motor you must consider this in the calculation. FLA x 1.75 = 29.75. This is the value you MUST use to size the OCPD and wire*. Since they do not make a 25 amp breaker or a 29.75 amp breaker, you need a 30 amp breaker. While you may legally install #12 wire on this 30 amp circuit*, I would not do it. I would use #10 following NEC Art.310.16.

The OP will have no external overload to adjust, (unless the compressor comes with one, or he installs one himself) so the breaker is the only protection he will have. The 30 amp breaker will allow this compressor to start. The internal (non-adjustable) thermal overload in the motor will protect at (running current).

I agree with Rob regarding the addition of the disconnect and the subsequent wiring after the disco. In this application we are addressing a significant motor load. You cannot just look at the name plate and size the OCPD and wire to this value. I would be willing to bet, if you put this motor (17 amp) on a 20 amp breaker and push the start button the OCPD (breaker) would trip instantly. It will not work, period.

*The NEC has exceptions for wire sized smaller than allowed in Art.310.16. For the sake of not confusing the OP we can save this for another discussion.

Note: Yes 10/3 for future expansion.
I figured the "15A Minimum" statement on the nameplate was referring to the circuit ampacity, not the FLA rating of the motor. If it were the FLA rating, then your calculation would of course be correct - but these consumer compressors rarely state FLA, they state the required circuit ampacity. If the "15A Minimum" does refer to the OCPD and not FLA rating, then a 30A breaker on #12 wire would not be permissible in this application, would it?
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Old 08-06-2009, 11:37 AM   #19
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240 Volt Air Compressor


On a 30 AMP Why risk a possible fire?!? Run new 10/3 or 10/2 copper (which is rated for 30 Amps) and be done with it. Physics is physics.. try to pull too much current through a piece of copper wire and it WILL overheat and possibly burn.
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Old 08-06-2009, 12:42 PM   #20
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240 Volt Air Compressor


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Originally Posted by mpoulton View Post
I figured the "15A Minimum" statement on the nameplate was referring to the circuit ampacity, not the FLA rating of the motor. If it were the FLA rating, then your calculation would of course be correct - but these consumer compressors rarely state FLA, they state the required circuit ampacity. If the "15A Minimum" does refer to the OCPD and not FLA rating, then a 30A breaker on #12 wire would not be permissible in this application, would it?
No. The circuit ampacity includes the motor.
I went to the link someone posted and you verified it was your compressor, but saw no electrical specs at all. It only said that it must be installed by a qualified electrician. So I have no idea where this 15 amp min. comes from. You need to know what the full load amps are. The whole unit (amps) are fine. Call them if you have too. It's Sears right? They will discuss this with you, I would hope.

I just located NEC Table 430.52. It actually requires a multiplier of 2.50 for an inverse time breaker. Thats whats in your panel. These are maximum values, not minimum. So, just go with 2.50 x (amps) all around and you should be just fine. (Breaker and Wire)
Still contact the manufacturer and make sure they agree.
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Old 08-06-2009, 06:45 PM   #21
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240 Volt Air Compressor


Its been my experience that there is often a lot of confusion figuring out the correct branch circuit to things like air compressors that do not have a cord and plug on them. So i'm going to give you my thoughts on this situation and see how it flies.....

This air compressor has a single phase motor rated 3.2 horsepower and 13 amps. This is nameplate information according to the OP.

If it carries a ul listing I would use the nameplate 13 amps to size the conductors for the branch circuit and the ocpd. I'll explain if you want.

If it doesn't carry a ul listing I would use 17 amps from table 430.248.

Based on that

Unit is listed: Branch circuit conductors 13 x 1.25 = 16.25 amps 14 awg thhn rated 20 amps table 310.16 in conduit or 14 awg nm-b rated 20 amps 60C column table 310.16.
Breaker can be 13 x 2.5 = 32.5 next size up 35 amps (provided this air compressor is not on a multioutlet branch circuit)

I would run a 14 awg branch circuit on a 30 amp double pole breaker.

Unit not listed: 17 x 1.25 = 21.25 amps... 12 awg thhn rated 25 amps or nm-b 60C rated 25 amps.
OCPD 2.5 x 17 = 42.5 amps next size up 45 amps

I'd run a 12 awg branch circuit with overcurrent protection of 30 amps.

The motor is overload protected as per manufacturer specifications so we are only concerned with protecting the branch circuit from ground faults and short circuit. A 13 amp running load is absolutely no problem for a conductor rated 20 amps. A 17 amp running load is no problem for a 25 amp rated conductor. So I'm not one to advocate overkill to a 10 awg conductor.

The fact this motor comes from the factory without a plug on its cord and noted that it is to be hard wired... pretty much puts us into the motor rules of article 430 and a bit into 422 for appliances. IMO Article 430 is primarily concerned with branch circuits to a motor or motors that are not considered appliances, a compressor IMO is an appliance as it comes as a unit with motor and performs a specific function just like a washing machine. This particular compressor by virtue of no plug on it's cord and hard wiring required puts you into that little niche of article 430 revolving around 430.6 (A)(1) exception 3.

If you wanted to stick a plug on it... say a nema 6-20...or a 6-30 for that matter.... I think you would find it will not suffice as the disconnect for a 3 hp 230 volt single phase motor. You would not be able to meet the requirements of 430.109F.

Like Rob I would hard wire this compressor into a properly sized disconnect located next to the compressor. I realize that the circuit breaker can be allowed to meet disconnect rules but to me that is just being lazy.

As for the horsepower rating I think peak horsepower for this thing would be 6 or 7 hp so I'm inclined to think the motor is indeed around 3 horsepower.

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Last edited by Stubbie; 08-06-2009 at 08:45 PM.
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