Receptacle HP rating for air compressor

reyetwinger · Jun 11, 2018

Hi, I am new to this forum. I have been reading through it and have found it is a great place for info. I have been looking through threads on air compressor wiring and can't seem to understand a few things
First, I have very limited electrical experience which is probably why I am having difficulty.

I received a new 3 HP air compressor, it has no cord. It is 60 gallon and not portable. I have seen it stated that 3 HP or less motors allow the use of a cord and plug/receptacle for power. I would not be using the plug/receptacle as a disconnect.

The specs on the motor state 3 HP, volts 208-230, amps 13.1-12.3. In the manual for the compressor it states under power requirements: amps 20, dedicated NEMA receptacles 6-30R. I thought this receptacle might be listed incorrectly and upon writing to them they claim that is a typo and it should read 6-20R.

I have a 30 amp dedicated circuit. I would have the breaker changed to a 20 amp.

What I can't seem to understand is I have read in many threads that the receptacle should be rated at a minimum of the motors HP, but all the 6-20R and even 6-30R receptacles I have found are only rated at 2 HP.

I am lost and can't seem to understand how this can be hooked up with the receptacles and plugs available and listed in the compressor literature. Any input would be appreciated as I just don't get it. Thanks.

pattenp · Jun 11, 2018

The thing is does the motor have a data label which states 3HP or does the compressor tank have an overall marketing label that says 3HP? I know you said motor spec but if the latter and the manufacturer states 20A circuit using a 20A plug and outlet then that's what you should use. Any reason not to just hardwire it?

Edit: At 12.3A that's only about 2HP.

reyetwinger · Jun 11, 2018

The 3.0 HP is taken from the label on the motor itself. I have been trying to figure this out for a few days now and I just can't seem to find an exact answer to this question. It frustrated me enough to join the forum and ask the question. At this point I really want to understand how some say you must have a HP rated outlet and others don't seem to think that's an important issue. It's kind of a challenge for me now to understand what's right and what's wrong. If that's the code I don't understand how the manual doesn't agree with it. It could also be that there is a simple explanation that I not being experienced in this field am ignorant of. Thanks again.

Norcal · Jun 11, 2018

If the comp is not portable then there really is no need to have a receptacle for it, use 30A fusible, or a 60A non fusible pullout A/C disconnect instead. They are cheap.

American Locomotive · Jun 11, 2018

The correct, legal, code way is to either use a special outlet rated for the motor's power, or to hardwire it. 6-30R plugs are typically rated for 2HP, and 6-50Rs are often rated for 3HP.

The issue is when it comes to disconnecting the compressor. Unplugging a compressor that's under full load with a standard NEMA outlet could possibly result in an arc across the contacts - possibly leading to arc flash injuries or worse. A standard NEMA 6-50 or 6-30 plug is perfectly capable of delivering the full rated current of the motor, plus its starting current without issue. Many people have their compressors on standard plugs like that, as it allows to more easily move the compressor for servicing or whatever.

What's bizarre, is that NEMA 6-50R outlets are typically rated for 3HP, while NEMA 6-30R outlets are often only rated for 2HP. It's bizarre because the outlets are basically constructed exactly the same. I've even seen plugs that are convertible between 30 and 50A configurations.

Rbreddin · Jun 11, 2018

Hard wire that compressor with a couple feet of 12/4 SOOW wire (or a section of 12/3 MC cable) from your local orange box store..

No need for a plug or receptacle if it's a dedicated circuit.

Use a 20A breaker.

make sure that it's grounded to either the ground wire in the SOOW or the metallic conduit of the MC cable..

alfredeneuman · Jun 11, 2018

Rbreddin said:
Hard wire that compressor with a couple feet of 12/4 SOOW wire (or a section of 12/3 MC cable) from your local orange box store..

"Hardwiring" of cords is prohibited except for elevators, cranes, and lifts.

12/4 cord and 12/3 MC have 3 current carrying wires plus the ground, the compressor only needs 2.

Rbreddin · Jun 11, 2018

alfredeneuman said:
"Hardwiring" of cords is prohibited except for elevators, cranes, and lifts.

12/4 cord and 12/3 MC have 3 current carrying wires plus the ground, the compressor only needs 2.

then 12/2 MC will work for him, even better.

reyetwinger · Jun 11, 2018

Thanks for the replies. So just for arguments sake if the 6-30R were used with a separate disconnect that receptacle would be permitted? I am trying to figure out just when that 6-30R that is mentioned in the compressor literature would be code compliant. I know that isn't the optimum way to have it wired but I am curious why it's listed in the manual. What circumstance would you be able to use it? Thanks.

pattenp · Jun 11, 2018

Based on the NEC a nema 6-30R rated at 2HP would not meet code being used on a 3HP motor. A plug and outlet is considered a disconnect and needs to be properly HP rated. I'm guessing the literature is written based on the amp listing and not the stated HP. The NEC requires you to use the listed HP to get the amps from the NEC tables. The NEC gives leeway to the manufacturers requirements.

