Compressor amp rating?

[KPack]

I'm still an electrical newbie, so keep that in mind.

I have my neighbor's old compressor sitting in my garage waiting to go into the shop. It's a Snap-on 70gallon, 5 HP compressor (BRA517V). The motor on it says 22 amps at 230 volts.


On the Snap-on website it says that the compressor needs a 40 amp circuit. But looking at the motor, I would assume a 30 would be enough to power it. Why would a motor only drawing 22 amps need a 40 amp circuit? And will a 30 amp circuit be sufficient or am I totally off base here?

 

[mike93lx]

It will draw far more than 22 when starting. Why wouldnt you follow the manufacturer's recommendation?

 

[KPack]

It will draw far more than 22 when starting. Why wouldnt you follow the manufacturer's recommendation?

Don't forget, I'm an electrical newb. I had assumed the 22 amps was the max draw of the motor. I didn't realize that initial start would be far more. So that means I'm going to have to run #8 wire instead of #10. And maybe rethink where I was planning on putting the compressor.

 

[u2slow]

Wire and breaker sizing is unique to motors. It's not a matching game like general-use circuits.

Inductive loads (motors, welders, etc) all draw extra current (inrush) at startup.

#10 wire is sufficient for *that* motor with a 40A breaker. NEC has an odd quirk where you ignore the nameplate FLA and use a generic table value instead. CEC goes by nameplate when the equipment is present. I'm not sure where you live.

 

[wyliesdiesels]

Don't forget, I'm an electrical newb. I had assumed the 22 amps was the max draw of the motor. I didn't realize that initial start would be far more. So that means I'm going to have to run #8 wire instead of #10. And maybe rethink where I was planning on putting the compressor.

It’s a little more complex than that.

For 5HP, you need 35a rated wire. This means either #10 THWN or #8 NM-b.

It will need to be hardwired as standard nema plugs arent rated for more than about 3.5HP.

If the compressor is not within sight of and more than 50’ away from, the breaker pane, then you will need a local disconnect.

Breaker can be max 250% of FLC which is 28a * 2.5 = 70a.

Does the compressor have a mag starter?

 

[wyliesdiesels]

Wire and breaker sizing is unique to motors. It's not a matching game like general-use circuits.

Inductive loads (motors, welders, etc) all draw extra current (inrush) at startup.

#10 wire is sufficient for *that* motor with a 40A breaker. NEC has an odd quirk where you ignore the nameplate FLA and use a generic table value instead. CEC goes by nameplate when the equipment is present. I'm not sure where you live.

What if the equipment isnt present? How do you size the circuit then?

 

[Terry D]

You do not use the data on the motor to size the circuit. You get the info out of the NEC article 430 table 430-248 to find the FLC. For that 5 hp motor @ 240 volts, you would start with 28 amps. Then you multiply that by 125% because it is a motor load. That will give you 35 amps. That is what you use to size the branch circuit conductors. If using THWN wire, you would need a #10. If using a NM-b, you would need a #8-2 cable. You can over size the breaker up to 250% of the 28 amp (FLC) . I would use a 50 or 60 amp. The reason for over size the breaker is to prevent nuisance tripping because of start up current. You compressor should have overload protection on it to protect the motor.

What you do use the name plate current (FLA)for is to size the overload protection. It would be either 115% or 125% of the nameplate current depending on the service factor and temperature rise of motor

 

[KPack]

Thanks for the additional clarification. I'm learning a ton.

The compressor does not have a magnetic starter. My original plan was to put it in a small shed behind the shop so I don't hear it while it is running, so it will for sure need a local disconnect. The run is about 75' from the panel. I've pulled #10 THHN from the panel to the rear of the shop in conduit under the slab (red, black, and green all #10). If it needs to be larger gauge wire then I will either need to pull it out and do #8 or move the compressor inside the shop and build a room for it closer to the panel.

I was originally going to do a 30 amp breaker, but I'll upsize it to a 50. I believe the compressor has overload protection, but I'll have to look into it.

 

[wyliesdiesels]

Thanks for the additional clarification. I'm learning a ton.

The compressor does not have a magnetic starter. My original plan was to put it in a small shed behind the shop so I don't hear it while it is running, so it will for sure need a local disconnect. The run is about 75' from the panel. I've pulled #10 THHN from the panel to the rear of the shop in conduit under the slab (red, black, and green all #10). If it needs to be larger gauge wire then I will either need to pull it out and do #8 or move the compressor inside the shop and build a room for it closer to the panel.

