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12/2 and 12/3 wiring.

#1
05-30-05, 08:31 PM
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Join Date: Aug 2004
Location: Michigan, USA
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12/2 and 12/3 wiring.

I would like to know what the difference is between 12/2 wiring and 12/3 wiring. I may know, but is 12/3 240 Volts?

#2
05-30-05, 08:47 PM
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There is one more conductor wire in 12/3

#3
05-30-05, 09:42 PM
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either of them could be used for 240V

#4
05-31-05, 04:15 AM
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What is this conductor used for?

#5
05-31-05, 04:22 AM
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There are many possible uses for the third (red) conductor in a 12/3 cable:

(1) For a 240V with neutral circuit black would be one hot, red would be the other hot, and white would be neutral;

(2) For two shared neutral 120V circuits black would be hot for one 120V circuit, red would be hot for the other 120V circuit, and white would be the shared neutral. (This is really the same as (1) but with slightly different terminology.)

(3) 12/3 is (probably) most commonly used in three (or four) way switch circuits. Exact use of each wire will depend the particular 3-way configuration, but generally two of the wires will be "traveler" wires (one of the two is hot depending on the position of the switches) and the third wire is either neutral, continuous hot, or switched hot.

(4) 12/3 is also used to run both switched and unswitched power from a wall switch box to a switched outlet.

(5) 12/3 can also be used to run two switched lines to a ceiling fan, one line for the fan and the other for a light kit.

I'm sure there are other uses for 12/3, these are the ones that come to mind.

#6
05-31-05, 06:41 AM
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Nice summary Mike.

Bottom line is that you use 12/2 when you need two conductors plus ground, and 12/3 when you need three conductors plus ground. This is somewhat like asking the difference between a 2-inch screw and a 3-inch screw. You use whichever one the project calls for.

#7
05-31-05, 12:29 PM
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Originally Posted by mikewu99
I'm sure there are other uses for 12/3, these are the ones that come to mind.
(6) 12/3 can be used to equalize the voltage drop when feeding a long run of incandescent lighting outdoors.

Use 12/2 from breaker to switch, and from switch to first light (or use 12/3 if you don't have 12/2 and leave the red capped off), connecting wires in the normal way. Run 12/3 from first to last light through all the other lights. Connect ground and neutral in the usual way at each light. Connect the black lead from the light itself to the RED wire of the 12/3 run. Connect the black wires of each 12/3 so it just passes stright through. At the very last light, connect the black and red wires together, along with the black wire from the light itself.

This method makes every light have an equal length of wiring so that the voltage drop involved dims each light the same proportion, and thus each light can be of equal brightness. The disadvantage is that every light will be as dim as the last light would be.

#8
05-31-05, 12:54 PM
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3 wire is also used in smoke detector circuits where the units are interconnected (which is required these days). Red usually acts as the inter-connect wire.

#9
05-31-05, 02:18 PM
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3 phase,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ha And feeding panels.