gauge vs breaker

12 awg on 15 amp breaker isn't bad but yes you are right 14 awg on 20 amp breaker would be an issue

12/2 is rated for 20A, so it's perfectly ok (but maybe wasteful material wise) to put it on a 15A or 20A breaker.

14/2 is of course only rated 15A, you put it on a 20A breaker and draw maybe 18A through, and the wire will start to melt while the breaker keeps on truckin, turning your house into a toaster

Thanks guys

johnnyboy said:

12/2 is rated for 20A, so it's perfectly ok (but maybe wasteful material wise) to put it on a 15A or 20A breaker.

14/2 is of course only rated 15A, you put it on a 20A breaker and draw maybe 18A through, and the wire will start to melt while the breaker keeps on truckin, turning your house into a toaster

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A little harsh don't you think? Yes, 14ga wire should be protected at 15A, but 14ga wire will carry 20A all day every day without "melting". It's the insulation that may suffer damage from overheating.

Remember also that just because the wiring at the breaker is 12 ga, it may be connected to 14 ga. wire in a junction box and the breaker size needs to match the smallest wiring in the circuit.

Jonnyboy's statement is a way over the top, 14 ga. wire can carry 30 amps all day without melting the insulation or reaching a combustible state except in high temp applications. The 15 amp code restriction has a large safety margin built in.

hankscorpio said:

Hi guys,

Most of my house is wired with 12-2. Some of the breakers are 20 but a lot are 15amp? Is this a problem? Or would it be a problem if it were like 14 guage wire on a 20amp breaker?

Thanks guys

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Not a problem - the electrician may have upsized the home runs for voltage drop - or just likes wasting copper.

If the wiring is AL or copper clad AL than 12awg must be protected by a 15 amp breaker, 10 awg can be protected by a 20. Look at the cut ends to determine if its copper clad.

Just FYI, there is situations that you may see a smaller wire then what the breaker is(i.e #14 on a 20A breaker) Air conditioning is one example, the breaker may be upsized for the inrush at starting. In Canada(not sure about the US) pure heating loads(baseboards, electric furnaces) are wired to the load but the breaker is upsized for continours use. So in that case you could see a #12 on a 30A breaker, probably not common but is legal.

Not sure about here in the us. I think you must have the breaker 15 for 14awg. If you need a 20 for inrush, 12 is the way to go.

There are breakers designed for motor start devices. HACR breakers are for heating, air conditioning, and refrigeration units.

FWIW
http://www.jhlarson.com/ind_tables/conductor/table310-16.htm

Plotting AWG# vs. I squared gives you very nearly a straight line, so you can use this graphical method to fill in the missing current values for #14, #16 and #18. Sort of.
Or you can figure out the temp-rise-above-amb. if you're given the current, or vice versa, within reason.

Decayed wood ignites at 150C.
http://www.tcforensic.com.au/docs/article10.html
So working backwards I figure you need ~35A (+/- 5A?) though #14 copper to get this hot.

darren said:

Just FYI, there is situations that you may see a smaller wire then what the breaker is(i.e #14 on a 20A breaker) Air conditioning is one example, the breaker may be upsized for the inrush at starting. In Canada(not sure about the US) pure heating loads(baseboards, electric furnaces) are wired to the load but the breaker is upsized for continours use. So in that case you could see a #12 on a 30A breaker, probably not common but is legal.

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I am not very familiar with the CEC, but that sounds suspect, especially for a heating load. I suggest you check that again. I am not saying you are wrong, but it sounds odd that a resistive load would be allowed to have an oversize breaker.

rgsgww said:

Not sure about here in the us. I think you must have the breaker 15 for 14awg. If you need a 20 for inrush, 12 is the way to go.

There are breakers designed for motor start devices. HACR breakers are for heating, air conditioning, and refrigeration units.

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Yes the Code allows for a few instances of oversizing the breaker compared to wire size. Motors and HVAC compressors can have oversized breakers in certain circumstances.

And almost all general purpose breakers in your home are also HACR breakers.

Inphase I know it sounds strange but its true, not sure people actually do it that way but if it was me i would just upsize the wire.

The code allows us to wire to the load of the heater. Then we have to consider that load as continous for the breaker so we bump it up.

So lets say you have 240V 3000W heater. 3000/240=12.5A. We can wire to this amperage so a #14 would work. That load is considered continous so now we have to bump it 12.5/.8(80%, in Canada at least)=15.625A so now you have to use a 20A breaker. Like i said if it was me i would just use #12 for it but you can get away with it.