I was originally going to do a 30 amp breaker, but I'll upsize it to a 50. I believe the compressor has overload protection, but I'll have to look into it.

75' might be a little long for the wire.

Looks like it does have integral overload protection, as it says "thermally protected" so you could get by with just a pressure switch. however having said that, pressure switches can weld closed due to the high current of the motor, causing it to run non-stop. I would get a mag starter to run that motor....

 

[u2slow]

75' of #8 copper for one compressor... wow. I put 3 smaller sub-panels around my shop to keep the individual circuits small and short.

With only a 22A nameplate FLA, a quality 5HP@240V-rated pressure switch should handle it just fine for a good long time. These switches have an mechanical "OFF" setting (lever or knob).

What if the equipment isnt present? How do you size the circuit then?

When the equipment is not present or directly specified, then you use generic table values. Motor nameplate trumps the table - in Canada.

 

[wyliesdiesels]

75' of #8 copper for one compressor... wow. I put 3 smaller sub-panels around my shop to keep the individual circuits small and short.

With only a 22A nameplate FLA, a quality 5HP@240V-rated pressure switch should handle it just fine for a good long time. These switches have an mechanical "OFF" setting (lever or knob).


When the equipment is not present or directly specified, then you use generic table values. Motor nameplate trumps the table - in Canada.

Who says the pressure switch from the factory is quality?

Another member on the forums just recently said that he walked into his shop with the compressor running non-stop and discovered that the contacts on the PS had welded closed....

Who says the OP’s PS is even rated for 5HP?

 

[u2slow]

Who says the pressure switch from the factory is quality?

Another member on the forums just recently said that he walked into his shop with the compressor running non-stop and discovered that the contacts on the PS had welded closed....

Who says the OP’s PS is even rated for 5HP?

But its Snap-On!

Turning off the compressor when you leave is as valid with a simple pressure switch as it is with a mag starter.

It seems Snap-On's catalog shows the OP's compressor as requiring the mag starter... so that's the route to take. I looked for a manual, but didn't have any luck.

 

[KPack]

I may just end up putting the compressor in the garage portion of the shop, but will need to build out a semi-sound proof room for it. I really don't want to be listening to it while working. And unfortunately building out that room will take up more space in the garage.

And by the way I won't have the compressor turned on all the time. I plan to only turn it on when I'm going to use it and shut it down afterwards. No reason to keep everything pressurized if I'm not using it.

 

[fitter30]

Any air compressor motor pulls full load amps at startup. But starting under a load they pull full load longer to get up to running speed unless they have a unloader that the compressor starts unloaded. Normally mounted on a belt drive compressor.

 

[wyliesdiesels]

Any air compressor motor pulls full load amps at startup. But starting under a load they pull full load longer to get up to running speed unless they have a unloader that the compressor starts unloaded. Normally mounted on a belt drive compressor.

Ummm they pull way more than just the FLA on startup due to in-rush current which can be 4x-8x FLA.

Heres what a 7.5HP 3 phase 208v motor pulls on startup and running. nameplate says 18a.... so almost 8x FLA

Also, I've never seen a compressor that didnt have an unloader.... wouldnt make sense to not have one

 

[mike93lx]

I may just end up putting the compressor in the garage portion of the shop, but will need to build out a semi-sound proof room for it. I really don't want to be listening to it while working. And unfortunately building out that room will take up more space in the garage.

And by the way I won't have the compressor turned on all the time. I plan to only turn it on when I'm going to use it and shut it down afterwards. No reason to keep everything pressurized if I'm not using it.

Building a sound-controlled room is cheaper and easier than 3 pieces of #8 thwn?

 

[u2slow]

I may just end up putting the compressor in the garage portion of the shop, but will need to build out a semi-sound proof room for it. I really don't want to be listening to it while working. And unfortunately building out that room will take up more space in the garage.

And by the way I won't have the compressor turned on all the time. I plan to only turn it on when I'm going to use it and shut it down afterwards. No reason to keep everything pressurized if I'm not using it.

Since you're not using it all the time... that's why I'd keep it in the shop. Be more secure, perhaps close to your panel, and probably stay in better condition. Typically the air tools I'm running need hearing protection anyway, so compressor noise is moot.

My air setup will move up onto the mezzanine as I get things figured out. One less thing on the shop floor.

 

[jar944]

I'm still an electrical newbie, so keep that in mind.

I have my neighbor's old compressor sitting in my garage waiting to go into the shop. It's a Snap-on 70gallon, 5 HP compressor (BRA517V). The motor on it says 22 amps at 230 volts.


