Compressor Hard Wire

[Jagmandave]

Can anyone explain why a 5 hp 240V compressor has to be hardwired - besides "It's code"? What I'm looking for is the rational behind the requirement?

I'm not arguing the need or the code, I just wanna know the why?

 

[sberry]

Because someone with minds greater than our own figured it out.

 

[PCustoms]

Because if it's not, someone somewhere will unplug while it's running.

 

[The Cobbler]

Because if it's not, someone somewhere will unplug while it's running.

or plug it in when it's calling for air, huge load on the plug/receptacle.

 

[urchinhead]

or plug it in when it's calling for air, huge load on the plug/receptacle.

That is the most reasonable explanation. Anything under load drawing that much power is dangerous - your body can easily become the conductor.

You may be able to find an exception for hardwiring in the NEC, but I would strongly recommend a disconnect switch above a receptacle if the panel is not within sight.


Sent from my iPhone using The Garage Journal mobile app

 

[Stuart in MN]

You can connect a 5hp compressor with a plug and receptacle, but they have to be designed and rated for the horsepower of the motor. The plugs and receptacles you'll commonly find at the big box store aren't. Ones that are rated for that size are designed to prevent arc flash, are very expensive, and will require a trip to your local electrical parts house.

 

[Jagmandave]

Why would you get an arc flash, if it's plugged in and being controlled by the pressure switch?

Again, just trying to understand the reasoning. I appreciate that having a service disconnect is the "right" way to do it, but if you shut it off at either the pressure switch or the breaker, what's the difference?

Also, why would the inrush current on this be any different than an air conditioner that's rated for the same amp draw - and has a wall plug?

 

[brewchief]

Picture this scenario, you're welding in the garage so compressor is unplugged because you are sharing a 50 amp receptacle. The next morning after you leave for work your wife goes to leave but has a low tire on her car. Luckily she knows how to air it up but the compressor is off, she flips the pressure switch a couple times before realizing it's not plugged in, she unplugs the welder and plugs in the compressor and boom gets sparks and maybe even arc flashed because the pressure switch was on and the compressor was drawing full locked rotor amps trying to start. Now she may have even gotten the plug in and tire aired up but now there is a bad connection at the plug because when it arced it burned the plug and left some splatter on it, the heat also may have taken some temper out of the receptacle so it can't grip the plugs blades as well. Now each time the compressor runs in the future it gets a bit hotter, not a big deal if it's a metal box on a concrete wall but if it's a plastic box in a wood wall it could be a huge problem.


Would it take the right chain of events to happen? Absolutely. If that chain of events did happen would you want it in your garage? I wouldn't.

Sent from my SM-G965U using Tapatalk

 

[Dave88LX]

If hardwiring, what is the proper connection/junction?

I'll be running 10/3 Romex (want to be able to power a 120V light in the compressor closet). I imagine it will run into a metal in-wall box?

Do you just run 10 AWG THHN from the compressor, through conduit, to the in-wall box? A knockout faceplate? I plan to have the 10/3 Romex in-wall. I'm accustomed to residential wiring but not really "industrial".

Thank you.

 

[SGKent]

lots of 5 hp units aren't 5 hp. UL approved cords for 120V and 240V are available for those. Devilbiss made hundreds of thousands of those.

 

[cvairwerks]

Arc flash can happen even if the switch is off, under the right conditions. We had a hydraulic mule in the hangar that just came back from repair. Repair shop miswired the big plug and one of our guys got the surprise of a lifetime... 480, 100 amp leg shorted to ground. He was lucky he didn’t have both hands on the plug when it threw him about 15 feet. Dumb schmuck got up and was trying to plug it in again when another guy on the crew knocked him down and drug the cable away from him.

 

[sberry]

If hardwiring, what is the proper connection/junction?

I'll be running 10/3 Romex (want to be able to power a 120V light in the compressor closet). I imagine it will run into a metal in-wall box?

