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Old 12-01-2009, 01:50 PM   #1
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Two Different Wire gages on Same Circuit OK?


Residential interior wiring.
Can two different gage wires be used on the same circuit? Can 12 gage BX and 10 gage BX be used together, assuming the circuit breaker is rated for the lower capacity wire and all fittings are sized correctly?

An electrician in a past upgrade here ran a 12/4 copper stranded BX cable through the wall for a possible future install, but it wasn't connected to the breaker box...it was just installed inside the wall in the event I may want to add a circuit at a later date. The wires are presently inert. However, I now want to design the circuit.

The circuit will start with the 12/4 stranded copper BX on a 15 or 20 amp breaker at the panel. It will run for about 18' where it terminates in a junction box [let's call this junction box 'A'].

Here's my question:
I have a spare 100ft of 10/5 solid copper BX here. Can I continue two legs from junction box 'A' with the 10/5 (and not utilize the fifth wire from the 10/5 and leaving it inert)? Can two different gage wires and also solid/stranded be used in the same run?

The circuit plan consists of two legs continuing from junction box 'A', both legs wired with the 10/5 BX. One leg will run for about 12' until I hit another room, where it will go into a junction box and continue from there with 12/2 stranded BX to layout the room receptacles.
The second leg will be 10/5 BX starting at junction box 'A' and run for about 12' where it will terminate in a single box with a duplex receptacle.
That's the circuit.

I would just use all 12 gage BX, but as I said, I have 100' of 10/5 just taking up space here. Can it be incorporated into the circuit?

I have yet to buy the junction boxes and cable clamps.

Thanks a bunch.

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Old 12-01-2009, 06:30 PM   #2
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Two Different Wire gages on Same Circuit OK?


Yes you can as long as you don't go over 20 amps. It's kinda shoty work but it can be done.....

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Old 12-01-2009, 08:10 PM   #3
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Two Different Wire gages on Same Circuit OK?


Thanks for the reply.

Everything else here is wired with 12 gage.

I’m seeking only the electrical supply capacity a 12 gage circuit can provide, no more.

I realize it is an ‘inelegant’ solution, but are there any technical, functional, or performance drawbacks to using some sections of 10gage as planned? Any at all?

[As mentioned, I have the 10 gage on hand and would like to use it up…I realize 10/5 is more costly and more capacity than this circuit requires, however, if I do not use up some of this 10 gage it will only serve me as a large paperweight another 10 years.]
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Old 12-01-2009, 08:13 PM   #4
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Two Different Wire gages on Same Circuit OK?


Drawback is someone in the future may see 10g & decide to install a bigger breaker
Can you do the whole run in 10g ?
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Old 12-01-2009, 10:08 PM   #5
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Two Different Wire gages on Same Circuit OK?


actually, since the #12 is at the panel, I would tend to worry less about somebody upsizing the breaker due to the #10 in the circuit.

It's still possible but since the #12 is at the panel, hopefully they will make their call on that rather than the #10 being in the circuit down the line.
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Old 12-02-2009, 06:00 PM   #6
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Two Different Wire gages on Same Circuit OK?


Sounds logical and makes sense but you know what they say.

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