Double check my thinking on compressor wiring, please

[ncfireman1918]

I thought that I had a good plan for wiring my compressor, but now I'm second guessing my idea. My electrician wired the 220v line (12/2) into a plastic J box, and capped off the wires until I was ready to wire it. I'm about 25 ft from the breaker box, with a clear line of sight. As I understand it, I don't need a disconnect.

I was at Lowe's the other day, looking for some other stuff and found an 8 ft MC whip (also 12 awg), and grabbed a metal faceplate with a knockout. It all went together just fine, but I'm concerned that I have MC conduit and a metal faceplate without a ground. If I had a metal J box in the wall, it would be grounded by the green screw that bonds the box and conduit, but with the plastic box, there is no grounding. Am I screwed on the off chance that the MC becomes energized? Is there a better option that I'm overlooking? I might be able to use a grounding wedge or bonding bushing, but that starts to feel like trying to make the wrong parts work.

I like the clean look of the flex into a faceplate, so I'd like to stick with that approach, unless there's a compelling reason not to. I'm wondering if using liquid tight (unless there is a different/better suited NM flex conduit) through a plastic faceplate that mates up would give the same clean look, without the potential hazard. Am I thinking about this correctly, or what? Thanks for gut checking my *******-ery.

Basically I've got one of these:
View media item 101706
And I bought one of these:
View media item 101708
And I'm connecting it with one of these:
View media item 101707

 

[u2slow]

That's an odd plastic box... usually there a metal tang between the bonding screw to one of the device screws, thus assuring continuity of ground for the cover plate and attached cable armour.

I would switch boxes, and/or go with a 6-20R cord connection.

 

[Terry D]

What size your compressor

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[Terry D]

If your wall is drywalled. They make a metal 1900 box that fits over a single gang box. You would be able to bond this box to your EGC. You could then use any of the knockouts on the box. Just put a blank cover on it. No need for a 90. If the wall isn't drywalled , just change it to a metal box.


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[u2slow]

If your wall is drywalled. They make a metal 1900 box that fits over a single gang box. You would be able to bond this box to your EGC.

I have yet to encounter an extension ring with a terminal for the EGC. Is that a new thing?

 

[ddawg16]

Your MC whip has a ground wire. I wouldn't worry about it. Assuming everything is wire nutted together, the MC will get it's ground at the compressor.

If your really worried about it....drill a 10-32 hole in the cover....run a 10-32 screw through it....and attach a crimped ground wire on the inside and wire nut that in with your other greens.....there....grounded.

 

[Norcal]

Drill a hole in the single gang cover and secure a ground wire with a bolt and nut, then it’s done. I hate plastic boxes but there is no sense making harder then it has to be.


The MC armor is not rated as a grounding path.

 

[Terry D]

I have yet to encounter an extension ring with a terminal for the EGC. Is that a new thing?

If it doesn't have a 10/32 tapped hole in it. You can always drill and tap your own

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[Bert_]

That's an odd plastic box... usually there a metal tang between the bonding screw to one of the device screws, thus assuring continuity of ground for the cover plate and attached cable armour.

I would switch boxes, and/or go with a 6-20R cord connection.

South of the border the blue plastic box pictured is probably the most common box used in residential construction. They are extremely common and cheap to purchase.

MC isn't really the right thing for a whip to power equipment. As far as the code is concerned MC is treated like romex. I'd be looking at flex. Could probably buy an AC whip. It will be sealtight which is a little overkill but it's common.

It would be good to know the size of the compressor. A #12 is only good to about 3 hp

 

[wyliesdiesels]

Whats the HP rating on the compressor motor nameplate?

 

[ncfireman1918]

Compressor is 3hp on the nameplate. I asked my electrician if I needed 10 awg, and he says 12 is good for that motor. He spent his career doing electrical for RJR tobacco, so I assume he’s pretty familiar with wiring machines. As a side note, the AC/MC/whatever you want to call it setup was all me. He’s hard to get out here sometimes, and I’m slightly impatient.

 

[sberry]

None of it is gonna burn the shack down, I would buy a nut and bolt too if it bothered me to add another connection, wouldn't hurt anything.

