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I was doing some work in an old house I recently moved into, and found that my dishwasher and garbage disposal (and some outlets near them) are on a 30 amp breaker. However, some of the electrical wiring going between the disposal and the wall switch is yellow, which implies 12-gauge, 20 amp-rated wire (right?). How dangerous is this situation? What should be done, if anything?

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What size wire is connected to the breaker? –  Jay Bazuzi Jan 15 '12 at 18:30
    
Is there a sub-panel between the 30 amp breaker and the kitchen appliances? –  Jeff Widmer Jan 15 '12 at 18:46
    
No subpanel. I believe the wire directly connected to the breaker is an appropriate gauge. The garbage disposal and outlets were newly wired into this circuit with a recent kitchen remodel. –  Scott Stafford Jan 15 '12 at 19:22
    
It sounds like the breaker was tripping and to stop the random tripping. Is the wire coming from breaker 10 gauge. –  lqlarry Jan 16 '12 at 3:53
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There is a special case in the code that allows something similar. I think the rule is that 2 hard-wired devices on the same circuit can use a conductor & breaker sized to the shared load from the breaker to the fork, and then conductors sized to the separate load from the fork to the device. But I don't remember where in the code this is written, and what the details are. –  Jay Bazuzi Jan 17 '12 at 0:05

2 Answers 2

up vote 10 down vote accepted

If the #14 or #12 wire is a branch going to specific loads, then it is "safe" with respect to that load. It won't overload that wiring regardless of the breaker capacity use (or even if the breaker is bypassed entirely). However, this is unsafe in the context of changes in usage. The electrical code focuses on safety and rightly prohibits this. The circuit breaker is required to be sized to protect all wiring in the branch circuit it feeds to (and the receptacles, too). This is because someone could overload the circuit (for example plugging in a 12 amp appliance in one outlet and another 12 amp appliance in another outlet on the same circuit, which should trip a 20 amp breaker and would not trip a 30 amp breaker).

In USA, all 15 amp receptacles are actually rated at 20 amps (just not configured to accept a 20 amp plug). So if you have 20 amp wire (usually #12 if the special cases requiring derating do not come into play), you can use a 20 amp breaker (one more exception in the code is if the branch circuit has a single dedicated outlet, it must be protected at the outlet designated amperage).

Since the normal type of outlets don't have a 30 amp capacity, you can't use a 30 amp breaker on them even if you have 30 amp wiring (usually #10), and meet the electrical code. The receptacles can overheat, too. You may need to split circuits if this branch is pulling too much current.

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How is it safe? A dead short in the #14 branch will draw up to 30 amps of current through the #14 wire. –  Bryce Dec 5 '13 at 6:21
    
@Bryce: A dead short on the #14 branch would pass a lot more than 30 amps, but would only have to do so long enough to trip the breaker. A worse case would be a load that took about 45 amps--severe enough to overhead the wire, but not trip the breaker quickly. There are some cases where a circuit may contain wiring which would be insufficient to carry the full circuit current (e.g. many chandeliers use 18ga wires to the individual lamps), but it's unlikely that they would have to carry anything between rated current and instant-trip current. –  supercat Jul 18 at 16:42

One important thing to remember is that appliances don't always run at full load and therefore don't always consume full power. So it's quite likely that the described setup will run okay for ages. I've seen many circuits running on undersized wires - they didn't even heat up enough to be notices by touching the insulation with one's hand.

Yet the code is there to ensure safe operation with some safety factor. In your case maximum ampacity is 20 amps, so the wire will run at up to 50% overload sometimes. You may be lucky and get away with it or you may be unlucky and get a fire started.

The only way to ensure safe operation is to use wire with at least the required ampacity. In your case it is to replace the undersized segments of the wire.

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Most likely the breaker is oversized in which case it would be easier to just replace the breaker with a smaller one, verus ripping out all of the wiring and replacing that. –  Steven Jan 16 '12 at 16:24

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