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 (http://www.sawmillcreek.org/showthread.php?t=153271)

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.

169370

[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.

[Tom Godley]

We were not allowed to do this -- you get into a gray area with some of this stuff because the ability to put a plug on a wire can get you into trouble.

Its a different situation than a direct wired appliance when you are following a manufactures instructions. For most homeowners this is encountered with the A/C condenser where you can see 25amps on a 40amp breaker.

We would have needed to upgrade the wire to the appliance or provide protection with a flexible cover (Like the outside A/C plastic whip) -- this would have not been practical and at the time I believe you could only go six feet. We could also have put in a fused switch.

The concern in this case was the possible damage to the feeder cable after the plug -- that was not designed/ approved for the amps. This was even with a thermal protected motor.

In many countries they have fused plugs that protect downstream cords -- like the fuses in our Christmas light plugs. They need to do this because the volts/ amps are higher at the outlets then in the US.

[Anthony Whitesell]

Let's also look at this fact. How many people have 15A or 20A outlets and matching circuit breakers? How many of the off the shelf kitchen appliances come with 14g and 12g cords. Personally, none of the small kitchen appliances I have, has a cord larger than what is required for the appliance's current draw itself.

Now I do note that this is a special case of a dedicated circuit, thus there will be only one tool in use on the circuit. I do see the size drop as a potential issue. If you would like to mitigate it, install a lower amperage circuit breaker. The heavier wire will not care, just the welder won't be able to be used.

[Mark Bolton]

Let's also look at this fact. How many people have 15A or 20A outlets and matching circuit breakers? How many of the off the shelf kitchen appliances come with 14g and 12g cords. Personally, none of the small kitchen appliances I have, has a cord larger than what is required for the appliance's current draw itself.

Now I do note that this is a special case of a dedicated circuit, thus there will be only one tool in use on the circuit. I do see the size drop as a potential issue. If you would like to mitigate it, install a lower amperage circuit breaker. The heavier wire will not care, just the welder won't be able to be used.

The answer has already been correctly stated that the breaker is protecting the wire run between the breaker and the receptacle and the receptacle. What happens beyond the receptacle doesn't factor in to the equation.

There is nothing to stop anyone from plugging anything from a 20A table saw or a .5 amp string of christmas lights into a 20A receptacle. The breaker is not there to protect the lights or the saw. Its there for the wire run and receptacle.

Mark

[Anthony Whitesell]

I know that. It appeared there was still some skeptism about the setup and I was just pointing out the pre-installed cords on off the shelf items are sized for the item's own power requirement or minimum UL sizing. Obvious a 28g wire for a wall wart won't hold up very well or meet UL requirements.

I do have a NEC question. Does a circuit without only one outlet (ie., not hardwired to the equipment) constitute a dedicated circuit?

[Tom Godley]

The problem with this setup is not with the supply circuit -- but rather with the decision to install a 50amp plug to supply the tool.

The installed plug that comes on an appliance is the protection against improper installations. In this case the OP is making the decision.

When you design a product to plug into a standard 15amp outlet it must meet all types of safety rules - that is why many items now come installed with fused plugs and fusible links in the supply. Most standard Christmas lights must have them because the supply wires when shorted out often will not cause some 15amp and especially 20amp breakers to trip -- the light string becomes a resistance heater. Not a good thing on a Christmas tree. This is what happens with extension cords all the time. This has actually become more of a problem since many houses are wired with 20amp circuits in many rooms vs 15amps years ago. The newer style breakers are addressing this.

The same goes with the OP's setup. The wires from the plug onward need to be sized and designed in order to make for a safe installation. It is not a question of if the tool will operate but rather if it is safe and proper.

This is after all a 50amp supply he is working with!

[Chris Friesen]

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 (http://www.sawmillcreek.org/showthread.php?t=153271)

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

Are you sure that plug-and-cord connected equipment (even if it's a motor load) falls under the motor section? For the canadian code at least one instructor has said that it doesn't, and the general purpose section applies.

[Dan Friedrichs]

The problem with this setup is not with the supply circuit -- but rather with the decision to install a 50amp plug to supply the tool.

That doesn't make sense. So if I cut the 5-15 plug off a lamp and install a 5-20 plug, I have made it less safe? :confused:




The newer style breakers are addressing this.

How? :confused:



The same goes with the OP's setup. The wires from the plug onward need to be sized and designed in order to make for a safe installation. It is not a question of if the tool will operate but rather if it is safe and proper.

So...you want to see a 50A plug connected a 6AWG cord running to the motor? :confused:



I stand by my earlier assertion that the OP's suggestion is perfectly safe and code-compliant. If you disagree, please provide some explanation of why it is unsafe - I'm just curious why anyone would argue that this common practice is unsafe.

[Tom Godley]

Dan - The new Arc breakers will detect and trip in situations where a standard breaker will not. Most fires that are traced to electrical problems are caused by some arc not stopped by a standard breaker.

This can happen when a wire is damaged in some fashion or a connection partially fails and you get overheating. So as the amps increase the potential also increases for this to happen. If you look at most modern appliance cords as well as what is called a "zip" extension cord you will see that they are heavier/ tougher than they were years ago in both wire size and insulation. Or they will have a special current limiting plug.

