12-3 wire on 30amp circuit breaker?

[Tom Burgess]

I want to run a 25' flexible 12-3 wire from a 30amp 220v junction box in my garage.
I'm setting up a Delta table saw with 3hp 220v single-phase motor.
This is a temporary situation, less than a year until I sell the house.

The circuit handles the central air conditioning (which I almost never use) and the clothes dryer (used only occasionally). So I'll make absolutely sure that the table saw is never running while either of the other appliances are.

The question is whether the 12-3 wire will suffice in this instance.
It appears that 10-3 is recommended for a 30amp circuit,
but 12-3 is recommended for this motor (as I understand it...)

Thanks

[Anthony Whitesell]

No it will not suffice. Breakers are there to protect the wiring not the equipment. Useful if they do both, but are there for the wire. The 12g wire can only handle 20A, above that is a risk of fire, until the breaker trips at 30A. The motor requires 12g as it will be pulling more than 15A but less than 20A (less than 15A would have asked for 14g wire). To use the 30A breaker, you will need 10g wire. I suggest you find or install a 20A double pole breaker in the box.

[Mike Henderson]

It sounds like you want to piggyback on an existing 30 amp breaker by adding the 12 gauge circuit to your saw. Your saw only requires a 20 amp breaker so you want to know if this will work.

It will work, but there's a safety problem. If you were to draw over 20 amps, the wire would overheat and you could have a fire.

Also, if you are only going to take 240V from that circuit, you only need two wires, plus ground. You don't need a third (neutral) wire.

Mike

[Ole Anderson]

Ok, I am going to go contrary here on the condition you have a receptacle at your junction box. As long as you aren't hard wired you should be ok. We plug lots of smaller wire tools into 20 amp circuits all of the time. Your Dremel tool that you plug into a 20 amp receptacle probably has an 18 gauge wire cord. As long as your saw doesn't draw more than 16 amps (80% of the capacity of a 12 ga cord). Ok that last sentence may get me into trouble as the ampacity of cords isn't always the same as buried conductors and depends on the insulation type and temperature rating, but 20 amps is usually the minimum ampacity of a 12 gauge conductor.

I won't comment on the A/C and dryer being on the same circuit as I, too, double up on my 220 circuits, just not those two.

[John McClanahan]

12/3 with a 30a breaker is against code. If your saw is running properly, it will never draw enough current to be unsafe. If you use metal flex conduit, the fire that will probably never happen would be contained in the conduit. If you run 10/3,the wire will then be up to code. The motor should have an internal overload protector. You did say this was a temporary setup. The choice is up to you.


John

[Anthony Whitesell]

Ok, I am going to go contrary here on the condition you have a receptacle at your junction box. As long as you aren't hard wired you should be ok. We plug lots of smaller wire tools into 20 amp circuits all of the time. Your Dremel tool that you plug into a 20 amp receptacle probably has an 18 gauge wire cord. As long as your saw doesn't draw more than 16 amps (80% of the capacity of a 12 ga cord). Ok that last sentence may get me into trouble as the ampacity of cords isn't always the same as buried conductors and depends on the insulation type and temperature rating, but 20 amps is usually the minimum ampacity of a 12 gauge conductor.

I won't comment on the A/C and dryer being on the same circuit as I, too, double up on my 220 circuits, just not those two.

True. True. I absolutely agree. The OP does state whether he is adding a plug to the circuit or just wants to plug in his saw. Using the 12-3 and 10-3 description for the wire, it would appear a new branch is being added to an existing circuit. In which case there should be 10-3 wire and a 30A receptacle used. If the 12-3 wire is describing the cord attached to the tool, then the OP is good to go.

[Steve Meliza]

There are not enough details to go on, but it is difficult to imagine a scenario in which you can power the saw from this circuit without a code violation as a minimum. If you proceeded to perform this work and not have an inspection and violated code you would have difficulty with homeowner's insurance paying out should that circuit cause a fire. And you'd need to undo the work before listing the home for sale or it would get caught in a home inspection and the buyers would have you paying an electrician to strip it out. You can ask the previous owners of our home if they enjoyed spending just over $1000 to a licensed electrician to come and put things to code.

It sounds to me like you have a junction box that you plan to tap into with a power cord, effectively hard wiring the table saw via a 25' extension cord. If so that is bad on many levels you should just forget it. If you intend to add a plug be advised that code requires only 30A plugs on a 30A circuit. If you are extending that circuit from the existing junction box to a new junction box near the saw then you'd use NM #10 cable to do so and end with either a 30A plug or hard wire the saw into the new junction box using the manufacturer provided power cord.

If it were me I'd pull a permit and drop in a new 2 pole 20A breaker and run NM 12/2 to a 20A/220V outlet on the wall near where I planned to use that table saw, about 50" off the floor. If I didn't have the space in my panel for the new breaker I'd add a new sub panel to provide the necessary space. It's cheap if you do it yourself and if done well you can take pride in the work and maybe the buyer will also appreciate a properly done 220V outlet.

[Chris Padilla]

I say it is fine as the motor will only draw what it is supposed to draw and if there is any issue with this extension cord, you will likely SEE and SMELL it. As mentioned, the circuit breaker protects the wiring in the wall, the wire you don't see or have easy access to.

[Dan Friedrichs]

12/3 with a 30a breaker is against code.

