Air compressor circuit sizing

[dhalleron]

I'm a little slow when it comes to motor circuit sizing.

I have a friend that had a 2 HP air compressor on a 20 amp 230 volt circuit in his residential garage. He upgraded to a 5 HP air compressor and is tripping the breaker.

I haven't been there yet and don't know the exact name plate info. From what he tells me it's installed in UF cable on #12 wire that runs under some concrete and can't easily be replaced. He wants to just increase the size of the breaker.

NEC 2008
Table 430.248 tells me a 5 HP motor draws 28 amps
Table 430.52 tells me the max Short-Circuit protection is 250% of 28 amps or 70 amps max breaker
Table 310.16 tells me #12 can handle 25 - 30 amps depending on the insulation.

I know I can size the breaker more that the current carrying capability of the wire for a motor. What I don?t know for sure is what current to figure the motor actually draws and what the #12 UF can handle.

This air compressor might be pushing the limit of #12 UF if I understand this right. Any pointers such as a duty cycle thing I am missing that might save the day?

Thanks,

 

[stew]

at 28 amps x1.25 you will need wire rated for 35 amps. min of 10 thhn or 8 nm. 50 amp breaker should hold but you can use up to 70. 12 wont cut it in any case.

 

[dhalleron]

I'm going to check it out tonight. Any chance a 5 HP air compressor will have a lower current on the nameplate than the tables in the NEC?

 

[stew]

you must use the table in the code and not the nameplate rating.

 

[renosteinke]

I would ordinarily agree with Stew- after all, the NEC says so - but it should be pointed out that your smaller air compressors are like 'power tools,' where they wildly exaggerate their 'horsepower' claims.

Before I jump to any conclusions about such a motor, I'd look at the 'service factor' and FLA for some guidance. A five hp. label on motor that claims a service factor of 5.00 is really just a one-horse motor, and the FLA won't be much lower than the LRA!

 

[suemarkp]

Many times a "5 HP compressor" won't have a 5 HP motor, especially if its a homeowner type of compressor. I've seen many 5 HP compressors with a 15A 240V motor. So you'll need to look at the motor nameplate (both HP and current rating). If it just lists FLA, then take that times 1.25 to get your wire ampacity rating. UF cable is limited to the 60C column of 310.16. In 2011, I think that number shrunk to 20 from 25.

You may be able to just upsize the breaker and be OK.

 

[infinity]

I agree most HP ratings on small compressors are bogus. Mine says in big letters on the side 6 HP, yet it comes with a cord and 15 amp plug. The motor itself has a little more information:
Voltage: 120 VAC
HP: blank
FLA: 15 amps

According to table 430.238 that motor is about 1 HP.

 

[stew]

Like the Sears compressors rated 5 hp that are 18 amps. The 5 hp is only if you add the 100 unit hamster wheel to the compressor pully!

 

[texie]

I've been waiting for a post like this OP. All these consumer grade shop tools have gone to bogus HP stuff. I'm inclined to think that since they typically come with a cord set already attached that these are not really Art. 430 applications, rather maybe they really should be installed under appliance rules. As you can see, if you use Art. 430 you end up with a wildly over sized circuit. Using appliance rules you can use the name plate. And, if you use Art. 430 how do you put a 15 amp recep. in for one of these typical compressors to match the factory cord? I really think this is a situation that really needs to be hashed out here!

 

[Ponchik]

How about if you turn it on and measure amp draw. You will probably have a better idea as to how much it actually draws.

Other than that, you need to use the table of article 430.

 

[renosteinke]

Texie, thank you. I was uneasy about my post, where I came awfully close to saying 'ignore the NEC.'

I think you hit on the key: if the unit is UL listed, and has a 'normal' plug, it ought to be fine to use any circuit that fits the plug.

I'm taking the position that UL looked at the unit, and considered all factors, before agreeing to list the item with a cord & plug. Naturally, if the cord and plug are field additions, one cannot make this assumption.

 

[iwire]

I think you hit on the key: if the unit is UL listed, and has a 'normal' plug, it ought to be fine to use any circuit that fits the plug.

I agree, if the unit is listed as a whole and came with a certain plug on it I would size the circuit based on the plug rating and be fine with it.

 

[kwired]

I've been waiting for a post like this OP. All these consumer grade shop tools have gone to bogus HP stuff. I'm inclined to think that since they typically come with a cord set already attached that these are not really Art. 430 applications, rather maybe they really should be installed under appliance rules. As you can see, if you use Art. 430 you end up with a wildly over sized circuit. Using appliance rules you can use the name plate. And, if you use Art. 430 how do you put a 15 amp recep. in for one of these typical compressors to match the factory cord? I really think this is a situation that really needs to be hashed out here!