The thinner the cord the less damage it can sustain and still safely carry the load without overheating - a more serious problem is when a cord is severely damaged and it actually would trip a 15amp breaker but because it can be installed in a 20amp circuit it does not trip and starts a fire.

So going to the OP's point what is a safe size cord for a 50amp potential circuit -- I have no direct knowledge. Many cords that power tools are 16g and they can burst into flames connected to a 30amp breaker that is not up to spec (and many are not) and a short occurs. So is a 14g safe for 50amps? I just don't know what the safety head would be because I have never been involved in a higher amp appliance. But why would you want to do it?

[Dan Friedrichs]

Dan - The new Arc breakers will detect and trip in situations where a standard breaker will not. Most fires that are traced to electrical problems are caused by some arc not stopped by a standard breaker.

Well, we need to differentiate between arcing faults and purely ohmic faults. A dirty plug inserted into a socket may present a high resistance, but not arc, so it may cause a fire even if on an AFCI.


Or they will have a special current limiting plug.

I have actually never seen a current limiting plug (aside from fused christmas tree lights). I tried googling it, and didn't find anything. Can you show me an example of these? I'm genuinely curious - maybe I've seen one and didn't even realize what it was.


Many cords that power tools are 16g and they can burst into flames connected to a 30amp breaker...

I'm not sure I buy this. Say, by some miracle, you happened to have a load fault that caused it to draw 29.9A. I'm not going to do the calculation, but I don't think 30A through 16AWG will get hot enough to melt it.




But why would you want to do it?
What's the alternative? Installing a dedicated outlet with appropriately-sized circuit breaker for each appliance you want to plug in?

Do you have 11.2A circuit breakers on the convenience circuits in your kitchen so that you can safely plug in your toaster that only has a 16AWG cord? :confused:

[Tom Godley]

Dan - I have no desire to argue over any of this. Just trying to point a few things out.


In the USA its easy to forget that many parts of our electrical code are legacies from the beginning of electrification. The need to be backward compatible has driven much of what we see in the US.

Much of the world uses fused plugs -- they are all over Europe/ Asia and Africa. Most of the world also uses true 220/240 power. Many electrical fires are caused by compromised feeders rather than an overload from the primary. The ability to change out the fuse size in a standardized plug allows many different loads to be safely installed on a variety of feeder cables


And "Yes" the electric code is designed to do just what you said -- provide a dedicated circuit amp dependent on the need controlled by the plug installed. This is different than breaker size.


Example: Lets say we have a 12 foot 16g cord feeding a tool. This cord is sized correctly for the tool load. Going by your advise I would be able to install any number of plugs of varying amps to provide power to this tool. It only needs 15amps - but I could install a 30amp plug or even a 50amp as long as I plug it into a properly installed outlet of the matching size. And let say while working in the shop I leave a garden tool standing up against the wall and after I leave for the day it falls on the wire. At what point is the protection compromised at 15 amp vs 50 amps with a 16g cord. The danger of a fire at 50amps is much higher. Its not the load thats the problem. That is why the protection covering a dryer cord is so strong and the length of the cord is strictly limited. A 50 amp feeder cable needs the proper protection.

[Rollie Meyers]

Dan - I have no desire to argue over any of this. Just trying to point a few things out.


In the USA its easy to forget that many parts of our electrical code are legacies from the beginning of electrification. The need to be backward compatible has driven much of what we see in the US.

Much of the world uses fused plugs -- they are all over Europe/ Asia and Africa. Most of the world also uses true 220/240 power. Many electrical fires are caused by compromised feeders rather than an overload from the primary. The ability to change out the fuse size in a standardized plug allows many different loads to be safely installed on a variety of feeder cables



And "Yes" the electric code is designed to do just what you said -- provide a dedicated circuit amp dependent on the need controlled by the plug installed. This is different than breaker size.


Example: Lets say we have a 12 foot 16g cord feeding a tool. This cord is sized correctly for the tool load. Going by your advise I would be able to install any number of plugs of varying amps to provide power to this tool. It only needs 15amps - but I could install a 30amp plug or even a 50amp as long as I plug it into a properly installed outlet of the matching size. And let say while working in the shop I leave a garden tool standing up against the wall and after I leave for the day it falls on the wire. At what point is the protection compromised at 15 amp vs 50 amps with a 16g cord. The danger of a fire at 50amps is much higher. Its not the load thats the problem. That is why the protection covering a dryer cord is so strong and the length of the cord is strictly limited. A 50 amp feeder cable needs the proper protection.

The Brits use fused plugs, the reason for that is they use 32A ring circuits, they are mostly a Brit thing also as they are prohibited by many codes including the NEC.

[Bill Berklich]

If #10 is the spec wire for the compressor then you will be ok. The worry is that if the run is too long or wire too small (or combination) the the resistance in the wire will heat it and potentially cause a fire. Basically smaller wire or longer run equals higher resistance and a lot of heat. So make it big and keep it short.