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Old 04-29-2013, 06:44 PM   #16
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Wiring a new air compressor


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Originally Posted by stickboy1375 View Post
What changes this fact? I want to hear both sides of the story.
My position is that a motor which is incorporated into a complete device (the motor along with its driven machinery) which has a nameplate, is no longer just a motor. It's a piece of utilization equipment like any other, and should be installed according to its ratings. The point of the NEC motor tables is to size circuits for generic motors which have no defined mechanical load. This generic approach takes the guesswork out of it, and is appropriate for motors used in industrial installations or other ad-hoc applications where there's no equipment rating plate to work from. But you can't (and shouldn't) just ignore the rating plate on a complete machine because it happens to contain a motor inside it! The rating plate data was set by the engineers who made the thing, and is part of the UL listing for the machine. They know better than the NEC tables what the actual electrical requirements are. Sizing the circuit based on the tables when the equipment has a rating plate right on it seems to be a violation of the listing, the manufacturer's instructions, and also the NEC.

A motor contained in a complete air compressor is not a motor. It's an air compressor. Its actual electrical requirements could be far below the motor's ratings, if the pump is under-sized. Could also be far above, if the manufacturer is playing games with duty cycle and the time-variant loading of a compressor. It is the manufacturer's prerogative to do this if they want. Just because the motor they bought to install in their product says "5HP" on it does NOT mean they are actually using it as a 5HP motor. The homeowner-grade compressors are a great example of this. They may say 4HP on them, but the motor is not a 4HP motor and does not need to be wired as if it were. Follow the nameplate on the whole device. The same applies to garbage disposals. 3/4HP advertized power? That doesn't mean it's actually a 3/4HP motor load for electrical purposes. It's a garbage disposal, not a standalone motor for generic use.

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Old 04-29-2013, 06:56 PM   #17
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Wiring a new air compressor


As in
NEC 2011 430.6(A)(1) Exception No. 1
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Old 04-30-2013, 07:23 AM   #18
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Wiring a new air compressor


I only have the 2008

Last sentence in 430.6

Then .....

430.6(A) ......


Then ....

430.6(A)(1)....

430.6(A)(2)
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Old 04-30-2013, 07:36 AM   #19
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Wiring a new air compressor


I suppose it sounds like it's a matter of interpretation, and in the end the case could be made by an inspector to go with the more strict interpretation... It's possible that some day my motor might wear out, and I'll go to harbor freight and buy a motor with an even bigger current draw. Not an unreasonable interpretation for somebody walking into my garage and seeing that I've done similar acts with my cars where I've put a big old FE block Ford into a '55 Mercury. Still... When doing things with motors, it's proper to upgrade other things as well so you don't start breaking things. It's true in cars. And with an electrical motor, upgrading it should necessarily include making sure that your wires are sized properly.

But with the motor I have right now, where the manufacturer provides an air compressor assembly with a 12 AWG wire from the motor to the switch, and the assembly is UL rated... It doesn't seem intuitive to require an 8 AWG wire for the rest of the electrical circuit.

Personally, my goal is always to try to conform to the strictest possible interpretation. I can see where a future replacement motor might merit that, but with everything as it is right now it seems that #8 wire is heavier than needed and I've already put #10 in place. Doing this is a matter of buying 2 spools of THHN (black and green) at $30 each, and $8 for a breaker. I'm between jobs, and dropping $70 is going to have to wait until after I have a job.

Here's the info from the motor data plate:
Emerson Definite Purpose Compressor Motor
Model T63CWDKH-1785 MFG No F11C
HP SPL Duty CONT Frame 56 Hz Type T
Rot CCW Ins Class B Max Amb 40 degrees C KVA Code -
Volts 208-230 Hz 60 RPM 3450 PH 01
Amps 18.6-17.5 SF 1.0 SFA -
L.R. Amps = 95 Output 2.98 KW
Prelubricated bearings
Thermally Protected Manual
Customer PN 160-0325
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Old 04-30-2013, 07:44 AM   #20
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Wiring a new air compressor


If you replace the motor with a larger motor, you have voided the listing of the compressor. At that point, you would have a code violation either way.
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Old 04-30-2013, 07:50 AM   #21
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Wiring a new air compressor


Quote:
Originally Posted by Philly Master View Post
I only have the 2008

Last sentence in 430.6

Then .....

430.6(A) ......


Then ....

430.6(A)(1)....

430.6(A)(2)
2005 has:
Quote:
Exception No.1: Multispeed motors shall be in accordance
Quote:
with 430.22(A) and 430.52.


My motor isn't multispeed though.