 

[ncfireman1918]

As usual, I was overthinking it. Not sure why it didn’t occur to me to drill a hole, and add a ground wire to the faceplate. It’s not like I don’t have a ton of nuts and bolts to choose from. I’m fairly confident that it would be fine without it, but the ground isn’t gonna hurt a thing, and eliminates that little percentage chance that something will go wrong. In hindsight, I should probably have grabbed an AC whip, instead of an MC whip, but I think it’s gonna be just fine. Thanks for the quick input. Best forum on the Interwebs.

 

[Kevin Essiambre]

That's an odd plastic box... usually there a metal tang between the bonding screw to one of the device screws, thus assuring continuity of ground for the cover plate and attached cable armour.

I would switch boxes, and/or go with a 6-20R cord connection.

The USA does not require a ground clip/bracket in their plastic boxes. Here in Canada the boxes need them to get CSA or ULc listings.... USA is a weird place I tell ya.

Sent from my new phone. Autocorrect may have changed stuff.

 

[wyliesdiesels]

Compressor is 3hp on the nameplate. I asked my electrician if I needed 10 awg, and he says 12 is good for that motor. He spent his career doing electrical for RJR tobacco, so I assume he’s pretty familiar with wiring machines. As a side note, the AC/MC/whatever you want to call it setup was all me. He’s hard to get out here sometimes, and I’m slightly impatient.

Technically code requires #10 NM-b for 3HP.

17a * 1.25 = 21.25a wire.

12/x NM-b limited to 20a

Or you could use #12 THWN

 

[Bert_]

I wasn't very clear in my previous post. By AC whip I meant air conditioner whip. The are commonly available.

AC cable is not the stuff to use.

 

[ncfireman1918]

Thanks for the clarification. I knew what you meant. I’m not gonna lose sleep over the 12ga wire. The motor is rated at 15amps, with a 20 amp breaker recommended by the manufacturer. I had the compressor when the electrician was doing the wiring, so he was aware of what the requirements are. He’s been doing this a long time.

 

[wyliesdiesels]

Nameplate amps cannot be used for sizing circuit if HP is listed. Must use HP.

Its technically not correct but it wont burn up...

 

[ddawg16]

Nameplate amps cannot be used for sizing circuit if HP is listed. Must use HP.

Its technically not correct but it wont burn up...

That is the most common mistake people make.

I've had more than one contractor get really pissed at me for making them replace local disconnects because they based the size on amperage instead of HP rating.

It's actually a good gauge as to if a contractor knows their stuff or not.

 

[sberry]

Yes, should be to in cable. My old 3 draws 18. They have now come up with a spl label on some new motors to get around this. Hard wire has the potential to use a different breaker than if wired to an outlet, it's kind of a plus in a way.
I had a customer recent want me to wire 5 to a 12 cable cause,,, it's the way hid dad or someone did it. He is still waiting. I ain't above being reasonable for a garage bud and millions of 5 run from 10/30 but this was a pay for gig from a guy can absolutely afford it and just thinks ground wires, box clamps are optional only for people that obviously don't know like they do.
I try to splain,, you can get anyone to do it wrong, one and main reason to hire someone is to do it right. I picked up a new customer a while back with a service call when I straighten out a couple real boogers while he was there and can see the difference to design and common detail.

 

[wyliesdiesels]

Yes, should be to in cable. My old 3 draws 18. They have now come up with a spl label on some new motors to get around this. Hard wire has the potential to use a different breaker than if wired to an outlet, it's kind of a plus in a way.
I had a customer recent want me to wire 5 to a 12 cable cause,,, it's the way hid dad or someone did it. He is still waiting. I ain't above being reasonable for a garage bud and millions of 5 run from 10/30 but this was a pay for gig from a guy can absolutely afford it and just thinks ground wires, box clamps are optional only for people that obviously don't know like they do.
I try to splain,, you can get anyone to do it wrong, one and main reason to hire someone is to do it right. I picked up a new customer a while back with a service call when I straighten out a couple real boogers while he was there and can see the difference to design and common detail.

Umm the SPL label doesnt let anyone get around anything.

If HP isnt listed or spl is listed then one must take the nameplate FLA amps and compare it to the FLC tables in the NEC to figure out the HP rating. Then size the wire based on the FLC for that HP.

 

[sberry]

A lot of "maintenance" electricians, factory guys do not know. Many have talent for fix, lots of them are not bad, some terrible but simply tend to copy some **** they see at work. Old handyman types are notorious too. I agree with dawg, this is a good place to note, a guy trained and tested should know the fundamental difference here.

