Yet another compressor wiring question with pics...

[pault28]

I just purchased a used four cylinder 60 gallon 7hp compressor. the first pic is the compressor itself. The second pic is the label on the compressor, the third pic is the label on the motor. My confusion begins with the fact that the label on the compressor calls for a minimum conductor (wire) ampacity of 35 amps. That means I cannot use 10 and have to use 8. However, if I look at the motor, it said it only pulls 21 amps. If that is the case, why can I not use 10 gauge wire. I currently have a 50amp circuit which is about 2 feet from my subpanel wired with 8 gauge wire and a 6-60 receptacle. Id like to run a 10/3 cable terminated with a 6-50P plug to go into my current receptacle.

Is this safe having this particular compressor wired with a 10-3 cable and a 6-50 plug into a 6-50R receptacle on a 50amp dual pole circuit breaker wired to the panel with 8 gauge wire?

 

[rockwithjason]

can you wire it that way? sure you can. is it safe? well you will have short circuit protection but you won't have the degree of overload protection that a proper dedicated circuit would provide. would it work? yes it would but i would not be a "legal" install. what are the real risks? minimal, i would bet that the motor has it's own thermal overload. if that is the case i would run it the way you describe until you get a dedicated outlet for it.

 

[Aceman]

The problem with that motor is it's only rated in amps, not HP. The 7HP the compressor company says it is doesn't mean anything, it's a made up number as far as anyone is concerned. The NEC requires you take a motor that's only rated in amps and make a best "guess" on the HP per 430.6(A)1. I'd say 5 hp based on the 21 amps.

According to the NEC tables a 5HP draws 28 amps. 28 amps x 125% for continuous loads=35 amp rated conductor.

Motors of the same HP can vary by a few amps. I believe the NEC's intention is to make sure the wire size can handle the worst case scenario, say that motor burns up and you swap it with another 5HP. This time it actually does pull 28 amps. You're covered because your wire size was taken from the tables, rather than the original motor.

A couple other things to think about, a 50 amp cord end most likely isn't rated for 5HP. Probably 3HP. You'll probably need to hardwire the compressor.

Check the HP rating on the pressure switch under the cover as well. Most pressure switches aren't rated for 5HP.

 

[pault28]

can you wire it that way? sure you can. is it safe? well you will have short circuit protection but you won't have the degree of overload protection that a proper dedicated circuit would provide. would it work? yes it would but i would not be a "legal" install. what are the real risks? minimal, i would bet that the motor has it's own thermal overload. if that is the case i would run it the way you describe until you get a dedicated outlet for it.

So how would you ideally wire it? Would you do 8 gauge cable all the way into the 50amp circuit or do a new circuit and go 10 gauge from the compressor all the way to the panel on a 30A breaker?

I should state that when I did the subpanel I wired the 50A circuit with the intentions of getting a larger welder, however I am happy with my Hobart 140 so I do not have a need to keep the 8 gauge 50A circuit, I can very easily switch to a 30A 10g circuit. Basically I am not sure which route I should go to make this as safe and legit as possible. Since the outlet is literally 2 feet from the subpanel, wire is cheap and negligible in price. I want to wire this compressor safely and correctly.

 

[pault28]

The problem with that motor is it's only rated in amps, not HP. The 7HP the compressor company says it is doesn't mean anything, it's a made up number as far as anyone is concerned. The NEC requires you take a motor that's only rated in amps and make a best "guess" on the HP per 430.6(A)1. I'd say 5 hp based on the 21 amps.

According to the NEC tables a 5HP draws 28 amps. 28 amps x 125% for continuous loads=35 amp rated conductor.

Motors of the same HP can vary by a few amps. I believe the NEC's intention is to make sure the wire size can handle the worst case scenario, say that motor burns up and you swap it with another 5HP. This time it actually does pull 28 amps. You're covered because your wire size was taken from the tables, rather than the original motor.

A couple other things to think about, a 50 amp cord end most likely isn't rated for 5HP. Probably 3HP. You'll probably need to hardwire the compressor.

Check the HP rating on the pressure switch under the cover as well. Most pressure switches aren't rated for 5HP.

Here is a picture of the pressure switch. I cannot see any rating on it.

 

[Aceman]

Flip the cover over....

 

[pault28]

Flip the cover over....

