220v Plug Question

[MayerMR]

Hey fellas,

I've just picked up an old air compressor off Craigslist that I thought was a 110v unit, due to the plug, but turns out that it's a 220v unit - makes sense why the motor would barely turn over...

Anyway, I just so happen to have a 220v outlet in my garage which was used for a welder (based on the tag in the junction box). The female receptacle is a NEMA 6-50 R type outlet and the plug face (see attachment) says it's 250v/50amps. Can I purchase a length of appropriately sized cord and the proper male plug and use my compressor with this outlet? The motor says it's 220v/10amps. I can't imagine the difference wouldn't work, but better to ask than to mess around with 220v...I don't want to end up dead for a $40 compressor.

Thanks gents!

-Matt

 

[R_einan]

You can make an adapter cord, but the issue with that is the circuit protection isn’t sized to the device. Meaning, if there is a compressor issue, the breaker won’t trip unless it exceeds 50a. I did it for my table saw until I could replace the 50a receptical with a 30a. Just be aware of the risks.

 

[The Cobbler]

properly done youwould replace the breaker with a 15 amp double & replace the outlet with 240v 15 amp

 

[MayerMR]

You can make an adapter cord, but the issue with that is the circuit protection isn’t sized to the device. Meaning, if there is a compressor issue, the breaker won’t trip unless it exceeds 50a. I did it for my table saw until I could replace the 50a receptical with a 30a. Just be aware of the risks.

Ah, so basically, I would need to replace the breaker with a 30a breaker? This poses a problem in that I have an old-style (can't recall right now the brand) breaker box and the breakers are a pain to replace (and expensive). Could I put a fuse or a trip breaker in the plug/cord/compressor in-line to avoid that situation?

 

[pstemari]

Sure. Nothing wrong with making an extension cord with a utility box and appropriate size breaker. Size the breaker to match the outlet you put on the cord; eg a 20A breaker for a 20A outlet.

I wouldn't do that without an appropriate breaker, and 240 VAC requires a two-pole breaker, one side for each phase.


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[R_einan]

Ah, so basically, I would need to replace the breaker with a 30a breaker? This poses a problem in that I have an old-style (can't recall right now the brand) breaker box and the breakers are a pain to replace (and expensive). Could I put a fuse or a trip breaker in the plug/cord/compressor in-line to avoid that situation?

My compressor needs a 30a, depends on what your compressor requires it will tell you what size breaker you should be using (should be on the motor data plate). An inline breaker for the extension is essentially a sub panel. Honestly, with the cost of the parts to make an extension cord with an inline breaker, it would not be cost effective to just swap the breaker and change the outlet.

 

[MayerMR]

Sure. Nothing wrong with making an extension cord with a utility box and appropriate size breaker. Size the breaker to match the outlet you put on the cord; eg a 20A breaker for a 20A outlet.

I wouldn't do that without an appropriate breaker, and 240 VAC requires a two-pole breaker, one side for each phase.


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Thank you!

My compressor needs a 30a, depends on what your compressor requires it will tell you what size breaker you should be using (should be on the motor data plate). An inline breaker for the extension is essentially a sub panel. Honestly, with the cost of the parts to make an extension cord with an inline breaker, it would not be cost effective to just swap the breaker and change the outlet.

It *would* or *would not* be more cost effective to swap the breaker and change the outlet?

 

[saryon7]

I will offer some different advice than what you have been getting. You first need to verify these things:
1) Plug needs to say 220v 50 amps ( you already checked this)
2) Breaker needs to be 50 amps
3) Romex or wire needs to be 6 gauge ( this is the size to safely carry 50 amps)

If these 3 things are all correct, than you have a safe 50 amp circuit. If this is the case than you can put a 50 amp male side plug on your compressor and everything will be safe. It is ok to have items on circuits that draw less than the max amps. Your circuit will be safe for all amperage draws up to 50 amps. The advice you have been getting would be like reducing your 20 amp circuits to 3 amps because you only have 1 tv plugged into the circuit.

 

[rcjoy]

saryon7 is correct. The purpose of the breaker in the main panel is to protect the wiring, not necessarily the device connected to it.

 

[sberry]

saryon7 is correct. The purpose of the breaker in the main panel is to protect the wiring, not necessarily the device connected to it.

Neither one of you are right. This comp has a max over current limit, it's the plug that came on it, I suspect it's 20.
The Max over current does not protect the device in a thermal sense but for fault or short circuit. If it has a 16 cord max is going to be 20, a 14 will be 30.
Many machines rely on the breaker and do not have additional internal, control wiring etc must be fault protected. Many allow for 10 wire , some circuits 12 on 50A depending on the machine connected to these dedicated special circuits.

 

[sberry]

I will offer some different advice than what you have been getting. You first need to verify these things:
1) Plug needs to say 220v 50 amps ( you already checked this)
2) Breaker needs to be 50 amps
3) Romex or wire needs to be 6 gauge ( this is the size to safely carry 50 amps)

If these 3 things are all correct, than you have a safe 50 amp circuit. If this is the case than you can put a 50 amp male side plug on your compressor and everything will be safe. It is ok to have items on circuits that draw less than the max amps. Your circuit will be safe for all amperage draws up to 50 amps. The advice you have been getting would be like reducing your 20 amp circuits to 3 amps because you only have 1 tv plugged into the circuit.