Do you just run 10 AWG THHN from the compressor, through conduit, to the in-wall box? A knockout faceplate? I plan to have the 10/3 Romex in-wall. I'm accustomed to residential wiring but not really "industrial".

Thank you.

Run the light from another circuit. It's not designed for 30A and turning off the comp circuit would turn the light off.

 

[sberry]

lots of 5 hp units aren't 5 hp. UL approved cords for 120V and 240V are available for those. Devilbiss made hundreds of thousands of those.

They ain't 5 hp is right. They may have that painted on the side but not on the motor plate.

 

[exranger06]

If hardwiring, what is the proper connection/junction?

I'll be running 10/3 Romex (want to be able to power a 120V light in the compressor closet). I imagine it will run into a metal in-wall box?

Do you just run 10 AWG THHN from the compressor, through conduit, to the in-wall box? A knockout faceplate? I plan to have the 10/3 Romex in-wall. I'm accustomed to residential wiring but not really "industrial".

Thank you.

10 AWG Romex is the wrong size wire for your compressor. You need 8 AWG. You can use 10 AWG THHN wire in conduit because it has a different temperature rating. I have the same compressor as you and I wired it mostly with 10 AWG THHN in conduit from the breaker panel to a junction box, then 8 AWG Romex to the disconnect.

And you can't put a light on this circuit. The light fixture is rated to be on a 20A max. circuit.

 

[Terry D]

Can anyone explain why a 5 hp 240V compressor has to be hardwired - besides "It's code"? What I'm looking for is the rational behind the requirement?

I'm not arguing the need or the code, I just wanna know the why?

Because they do not make Plug / receptacles rated for 5 hp and up

 

[Aceman]

Because they do not make Plug / receptacles rated for 5 hp and up

They do make them.

From what I have seen, the folks on this forum do not want to spend the higher cost on properly rated receps/cord ends, and choose to hardwire instead.

 

[EricVonHa]

Hardwire to the breaker, on/off switch built-in

Keep the arc in the box, not the shop

 

[Terry D]

They do make them.

From what I have seen, the folks on this forum do not want to spend the higher cost on properly rated receps/cord ends, and choose to hardwire instead.

I did not know that

 

[Jagmandave]

It's interesting to me that the wires from the pressure switch to the motor itself look a lot smaller that the supply wires?

This one does say 5hp on the label....

 

[Dave88LX]

Run the light from another circuit. It's not designed for 30A and turning off the comp circuit would turn the light off.

10 AWG Romex is the wrong size wire for your compressor. You need 8 AWG. You can use 10 AWG THHN wire in conduit because it has a different temperature rating. I have the same compressor as you and I wired it mostly with 10 AWG THHN in conduit from the breaker panel to a junction box, then 8 AWG Romex to the disconnect.

And you can't put a light on this circuit. The light fixture is rated to be on a 20A max. circuit.

That's a good point, I don't know why I didn't think of that -- it does not make sense to lose your lighting if you shut off the breaker circuit. I'm about to need a subpanel...and I have a 40-breaker panel.

I was thinking I could run a 120V circuit in the same way a magnetic starter or stove clocks...but, no.

I also "thought" that most everyone was running these compressors off 10/2 w/ a 30A breaker...maybe I am misunderstanding.

Do you have a good resource to read? Thanks.

 

[Dave88LX]

They do make them.

From what I have seen, the folks on this forum do not want to spend the higher cost on properly rated receps/cord ends, and choose to hardwire instead.

I don't think it's completely about the money. There are tons of people on here stating that a motor over "X" amount of HP needs to be hardwired...but I definitely don't know. I'm not opposed to either/or.

What would be the proper pigtail and receptacle for this application?

 

[exranger06]

That's a good point, I don't know why I didn't think of that -- it does not make sense to lose your lighting if you shut off the breaker circuit. I'm about to need a subpanel...and I have a 40-breaker panel.

I was thinking I could run a 120V circuit in the same way a magnetic starter or stove clocks...but, no.

I also "thought" that most everyone was running these compressors off 10/2 w/ a 30A breaker...maybe I am misunderstanding.