On the Snap-on website it says that the compressor needs a 40 amp circuit. But looking at the motor, I would assume a 30 would be enough to power it. Why would a motor only drawing 22 amps need a 40 amp circuit? And will a 30 amp circuit be sufficient or am I totally off base here?

Funny, the exact same motor was used on the harbor freight 60gallon 2 stage compressors.

 

[KPack]

Building a sound-controlled room is cheaper and easier than 3 pieces of #8 thwn?

I'm not worried about the cost of the wire. My only two concerns for putting the compressor behind the shop are 1.) will I have enough room in the under-floor 3/4" conduit for 3x #8 wires (2 circuits #12 THHN are in there as well), and 2.) building a shed behind the shop is way more work than building a room in the shop.

Still haven't decided yet, but looking at options. I can always do it later since I have 3/4" underground conduit going to both places I would consider put a compressor.

Since you're not using it all the time... that's why I'd keep it in the shop. Be more secure, perhaps close to your panel, and probably stay in better condition. Typically the air tools I'm running need hearing protection anyway, so compressor noise is moot.

My air setup will move up onto the mezzanine as I get things figured out. One less thing on the shop floor.

Good point on hearing protection with other tools. Angle grinders, needle scalers, etc for sure. So maybe it's not so important to have the compressor far away. Lots to consider for sure.

 

[Norcal]

I'm not worried about the cost of the wire. My only two concerns for putting the compressor behind the shop are 1.) will I have enough room in the under-floor 3/4" conduit for 3x #8 wires (2 circuits #12 THHN are in there as well), and 2.) building a shed behind the shop is way more work than building a room in the shop.

Still haven't decided yet, but looking at options. I can always do it later since I have 3/4" underground conduit going to both places I would consider put a compressor.


Good point on hearing protection with other tools. Angle grinders, needle scalers, etc for sure. So maybe it's not so important to have the compressor far away. Lots to consider for sure.

Only need 2-8AWG & 1-10 AWG conductors.

 

[wyliesdiesels]

Building a sound-controlled room is cheaper and easier than 2 pieces of #8 thwn and 1 piece of #10 thwn?

fixed it for ya

 

[mike93lx]

fixed it for ya

You guys and your details…

 

[nadogail]

Trying to run a compressor on an undersized circuit will leave you frustrated; avoid frustration and disappointment by putting in an adequate electrical supply for your compressor.

 

[Bert_]

3/4" pipe is fine for those wires. It's not undersized...

 

[Citation]

I may just end up putting the compressor in the garage portion of the shop, but will need to build out a semi-sound proof room for it. I really don't want to be listening to it while working. And unfortunately building out that room will take up more space in the garage.

And by the way I won't have the compressor turned on all the time. I plan to only turn it on when I'm going to use it and shut it down afterwards. No reason to keep everything pressurized if I'm not using it.

Rather than moving the compressor or building a room around it, you might consider something like Eaton's intake muffler.

Intake Noise Suppressor Installation Manual by Eaton Compressor

Here are the installation instructions for Intake Noise Suppressor in Whisper 100 compressors along with the parts list. You will start by

www.yumpu.com

If you look later in the install manual it's rather clear this is little more than a box with baffles and the air filters mounted inside. It wouldn't be hard to build a similar box from wood, add some sound deadening boards, and plumb it into the compressor's intake. I did something similar but simpler with my 120V belt drive compressor.
Here is a similar setup using car mufflers.

On my own compressor I had one, similar air intake. I found that adding 2ft or so of rubber hose did a lot to further reduce noise levels. Not California Air Tools quiet but good enough that I'm no longer worried about noise levels.

 

[haveissues]

Who says the pressure switch from the factory is quality?

Another member on the forums just recently said that he walked into his shop with the compressor running non-stop and discovered that the contacts on the PS had welded closed....

Who says the OP’s PS is even rated for 5HP?

You are probably referring to me. It was running that way for at least 24 hours and you could smell hot compressor oil in the air. Luckily that compressor was rated for running continues and the poppit valve worked so it survived but I would never run any decent sized compressor without a mag starter again.

 

[alfredeneuman]

may just end up putting the compressor in the garage portion of the shop, but will need to build out a semi-sound proof room for it.

If you do that you'll also need some form of ventilation to remove the heat from the room.

 

[PoorUB]

I have my 5HP 80 gallon compressor in a 4x4 foot closet, sheet rocked inside and out with a small exhaust fan on a thermostat. No issues.

