Can I replace 12 gauge line to attic with 10 gauge when distributed wires will remain 12 gauge. Can I then increase Circuit breaker amps as each distributed line handles little amperage and only short main line to circuit board may have higher amperage. Changing wire from board to attic distribution point is easily done. Would this be Code? Would this endanger House insurance? Dan B.

  • Where are you on this planet? Also, what are these 12 gauge wires feeding? – ThreePhaseEel Dec 5 '17 at 0:49
  • What are you trying to accomplish by doing this, anyhow? – ThreePhaseEel Dec 5 '17 at 0:50
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    Is there a reason you don't want to add a new 30A circuit? – Dotes Dec 5 '17 at 1:56
  • Would this be Code? Not in the USA. Would this endanger House insurance ? WORSE it will endanger the occupants of the home! – Ken Dec 5 '17 at 2:01
  • The thinking behind this question is based on the old analogy of electric circuits with water piping. Water pipes can and are downsized as one proceeds through a branching distribution network. The reason this is OK for water pipes is that there is no water equivalent of a current spike from a short circuit which damages the pipes. Water pipes don't need "over flow limit protection". Water flowing through a pipe does not raise the temperature of the pipe as it flows through. More erosion of the inner surface of the smaller pipe must occur due to higher speed, but this is a slow process. – Jim Stewart Dec 5 '17 at 8:19

You haven't said how you will manage the transition from 10 gauge to 12 gauge. If you are thinking of wire nuts in a junction box, then the answer is that you cannot do this safely. The 20A breaker's job is to protect the load wire connected to it. A 12 gauge wire needs a 20A breaker to protect it. You cannot guarantee that a device at the end of one of these runs will not draw in excess of 20A (the rated capacity for 12 gauge wire). Your larger breaker (30A, the maximum 10 gauge is rated for) will not trip in this situation and the 12 gauge wire may start a fire. In addition to being unsafe, it is also not to code. Code requires the breaker to be no larger than the ampacity of the wire and since you're using 12 gauge wire at some point that's your limiting factor.

However, if you are thinking of installing a sub-panel then you may be able to do this. It would be fine (if you follow all the rules for sub-panels) to run 10 gauge wire protected by a 30A breaker to a sub-panel and then run 12 gauge wires each protected by a 20A breaker from the sub-panel. However, this seems a little silly (if you're going to all this work you probably want to run more than 10 gauge to the sub-panel).

If you explain more about your goals you might get more useful advice about what approach to take.

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  • Hit return to go down a line - not so Anyway, I will need to add new wiring. At least one breaker has numerous all-the-time appliances etc (each small amps), but when I occasionally add another higher amp item, the circuit goes. My solution would only work if I am certain of the load on each 12 gauge line, so I agree it would not be an acceptable solution. Dan B. – Dan B Dec 5 '17 at 17:59

You can do that. You must use a 20A breaker since there is any 12AWG in the circuit.

I gather that's not what you're after.

A single large load is typically 1440W, leaving 960W remaining on the 2400W breaker. Clearly the other loads are not small!

Since you do have an easy time running new cable from the service panel to the distribution point, run additional cables and use the distribution point to split the circuit into several circuits with additional breakers.

You were going to buy 10AWG and scrap 12AWG. I'd have you buy 12AWG and keep the other. Ideally run two 12AWG (for about the price of one 10) - wire and breakers are cheap, and now you have a legal 40A (20 + 20) instead of illegal 30A.

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Upgrading your wire is allowed.

But remember, the role of the breaker is to protect the wires connected to the breaker. Breakers are not protecting the loads, or whatever is connected to the outlets, breakers protect the wires.

So, that's simple: look at the wires which are downstream of the breaker, and size your breaker to protect those wires.

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  • Many small appliances were also designed to be "protected" by the breaker, i.e., they have no internal over-current protection and (for plug-connected devices) the manual may say to plug into "only" a 15- or 20-amp receptacle. – Upnorth Dec 8 '17 at 6:16

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