 

[sberry]

Why would the mfg bother with this? Why not simply list the hp? Ok, it's been a while since I read but what does it say the hp is for 15A?

 

[sberry]

I will agree it's proper to use a 10 cable here and for good reason,,, let's assume someone puts a 5 on this in the future,,, now we are 2 sizes too small.... ha

 

[ncfireman1918]

So... do we think that I should pull out the 12/2 and install 10/2? The building construction makes it pretty easy to do so. Walls are OSB, so 15ish screws and that panel is off the wall. It’s about a 40 ft run from the breaker box, so it really wouldn’t be anything to replace. If a 5hp compressor goes in at some point, it will be me putting the 5hp in (and running appropriate sized wire at the same time) We built on my wife’s family land (against my many objections in the beginning), so this place won’t likely be sold in my lifetime. I get that **** happens, but everything is paid for, so that wouldn’t make much sense.

 

[sberry]

An 8 cable is what a 5 calls for. Personally,,,, not saying I would do this for a hire job,,, but if it was mine would probably leave it alone. There are some valid reasons it doesn't meet the letter but due to the fact this is a known load, well within any real mechanical limits its not a danger. I might catch it for saying that but its the reality.
Millions of 5 are wired incorrectly, lots of them on 10/30 and we often hear there will be a start issue they seem to work,,, had a bud with a full 5 run from a 30 breaker for 30 years. My pressure washer is listed 30, same deal, heavy duty 5. I have never had a call out about fixing one that tripped a modern 30 where there wasn't something wrong with the motor.
I have seen a couple 3 trip up a 20. since this is hard wired,,, no recept you could wire it 30 with the wire that is there.

 

[Bert_]

I've got one customer with a 5 hp auger wired to #12 THHN.

It's in a conduit that goes under about 30 foot of cement. Wires wouldn't budge so I hooked it up. The rest of the circuit got wired with #10. It hasn't been a problem for 5 or so years now. If it burns up then I guess we'll have to cut the concrete and replace it.

In that case it can't burn anything down. But I wouldn't lose any sleep over 3 hp on #12.

 

[u2slow]

So you wire your shop for generic 240V considerations and put in #12 wire with 6-20R outlets. Now you buy a new 10cfm air compressor that comes with a factory 6-20P cord set on it.

How are you not allowed to use that machine on that circuit?

 

[TRWham]

So you wire your shop for generic 240V considerations and put in #12 wire with 6-20R outlets. Now you buy a new 10cfm air compressor that comes with a factory 6-20P cord set on it.

How are you not allowed to use that machine on that circuit?

If it's UL listed as an appliance and came with that cord and plug from the OEM, then it is governed by Article 422 rather than Article 430. In the case of a listed appliance, if the installation complies with Article 422, then it probably is compliant.

 

[sberry]

Yes, if it comes cord and plug then it must have enough wire and is limited to a max breaker, the plug rating,, or recept rating. Comes with a 20 then the max ocpd is 20.

 

[Terry D]

So you wire your shop for generic 240V considerations and put in #12 wire with 6-20R outlets. Now you buy a new 10cfm air compressor that comes with a factory 6-20P cord set on it.

How are you not allowed to use that machine on that circuit?

Maybe Im not seeing it. Where does it say the compressor has a cord on it. And the first post says the wires in the box are capped off, so no receptacle.
Unless your are just giving a example.

 

[sberry]

2 different senerio. The second was hypothetical.

 

[Terry D]

2 different senerio. The second was hypothetical.

 

[u2slow]

Maybe Im not seeing it. Where does it say the compressor has a cord on it. And the first post says the wires in the box are capped off, so no receptacle.
Unless your are just giving a example.

I'm suggesting installing a receptacle instead of hardwiring it. Seems to be the most common setup with smaller/portable equipment up to 3HP.

 

[ncfireman1918]

I'm suggesting installing a receptacle instead of hardwiring it. Seems to be the most common setup with smaller/portable equipment up to 3HP.

That’s what my electrician was planning. I honestly was not thrilled with the idea (no particular reason, I just wanted it hardwired). Plus it was cheaper this way. I’ve laid out a bunch of cash over the last 6 months, so every bit counts.