12/3 ran as a permanent installation in the wall with a 30A breaker is not code-compliant. Running a 12/3 extension cord from a receptacle at the junction box is fine. OP - are you talking about using the "flexible" 12/3 as an extension cord? If so, install a receptacle at the J-box and you'll be good to go. Just don't run the 12/3 in the wall or otherwise make it a "permanent" installation.

[Tom Burgess]

OP here ==

Thanks, fellas, for such a detailed, thoughtful discussion.

To clarify: There's a 4" square metal junction box on the wall of my unfinished garage. My intention is to remove the cover and add an extension box atop that box. My earlier comments describe using wire nuts inside that expanded box to connect a 25-foot 12-3 flexible wire (black plastic shielded against abrasion, petrol, etc) with a clamp-on receptacle attached to its end (female configuration that matches the plug on the saw's 5-foot cord).

My (sketchy) understanding is that an extension cord is always a bad idea, and what I'm proposing would avoid any concerns associated with extension cords...

The flexible wire would just hang looped on the wall when not in use.

The saw motor is rated at 220V 12 amps so I don't think it will tax either a 12-3 wire or the 30amp circuit breaker.

The saw is just for occasional use at present, nothing anywhere close to commercial use.

I will remove the 25' flex wire arrangement prior to listing the house.

So where do things stand now, based on this additional information?
Is 10-3 (or 10-2) a necessity, or will 12-3 do?

Again, thanks to you all.

[Duane Meadows]

Fact still stands... you don't use 12ga wire with a 30 amp breaker. Try explaining to your insurance agent, "but it only got used occasionally". Don't think they will be too understanding! Don't think they will by "but some guy on a forum told me I could", either.

Or to put it another way, you would be using it as a hard wired extension cord.

[Mike Cutler]

Tom

I'm going to go against the grain here and tell you that if it's done correctly, and if you're local codes/inspector will permit, you are very close to installing a "feeder tap", which can be installed in a code compliant manner, and be permanently left in place. (But, the part about having SO cord coming out of a 4" box seems a little sketchy. There are ways to have flexible conductors exit boxes, but that doesn't seem to be the "best way".
From your description you have a 4" box in the wall, and in this box are 10/3 conductors? Are these conductor passing through this box, or are they terminated to something?
If done correctly a separate box can be fed from the conductors in the currently installed box, and a 240/20 amp plug can be installed that box, and from here you would plug in an SO extension cable, just in the manner you are explaining.

However, It seems as if you may be doing some extra stuff that you don't need to,and don't need a tap at all.
Why not just put a 30 amp plug on the current box, and make a 240 breakout connector box, on the end of the SO cable, that plugs into the 30 amp receptacle? The NEC stops at that receptacle.
The breakout box in something you will make, or buy (There are UL approved breakout boxes.) Your saw then plugs into the breakout box. A licensed electrician can probably install that 30 amp receptacle for an amount that isn't going to break the bank, and it can be left in place when you move.

There is nothing wrong with extension cords. As long as they are sized properly, and kept out of harms way.

[Dan Friedrichs]

Fact still stands... you don't use 12ga wire with a 30 amp breaker.

That is NOT TRUE. You can't use 12ga in a permanently installation on a 30a breaker. The OP is using an SO ("extension cord") cable. That same "rule" doesn't apply.

OP - what you're proposing will work fine, but install a receptacle and make your cable into a proper extension cord. You can use high-quality twist-lock plugs on the junction box end if you like. The general prohibition against "extension cords" has more to do with voltage drop over long cables, rather than anything being inherently wrong with extension cords.

For those who have concerns about this idea, I would ask: Do you always check that the extension cords you use are appropriate gauge for the breaker feeding the circuit you plug in to? Of course not. Wires sized for permanent in-wall installation are subject to much more conservative estimates of heating potential BECAUSE they are often hidden and inaccessible. The OP is going to have a visible, exposed cord running along the floor (which is perfectly fine) - he'll notice if it starts on fire (which, for the record, it certainly won't).

[Duane Meadows]

That is NOT TRUE. You can't use 12ga in a permanently installation on a 30a breaker. The OP is using an SO ("extension cord") cable. That same "rule" doesn't apply.

OP - what you're proposing will work fine, but install a receptacle and make your cable into a proper extension cord. You can use high-quality twist-lock plugs on the junction box end if you like. The general prohibition against "extension cords" has more to do with voltage drop over long cables, rather than anything being inherently wrong with extension cords.

For those who have concerns about this idea, I would ask: Do you always check that the extension cords you use are appropriate gauge for the breaker feeding the circuit you plug in to? Of course not. Wires sized for permanent in-wall installation are subject to much more conservative estimates of heating potential BECAUSE they are often hidden and inaccessible. The OP is going to have a visible, exposed cord running along the floor (which is perfectly fine) - he'll notice if it starts on fire (which, for the record, it certainly won't).

If the OP does it that way(with a plug to make it an actual extension cord), I agree. If it is permanently installed in the box, intent to remove later doesn't change that. Certainly it will function. Just because it will function doesn't mean it should be done that way.

[Rollie Meyers]

Because of the fact that the OP is using NM cable, 30A is not OK with 12/3 , there are rules that would allow 12 AWG on a 30 or 40A breaker but this is not one of them.