NEC Rules for Sizing Wire, Breakers, & Outlets for 220V

[Brett Bobo]

I'm certain I've read this somewhere here before but I couldn't find those same threads so my apologies if this is a redundant question. Even though the title is fairly generic, I'm specifically curious about sizing the breakers and outlets for 10 gauge wire.

After a recent move, I'm wiring the entire finished garage for 220 outlets via EMT, metal boxes, etc. So, I'm planning to pull all 10 gauge THHN wire for more versatility down the road if I acquire more 30A rated tools. Currently, all of my tools either require 20A or 30A breakers. With some minor electrical issues with the new house (GFCI related), I was able to speak with the builder's electrician, who is a journeyman, about the wiring for the garage. He indicated that it was acceptable to use 10 gauge wire with both 20A and 30A outlets; however, all of the breakers must be rated for 30A. Originally, my plan was to size the breakers to match the outlets, e.g. 20A breaker with 20A outlet, to avoid surges but he indicated that wasn't allowed per the NEC.

Is it acceptable to size the breaker to the wire for 30A and downsize the outlet to 20A, if needed? What are the limitations on mixing and matching wire, breakers, and outlets? I realize there are limited combinations but it seems there is some flexibility here.

Thanks in advance,
Brett

[Steve Meliza]

With #10 wire you can have 30A circuits or less. The breaker must match the outlet and in no case be greater than the 30A rating of the wire. You cannot put a 20A outlet on a circuit with a 30A breaker unless your local code allows it as it is not allowed by the 2011 NEC.

[Chris Friesen]

Pretty sure the electrician gave you bad advice. The breaker must be sized to match the receptacle*, the wire can be oversized as long as both the breaker and receptacle allow the larger wire.

*The exception here is that you can have multiple 15A receptacles on a 20A circuit. There are other exceptions for dedicated motor circuits, but that gets complicated and it's simpler to avoid it for home use.

[Ken Fitzgerald]

Contact the building permit and code inspection authority in the community where you live. I always pull a permit, perform the work to code and have it inspected.

Different communities adopt, adapt and interpret the building codes differently. As such, it's better to find out what they require before doing the work. While some people will blow off performing work with a permit and to code, I have known several people who didn't follow code and when they got ready to sell their house in the future, the property failed inspection. It was much more expensive to bring up to code than it would have been to do it to meet local code when the work was initially performed.

[David L Morse]

I don't understand what all the fuss is about. If you're going to pull 10ga then just go ahead and put in the 30A breakers and receptacles. The big cost is the wire, there's no cost difference in the breakers and only a small difference in the receptacles. No inspector will complain about that combination. You might have to replace a few plugs on your tools but that's fairly simple. Just standardize on 6-30 or L6-30 in your shop and everything becomes very simple.

[Chris Padilla]

FWIW, Brett, I did EXACTLY as you are doing...just ran #10 for everything in my garage whether it was 30A or 20A circuits. The city was fine with it...you should check with yours. To save a bit of money and size, you do not need to run a #10 ground with all this...it can be smaller. The question is, I do not know how small the ground wire can be. I ran #10 ground but it wasn't necessary. As the years have gone by, I've found this to be a very convenient thing I've done. I can move my 30 A circuits wherever I want.

[John Coloccia]

I ran 10awg for everything, including the 120V outlets. I can convert every outlet in the shop into 120/20A, 240/20A and 240/30A with nothing but an outlet change and a breaker change. Very convenient.

[Brett Bobo]

FWIW, Brett, I did EXACTLY as you are doing...just ran #10 for everything in my garage whether it was 30A or 20A circuits. The city was fine with it...you should check with yours. To save a bit of money and size, you do not need to run a #10 ground with all this...it can be smaller. The question is, I do not know how small the ground wire can be. I ran #10 ground but it wasn't necessary. As the years have gone by, I've found this to be a very convenient thing I've done. I can move my 30 A circuits wherever I want.

Hi Chris, I assume you do match the sizes of the breakers with the outlets, correct? That appears to be the consensus with this arrangement. Maybe I misunderstood the electrician because he had other accurate and helpful advice.

Also, good point on the ground wire being smaller but like you, I've already purchased the spool and will just keep everything consistent.

Thanks for the input thus far from everyone.

