compressor hook up

[Mike Honcho]

me again....

so i bought this compressor

http://www.tractorsupply.com/en/store/porter-cablereg;-stationary-belt-drive-air-compressor-60-gal

which says 15 amps, which 12 thhn and a 20 amp dp breaker should suffice....it will be about a 35-40 foot run of wire. ive attached the plate on the motor which shows 17.2 FLA. will the 20 amp/12 gauge be enough?

 

[Falcon67]

I have the older CH version in blue. I run mine on the end of 50ish feet of 12-2 Romex. Mine is mounted to a skid and parked in a compressor closet, so I use a twist lock 20A plug/socket to disconnect the compressor for service/removal.

 

[Mike Honcho]

what did you use for the cord from the compressor to the socket? just 12/2?

 

[pattenp]

Over 3HP should be hardwired. Period.

 

[C96]

In my opinion you’re pushing it just a tad bit. The 12 gauge wire may be ok, but the 20 amp breaker might give you problems at start-up. If it should trip the breaker install a 25 amp.

Yes, many will tell you that you can’t put 12 gauge wires on anything over 20 amps, but there are exceptions to the rule when it comes to wiring electric motors.

Personally, I would use 10 gauge wire on a 30 amp 2 pole breaker over to a 30 amp 2 pole fusible disconnect switch adjacent to the compressor. Install 20, or 25 amp slow blow fuses, this will allow the compressor to start-up and offer motor overload protection as well.

 

[pattenp]

By NEC it should be on #10. You should use at least a 30A DP breaker. I assume the motor has its own overload protection (Red reset button on motor housing). You need to size circuit by the 3.7HP on the motor. Forget the 15A and 17A listed in various places.

 

[Mike Honcho]

By NEC it should be on #10. You should use at least a 30A DP breaker. I assume the motor has its own overload protection (Red reset button on motor housing). You need to size circuit by the 3.7HP on the motor. Forget the 15A and 17A listed in various places.

So, 10 thhn to a 30 amp ac disconnect, and a liquid tite whip to the compressor?

 

[pattenp]

So, 10 thhn to a 30 amp ac disconnect, and a liquid tite whip to the compressor?

That's fine. I like this disconnect, it's a non fused, just a switch.

http://www.homedepot.com/p/Square-D...t-Protection-QO200TRCP/202353314?N=5yc1vZbm0h

If the breaker that feeds the circuit to the compressor is in site of the compressor you don't need the disconnect.

 

[Mike Honcho]

It's on the other wall of the garage, about 15-20 feet from the sub. I don't have a knockout in the switch box, only a clamp for a cord. Am I stuck drilling a hole for a flex conduit?

 

[wyliesdiesels]

Heres the compressor hookup low down:

Wire sized @ 125% of FLC from NEC tables

Breaker can be max 250% of FLC; this usually results with a breaker rated higher than wire ampacity and many people get confused with this. This is because the breaker isnt used for overload protection in a motor circuit. That is the job of the overloads. The breaker only provides ground fault and short circuit protection.

Disconnect required if farther than 50' and not within sight of panel

Motors larger than 3HP must be haddwired because NEMA plugs and recepticles arent rated for more than 3HP despite being rated for the amperage. This has to do with in-rush currents...

Motor starter is required if pressure switch isnt rated for the HP of the motor. A motor starter is also required for a motor that has no integral overload protection such as a red reset button on te back of the motor.


Now one could argue that your motor isnt 3.7HP because the FLA of the motor is only 17.2a, a true 3HP motor is 17a, so 3.7HP would be a little higher...this would mean cord and plug is acceptable. Keep in mind motor and compressor manu. grossly overate their product most of the time....

 

[pattenp]

Heres the compressor hookup low down:

......
Now one could argue that your motor isnt 3.7HP because the FLA of the motor is only 15a and a true 3HP motor is 17a...this would mean cord and plug is acceptable. Keep in mind motor and compressor manu. grossly overate their product most of the time....

The thing is the specs on web page at Tractor Supply says 15A, but the data label on the motor says 17.2 amps and 3.7HP.

 

[wyliesdiesels]

The thing is the specs on web page at Tractor Supply says 15A, but the data label on the motor says 17.2 amps and 3.7HP.

I didnt look at the nameplate(i will edit my previous post) and it sounds like the webpage is a typo.

U know that 17a is 3HP so those numbers dont jive, ie 3.7HP...