 

[KPack]

Thanks for the advice everyone. Still not sure if I will put the compressor inside or outside, but I don't need to decide that now.

But speaking of 50 amp breakers.....I had an inspection on my work so far in the walls to see if I was good to insulate/sheetrock. There were a few things to fix, but one that I didn't expect was referenced in NEC 210.8 (A). Apparently now ALL 125-250 volt receptacles are required to be GFCI protected. So that means for my two 50 amp plugs (welder and RV) and my compressor, I will need to get 3x 50 amp GFCI breakers. The Eaton BR ones I need are $125+ a piece if you can find them. So, yay. Unfortunately the normal breakers I just bought a couple weeks ago are all useless now.

I went to several electrical suppliers today to find the GFCI breakers. They all looked at me like I was crazy.

 

[wyliesdiesels]

Thanks for the advice everyone. Still not sure if I will put the compressor inside or outside, but I don't need to decide that now.

But speaking of 50 amp breakers.....I had an inspection on my work so far in the walls to see if I was good to insulate/sheetrock. There were a few things to fix, but one that I didn't expect was referenced in NEC 210.8 (A). Apparently now ALL 125-250 volt receptacles are required to be GFCI protected. So that means for my two 50 amp plugs (welder and RV) and my compressor, I will need to get 3x 50 amp GFCI breakers. The Eaton BR ones I need are $125+ a piece if you can find them. So, yay. Unfortunately the normal breakers I just bought a couple weeks ago are all useless now.

I went to several electrical suppliers today to find the GFCI breakers. They all looked at me like I was crazy.

Yes that is a new code however, a 5HP motor should NOT be on a 50a plug and receptacle because standard nema plugs are not rated for 5HP. Hardwire the compressor and save yourself the cost of the expensive GFCI breaker.

 

[KPack]

Yes that is a new code however, a 5HP motor should NOT be on a 50a plug and receptacle because standard nema plugs are not rated for 5HP. Hardwire the compressor and save yourself the cost of the expensive GFCI breaker.

Good information on the compressor circuit. I will leave that one without a GFCI breaker.

I'm having trouble finding any GFCI breakers, as they seem to be out of stock everywhere. I may just remove the wires from the current breakers, tuck them away and cap them off, label the breakers as spares, and remove the welding plugs from the boxes downstream. I don't currently have anything that requires a 50 amp plug so I may as well just "deactivate" those until some later date when I can actually use them. Can't satisfy the requirement anyways if I can't get a GFCI breaker.

 

[wyliesdiesels]

Good information on the compressor circuit. I will leave that one without a GFCI breaker.

I'm having trouble finding any GFCI breakers, as they seem to be out of stock everywhere. I may just remove the wires from the current breakers, tuck them away and cap them off, label the breakers as spares, and remove the welding plugs from the boxes downstream. I don't currently have anything that requires a 50 amp plug so I may as well just "deactivate" those until some later date when I can actually use them. Can't satisfy the requirement anyways if I can't get a GFCI breaker.

good luck.... the supply chain issues are only getting worse....

 

[KPack]

good luck.... the supply chain issues are only getting worse....

Yup. Hence my thoughts of just removing the plugs and disconnecting everything. Not sure if that will satisfy the inspector or not, but I don't really have much of a choice.

 

[budmur]

I'm having trouble finding any GFCI breakers, as they seem to be out of stock everywhere.

My hot tub is on a 50-amp GFCI breaker. Maybe a local dealer could point you in a direction.

 

[ycgoat]

all 250v outlets on GFCI in garages! good thing I am wiring my garage now before they adopt the 2020 NEC.

 

[sparky 1971]

that means for my two 50 amp plugs (welder and RV) and my compressor, I will need to get 3x 50 amp GFCI breakers. The Eaton BR ones I need are $125+ a piece if you can find them.

I went to several electrical suppliers today to find the GFCI breakers. They all looked at me like I was crazy

Look for Eaton 50 amp spa disconnects. They run about $75 and have the breaker in them. Just take the breaker out and put it in the panel. That might be easier said than done though with the broken supply chain.

 

[Steve_P]

I have a 7.5 HP Quincy QT7.5, it's 30-32A, can't remember. It's on a 50A breaker and has never tripped it over 20 years- the only reason it's on a 50A is because that was there and I figured I'd try it. If it wasn't there, then I'd go with a 60A. It will pull 100A+ when it starts- obviously just for a millisecond, but it does.