Jeez, that's embarrassing! Haha. Attached is a picture of the pressure switch label. Looks like at 230V single phase, it is rated at 2.4? Am I reading that correctly?

 

[Aceman]

It's rated 24 amps from what I can see, it looks like it'll handle the motor you have.

 

[pault28]

So should i hardwire with 8awg from the pressure switch to the subpanel with a 50a breaker or should I hardwire with 10 all the way from the compressor to the subpanel and put in a 30a breaker? Now I'm thinking I'll hook whatever whip I get right to the panel and turn it off or on with the breaker. Then I'll leave the 50A receptacle for a future welder.

 

[pattenp]

If the circuit is going to have an outlet to plug in the compressor and you are using NM-B wire, #8 is good to 40A not 50A.

 

[pault28]

I'm ok with running new wire and hardwiring it, but do I need 10 and a 30A breaker or or 8 and a 50A breaker? This will bechardwired, not plugged in. I am just going to keep my current receptacle for a welder in the future and will run a separate circuit for the compressor.

 

[Norcal]

Since the motor has overload protection, I would use 10 AWG & a 50A breaker, but this can only be done if hardwired (a disconnect is required) and no NM cable "Romex®", or flexible cord, or any receptacles are used. The reason this is OK is that NEC art. 430 allows table 310.16 ampacities to be used & 10AWG is 35A & the motor has overload protection, a fuse or circuit breaker is just for short circuit protection. Please note that any part of this does not apply to normal branch circuits under any circumstances.

 

[pault28]

Since the motor has overload protection, I would use 10 AWG & a 50A breaker, but this can only be done if hardwired (a disconnect is required) and no NM cable "Romex®", or flexible cord, or any receptacles are used. The reason this is OK is that NEC art. 430 allows table 310.16 ampacities to be used & 10AWG is 35A & the motor has overload protection, a fuse or circuit breaker is just for short circuit protection. Please note that any part of this does not apply to normal branch circuits under any circumstances.

NorCal,
Can I just wire it right to the subpanel. The compressor is right next to the subpanel, and would be less than 4 feet of wire. Can I use 10/3 BX? If not can I use greenfield with 3 10 gauge wires? I know it requires a disconnect close to the compressor when hard wired, but the panel is right there and easily accessible. I would flip the breaker on when I'm working in the garage and turn it off when I go inside the house. Id rather not get a disconnect box. This is hobby, non-pro use.

 

[Norcal]

The breaker would qualify as a disconnect, a breaker "lockout" should be used though, also greenfield or sealtite could be used....

 

[pault28]

Can someone post a link to a basic disconnect box that would suffice for my compressor? I'm not familiar with disconnect boxes. It is basically like a switch in line between the compressor pressure switch and the electrical panel?

 

[Milton Shaw]

iI notice that the black wire already shows overheating where the screw is not making a good connection, and there is a little discoloration on the white wire too. Wires should wrap around the terminal for better connection, not just a straight wire stuck in.

 

[pault28]

iI notice that the black wire already shows overheating where the screw is not making a good connection, and there is a little discoloration on the white wire too. Wires should wrap around the terminal for better connection, not just a straight wire stuck in.

I saw that too. I am going to disconnect them and crimp on some ring terminals for all the the screw terminals. I think I will end up doing a disconnect. I am looking into them and am not sure if I need a fused or non-fused one. I'll run 10AWG wire from the pressure switch to the disconnect and 10AWG from the disconnect to the subpanel. There will be a 50A breaker in the subpanel and nothing else will run on this circuit as it will be hardwired.

So...finally, fused or non-fused disconnect?

 

[pattenp]

This will work. It's rated at 60A.

http://www.homedepot.com/h_d1/N-5yc...oreId=10051&relatedSearch=square-d disconnect

 

[pault28]

And 10AWG from the pressure switch to the disconnect and from the disconnect to the subpanel is sufficient? I'll get a 50A circuit breaker too.

Thanks for the link to the disconnect. My local store has it in stock.

 

[Aceman]

You don't want a disconnect, there's no reason for one. Your panel is only a few feet away. Shut the breaker off at night if you're worried about the compressor kicking on.

I'd run 10-2 MC from the panel right to the pressure switch. Wrap the wire around the screws CLOCKWISE rather than just sticking it straight in like your existing wires. Nothing to it.

 

[pault28]

Ok,
I'll do that wired to a 50A breaker.