can breaker and wire be too big?

[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

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?

The newer style breakers are addressing this.

How?

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?



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?

[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.