Do you have a good resource to read? Thanks.

NEC Article 430 deals with motor circuits, and is the section to use to determine wire size and breaker size. Table 430.248 says that a single phase 230V motor that's 5 HP has a full locked rotor current (FLC) of 28 A. You need to size the wire to 125% of that. So whichever wire you use needs to have an ampacity of 35 A. If you go back to Article 310 and table 310.16, you'll find that #10 Romex has an ampacity of 30A (60 degree column), so it's too small. #8 Romex has an ampacity of 40 A. You use the 75 degree column for THHN, so #10 THHN has an ampacity of 35 A.

 

[exranger06]

Receptacles and plugs are good for portable tools that you need to disconnect and reconnect somewhere else frequently. Considering that compressors this big are meant to be stationary and bolted to the floor, there's not much need to have a receptacle and plug. Might as well just hardwire it in. It's kind of like putting a plug on your furnace or central A/C unit and plugging it in, instead of hardwiring it.

 

[Jagmandave]

Good point. A simple 35? 40? amp service disconnect hardwired in conduit with #10 THHN and then #10 3 wire service cord from there?

 

[exranger06]

I used this disconnect:
https://www.homedepot.com/p/GE-60-Amp-240-Volt-Non-Fuse-Metallic-AC-Disconnect-TFN60RCP/100674085
Super cheap and rated for up to 10 HP.
I ran #10 wires in 1/2" flexible metallic conduit. You could also use liquid-tight conduit. I'm not sure if cord is allowed in this application. Maybe it is.

 

[cvairwerks]

All of our ground support equipment has plugs....but you don't want to be buying these...480VAC, 3p, 175 hp capable....about 1200$ each.....

 

[marinusdees]

It's interesting to me that the wires from the pressure switch to the motor itself look a lot smaller that the supply wires?

This one does say 5hp on the label....

22 amps ain't 5 hp.

 

[Dave88LX]

22 amps ain't 5 hp.

I pulled a pic of my motor tag, if you could try and help me/everyone understand better.




I ran the listed numbers through this calculator:
https://www.inchcalculator.com/amps-to-horsepower-calculator/


230V (label input power)




240V (wall power)



Is it not really 5 HP based on what...that the 230V motor rating is just below 5 HP? Or is the motor mis-rated or something?

 

[wyliesdiesels]

Im a bit late to the party but here goes:

Can anyone explain why a 5 hp 240V compressor has to be hardwired - besides "It's code"? What I'm looking for is the rational behind the requirement?

I'm not arguing the need or the code, I just wanna know the why?

It doesnt have to be hardwired. It can be plug and cord connected as long as the plug and receptacle are rated for the same or higher HP as the motor.

When it come to motor circuits, its not just about the running amps. Motors draw in-rush current that is 4x-8x FLA.

This is an enormous amount of current.

Say there is an issue with the compressor while running and it needs to be disconnected. More on that below.....

You can connect a 5hp compressor with a plug and receptacle, but they have to be designed and rated for the horsepower of the motor. The plugs and receptacles you'll commonly find at the big box store aren't. Ones that are rated for that size are designed to prevent arc flash, are very expensive, and will require a trip to your local electrical parts house.

This

Why would you get an arc flash, if it's plugged in and being controlled by the pressure switch?

Again, just trying to understand the reasoning. I appreciate that having a service disconnect is the "right" way to do it, but if you shut it off at either the pressure switch or the breaker, what's the difference?

Also, why would the inrush current on this be any different than an air conditioner that's rated for the same amp draw - and has a wall plug?

Lets say the pressure switch gets stuck in the closed position. (I've seen it happen) Person goes to unplug the compressor and they then get arcs.

Or at the end of the night, the compressor is not running, theyre tired and forget that the pressure switch is on and they go unplug the compressor. In the morning, after all the air has bled out overnight, they go to plug it in and they get a massive arc flash due to the in-rush current the motor pulls.

Only way to prevent this, is with a plug rated for the same or great HP as the motor. Theyre designed to contain the arc-flash.