 

[Mike Honcho]

i have zero issues hardwiring it, as a matter of fact looking at the plug in cord price it looks like it might eve n be cheaper to hardwire it.

do i have any other options other than liquid tight, or metal flex conduit?

 

[wyliesdiesels]

Because of vibrations, not really...

 

[Mike Honcho]

ok, thanks for the help guys, much appreciated

 

[C96]

do i have any other options other than liquid tight, or metal flex conduit?

Absolutely, another choice is M/C cable. It’s basically like Romex but has an outer aluminum armor that wraps around the conductors. Installs very much like Romex and is smaller in diameter than traditional metal flex conduit. It comes with wires installed and you can basically bend it however you like not having to worry about the 360° bend rule that metal flex and other conduit systems must adhere to.
↓

 

[Charles (in GA)]

Heres the compressor hookup low down:

Wire sized @ 125% of FLC from NEC tables

Breaker can be max 250% of FLC; this usually results with a breaker rated higher than wire ampacity and many people get confused with this. This is because the breaker isnt used for overload protection in a motor circuit. That is the job of the overloads. The breaker only provides ground fault and short circuit protection.

Disconnect required if farther than 50' OR not within sight of panel, no matter the distance.

Motors larger than 3HP must be haddwired because NEMA plugs and recepticles arent rated for more than 3HP despite being rated for the amperage. This has to do with in-rush currents...

Motor starter is required if pressure switch isnt rated for the HP of the motor. A motor starter is also required for a motor that has no integral overload protection such as a red reset button on te back of the motor.


Now one could argue that your motor isnt 3.7HP because the FLA of the motor is only 17.2a, a true 3HP motor is 17a, so 3.7HP would be a little higher...this would mean cord and plug is acceptable. Keep in mind motor and compressor manu. grossly overate their product most of the time....

Fixed it for 'ya. By definition, anything over 50 ft is "out of sight".

What WD has posted above needs to be a standard post anytime someone asks about a compressor hookup. Its correct and to the point.

Charles

 

[Charles (in GA)]

I cheated somewhat. After running the ¾ EMT and pulling the #6 Red/Black/White and #10 green THHN wires to the 4 position subpanel next to the compressor, I had 7 ft of wire remaining. I went and bought a section of blue smurf tubing and end pieces and fed the Red/Black/Green wires thru it to the compressor from the sub panel breaker. Cheap, easy, good protection for the wires. Compressor is in a corner, no traffic so the smurf is not likely to get disturbed or damaged.

Charles

 

[C96]

Heres the compressor hookup low down:

Wire sized @ 125% of FLC from NEC tables

Breaker can be max 250% of FLC; this usually results with a breaker rated higher than wire ampacity and many people get confused with this. This is because the breaker isnt used for overload protection in a motor circuit. That is the job of the overloads. The breaker only provides ground fault and short circuit protection.

Disconnect required if farther than 50' and not within sight of panel

Motors larger than 3HP must be haddwired because NEMA plugs and recepticles arent rated for more than 3HP despite being rated for the amperage. This has to do with in-rush currents...

Motor starter is required if pressure switch isnt rated for the HP of the motor. A motor starter is also required for a motor that has no integral overload protection such as a red reset button on te back of the motor.


Now one could argue that your motor isnt 3.7HP because the FLA of the motor is only 17.2a, a true 3HP motor is 17a, so 3.7HP would be a little higher...this would mean cord and plug is acceptable. Keep in mind motor and compressor manu. grossly overate their product most of the time....

Fixed it for 'ya. By definition, anything over 50 ft is "out of sight".

What WD has posted above needs to be a standard post anytime someone asks about a compressor hookup. Its correct and to the point.

Charles

Not so fast, here’s another fix: A motor starter is not required even if the motor has no integral overload protection. NEC 430 states a separate overload device such as fuses may be used to provide overload protection providing they do not exceed 140% of FLA therefore a motor starter is not required.

 

[Mike Honcho]

im back, so after looking at it last night heres my issue, the first pic is the side the motor comes into the switch. the next to are where the power cord is meant to come in the back. no knockouts on either side, and on the power in side there is a small tube and only about 3/4 inch of clearance between the tube and switch housing. so i dont think flex will fit. can I use a SJO style cord from the disconnect to the switch? or is that a no no?

any other ideas will be apprieciated

 

[C96]

If the cord will not be subject to physical damage it should be fine.