PoorOwner · Jun 11, 2018

I don't think you will have any issues if you have a manual from the manufacturer that says you can use certain wire and rating. The manual for my compressor clearly says 30A breaker but if you go by NEC and nameplate rules I would have to use a 40A breaker. (and, it runs just fine with 30A like the manufacturer says)

nadogail · Jun 11, 2018

A pullout disconnect, like an Air Conditioner, and non metallic flex conduit with appropriate grounding and bonding conductors.

To paraphrase one of my favorite movie lines; "We don't need no stinking Inspectors"

AntonLargiader · Jun 12, 2018

alfredeneuman said:
"Hardwiring" of cords is prohibited except for elevators, cranes, and lifts.

12/4 cord and 12/3 MC have 3 current carrying wires plus the ground, the compressor only needs 2.

How are you using the term hardwire? Normally for compressors it refers to using a cord without a plug and receptacle.

Also, is MC actually allowed for use as a flexible cord? I didn’t think so.

alfredeneuman · Jun 12, 2018

AntonLargiader said:
How are you using the term hardwire? Normally for compressors it refers to using a cord without a plug and receptacle.
Also, is MC actually allowed for use as a flexible cord? I didnt think so.

Yes, I'm using the term hardwire as you said.

MC cable is subject to the same physical damage restrictions as NM. (It has to be protected against physical damage)
It contains solid conductors, and isn't really the best to use to for motor whips. (It may be against the Code too, depending on the location where it's used)
A better choice is to use flex conduit or sealtite with stranded conductors

AntonLargiader · Jun 12, 2018

alfredeneuman said:
Yes, I'm using the term hardwire as you said.

OK, FYI I wasn't referring to only a particular kind of wiring, but if you're talking specifically about SOOW-type cord, not allowed. Got it.

Rbreddin said:
make sure that it's grounded to either the ground wire in the SOOW or the metallic conduit of the MC cable..

I sure wouldn't use MC for a compressor and definitely wouldn't rely on its casing as a ground on something that vibrates. All of the MC I've seen has a ground and AFAIK the casing isn't even permissible as a ground. From AFC:

Unlike Type AC cable, the armor of interlocked Type MC cable is not
an equipment grounding means and traditional Type MC cable
requires a bare or green grounding conductor.

alfredeneuman · Jun 12, 2018

AntonLargiader said:
OK, FYI I wasn't referring to only a particular kind of wiring, but if you're talking specifically about SOOW-type cord, not allowed. Got it.

I'm referring to ALL cords. The type doesn't matter.

Rbreddin · Jun 12, 2018

The beautiful thing about this board is that if you ask for help from an electrician, they'll tell you to hire someone, you don't know what you're doing.

BUT, if you ask a question that's 80-90% right, they'll trip over each other to correct you and tell you exactly how "it should be done".. They can't resist an opportunity to correct someone, even if they wont help with the initial question

ask your 80% questions and get the answer you were looking for..

PoorOwner · Jun 12, 2018

Rbreddin said:
The beautiful thing about this board is that if you ask for help from an electrician, they'll tell you to hire someone, you don't know what you're doing.

BUT, if you ask a question that's 80-90% right, they'll trip over each other to correct you and tell you exactly how "it should be done".. They can't resist an opportunity to correct someone, even if they wont help with the initial question

ask your 80% questions and get the answer you were looking for..

Got no problem with that. Learn a lot through that kind of banter

Rbreddin · Jun 12, 2018

PoorOwner said:
Got no problem with that. Learn a lot through that kind of banter

I started the post with the BEAUTIFUL thing..
obviously, I don't have an issue with it either..

alfredeneuman · Jun 12, 2018

Rbreddin said:
BUT, if you ask a question that's 80-90% right, they'll trip over each other to correct you and tell you exactly how "it should be done".. They can't resist an opportunity to correct someone, even if they wont help with the initial question

Wrong is wrong, right is right, and never shall the 2 meet.
:bounce:

Edit: You didn't start the thread with a beautiful thing, in fact it was incorrect as it could be.

AntonLargiader · Jun 12, 2018

Rbreddin said:
BUT, if you ask a question that's 80-90% right, they'll trip over each other to correct you and tell you exactly how "it should be done"..