Thanks,
Brett

[Jim Neeley]

Brett,

You can definitely install circuits with 20A breakers and receptacles with 10ga wire, as long as your local area doesn't specifically forbit it. Your AHJ (Authority Having Jurisdiction [inspector]) can confirm this for you. There is one caveat.

The first requirement is the ampacity (current-carrying rating) of the wire must meet or exceed the protected load. 10ga clearly exceeds this.

The second (caveat) is that the terminals on the breaker and receptacle must be rated for the wire you are going to put in them. For example, you'd have to carve down a #4 solid copper wire to get it into the press-fit connection; this is not permitted.

The third issue provides exclusions specifically for connection to motors that let you use breakers greater than the ampacity of the wire (the 225% rule). This permits me to hook up my 5hp Unisaw with 10ga wire and a 40A breaker as recommended by Delta), but that doesn't apply here.

Thus you need to confirm is that the approved wire size range for the breaker and receptacle terminals are rated to clamp onto 10 ga wire. I just checked a spare 15A receptacle in my box of electrical goodies and, in little raised letters, it says 10-14ga. I don't expect you'll have a problem but you should confirm it for this application.

I'm wiring my shop just as John and Chris, and have thoroughly reviewed the NEC for conformance. I have also contacted my AHJ who confirmed there are no local codes prohibiting it. My work is also subject to inspection and I'm going to have it all inspected when I'm done. This will provide me some level of insurance protection should there be a fire in the future and someone try to blame it on the electrical work.

FWIW I'm also a registered Professional Electrical Engineer..

Jim

[Brett Bobo]

For the ground, is it necessary to maintain a home run from the outlet to the breaker for the ground per circuit, just as you would the two hot wires per circuit. Or, can the ground wires from several circuits in a series along the wall be pig tailed together, basically having one common ground for several circuits?

Reason being is that the 3/4" EMT is getting congested with pulling wire for three circuits for a total of 9-10 gauge wires. I'd like to add a fourth circuit in this series but I'm doubting there will be enough room to pull it.

Thanks for the help,
Brett

[joseph dake]

If you are running emt from the panel to the boxes it is grounded by the main ground anyways. You can pig tail all of the wires if you want too. Might cut down on the wire in the pipe. A 14 gauge ground should be fine but I would still run ten just for point of it.

[Brett Bobo]

If you are running emt from the panel to the boxes it is grounded by the main ground anyways.

I'm not sure if flex counts as continuous grounding but I'm running flex behind the wall from the surface mounted junction boxes where the EMT begins to the service panel. I've read that the EMT can serve as the ground by itself. Maybe no ground wire is required? Or, would pig tailing between circuits be a better option for redundancy?

Thanks,
Brett

[Ole Anderson]

Unless my motor is protected by appropriate overloads either in the motor itself or it's magnetic starter, I would be reluctant in running a small motor (less than 2hp) on a 30 amp breaker. Stall it and it may be a while before the breaker trips on overload.

[joseph dake]

I'm not sure if flex counts as continuous grounding but I'm running flex behind the wall from the surface mounted junction boxes where the EMT begins to the service panel. I've read that the EMT can serve as the ground by itself. Maybe no ground wire is required? Or, would pig tailing between circuits be a better option for redundancy?

Thanks,
Brett

If by flex you mean mc romex then it should be made up with the wire and ground inside of it. If it is just a flex tubing that you are running your own thhn in than I would run the ground in it because it does not qualify as a solid ground as far as I know. You are using thhn right? It has to be in a approved raceway to meet code. Maybe post some pics of what you are using and the space your working on.

[Rollie Meyers]

Brett,

You can definitely install circuits with 20A breakers and receptacles with 10ga wire, as long as your local area doesn't specifically forbit it. Your AHJ (Authority Having Jurisdiction [inspector]) can confirm this for you. There is one caveat.

The first requirement is the ampacity (current-carrying rating) of the wire must meet or exceed the protected load. 10ga clearly exceeds this.

The second (caveat) is that the terminals on the breaker and receptacle must be rated for the wire you are going to put in them. For example, you'd have to carve down a #4 solid copper wire to get it into the press-fit connection; this is not permitted.

The third issue provides exclusions specifically for connection to motors that let you use breakers greater than the ampacity of the wire (the 225% rule). This permits me to hook up my 5hp Unisaw with 10ga wire and a 40A breaker as recommended by Delta), but that doesn't apply here.