The tv has a wire suffecient to trip 20 should it short, following the wire it has additional internal protection. While it only draws 1 it is designed to be plugged to 20A.
That is for use on general circuits. The comp is limited by a couple factors, if it doesn't come plug and cord then it is up to the installer to follow code and install it on the proper circuit usually in the manual it suggests licensed electrician due to this exact reason.
In the case of dedicated circuits thermal protection may be by other methods, additional or by load limit applied by the device similar to fixture wires. The breaker in dedicated rarely protects the wire from thermal the way it does in general where there are multiple outlets where overload is possible.
The circuit is safe for loads to 50 but not all the equipment plugged in to it is safe. Breaker doesn't "need" to be 50 in all scenerios, that is the max. If it's a welder recept there may be special circumstances, if it's a range it needs 6 wire and 50. A 50 may not be motor rated for some comps that only draw 22.
Wylie wrote a sticky about grounding, this forum needs another one about fundamentals of circuit design.

 

[R_einan]

Sorry OP, I meant would be more cost effective, little tired last night.

Sberry explained it better than I had.

 

[MayerMR]

Fellas,

Thanks for all your assistance. I will try to get a photo of the motor plate tonight and check the gauge of the wires in the power cord. This is an old Sears compressor and as far as I can tell it's all original.

My intent is to get it set up and prove that it's functioning before doing a strip down and clean-up, repaint, tune-up and setting it up more permanently in a corner somewhere.

I appreciate everyone's advice and assistance!

S/F,

-Matt

 

[seber]

There is no need or reason to reduce the ampacity of the in house circuit. If it is wired and protected for 50 amps, then you have a proper circuit to the outlet. If the compressor needs protection, then do it at the compressor. A small box with breaker will do the job just fine.

 

[brownbagg]

yes, it be fine with the welder plug

 

[bubinga]

properly done youwould replace the breaker with a 15 amp double & replace the outlet with 240v 15 amp

Good advice
Probably could use the same wiring as long as it isn't too thick to be allowed in that size breaker? Correct?

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[bubinga]

Ah, so basically, I would need to replace the breaker with a 30a breaker? This poses a problem in that I have an old-style (can't recall right now the brand) breaker box and the breakers are a pain to replace (and expensive). Could I put a fuse or a trip breaker in the plug/cord/compressor in-line to avoid that situation?

15 Amp double pole breaker.
15 Amp per lug
It's still called 15 Amp double pole breaker. Not 15 + 15 equals 30 no. It's a 15 amp double pole breaker 15 amps per line.

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[The Cobbler]

if breakers are hard/ pricey to source,you could add an easier to source/cheaper breaker panel to the line at the receptacle, then run the outlet off that

 

[bob15]

Just curious, what brand/manufacturer is your circuit panel?

 

[2oolhound]

If it was me I'd be trying to use an appropriate extension cable to the dryer plug in the house for the compressor and keep that 50 amp for the welder you'll be getting

 

[beamrider]

Just curious, what brand/manufacturer is your circuit panel?

I wondered the same thing, hoping it's not Stab-Loc.

 

[Mr. T]

There is no need or reason to reduce the ampacity of the in house circuit. If it is wired and protected for 50 amps, then you have a proper circuit to the outlet. If the compressor needs protection, then do it at the compressor. A small box with breaker will do the job just fine.

I agree with this. The easiest and cheapest thing to do would be to buy a 50 amp fused disconnect to attach to the compressor. Use 6/3 SOOW from the receptacle to the D/C. Install the proper size fuses (per compressor manual) in the D/C and then wire load side of D/C to current cord of compressor.

Edit: actually the cheapest would be to buy a fuse block and install it in the enclosure for the compressor, assuming that it has an enclosure and that there is room.

 

[bubinga]

I agree with this. The easiest and cheapest thing to do would be to buy a 50 amp fused disconnect to attach to the compressor. Use 6/3 SOOW from the receptacle to the D/C. Install the proper size fuses (per compressor manual) in the D/C and then wire load side of D/C to current cord of compressor.

Edit: actually the cheapest would be to buy a fuse block and install it in the enclosure for the compressor, assuming that it has an enclosure and that there is room.

Good idea. [emoji362]

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[wyliesdiesels]

Wow 21 comments and not one started at the proper step one question which is to ask what the HP rating on the motor is.

 

[sberry]

It may not need 6 to the breaker, similar to a power strip. It could be a 10 or 12 from the supply to the breaker, this is the load limiting. Again the wire needs to be heavy enough to provide short circuit, the breaker downstream can provide the thermal.
Like a power strip the breaker after the wire prevents thermal overloading from multiple recepts.
Same as feed thru lugs on an old fuse service. 10 wire from the 60 main to a 1 armed bandit for additional equipment, often 3 hp mechanic compressors in shops or water heaters in houses.