Really, it's just people chiming in with what they know and what they have time to post. Different people focus on different things and you get a good answer.

reyetwinger · Jun 12, 2018

PoorOwner said:
Got no problem with that. Learn a lot through that kind of banter

Yup, the longer the discussion the more ideas and answers I get. Sometimes it's pretty entertaining too.

TRWham · Jun 12, 2018

Article 430 is where motors are addressed in NFPA 70 (aka the NEC) and where we most often look for guidance in wiring compressors around here. However, is that compressor of yours UL listed, by chance? It could be so under UL 1450 (covers compressors, vacuum pumps and paint sprayers up to 3 hp). If it is listed, Article 430 will no longer govern, but 422 will (though 422 points to 430 for some topics) as it is considered an appliance by UL. Equipment OEMs will configure units with components or in ways that may appear to violate NFPA 70, but nevertheless satisfy UL requirements. The difference is that the code is written to account for field conditions that vary significantly because they are implemented by thousands of different people with many different, sometimes insane, ideas about how things should be done, but listed equipment is tested in its actual manufactured configuration so the required tolerance for lunacy is smaller. The NEC defers to UL on many issues.

I have lived this first hand. When I was in manufacturing, UL (and VDE) testing for my products was one of the things I managed, though not directly. Those products were motor driven HVACR compressors, and the resulting ratings would make no sense if you only looked at them as motors per 430.

As far as I could see, UL 1450 was silent on horsepower ratings of plugs or receptacles for cord connected units. The UL Whitebook (look for categories QDFT and QDGS) did specifically say:

Products can be cord-connected or provided with means for permanent
connection in the field. Permanently connected products are intended to
be installed in accordance with ANSI/NFPA 70

Given all this, I see 2 options for you:
1. Plug it in as the manufacturer suggests. No manufacturer in the US is going to guide you to do something dangerous or illegal, so you can be comfortable this will work just fine and be safe (assuming it is done correctly and you don't trip on the cord).

2. Hard-wire it according to NFPA 70. This may be a little more difficult, and will not work any better nor be any safer in a practical sense, but might better satisfy the roving bands of electrical inspectors that apparently menace the populace in some parts of the nation. Around here they only come when I call them, so I only call them when I want them around. And when they are around, they are pretty reasonable fellows. They'll even admit when they are wrong if you handle it right.

reyetwinger · Jun 12, 2018

The compressor is a Northstar model 47500. The manual is readily available online with a Google search. It will wind up being hardwired....it was just frustrating me trying to reconcile the 6-30R in the manual specs with what I was reading relating to motor HP and receptacles. No matter how many searches I did I couldn't find anything stating that receptacle was code compliant with that motor. I wanted to find a logical answer to satisfy my curiosity but it wasn't to be. It will remain a mystery!!!

Rbreddin · Jun 13, 2018

alfredeneuman said:
Wrong is wrong, right is right, and never shall the 2 meet.

Edit: You didn't start the thread with a beautiful thing, in fact it was incorrect as it could be.

Got news for you sport, there are literally thousands of compressors hardwired all over the nation using MC, size the wire correctly and the breaker correctly and it'll never be an issue.

I didn't start the THREAD with "a beautiful thing", I started the post (that he was replying to) with "THE BEAUTIFUL THING"..

Wrong when it comes to the NEC is subjective, due to the fact that it's a living document that is constantly changing and MOST of the changes made only apply to "new construction or renovation" and are RARELY retroactive.
Know why? Because there's NOTHING wrong with the way "it WAS done", but the "new way" is safer.

There are probably millions (possibly billions?) of NEC code violations IN PLACE right now, all over the country, yet, there hasn't been a catastrophic conflagration of every city in the country..

Imagine that..

alfredeneuman · Jun 13, 2018

Rbreddin said:
Got news for you sport,

Had anyone followed your instructions, there would have been even more NEC violations than there already are.

It's time to get out the Troll-Be-Gone spray

LG63 · Jun 13, 2018

While the debate the continues here's some historical info for context. 20-25 years ago Coleman/Sanborn sold a 240V/15A portable compressor with a 14/3 cord and a 6-15P plug. This was back when horsepower numbers were wacky but the motor did indeed pull 13 plus amps because I checked mine with a clamp meter. I suspect codes/standards have changed over the years so they don't sell these anymore but I bought mine at Sams Club so you can bet there were thousands of them sold like this.
Mine is still in service and other than being annoyingly loud all seems fine.

Receptacle HP rating for air compressor

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