Thus you need to confirm is that the approved wire size range for the breaker and receptacle terminals are rated to clamp onto 10 ga wire. I just checked a spare 15A receptacle in my box of electrical goodies and, in little raised letters, it says 10-14ga. I don't expect you'll have a problem but you should confirm it for this application.

I'm wiring my shop just as John and Chris, and have thoroughly reviewed the NEC for conformance. I have also contacted my AHJ who confirmed there are no local codes prohibiting it. My work is also subject to inspection and I'm going to have it all inspected when I'm done. This will provide me some level of insurance protection should there be a fire in the future and someone try to blame it on the electrical work.

FWIW I'm also a registered Professional Electrical Engineer..

Jim

But you cannot use a 20A or 30A receptacle w/ a 40A breaker, it would have to be hardwired. There are rules in art 430 that have to be followed before the 40A breaker could be used & NM cable (Romex®) if used has to be used at the 60 degree ampacity.

[Jim Neeley]

Rollie,

I agree that Art 430 has a lot of governing rules. :-) I was surprised to learn that it is acceptable; let me share how I learned this.

Just as the 2011 NEC came out I purchased a "5hp" Delta Unisaw (20A FLA on the nameplate, I know, I know) that came with a 5-20 plug yet the instructions said to install to a 40A breaker. When I think 40A breaker I think #8 and a 5-50. This left me in a bit of a quandry as I did not want to cut the plug off a brand new (still unrun) saw just in case it had warranty issues given that I'd had it shipped to me in Alaska and didn't want to pay to ship it outside if it was bad and the factory wanted it returned for checkout. Meanwhile I researched a 5-20 receptacle but couldn't find one that would accept >#10 wire.

The next week I was attending Jim Stallcup's 2011 NEC Code Changes class so I held off and explained my quandry to him. He indicated that the 5-20 plug was sufficient since it met the nameplate rating. I explained the #10 issue on 5-20R's and he confirmed that with the thermal overloads in the motor that was still OK.

Next I went to my local inspector with the question who came to the same answer as Jim. He also signed off on the inspection.

A couple of months later I was working with a listing agent for Intertek Testing on a project at work and ran it by him, with the same answer.

Three code-masters, same independent answer from each. I may be slow, but I can be convinced!! <g>

I did install it with #10's and take the precaution of labeling the receptacle, however.

Jim

[Art Mann]

Unless my motor is protected by appropriate overloads either in the motor itself or it's magnetic starter, I would be reluctant in running a small motor (less than 2hp) on a 30 amp breaker. Stall it and it may be a while before the breaker trips on overload.

If you examine the trip curve of a typical household breaker, you will find that it will sustain twice the rated current for something like 30 or 40 seconds and 3 or 4 times the rated current for a few seconds. That is more than enough time to completely destroy a stalled motor. My point is that one should never count on a breaker to protect a piece of power electrical equipment.

[Ole Anderson]

If you examine the trip curve of a typical household breaker, you will find that it will sustain twice the rated current for something like 30 or 40 seconds and 3 or 4 times the rated current for a few seconds. That is more than enough time to completely destroy a stalled motor. My point is that one should never count on a breaker to protect a piece of power electrical equipment.

And if you look at the locked rotor amps of a motor it is typically on the order of four times the full rated running motor amps, so it may very well trip in a few seconds if the FLA (full load amperage) is near the breaker rating. But if the breaker is 2 or 3 times bigger than the FLA, then yes, you will be in trouble,.

[Art Mann]

I am not going to count on a breaker, even if it is sized near FLA. It may or may not be of any benefit under locked rotor conditions. If a motor is stalled for 3 or 4 seconds, that may very well be enough time to damage it. Most modern machinery is powered by motors with thermal overload protection anyway but I also try to be prepared to hit the E-stop quickly when I am doing something that might cause a stall.

[Ole Anderson]

I am not going to count on a breaker, even if it is sized near FLA. It may or may not be of any benefit under locked rotor conditions. If a motor is stalled for 3 or 4 seconds, that may very well be enough time to damage it. Most modern machinery is powered by motors with thermal overload protection anyway but I also try to be prepared to hit the E-stop quickly when I am doing something that might cause a stall.

I agree. The only motor I ever stall is my DP and it is only a 1/2 hp and has a switch I can bump with my chin or forehead if necessary.

[Jim Neeley]

Ole,

Same thing here except I've found mine is the belt slipping. :-/

Jim