can breaker and wire be too big?

[John Sheets]

Hey Folks,
First, my apologies if this has been discussed, but I had no luck with this specific situation after an hour of searching.
I have an IR 3HP, 220 volt, (running HP) compressor (motor plate says 15A) that I need to get power to. The motor has its own thermal overload protection. I realize that I could just run another 20A circuit, but we're looking at a lot of wire, drilling holes through I-joists that are not too easy to access, etc.
_However_, I do have an outlet for the welder (which I rarely use) about 3 feet from the compressor's location. It has the dryer type receptacle, and is a 50 amp breaker with #6 copper from the subpanel. What I'd like to do is use a short length of #10 rubber coated wire, get another dryer plug, and just plug the compressor in to the dryer outlet.
Am I violating any major principles here? I realize #10 wire is not the proper size for a 50 amp breaker, but also the 18 gauge wire on the lamp on my living room end table is not the proper size for the 20 amp breaker on the circuit that it's plugged into at the wall receptacle.
Thanks in advance for your expert opinions.
JS

[Mike Davis NC]

The number 10 wire will be attached to the motor? Nothing will be changed at the dryer outlet?

I see no problem with this.

[John Sheets]

Mike, that is correct. Dryer-type outlet will remain as is so I can still use the welder if necessary. The short length of #10 wire will have a compatible plug on one end and the other end is wired directly to the control on the compressor.

[Dave Beauchesne]

The number 10 wire will be attached to the motor? Nothing will be changed at the dryer outlet?

I see no problem with this.

John / Mike :

An inspector would find fault with this - the breaker is to protect the conductor, so a 10 gauge wire hooked up to a 50 amp breaker would violate that rule. Even though the motor has thermal protection, you can still ' overprotect ' the load ( use too large a breaker ).

Go with a dedicated 20 amp circuit for the IR and be done with it.

There will likely be more chime in on this subject.

JMHO - Dave Beauchesne

[George Clark]

John / Mike :

An inspector would find fault with this - the breaker is to protect the conductor, so a 10 gauge wire hooked up to a 50 amp breaker would violate that rule. Even though the motor has thermal protection, you can still ' overprotect ' the load ( use too large a breaker ).

JMHO - Dave Beauchesne

I disagree with your logic. Are you a licensed electrician? Does a 16 ga wire plugged into a 20 amp circuit violate that same rule? If so then an awful lot of lamps and other devices are in violation. I have been told by licensed electricians that the breaker is there to protect the wiring from the breaker box to the outlet, not the wiring from the outlet to the device or the device itself. That is a function of the overload protection on the device.

[Dan Friedrichs]

I disagree with your logic.

As do I.

The breaker protects the wire between the breaker and the outlet, not the cord leading to the appliance.

What the OP has suggested is perfectly fine, safe, and code-compliant.

[Mark Bolton]

I too agree that you would be fine to plug in directly. What you could do if the welder circuit is 4 wire is remove the welder receptacle, install a small 4 space sub panel and then two breakers. One for the welder one for the compressor. just have to remember to float the neutral.

Probably only worth it if you wanted air while welding but you say you rarely weld so plug it in.

[Dave MacArthur]

Since this topic comes up often, I just wrote an analysis of the NEC for shop motors last week that exactly addresses this issue via all the code:

Sizing Wires and circuit breakers for 3HP and 5HP shop motor circuits

In fact, if you are running a motor (compressor) off that circuit, then sometimes the CB does NOT protect the wire from overheat:
NEC 430-148: Full Load Current for 3HP compressor motor = 17A
Motor circuits have their own section of the NEC:
[Sec. 240-3(g)]. lists exceptions, detailed in Motors [Secs. 430-22(a) and 430-52(c)

Motor must be calculated as continuous duty 125% of FLC, = 21.25A.
Your wire to that circuit is NM cable, so you must use the 60C column from table 310-16 for ampacity restrictions for wire termination criteria (it's hookup to the panel). 12AWG wire will flow 25A and stay below 60C, good to go there. Next you look at current flow. 12 AWG will also exceed the 21.25 current flow requirement, including any bundling and temp deratings, probably at the 60C column and certainly at the 90C column you could use for NM THHN conductors for this test.

