mixing wire gauges

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  #1  
Old 05-11-06, 06:19 AM
magicalpig
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mixing wire gauges

Hi everyone

This is a great forum. I'm happy to have found it.

My question (first of many to come, I'm sure) involves the mixing of wire gauges. I will have some wiring to do for kitchen lighting and I hope to be able to use #14 wire since it's cheaper. The branch circuit the lights will tap into will be #12 guage. It will make the transition at the splices behind the light switch.

Is this okay to do? Or is this where the "10 feet" rule from the NEC has to be satisfied?

Thank you, anyone with advice.

magicalpig
 
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  #2  
Old 05-11-06, 06:35 AM
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Do not change wire size.

If the circuit is a 20 amp circuit then you MUST use 12 gage wire. Even if the the circuit is 15 amp, or you changed it to be 15 amp, mixing the wire gage would be a bad idea.

First, someone later on might not know you changed the wire size, and install a 20 amp breaker.

Second, a 20 amp circuit provides 33 percent more power than a 15 amp circuit.

Finally, if this circuit serves kitchen counter top receptacles, you cannot tap into it.
 
  #3  
Old 05-11-06, 06:54 AM
magicalpig
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Thanks racraft. I guess a more general question could have been : when wiring a circuit to serve a combination of receptacles and lighting, must all wire on the circuit consist of the same gauge? An example where you'd want to change gauge is when you've got a 20-amp circuit using #12 and you have to run a 3-way switch. It'd be much easier to run that switch with #14/3.

What you said about the breakers makes sense and is prudent advice. There's something I don't fully understand though about electricity:
If you have #12 for a circuit, but then splice #14 into it for a light (making the transition in the box for the light switch), is the current running along that length of #14 limited to the draw of the lights it serves? Or does the current drawn by the appliances plugged into the #12 portion of the circuit also visit this region?

Sorry if I'm unclear. I'm trying to rush this along to get into a meeting. I can try again later if I'm making no sense
 
  #4  
Old 05-11-06, 07:03 AM
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For a 20 amp breaker, the minimum wire size is 12 gage copper.

While there are certain limited situations where you can get by with smaller gage wire (called taps) , they don't apply in this case, and knowing when they DO apply is best left to an electrician.

Yes, 12 gage wire is harder to work with. It also takes up more room when doing box fill calculations.

For this reason, some electricians like to make their lighting (and only lighting) circuits 15 amp.
 
  #5  
Old 05-11-06, 08:21 AM
magicalpig
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Gotcha. I may do that (dedicated lighting circuits). I like the idea of the lights being on if a breaker trips or if I need to do any electrical repairs on the receptacle circuit or the appliances that tap into it.

Plus, I've read that a 20-amp circuit isn't good for lights because "lights have thin wires". I don't know if the book was referring to the assumed 14 gauge wire used to power the lights or to internal wires of the lighting unit. The former case reduces to "don't use 14 gauge wire on a 20-amp circuit" which is exactly what you're saying

Thanks a lot, racraft.
 
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