This has been hashed out here before. The overrated horsepowers are simply a short time duty rating. I don't really know how this rating can apply to a compressor but I can see it applying to the compressor motor. Better example is a saw. If it is truly a 1hp saw you should be able to load it to 1hp continuously and not overheat the motor. Now run some really tough stock through it for short period of time and it may be able to deliver up to 6hp to accomplish the task but only for a short time.

The 1 hp motor that is not marked this way can probably do the same thing. It is just a marketing thing. It can be misleading. But they know what they can and can't mark these tools with. If you have essentially the same tool as a competitor but his says 5 hp and yours says 1hp and they are similarly priced which one do you think most consumers will buy?

 

[dhalleron]

Solved - will be upsizing the wiring

Solved - will be upsizing the wiring

When I checked it out I found it was an Ingersoll Rand compressor, so maybe not some cheap junk that has to lie about the rating.

FLA on the nameplate was 21.5A. So multiply that by 1.25 and I get 26.875A which rounded up is what the table in the NEC said a 5 HP motor should draw. The instructions said hard wire only - no plug in.

So it will be too big for his #12 wire even if I upsize the breaker. My friend wants to do it right and he will be renting a ditch witch to go out the back of the garage and around the concrete to run a new circuit. This will add some distance, so instead of #10 we decided it should be #8.

Always another problem. He does not have proper disconnects for this garage. The 20A compressor circuit just comes straight in from underground and feeds the compressor with no disconnect. There is another small 4 circuit panel already there. We would need to run extra wire from the back of the garage where our new circuit comes in to place a disconnect near this panel so we can group our disconnects.

It makes more sense to use this new ditch to run a larger feeder for a new sub panel to replace all the feeds to the garage.

 

[sameguy]

Run a new feeder and add new panel now, he will be adding tools later.

 

[kwired]

FLA on the nameplate was 21.5A. So multiply that by 1.25 and I get 26.875A which rounded up is what the table in the NEC said a 5 HP motor should draw. The instructions said hard wire only - no plug in.

FYI that is not quite how it is supposed to work when sizing conductors for a motor.

Table 430.28 says a 5hp 230 volt motor draws 28 amps. The current values in these tables consider the worst possible efficiency and power factor so that if the motor is ever replaced with same hp but lesser efficiency the conductors are still suitable for the replacement. You probably will not find many motors that have same FLA marked on them as what is listed in tables.

Your minimum conductor ampacity before any adjustments is 28 x 1.25 = 35 amps. If you have 75?C terminations then 10 AWG is smallest conductor you can use. If this circuit is to be run with NM cable which is only allowed to be used at 60?C rating then it must be at least 8 AWG.

 

[texie]

OK, so this unit sounds like a real compressor with a standard motor so Art 430 obviously applies. Back to my original question, what about a typical Home Depot Husky that has a factory cord and a wild horsepower rating? I don't think 430 applies or could even be used but rather Art. 422.

 

[Little Bill]

Always another problem. He does not have proper disconnects for this garage. The 20A compressor circuit just comes straight in from underground and feeds the compressor with no disconnect. There is another small 4 circuit panel already there. We would need to run extra wire from the back of the garage where our new circuit comes in to place a disconnect near this panel so we can group our disconnects.

It makes more sense to use this new ditch to run a larger feeder for a new sub panel to replace all the feeds to the garage.

I think you are going to have to run a new feeder and install a new sub as you are not allowed to run more than 1 feeder to a separate building. Good time to do that while you have the trencher.

 

[kwired]

OK, so this unit sounds like a real compressor with a standard motor so Art 430 obviously applies. Back to my original question, what about a typical Home Depot Husky that has a factory cord and a wild horsepower rating? I don't think 430 applies or could even be used but rather Art. 422.

If it comes with a cord and plug all you really need is to supply with the matching receptacle most of time. If you go to 422 it will send you right back to 430 for most anything involving motor operated appliances.

I have probably seen more of these air compressors that do not have a cord already on them than I have seen that do. The ones that do have a cord usually also have wheels and are considered portable.

 

[tom baker]

For a listed appliance with no HP rating the NEC allows us to use the FLA for the motor branch circuit size. This change was due to so called 2 hp garage door openers on a 15 amp circuit. These and your ACs are probably rated on locked rotor amps...or ? If you want a real 5 hp AC go to Graingers.