You can not use a 30 amp breaker for 110V outlets, 20 is the maximum.

 

So what use is a 30A single pole breaker then?

Bob

 

Wanted to delete this,, screw up

 

Again Bob, 30 A breakers are not for general use circuits, they are for special circumstances on dedicated circuits not where you can plug just anything in to them. They are mostly used on compressor or motor circuits, sometimes small welders where that is the only appliance that can get plugged in to it. Group lighting, gas station canopy lights, banks of florecent lights where the stuff runs on 120V but they can wire more to a 30A circuit than a 20. They are not to be used where you can randomly plug in devices with a 15 or 20A common plug.

 

Ok - that makes sense. I've seen the special plugs for 30A 125V circuits (5-30R/5-30P). I guess this is what you're referring to. No problem - will get a 20A breaker for that outlet.

Since I've already got some 6/3 and the two 50A breakers for the two 230V circuits, I think I'll just stay with those. Lots of juice.

Bob

 

Bob, the weldors it wouldnt make much difference on, but the comp should be breakerd at no more than 30A. I think you will have problems fitting the 6 wire in a 30,, get a piece of 10. BTW, for that little comp 3 hp a 13 A correct? You could use a piece of 12 romex if you had one laying around also. That follows the 125% rule that was mentioned earlier in regards to motor wiring.

 

Sorry about the confusion but the point is that the breaker is never sized larger than the ampacity of the conductor. You would not be protecting the conductor to the motor. As far as nuisance trips, the breaker will not trip on start up unless there is a problem with it. It has to do with the thermal trip characteristics of the breaker. Using a breaker as a switch will eventually pit the contacts to a point where it will not hold for normal load current.

You are correct that the ampacity of the conductor needs to be 125% of the motor running amps however the breaker still must be sized less than the ampacity of the conductor (but more than the motor running amps).

I believe (I do not have my code book with me) that the code does not allow a #12 conductor with a 40 amp breaker. If memory serves the little asterisk by the #14, #12 and #10 conductors specifies just how large a breaker that you can use in each case regardless of the actual ampacity of these conductors.

By the way I did stay at a Holiday Inn Express last night.

 

ok. one last time (I'll try).

10/3 for two 30A 230V breakers, 10/2 for one 20A 115V breaker.

Is that right?

Bob

 

Mike, you are often allowed to upsize the breaker larger than the conductor size. Welding machines are rated this way as are motors. If you have a code book handy motors are in art 430.52. Some applications allow breakers 250% of fla for motors that have their own thermal. The thermal overload is what protects the wire fom overload, the breaker simply protects from short circuit and ground faults, doesnt concern itself with protecting the cord or feed wire. Welding machines are NEMA rated, this is another example of over breakering. A common AC225 Lincoln actually allows 12 wire on a 50A breaker,,,, providing that its a single circuit in pipe. (Too light for me even if its legal) I believe welding machine wiring is in 630.11 It is the duty cycle factor for machines. It allows less wire due to the fact that they do not run continious. If there is a inrush current problem and it is tripping the breaker they allow up to 200% of the primary current rating of the machine,, that means that there could be up to a 100 A breaker on these machines depending on the wire size being supplied to them. A common setup back in the day was a 50A fuse or breaker feeding 10 wire, look at some old installations on farms from fuse panels. This is still common to welder circuits.
Back to Bob,, cords are different than cables like romex types. A cord lists all the conductors, a 10/3 cord would have 3 wires. A 10/2 cable would also have 3 wires, 2 conductors and a ground. Welders use 3 wires, 2 conductor wires and a ground. You can use XX/2 with ground for the comp or welder to the recept. Cables with 3 wires and a ground are for multi-wire circuits (in residental type systems like yours) where the appliance uses multiple voltage or for shared neutral circuits, none of which apply to this equipment.

 

Of course, if you really want to get fancy you could just install a subpanel in your garage. That way if you trip one of your 20A 120v circuits you can reset the breaker locally. You can also shut the whole thing off and lock it so no one can "play" with your high powered welding equipment when you're not around; this is important for any workshop when there are kids at home.

And I'm just introducing this idea because if you don't have enough 20A circuits available right now and you need to add a 30A for a welder, then you may as well just run one low gauge cable instead of a bunch. Keep in mind that the lower the gauge of the conductors the more of a pain it is to work with when installing in an existing building.

 

Thanks for the dissertation. It was never my intention to bring in all the various codes for the different applications. It only serves to confuse. The "easiest" and most safe answer for those who want to do it themselves is to follow the general example that I gave earlier. It serves no pupose to the neophyte to confuse with the exceptions such as certain applications allowing "overbreakering" (which I will NEVER do) even though it may be allowed.

Your knowledge is extensive, I hope that the original question the poster had was answered by all of this.

 

I agree Mike that in most cases there is no real reason to mis-match for most applications covered in most topics on this board. The cost differences are so minor most of the time that it just isnt worth the effort to calculate it. My intent was to develope some understanding of how it works beyond reliance on general hearsay that we often see on forums similar to this.

 

One place this may be applicapble here is extension cords for welders. For the buzz box type machies in most cases number 10 is fine, especially at short distances. For up to 50 ft or so it works well and the ratings for wire are for wide open according to the mfg. These machines are rarely run that hard in small shops, very seldom over 120A. Sometime next winter I might run a couple tests and see just how much performance difference there really is. I have been told by engineering on the stick machines it will be negligable but may show up some with large wire feeders at high currents. It would be interesting to see. As far as overloading of wires go the point I was trying to make was that the device itself becomes the limiting factor for overload protection more than the breaker. Thats why they call for the 125% rule, you just cant load the motor any more than that and its own thermal protection kicks in. The breaker protects against shorts which will develope hundreds and in some cases thousands of amps. I was testing a fixture a while back and wired a temp plug on a number 16 cord. It didnt have the protector around the prongs and one tiny strand, not much bigger than a hair stuck out and when I plugged it in it touched the metal cover plate on the recept. POW! Not only did the hairlike strand kick the 20A on the branch circuit it knocked the 60A feeding the panel down. Good example of the protection working properly. This is somewhat true of light fixtures. They have small wire in them and are wired to 20A circuits. The load on them is known, limited by the lamp you can screw in to them. That becomes the overload protection. True for most general plug in devices. But,,, the whole circuit is protected by a breaker where there is several recepts on it because that factor is unknown,, how many things plugged in and running at the same time cant be predicted. Personally, in my shop most things are wired one recept to a breaker making it difficult to overload with multiple devices. Hope someone finds this interesting in some way.

 

I would agree with Sberry calcs on the 30 amp breaker for the compressor referred to in this thread.

I would have to disagree with mikes calculations on the amperage draw of a 3 HP motor based on the charts given in the 430 section of the NEC. A full load current based on a 3 Hp single phase AC motor operating on a 230 Volt circuit would have a draw of 17 amps, not 13.

 

Again another correct post! If you look at my "calculation" you will notice that I was following the 1 HP = 1 kVA "rule of thumb" and did not reference the NEC. Therefore, with all due respect, my calculation is correct although you may still have a disagreement with the method. Again I was trying to impart a basic understanding of the problem and illustrate that the calls for a 50 Amp breaker and number 6 wire was way out of the ball park.

The point is that I think that we are making this answer way too complicated for such a simple project.

Next time I'll go get my code book and in the mean time I just hope that he doesn't give up on his garage project. Carry on guys!