This may seem like a relatively simple question, but I wanted to get a professionals verification. I have always heard that if you have 15 amp breakers you are to use 14 gauge wire and if you have 20 amp breakers you are to use 12 gauge wire. Is this always the case and are there exceptions to this rule? Does it matter whether the wire is connecting to lights, an outlet, a switch, etc..

There are usually exceptions to any rule, and there are in this case also. But only for very specific reasons. These exceptions only apply to motors , refrigeration, etc. These exceptions do not apply to routine residential wiring of lights, outlets, switches.

If you can explain the reason for your question , perhaps I can provide more detail.

Tk - I believe that there is no reason that you can't use a larger (12-guage) wire on a 15 amp fuse. In fact, it is recommended on particularly long runs, in order to lower the resistance of the wiring in the wall.

Are there actually cases where you are allowed to use 14-guage wire on a 20 amp circuit? I replaced a breaker in my house when I bought it, because it was a 14-guage wire in a 20 amp breaker. It fed an exterior plug as well as the lights in my shed. I couldn't see any reason for the 20 amp breaker, and my house inspector told me that it was definitely a hazard (and against code).

Absolutely no disrespect or offense to home inspectors, they are usually not licensed electricians, nor electrical engineers. They are trained or learn the typical basics of home wiring and the standard typical codes.

As I stated, there are very specific reasons #14 wire would be used on a 20 Amp breaker, and none are related to residential receptacle or lighting wiring.

A specific instance thou, for example, I may have an air conditioner that has a running load of 10 amps, this value is called MCA, or Minimum Circuit Amps. Perfectly allowed to use #14 wire.

However, this unit can have a value called MOP, or Maximum Overcurrent Protection, that can be rated 20 Amps. What this means is I can use a breaker no larger than 20 Amps. This is due to the inherent high starting current, typical of all refrigeration units.

Article 440 of the NEC is where this is all explained. I can start by using a breaker 175% of rated load amps, or MCA, and if that is not sufficient to start the compressor, I am allowed to go as high as 225%.

This allows for the many compressors that are on the market. So there can be many instances where a small wire can be connected to a larger breaker.

Code allows for using the min size wire, since starting current is only for a very short period. Once the unit starts, the amps drop down to the running load, in my example, 10 amps. No need to waste copper for a load that would never get higher.

This practice is also allowed for standard motors. Most are not used nor relevant in residential wiring since these motors are so small. But when dealing with a 100 HP motor in an industrial application, again great savings is realized by this practice.

I reread your first statement, and I missed it at first.Yes you can always use larger wire on a smaller breaker exactly for the reason you mention, voltage drop.

Excellent! My knowledge increases. Soon I shall know enough to take over the world!

Er,

Forget you read that.

Anyway, thanks for the informative post!

Listen to tkrussell

Pass this,

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Originally Posted by TAHerrmann

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Onto my wife.