Circuit breaker: on a motor circuit, the thermal overload of motor protects the wire, the CB provides short circuit and GF protection, and you may size the CB between FLC and up to 250% of the FLC as required to start the motor, so 17A to 50A (42.5 rounded up to next normal CB available size NEC 240.6). Therefore:

50A breaker on a 12AWG circuit is acceptable for this circuit as a motor circuit, so 50A on a 6ga line is way acceptable. And the 10ga extension line also exceeds all requirements.

In summary, wire can't be too big, and while breaker CAN be too big, if you protect the wire it can't, and you can actually have special situations with motors where if you comply with all the requirements, your breaker is allowed to be hellacious big if needed.

[Rod Sheridan]

The only issue is if the "dryer type receptacle" really is a dryer receptacle.

In that case it cannot be fed from a 50 ampere breaker.

Perhaps it's a range receptacle? If so, it's fine.

Purchase a mating cord cap for the #10 flex and you're good to go.

Regards, Rod.

[John Sheets]

Rod,
It actually _is_ a welding receptacle, or at least that's what it was labeled when I bought it back when. The plug is also described as for a welder; two different sized vertical blades and the half-round grounding prong. I just referred to it as a dryer-type simply as a generic description of the larger blade-type connectors. JS

[Chip Lindley]

_However_, I do have an outlet for the welder (which I rarely use) about 3 feet from the compressor's location. It has the dryer type receptacle, and is a 50 amp breaker with #6 copper from the subpanel.

I would have to assume the OP is referring to his 3-prong 50A welding plug as a dryer type receptacle. No way it's a 4-prong range receptacle. Wrong terminology, but it is certainly not a 30A dryer circuit. There are several NEMA flat blade 50A plug configurations, and some do look like oversized dryer plugs. The 250V welding plug that came on my Lincoln is 6-50R.

NEMA flat plugs.jpg

[Dave Beauchesne]

As do I.

The breaker protects the wire between the breaker and the outlet, not the cord leading to the appliance.

What the OP has suggested is perfectly fine, safe, and code-compliant.

Gentlemen:

My apologies if my earlier response caused confusion - I just attended a three day Electrical Code Essentials ( here in Canada ) and a situation very similar to this was discussed. The instructor was an extremely knowledgable gentleman who sits on several sections of the Canadian Electrical Code board and is responsible for much of the group re-writing and updating of said code.

As I eluded to, this similar situation was discussed, and the ' kitchen appliance with a 16 gauge cord on a 20 amp circuit ' was discussed as well. The rules may well be murky to some degree; the bottom line is the ' authority having jurisdiction ' has the final say. Maybe a little different south of the border.

That being said, knowing Dan's credentials, I would take his word over my suggestion. As a HVAC tech that works with 24v to 575v and everything in between daily, I have seen all kinds of fires, fried wires, and more than a few motors has fused thermal protection and caused some excitement to be sure. I like to err on the side of caution, nothing else.

My apologies.

Dave Beauchesne

[ray hampton]

if the service is for 200 amperes to the c b panel and a machine wired into
20 amp. circuit breaker were to short out ,would the short pull the total 200 amperes before the breaker trips ?

[Dave MacArthur]

No. That's what a circuit breaker is for... it would trip at 20A pull plus some statistical variation.

On a motor circuit, the CB protects the circuit from short circuit and GF. And overload if it's not boosted up per specialized motor NEC rules. And the motor also protects itself and the circuit if it has thermal overload protection.

[Ken Fitzgerald]

I would think ....it the wire was hidden within a wall or conduit, the NEC code would apply. If, for example, the wire is not covered or hidden then it would be more of an "extension cord" or "appliance cord" and different code regulations would apply.