The code reasoning for this is obviously safety. You imply people will always turn off the compressor with the pressure switch but remember humans are not perfect and do stupid things or they dont use their brain when theyre panicking and dont do things the proper way.

So the plugs rated for more than 3Hp are the expensive pin and sleeve type and people dont want to spend that kind of money. So the only other option is to hardwire it.

As to the air conditioner question dont recall ever seeing a window unit with a 5HP motor. Even the large 18KBtu and 24KBtu units do not have that high of a current draw.

 

[Jagmandave]

I wasn't saying the A/C had a 5 hp motor, just that the current draw is rated at 21 amps @240V......same as the motor label.

Again, not arguing the points, just trying to learn.

The motor would only draw that heavy of inrush current when trying to start against full pressure, right? For example, it wouldn't do that starting on an empty tank, right? And I don't think they start against pressure anyway as it seems like when the motor stops there is a release of pressure from the pressure switch - as it bleeds off air for a few seconds after it stops the motor?

Maybe that's not how it works, I dunno....I'll have to go study it a bit more.

 

[u2slow]

Many compressor motors use capacitors to reduce the amperage requirements.

As I understand it, the NEC ignores the nameplate FLA - and only identifies with the horsepower - which you look up in a table (i.e. based on a std non-capacitor motor).

When the mfr gets that US-market 5HP compressor rubber-stamped with a ULC or CSA rating, and ship it to Canada, we wire that machine's circuit with #10 wire because the CEC stipulates to use the actual equipment's rating to size the circuit (when available).

IMO, #10 will work fine/safely for the circuit in either country. Its just not an NEC-compliant install.

 

[wyliesdiesels]

If hardwiring, what is the proper connection/junction?

I'll be running 10/3 Romex (want to be able to power a 120V light in the compressor closet). I imagine it will run into a metal in-wall box?

Do you just run 10 AWG THHN from the compressor, through conduit, to the in-wall box? A knockout faceplate? I plan to have the 10/3 Romex in-wall. I'm accustomed to residential wiring but not really "industrial".

Thank you.

You only need 2 conductor plus grnd NM-b. 10/3 NM-b is 3 conductor plus ground

As pointed out by exranger06, #10 NM-b is too small

As to what wire to use from the wall to the compressor, #10 THWN is fine in FMC. Or you may be able to find a premade whip at the store.

Also, if the compressor is more than 50' from and not within sight of the breaker panel, you will need a local disconnect.

lots of 5 hp units aren't 5 hp. UL approved cords for 120V and 240V are available for those. Devilbiss made hundreds of thousands of those.

yes because of inflated numbers. This is why one must check the motor nameplate. The OPs motor however, IS a 5HP rated motor so he must size the wire based on NEC FLC table amps of 28a.

10 AWG Romex is the wrong size wire for your compressor. You need 8 AWG. You can use 10 AWG THHN wire in conduit because it has a different temperature rating. I have the same compressor as you and I wired it mostly with 10 AWG THHN in conduit from the breaker panel to a junction box, then 8 AWG Romex to the disconnect.

And you can't put a light on this circuit. The light fixture is rated to be on a 20A max. circuit.

This

Because they do not make Plug / receptacles rated for 5 hp and up

actually they do.

heres one rated for 250v and 5HP. you wont like the price though ($200-$300+)

https://www.hubbell.com/hubbell/en/...e/IEC-Devices/HBL330P6W/p/159377?PN=HBL330P6W

https://hubbellcdn.com/literature/Wiring_WLBP005.pdf

It's interesting to me that the wires from the pressure switch to the motor itself look a lot smaller than the supply wires?

This one does say 5hp on the label....

you can't compare the 2. Thats a false equivalency. The wires on listed equipment have been tested in a lab, they aren't ran inside the walls of a building next to flammable materials and they often have a higher temperature rating (105° instead of 75° or 60°).

That's a good point, I don't know why I didn't think of that -- it does not make sense to lose your lighting if you shut off the breaker circuit. I'm about to need a subpanel...and I have a 40-breaker panel.

