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Discussion Starter #1
i replaced my electric dryer for a gas one and now i have a vacant 30A circuit with 10/3 gauge cable going through the house.

at the same time, i am putting up a powder room where the old dryer used to be and there will be a bathroom above as well.

i wanted to have a 20A circuit going to the powder room and the bathroom. i was wondering whether it is OK for me to reuse the legacy 10/3 cable if i do replace the 30 for a 20A breaker at the panel. i would do it just to the powder room and then i would, of course, go with a 12 gauge... just so i don't have to pull the old cable out and replace it with the adequate 12 gauge. i'm guessing smaller amperage should be OK going through fatter wire. obviously, i there would be an unused wire in the cable in this scenario but i don't care.
 

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Master Electrician
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The short answer is yes. The ampacities listed in the code are the minimum. You can always use a larger wire than necessary as long as the terminals will accept said gauge.
 

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i replaced my electric dryer for a gas one and now i have a vacant 30A circuit with 10/3 gauge cable going through the house.

at the same time, i am putting up a powder room where the old dryer used to be and there will be a bathroom above as well.

i wanted to have a 20A circuit going to the powder room and the bathroom. i was wondering whether it is OK for me to reuse the legacy 10/3 cable if i do replace the 30 for a 20A breaker at the panel. i would do it just to the powder room and then i would, of course, go with a 12 gauge... just so i don't have to pull the old cable out and replace it with the adequate 12 gauge. i'm guessing smaller amperage should be OK going through fatter wire. obviously, i there would be an unused wire in the cable in this scenario but i don't care.
Will this be feeding 2 bathrooms?
 

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Suggest you tag and annotate the wire in the panel so the next guy does not see the larger wire and think a larger breaker is ok.
 

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Master Electrician
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I might have jumped the gun...is that cable 10/3 with ground? (black, red, white, bare)?

Assuming it is....
As vsheetz suggested, it's a good idea to tag the 10/3 in the panel that it's a 20A circuit. You can use a 20A/2-pole breaker, have one 'hot' feed the powder room and feed the bath with the other, sharing the neutral. Do you know the rules about feeding a bathroom? If you cap off one of the wires in the 10/3, you'd probably have to run another homerun from the panel to feed the bath receptacle(s).
 

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Discussion Starter #10
alternatively, i can use the last remaining slot for a dedicated circuit to the bathroom but then i would need to branch out a non-bathroom circuit (mostly used for lighting) to go into the powder room. do you think that would be better?

thanks
 

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alternatively, i can use the last remaining slot for a dedicated circuit to the bathroom but then i would need to branch out a non-bathroom circuit (mostly used for lighting) to go into the powder room. do you think that would be better?

thanks
You can put all the outlets from both bathrooms on one 20A circuit. However; other items such as lights and fans need to be put onto some other circuit. Their load is likely low, so you might be able to sneak them on with something else.

Jamie
 

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Discussion Starter #12
You can put all the outlets from both bathrooms on one 20A circuit. However; other items such as lights and fans need to be put onto some other circuit. Their load is likely low, so you might be able to sneak them on with something else.
do you know the reasoning behind that?

thanks
 

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do you know the reasoning behind that?

thanks
Not really. But you can either have the outlets only from the 2 baths on one circuit and the lights / fans on a different (any shared circuit) or you can have a dedicated circuit for each bathroom.

It's often hard to know the exact reasoning behind many of the NEC rules.
 

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A circuit for a bathroom can either (1) supply everything in that bath only, as long as nothing fastened in place, other than lighting, exceeds 50% of the circuit rating (10 amps) (like a heater for example), or (2) can supply receptacles in that and other bathrooms, but only bathrooms and only receptacles.
 

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Discussion Starter #15
A circuit for a bathroom can either (1) supply everything in that bath only, as long as nothing fastened in place, other than lighting, exceeds 50% of the circuit rating (10 amps) (like a heater for example), or (2) can supply receptacles in that and other bathrooms, but only bathrooms and only receptacles.
thanks but that sounds really more like a code excerpt rather than an actual scientific backing thereof
 

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Discussion Starter #16
OK, does that circuit for bathroom outlets have to be 20A (i am in USA) and is the reason for that the proximity of water supply (the same reason you use GFCI outlets)? just curious
 

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OK, does that circuit for bathroom outlets have to be 20A (i am in USA) and is the reason for that the proximity of water supply (the same reason you use GFCI outlets)? just curious
Yes, it is required to be 20A, I doubt it has anything to do with water. It is much more likely due to blow dryers and curling irons.

If you really want to dig into more of the nitty gritty of why things are the way they are in the NEC, there are many such discussions on the Mike Holtz forum. Many of the people there are involved at least in submitting proposals for the NEC. Nothing wrong with asking, but most of us here are more hands on / real world, and we just work with the code the way it is, and don't have any secret insight on why it is the way it is. We can just make educated guesses.

Jamie
 
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