Since your going that route, SOOW cable is better with higher ratings.

 

[warren57]

me again....

so i bought this compressor

http://www.tractorsupply.com/en/store/porter-cablereg;-stationary-belt-drive-air-compressor-60-gal

which says 15 amps, which 12 thhn and a 20 amp dp breaker should suffice....it will be about a 35-40 foot run of wire. ive attached the plate on the motor which shows 17.2 FLA. will the 20 amp/12 gauge be enough?

Yes it will work. 35-40' has minimal voltage drop and 20 amps is 20 amps. Actually #12 wire with your 15 amp load at 40' length will have a voltage drop of 1.91 volts or 1.50 %. Normally you do not want voltage drop more than 3%, so you are good. You can expect to see 118 volts at the motor., which will have no effect on amps.
LRA (locked rotor amps) is the initial surge of power required to start a non moving motor. As the motor spins, the current goes down to the name plate amps. (Which is a fraction of a second at startup)
So, when it turns on it will draw 17.2 LRA and in a fraction of a second, it will be at 15 amps (or less in most cases).

 

[pattenp]

Yes it will work. 35-40' has minimal voltage drop and 20 amps is 20 amps. Actually #12 wire with your 15 amp load at 40' length will have a voltage drop of 1.91 volts or 1.50 %. Normally you do not want voltage drop more than 3%, so you are good. You can expect to see 118 volts at the motor., which will have no effect on amps.
LRA (locked rotor amps) is the initial surge of power required to start a non moving motor. As the motor spins, the current goes down to the name plate amps. (Which is a fraction of a second at startup)
So, when it turns on it will draw 17.2 LRA and in a fraction of a second, it will be at 15 amps (or less in most cases).

The OP's compressor motor is labeled as 17.2 FLA (Full Load Amps), not LRA (Locked Rotor Amps.). The 15 amps is an incorrect amperage figure on the Tractor Supply website. All that aside the motor should be wired based on the 3.7HP as NEC requires which is #10 wire.

 

[Mike Honcho]

If the cord will not be subject to physical damage it should be fine.

Since your going that route, SOOW cable is better with higher ratings.

Thanks, I may have to go that route if I can't fit a whip between threw tube. I'll be working on that today

 

[sberry]

There are wired with cord all the time.

 

[Mike Honcho]

That's fine. I like this disconnect, it's a non fused, just a switch.

http://www.homedepot.com/p/Square-D...t-Protection-QO200TRCP/202353314?N=5yc1vZbm0h

If the breaker that feeds the circuit to the compressor is in site of the compressor you don't need the disconnect.

i was looking at these today and there are only knockouts on the back and bottom. my conduit drop comes from the ceiling, can i cut a hole for an emt connector in the top? although i dont technically need it, i may put it in for conveinence.

 

[Slowgsr]

I wouldn't recommend it. A pro wouldn't. Looks Mickeymouse

They don't come with ko's in the top since they're wet location rated and you generally avoid going in the top in wet locations, I mean some seal rings and other means can make it do-able but it's just less desirable.

Come down beside it, the on the end of the emt go connector/rigid coupling/then convert it to sealtight, pull your wire with it straight (don't forget a bond) I never rely on emt anyways but it's required in sealtight, then make a nice little 180deg bend and go into the bottom.

You could get away using a short piece of flex as well rather then sealtight, cost savings only. Slightly less durable, but acceptable if less then 3ft.

Good luck.

You could just get a 30amp 250v rated disconnect, even 3ph, and just use it for single phase, they have ko's in the top slightly more cost vs the photo.

 

[Charles (in GA)]

Cord is generally not intended to be used for permanent installation, with the exception of certain specialized installations such as elevators, pendants, and gantry cranes and the like. It also is not allowed to be installed in raceways except where specifically permitted by the code.

Most pressure connectors/screw type connectors, as used on magnetic starters, pressure switches and such are not designed for use with the fine stranded wire used in cords.

Charles

 

[pattenp]

i was looking at these today and there are only knockouts on the back and bottom. my conduit drop comes from the ceiling, can i cut a hole for an emt connector in the top? although i dont technically need it, i may put it in for conveinence.

Mount it sideways. It's not being placed out in the weather so it doesn't matter and I don't recall any markings on it that indicates it has to be mounted upright. That way you can use 90 deg ells for connections. Will just look strange.