I was thinking I could run a 120V circuit in the same way a magnetic starter or stove clocks...but, no.

I also "thought" that most everyone was running these compressors off 10/2 w/ a 30A breaker...maybe I am misunderstanding.

Do you have a good resource to read? Thanks.

exranger06 already explained the wire sizing requirements for motor circuits. #10/x NM-b is too small for 5HP

As to the breaker sizing, you can use a breaker rated up to 250% of the FLC table amps. But a 30a breaker may very well work and not nuisance trip.

22 amps ain't 5 hp.

Umm, what? I've seen numerous 5HP nameplates and they all have an FLA between 20-23a. The NEC FLC table lists 5HP as 28a and thats what the wire must be sized by NOT the nameplate FLA.

 

[wyliesdiesels]

I wasn't saying the A/C had a 5 hp motor, just that the current draw is rated at 21 amps @240V......same as the motor label.

Again, not arguing the points, just trying to learn.

The motor would only draw that heavy of inrush current when trying to start against full pressure, right? For example, it wouldn't do that starting on an empty tank, right? And I don't think they start against pressure anyway as it seems like when the motor stops there is a release of pressure from the pressure switch - as it bleeds off air for a few seconds after it stops the motor?

Maybe that's not how it works, I dunno....I'll have to go study it a bit more.

FLA rating doesnt matter when it comes to wiring motor circuits when the HP rating is listed on the nameplate. The NEC requires the motor circuit wiring to be sized based on the corresponding FLC table amps...

Also, a 21a rated air conditioner will not exhibit the same characteristic arc flash that a 5HP motor will when being unplugged while running.

 

[Norcal]

These are HP rated, no idea on cost though.

https://www.meltric.com/html/ds-decontactor.html#


Click on ratings.

 

[sberry]

I also "thought" that most everyone was running these compressors off 10/2 w/ a 30A breaker...maybe I am misunderstanding.

Not everyone but a lot are. Its not compliant but pretty low on the risk factors of ways to burn down your house. Had a guy want me to run 12 for one a while back. Have seen them on 12 cable for decades.

 

[alfredeneuman]

These are HP rated, no idea on cost though

I hooked up a 200A 120/208 3Ø receptacle of theirs once and as I remember the cost was ~$400.00 - 1990's prices (Plug not included)

 

[mxdev]

Not everyone but a lot are. Its not compliant but pretty low on the risk factors of ways to burn down your house. Had a guy want me to run 12 for one a while back. Have seen them on 12 cable for decades.

I got my 5HP compressor for cheap because the previous guy was trying to run it on 14ga with a 15A breaker... According to him it would work for a while before popping the breaker. Didn't want to bother running a better circuit, so got rid of it.

He got it for cheap through work, and since it didn't work for him, he sold it to me for even less.

I added my own magnetic starter/overload and just use the pressure switch to signal that instead of relying on a disconnect.

 

[sberry]

Yes, good for you. One thing about that and this happens all the time. They get the "electrician at work" to come over and wire these up and at work a 480 3 ph runs a 5 on 14/15,,, but lots of those guys are maintenance men types, they work their way in to it learning hand to mouth from someone often don't know all that much about it to start with.
One of my Buds growing up was like that, either obsessive about v drop or did **** the way they did at work and never really had a formal apprenticeship where they got tested and made sure they knew about **** they thought they did. Never did inspected work. They might been talented enough most of the **** they did worked and no one notices they don't know squat.

 

[Bert_]

Also, a 21a rated air conditioner will not exhibit the same characteristic arc flash that a 5HP motor will when being unplugged while running.

I'm not sure if I agree with this. A 21 amp (240 1 phase) air conditioner is probably around 4 ton. The old rule of thumb was always one horsepower per ton.

So the two are actually pretty similar.

 

[Bert_]

22 amps ain't 5 hp.

22a is definitely 5 hp. This falls right into the range of all 5hp industrial or farm duty motors.