14/2 and 12/2 mix


Old 08-16-05, 12:00 PM
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14/2 and 12/2 mix

I was changing lights in my basement and found some 12/2 and 14/2 wiring. I traced back to the breakers and found that some of the wiring is 12/2 and some is 14/2 leading into a 15 amp breaker. Is this a problem. Can you mix both types of wiring?

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Old 08-16-05, 12:09 PM
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So long as the circuit breaker or fuse is sized for the smallest gauge wire on the circuit, you are safe and up to code. A 15A breaker protecting 14 gauge wire is acceptable. When in doubt, use a 15A breaker; do not put a 20A breaker on a circuit unless you know absolutely for sure that every wire on that circuit is at least 12 gauge or larger.

Although code-legal, mixing wire gauges on a circuit is usually considered bad workmanship for this very reason. It's confusing to someone working on the circuit, and could lead a less than cautious person to see a piece of 12 gauge and up the breaker size to 20A. This creates a dangerous situation if 14 gauge wire is hidden somewhere in the circuit.
Old 08-16-05, 09:06 PM
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Originally Posted by ibpooks
This creates a dangerous situation if 14 gauge wire is hidden somewhere in the circuit.
Improper, certainly. Not too dangerous. #14 THHN (as found in most Romex) is rated for 25A of current, it's just SUPPOSED to be limited to 15A of overcurrent protection. While it would be against the Code to raise that to a 20A circuit, it's not dangerous.

I'm not trying to bust you; I just don't want someone else thinking their house is going to burn down tonight.
Old 08-17-05, 08:15 AM
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An individual #14 conductor in free air is rated at 25 amps. A #14 conductor in a cable assembly inside a wall is definitely not.

The terms "dangerous" and "safe" are not black and white. There is a huge gray area between the two. The electrical code draws a line through this gray area to say what is allowed and what is not. Considering that the electrical code is based on safety and nothing but safety, the line is drawn by a committee of experts using their best (but not perfect) judgement. What falls on the "not allowed" side of the line is the industry-accepted view of "dangerous" as best we know today.
Old 08-17-05, 10:34 AM
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Caution: how many angels can dance on the head of a pin discussion

I am entirely in agreement with John on the general concept of safety as a grey area, with code drawing a sharp line between safe enough and not safe enough. Sometimes we even disagree with the code makers on this, for example, 'backstab' receptacles are _listed_ (by UL, not the NEC, but that is for a different discussion...) and thus considered 'safe enough', but the consensus here is to never use them and remove them when you find them. And very often an installation will 'work', but not meet code, and really not be 'safe enough' in the event that something goes wrong. Most of code is about keeping things safe when something goes wrong, and if things never went wrong most of code would be a useless waste of time.

Just on a technical point, however. A 14 ga wire has a continuous ampacity (60C rating, in a cable assembly, so this applies to standard romex installations) of 20A, and if nothing ever went wrong, could safely carry 20A continuously without overheating. This has nothing to do with cable assemblies or free air flow.

My understanding is that the restriction on using 15A circuit breakers with 14ga wire has to do with how the system responds to things going wrong. Circuit breakers and fuses do not have a magical _exact_ trip point where they stay untripped forever below a certain current, and instantly trip above that current. Instead they have a 'trip curve', which described how quickly they will trip at different current levels; the higher the current, the faster they trip. In addition this trip curve has 'tolerance', meaning that there is a certain amount of allowed variability in this trip curve.

In the event of a short circuit or overload, extremely high current will flow for a short period of time until the breaker trips or the fuse blows. The restriction to using 15A breakers for 14ga wire, 20A for 12ga, and 30A for 10ga has to do with protecting the wire from damage in the event of an extreme overload or short circuit, and to insure that the breaker trips rapidly in the event of a short circuit. A wire that could safely carry 20A continuously might be damaged by the short circuit energy that a 20A breaker would let through, so a smaller breaker is mandated.


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