 

[Mr. T]

Wow 21 comments and not one started at the proper step one which is to ask what the HP rating on the motor is.

Wild guess... 1 ish hp.

Maybe up to 2?

Rectally sourcing that btw.

 

[bubinga]

Wow 21 comments and not one started at the proper step one question which is to ask what the HP rating on the motor is.

doesn't the amp draw give a pretty good indication?
Serious question, not being wise.

 

[Stewart Warner]

Wow 21 comments and not one started at the proper step one question which is to ask what the HP rating on the motor is.

If it's 10 amps at 240 it's a 2 HP motor, a double 15 amp breaker with the correct outlet is the only way to do it, 25 bucks, half hours work, done.

 

[MayerMR]

I wondered the same thing, hoping it's not Stab-Loc.

Just checked, it's a Zinsco panel box

Wow 21 comments and not one started at the proper step one question which is to ask what the HP rating on the motor is.

Wellllll, in the original post I did say it was 230v at 10amps...that said, I was off by 10 volts and a half an amp, after cleaning it's 230v @ 10.5amps

If it's 10 amps at 240 it's a 2 HP motor, a double 15 amp breaker with the correct outlet is the only way to do it, 25 bucks, half hours work, done.

You nailed it Stewart, it's a 2hp motor.

You're saying to source a new breaker for the panel and replace the receptacle?

All-

I've attached a photo of the motor data plate.

 

[Norcal]

Zinsco and FPE Stab-Lok are the two most reviled names in electrical equipment, Zinsco was used because they were the cheapest (like Eaton BR is now or as I call them "Zinsco II") but now Zinsco breakers are very expensive.

Edit: 2-pole circuit breakers that are UL classified replacements for Zinsco/Sylvania are close to $60.00 each.

 

[pstemari]

Well, I don't know about that. I think Lucas has them beat for notoriety, if not the consequences of the screw-ups.

The protection of downstream equipment is a bit complicated. I dislike items with no internal protection, unless they are something totally trivial like a lamp.

Generally most consumer items should withstand 15–20 amps without catching on fire if they develop an internal short. Will they handle 50A? Probably not, unless they have internal protection. This is why you can't plug 15A plugs into 50A outlets. Note that you *can* plug 15A plugs into 20A outlets (which have a t-slot on the hot), which is a bit of a special case.

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[shaggyant]

Wylie wrote a sticky about grounding, this forum needs another one about fundamentals of circuit design.

That would be awesome. I have questions about derating of circuits for continuous duty and multi wire branch circuits for lighting.

 

[sberry]

The t slot simply says that you can plug specific equipment in here. Many if not most 15A recepts are on 20A circuits. The 20 end on an appliance simply prevents it from being put on a general not intended for it. These are usually found in instatutions. It's not always because the tool actually needs it but to eliminate some stuff from being plugged in to other general or sensitive circuits and to force the janitor to use a dedicated circuit instead of a computer or circuit with science experiments on it. See it on vacuum and floor equipment. They even change factory plugs to it in many cases. Often add long cords to reach these dedicated circuits.

 

[sberry]

Lots of stuff that uses 20A comes with 15A ends. Welders, chop saws etc, most of it with a short duty cycle and 14 cord. It's usually up to the operator when using them on general circuits to decide if a trip is acceptable. This is used in hospitals, some of each, some dedicated. They do not want additional history demand equipment plugged in to the same circuit as the heart monitor. They prevent this by changing the end on the tool.

If it was just about overheating the wire we would simply use 10 and 30. It would actually be hard to overload a 12 on 30A but the equipment being plugged in to it isn't rated to be on 30. It isn't that it needs 20A or even 1 but the fixture wires etc need to be short circuit limited.

 

[alfredeneuman]

doesn't the amp draw give a pretty good indication?
Serious question, not being wise.

The nameplate amp draw is not the criteria for choosing correct the wire size.
The horsepower (listed in the NEC) is.

 

[TRWham]

That motor IS thermally protected according to the nameplate. If that cord is factory installed, there should be a nameplate in addition to the one on the motor that specifies what the electrical requirements are for the appliance. There is a chance that any compressor 3 hp or less is UL (or ETL) listed as an appliance and the appliance nameplate is your guide rather than the motor nameplate. Articles 400 and 422 govern in lieu of 430.

 

[bubinga]

The nameplate amp draw is not the criteria for choosing correct the wire size.
The horsepower (listed in the NEC) is.

0h OK... [emoji56] Thank you!

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[KenC]

My 2 pennies: Since the motor has thermal protection and the circuit has proper overload/short protection, a new plug would be safe.

But, I'd personally put in a small panel splitting the service into 240/120, IF the wiring provides a ground and neutral to the recept.

 

[sberry]

If it's thermal I spose it could be safe provided the cord was large enough. It would be 12 for 50. It's been so long since I read motor stuff most of it I forget if I did know it in the first place.
Does the thermal also provide short circuit?

 

[TRWham]

...
Does the thermal also provide short circuit?

No, unless the short is inside the motor itself. The circuit breaker covers anything that happens upstream of the internal overload protection.