Then 430.6(A)(1) says

Quote:

Exception No.3: For a listed motor-operated appliance that
is marked with both motor horsepower and full-load current,
the motor full-load current marked on the nameplate of the
appliance shall be used instead of the horsepower rating on
the appliance nameplate to determine the ampacity or rating
of the disconnecting means, the branch-circuit conductors, the
controller, the branch-circuit short-circuit and groundjault
protection, and any separate overload protection.
(2)
Nameplate Values. Separate motor overload protection

shall be based on the motor nameplate current rating.


Also on the compressor itself is a label with this data:
Motor RHP -
Voltage 230
Amp 17.5
Hz 60
Phases 1
Complies UL 1450 Standard

There's also a data plate for the Sanborn tank, but that doesn' have any data relevant to the electrical discussion.
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Old 04-30-2013, 10:02 AM   #22
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Wiring a new air compressor


Quote:
Originally Posted by WillK View Post
But with the motor I have right now, where the manufacturer provides an air compressor assembly with a 12 AWG wire from the motor to the switch, and the assembly is UL rated... It doesn't seem intuitive to require an 8 AWG wire for the rest of the electrical circuit.
Resistance and heat dissipation. Resistance rises with distance, so a short little 3' of cord won't present a lot of resistance, but 30' or more to the panel has a lot more. Also, the cord is hanging in free air so it's able to cool via convection, while your 30' run to the panel is buried in drywall and possibly insulation so it can't get rid of heat as fast. (Even if you don't have drywall or insulation, you have to assume that there is, as someone may come along and add it later.)

Not intuitive... but still a good reason for using fat wires to feed skinny cords.
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Old 04-30-2013, 04:34 PM   #23
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Wiring a new air compressor


I found this article, really worth a read for anyone still wondering....

http://www.ecmag.com/section/systems...ted-appliances
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Old 05-02-2013, 01:59 PM   #24
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Wiring a new air compressor


Quote:
Originally Posted by brric View Post
As in
NEC 2011 430.6(A)(1) Exception No. 1
Sorry for the delayed response. 430.6(A)(1) Exception No. 1 does not apply to air compressors. A fact I have pointed out in the past. It applies to "listed motor operated appliances" which are covered by UL 73 found here.

Scope 1.4 These requirements do not cover equipment intended particularly for the control of electric motors; separator motors; nor electric clocks, fans, clothes dryers, washing machines, hair dryers, tools, waste disposers, dishwashers, office appliances and business equipment, refrigerators, air conditioners, vending and amusement machines, hair clippers and shavers, snow movers, automotive and garage equipment, or other motor-operated appliances that are covered by individual requirements.

Since air compressors are covered under a different standard (mentioned earlier) they are not considered a "listed motor operated appliance" and therefore, 430.6(A)(1) Exception No. 1 is not applicable.

ARticle 422 does apply and references 430 for motors that run continually. I doupt this compressor is going to run continuosly, but nobody can say for sure. NEC 422 does require that the motor tables be used for sizing.

Anyway, my original post was based on the specs from the unit in the link I provided and not the OP's unit. And as mentioned in my post I stated 8/2 romex based on 60 degree, or #10 THWN/THHN in a raceway. I still stand by my comments. As for the ITCB, I stated "up to" for the breaker. Leaving the OP the option of increasing it if starting current becomes a problem.

Finally, my last comment in the post instructed the OP to follow the manufactures installation instructions.

To the OP - is there a duty cycle mentioned on the motor?
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Old 05-03-2013, 05:32 AM   #25
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Wiring a new air compressor


I posted every spec there is. And the manufacturers instructions don't say a single thing about wiring, no wiring diagram or anything, it only says use a qualified electrician for installation.

I have any problem with interpreting as continuous use... If the compressor is being used for sandblasting, and that's one of the intended uses for me, it will run continuously as long as I'm sandblasting continuously. I'd have a hard time motivating myself to stand there squeezing a trigger for more than an hour, but who knows how ambitious I might get or if I get a foot operated control to make it easier to go for longer.

Sanding equipment, particularly an air file, are pretty heavy draws as well, but you can only go so long on that before it's time to pause and apply some more body filler then wait for it to harden.

Hey, let me ask this... I've been assuming all along here that if I went up to heavier wire gage due to current rating, I'd also need to go up in rating on the circuit breaker and switch. The switch has fuses at 30 amps. Could I legally just go with 8 AWG wire and stick with the 30A switch and breaker?
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Old 05-03-2013, 05:53 AM   #26
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Wiring a new air compressor


Quote:
Originally Posted by WillK
ICould I legally just go with 8 AWG wire and stick with the 30A switch and breaker?
Yes you can

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