CompressorHook-up Question

[kc10a]

I am getting ready to hook up my Quincy 5hp 2 stage compressor. The plate on the motor says 21 amp. Is a 30 amp 220v breaker adequate? Is 10 gage wire adequate? The manual just says to have it wired by a licensed electrician but I can connect wires. Thanks for any advice.

 

[geartow]

If you are asking for help you may not be able to conneect the wire yourself in a safe manner, seek professional assistance.

 

[RickP]

I just wired a compressor of the same size. If you're using romex, you need 8awg cable from the panel. With conduit, you can use 10awg wire. You should wire the last 6 feet with a pre-made 10 gauge liquid-tite whip or 8-3 SJ cord. Don't use a plug (5 hp is too big for a plug under the NEC code). You could use a 30amp breaker, but I'd recommend upsizing it to a 40amp.

Just make sure you know what you're doing before you open the panel. Have you considered running the cable yourself and then hiring an electrician to hook it up in the panel?

 

[myredracer]

21 amps is the full load rating or FLA which is at full running rpm (ie. not at startup). By NEC, multiply by 1.25 for wire size which means min. #10 AWG. Breaker sizes are as per a table in NEC. Without looking it up, it is probably 40 amps or maybe 50 amps. Breakers for motors are usually larger in order to handle the inrush current at startup.

I have a 5HP motor on my compressor and FLA is 22. I have a 40 amp breaker because I had a spare kicking around and it works fine but IIRC, our code calls for a 50 amp breaker.

There's no problem using a smaller breaker than code says to use if it works (but not less than required wire size), but if it does trip on startup, you'll have to get a new one and may not be able to return it...

If you hardwire the compressor, you will need a disconnect switch at the motor or I believe the NEC allows the breaker to be the disconnect if within sight and within a certain distance.

You can use a receptacle and cord connector as long as horsepower rated. (See NEC 430.109(F)). If using a receptacle and connector, this functions as the disconnect means.

Not sure why you'd have to go to #8 if Romex? What NEC articles require that?

Someone with a Quincy will probably come along to say what they are using.

 

[kc10a]

Thanks for the help. There is a switch on the compressor. The panel is only about 4 Ft away.

 

[CNGsaves]

Pretty sure the GJ Sparkys have pointed out many times that HORSEPOWER is what determines the breaker size for compressor that must be direct wired when 5 HP or more. You don't use FLA for 5 HP or larger motors.

Soon they'll chime in and provide details.

 

[manwithtools]

Pretty sure the GJ Sparkys have pointed out many times that HORSEPOWER is what determines the breaker size for compressor that must be direct wired when 5 HP or more. You don't use FLA for 5 HP or larger motors.

Soon they'll chime in and provide details.

Nonsense....

 

[pattenp]

Nonsense....

Not total nonsense. NEC requires motors to be wired according to the listed HP on the motor data plate, not the listed amps. NEC table 430.248 list 5HP as being 28 FLA.

 

[manwithtools]

Yep, I should have declared that it's my opinion this portion of the code is nonsense. It also applies to smaller motors BTW.

The whole reasoning behind this requirement is that some idiot "might" "someday in the future" replace this motor with some mythically inefficient motor that draws 33% more current than the most typically available motor on the market.

I swear the NEC is written by folks with significant investment in the copper industry.

Thanks for holding my foot to the fire on this subject. I should not spout off on my opinions without acknowledging the proper sections of code.

 

[matt_i]

I'm a violator.

I have a 50A oven plug/welding receptacle that plugs into the compressor @ 5hp.

I have a CNC milling machine that's on a 60A x 4 pole pin & socket connector. 7.5hp spindle motor.

Neither have overcooked.

 

[Cyberbear]

There are lots of opinions out there, so talk with a professional electrician and ask how he would hook up your a/c. I spent more than 12 years in the electrical trade building motor control panels and wiring them up. Back then we always used a properly sized motor starter to operate motors 3 hp and larger. These systems provide all the protection you need to simply operate an air compressor. Once you gather your information, decide how much you are capable of doing yourself and go from there.

 

[kc10a]

Thanks for all the advice. I appreciate it.

 

[matt_i]

The glaring issue that I see with a plug is that its easy to uneducatedly unplug it under load and draw an arc. The pin & socket connectors have a mechanical interlock to the disconnect above requiring it to be open circuit before the plug can be released from the receptacle.

 

[manwithtools]

Thanks for all the advice. I appreciate it.

I just looked at my new Quincy 5hp two stage. It looks like Quincy thinks that 10 ga wire is adequate (which would indicate a 30 amp circuit). I say this as the conduit inlet to the pressure switch is 1/2" and the supplied fork terminals are for 12 -10 ga wire - this is where they intend you to hook up power to the compressor.

I'm going to hook mine up with 10 ga wire and a 30 amp breaker. The worst thing that can happen is the breaker will nuisance trip. I've got a hunch it won't and all will be fine. The 30 amp breaker will protect the 10 ga wire as it's intended to do. If I get nuisance trips; I'll put a 5HP motor starter on it and run 8ga and a 40 amp breaker to it.

 

[manwithtools]

The glaring issue that I see with a plug is that its easy to uneducatedly unplug it under load and draw an arc. The pin & socket connectors have a mechanical interlock to the disconnect above requiring it to be open circuit before the plug can be released from the receptacle.

Even better are Meltric plugs, but what an expensive solution for the home shop. If you have a plug you will most likely never unplug it on a big compressor like these. If you do unplug and reconnect, just be sure to turn the breaker off before doing so.

Sometimes we just have to be in control of our own actions. My shop, my responsibility to act accordingly. If I don't, then I deserve what I get. I've got the scars to prove that I've gotten it a few times, I tend to remember not to do those things again.

 

[manwithtools]

I thought the OP might like to know that I got mine wired up today, 10 GA - 30 Amp breaker and so far it seems fine. No labor on starting or when nearing full pressure. If I think about it, tomorrow I'll throw an ammeter on it to see what amperage draw really is. I suspect it's substantially lower than 21 amps other than startup.

 

[manwithtools]

Oh, BTW this is sooooo much quieter than my 15 year old 3 HP 60 Gal Porter Cable single stage. I can't wait to get to where I can use the air tools on a regular basis.