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Forum topic by live4ever posted 05-07-2010 06:27 PM 1254 views 0 times favorited 7 replies Add to Favorites Watch
View live4ever's profile


983 posts in 3010 days

05-07-2010 06:27 PM

My new tablesaw has arrived! Unfortunately, I haven’t had time to complete the 220V upgrade in my garage, which is where I need your help.

I’ve done a fair amount of wiring in our home, but always with 120V. I wanted to check a few of my assumptions regarding the job at hand.

The basic situation is that there appears to be an unused 220V circuit in the garage already and that all I would need to do is tap into it and install the proper receptacle at the end.

Here are the facts…
1) There is a 20A double-pole breaker in the panel for this circuit.
2) From the panel, a run of 12/3 cable (15 ft) heads to a junction box on the exposed ceiling and ends there. Originally, from this junction box, more 12/3 fed up through the ceiling to an old built-in electric grill, but we removed this when we remodeled our kitchen last year. So the only entry/exit into that junction box is the 12/3 from the panel.

Here are my questions…
1) Can I simply branch out of that box with 12/3 and connect my receptacle (6-20R)? The run from box to receptacle would be about 10-15ft.
2) Where I am confused is the number of wires. The fact that 12/3 is coming off the breaker seems to indicate that the circuit is 120V/240V (as many ranges require) as opposed to pure 240V, which you would expect to be wired with 2-wire. Is this a problem?

Thanks for your help!

-- Optimists are usually disappointed. Pessimists are either right or pleasantly surprised. I tend to be a disappointed pessimist.

7 replies so far

View Nomad62's profile


726 posts in 2958 days

#1 posted 05-07-2010 07:03 PM

I would think your circuit should work fine, but for one exception, and that’s providing I am reading it all correctly. The wires coming from your power box should contain 4 wires, 2 hot (black and red), 1 main ground (white) and a safety ground (green). The safety wire is generally unshielded and uncolored, but is connected to a green point in an appliance. The black and red are both “hot”, but out of sync; they are 120 volts each, which works against each other to create the 240 volt potential. The white ground is a stabilizer, and the green is a safety to aid the white. The green and white end up joining each other, but that’s another story. To be safe, and legal, you need them all. If it isn’t too difficult, I would jump up to a 10 gauge to help in keeping the voltage up when turning your saw on as well.

-- Power tools put us ahead of the monkeys

View uffitze's profile


199 posts in 2955 days

#2 posted 05-07-2010 07:21 PM

Run 12/2 from the JBox to the location that you want to put your table saw receptacle. There is no need to use 10 guage wire. Wrap the white wire of the 12/2 with black tape at the JBox and at the receptacle. In the JBox, connect the 12/2 to the black and red wires of the 12/3, and to the bare equipment ground wire. Put a wire nut on the white wire of the 12/3.

This assumes that you can for sure trace the 12/3 directly back to the panel, and that it is connected to a 2 Pole 20A breaker.

If what I just typed doesn’t make sense, call an electrician.

Have fun with your new saw.

View live4ever's profile


983 posts in 3010 days

#3 posted 05-07-2010 08:07 PM

Awesome, thank you guys. Uffitze – the explanation on what to do with the white wire in the Jbox was exactly what I needed. Thanks much!

-- Optimists are usually disappointed. Pessimists are either right or pleasantly surprised. I tend to be a disappointed pessimist.

View hokieman's profile


185 posts in 3754 days

#4 posted 05-08-2010 03:44 AM

You probably ought to let us know if you have a 3 or 5 hp motor. I ran 12 gauge for my 3 hp table saw at a couple of houses. Used 20 amp breakers for the two hot leads. Works well. If you have a 5 hp motor, then you might need bigger breakers and heaver wire. Your saw manual ought to have wiring information on the motor if it is 5 hp.

View Gofor's profile


470 posts in 3787 days

#5 posted 05-08-2010 04:35 AM

I am assuming you are running romex.

In a 220/240v circuit, the voltage runs on the two hot wires.

If you have three wires (white/black and bare) it is called 12-2. You have 110v/120v input on both the white and black, and the voltage returns on the ground (bare).

In 12-3 wire, you have four wires (black,red, white, and bare). 120v input on the black and red, white is the neutral return for the current, and the bare is the structure ground.

In a 4 prong plug, the voltage input is 120v on the black and red. The current returns on the white (neutral)and the ground is there as a redundant return to guard against current going to the structure of the machine. (IF the machine/appliance has a green wire, it is connected to the bare wire or bare wire terminal). The neutral and ground are tied together at the main service. Updated electric codes now require the 4 prong, 12-3 wiring for many appliances that previously were wired 12-2.

Realize that when saying 12-2 that means 2 insulated wires and one bare ground, and 12-3 means 3 insulated wired and one bare ground.

12 gauge wire will carry 20 amp per leg. If your saw is 5 HP, you may need to upgrade to #10 wire, all the way back to the breaker, and you will need a larger circuit breaker. Follow mnfgrs recommendations and check the amperage rating on the saw. There is no advantage to running heavier (#10) wire from the junction if it is fed by #12.

As for 220/240, do not worry. The difference is how the circuit is connected to the transformer at the power source (wye or delta) and will only become a factor if your are using multiple phase. House wiring is single phase.


-- Go

View uffitze's profile


199 posts in 2955 days

#6 posted 05-08-2010 06:52 AM

Of course, based on the NEMA number of that receptacle, I’m assuming that you have a 3HP or less motor.

If it is 5HP or larger, you will need larger wire all the way to the panel, a different receptacle, and a bigger breaker.

View live4ever's profile


983 posts in 3010 days

#7 posted 05-08-2010 07:56 AM

Thanks hokieman and Go. No worries – it’s a 3hp motor and I was already certain 12ga/20A is fine for it.

It was just a matter of knowing what to do with the 12/3 going into the Jbox (for the old 120/240 circuit) versus needing a pure 240V branch now.

Thanks again for the help – much appreciated! Now I just need to get a length of conduit for the drop from ceiling to receptacle.

-- Optimists are usually disappointed. Pessimists are either right or pleasantly surprised. I tend to be a disappointed pessimist.

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