Teeing Off of 12 AWG with 10 AWG?

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Old 03-22-18, 06:02 PM
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Teeing Off of 12 AWG with 10 AWG?

This is all old wiring that I'm trying to just reconfigure without running any new lines. One run goes through the junction box and out to switch 1, then to several lights all using 12 gauge. Today I tied a 10 gauge (also already in the junction box) into the 12 at the junction and it runs to switch 2 then to one light. If switch 1 is off and switch 2 is turned on the light comes on. If switch 1 is on and switch 2 is off the lights after switch 1 come on, but if switch 2 is turned on while switch 1 is on, its light does not come on... but if you then turn switch 1 off while switch 2 is still on the light comes on. Is this some sort of problem with a difference in resistance between the 12 and 10 gauge wire, and is there a simple fix without running 12 gauge where I currently have 10?
 
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Old 03-22-18, 06:12 PM
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Welcome to the forums.

Although it's preferred to keep the same size wiring on a circuit.... the use of #10 there is not causing your problem. It sounds like you tried to connect to a switch loop. That would mean there is no neutral available and you are connecting your new load in series with the existing load.
 
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Old 03-22-18, 06:15 PM
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That is not an issue with the combination of 12 and 10. It's because of wacky wiring.
 
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Old 03-23-18, 03:20 AM
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A little clarification please if you will.

You mention two switches. What you are describing can be either that you tapped into this jbox for power to run two separate sets of light with two completely separate switches and maybe the way you wired in the jbox may be causing your issue

or

when you refer to two switches you are referring to a set of three way switches that are meant to turn on and or off all the lights using either of the switches not dependent on what position either switch is in at the time of switching.

Although using #10 would be fine physically/theoretically it is a waste of money to do so and #10 is far more difficult to work with. #10 is rated for 30amp while 12 is rated at a max of 20amp. Do not connect this to a larger than 20amp breaker.
 
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Old 03-24-18, 05:54 PM
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Thanks for all the quick replies. I still haven't been able to diagnose the problem. Here is a little clarification for AFJES. Both switches are single pole. A neutral runs through the junction box and to the switches where it obviously just bypasses the switch in the box and runs on to the lights.

One more observation, but do not try this at home. Within the junction box if I touch the hot coming into the junction to the 12 AWG going to switch 1 (switch 1 in ON position) those three lights come on, then if I separately touch the hot to the 10 AWG heading to switch 2 (with switch 2 in the ON position) its light comes on, but if I touch the hot to both the 12 and the 10 at the same time (with switch 1 and 2 both in ON position) the three after switch 1 come on, but the one after switch 2 does not.
 
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Old 03-24-18, 09:22 PM
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Can you explain what you have wiring a little clearer.

You have a 12-2 coming in for power.
Then you have a 12-2 out to the switch which then goes out to the lights ?
That's it..... just two cables ?
That would mean.... in that box.... you only had a white to white and black to black.

We know about the new #10 addition.
 
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Old 03-25-18, 03:53 PM
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There is no addition per se. All wires were preexisting, I'm just utilizing them differently. Currently the junction box has an incoming 12/2 bringing hot and neutral, an outgoing 12/2 taking hot and neutral to switch 1, and a 10/2 taking hot and neutral to switch 2. There are 3 sleeves stubbed into the box bringing a total of 6 wires into the box (9 if you count the uninsulated ground wires).
 
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Old 03-25-18, 05:28 PM
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Not sleeves.....cables.


I thought I made my post pretty clear.

Originally..... one cable in and one cable out. That cable in cannot be an always live cable. That wire in sounds like a switch loop.

Ignore the #10 cable.

Open the splices..... check the cable going out to the switch and the lights. If that cable shows any power.... you have a confirmed switch loop and cannot connect there.,
 
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Old 03-31-18, 05:20 PM
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Well, thanks for trying guys! I appreciate it.
 
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