I've got a situation where I have mismatched wires to circuit breakers in my panel. The breaker for my range is 40A and heat pump is 50A. The electrician who discovered this (during an inspection) advised that they need to determine if the wires (30A) are undersized or if the breakers are oversized with considerable cost difference ($1250) if they have to run new wire.

I'd like to see if I can determine this myself in order to make an educated decision concerning the repair. Is this as simple as looking at the specs of the heat pump/range to see what they should be pulling? Or is it more involved.

How do i determine the amp rating of the wires themselves? A quick Google search indicates it's not only based on Guage but also length but how do I determine the length without knowing where the wires run in the walls/ceilings?

  • 1
    It's based on gauge, basically -- length is by far a secondary concern, and only really one for long runs such as to things way off in the yard, or to a detached structure (shed, garage, etal) Commented Nov 23, 2016 at 4:26

2 Answers 2


Look at the cable or wire. Any cable or wire legal to use in the US must be labeled at regular intervals. There's usually a lot of gibberish but amongst it will be something like 10 AWG or 8 AWG or 6 AWG.

You should also double-check to see if the wire is copper or aluminum. Look at the ends at the bare wire. Copper is the color of a penny. Aluminum is the color of a nickel. Aluminum wiring may also have markings like AL or AA-8000 or AA-1350. The AA-1300 series alloys are illegal for new work and widely considered dangerous. The AA-8000 series are legal but must be terminated in a special way.

The allowable circuit breaker size is defined by the size of the wire. It's fairly simple:

  • 14 AWG copper wire -- 15 amp breaker
  • 12 AWG copper wire -- 20 amp breaker
  • 10 AWG copper wire -- 30 amp breaker
  • 8 AWG copper wire -- 40 amp breaker
  • 6 AWG copper wire -- 50 amp breaker (higher in some cases)

(the pattern is not linear after this, it is also different for aluminum.) You cannot use a larger (more amps) breaker than the wire can support. Smaller is OK.

So step 1 look at your wire. Step 2 check your breaker. If your breaker is not allowed for the wire, turn the breaker OFF until you change it to the value allowed for the wire. Don't delay, shortcut or fool around. Do it right away. At this point the circuit is safe from overload.

Now that you know the allowed amps on the circuit, and have replaced the breaker to that value, it's time to look at the receptacle. Here's the table right out of the National Electrical Code.


Look at your circuit amps, and change your receptacle to one of the allowed sizes. (by the way, 50A receptacles are allowed on 40A circuits because there's no such thing as a 40A receptacle. If it ever did exist, code would allow it, but it doesn't. That's because there's already a glut of receptacle sizes, we don't need another.)

Your fixed wiring is now legal, in a safety sense. (it may not be provisioning the circuits that Code requires for new construction, but it won't burn your house down.)

Now, let's look at distance. All wires have resistive losses at high loads. The loss is "per-foot", and the above steps have made sure the loss is small enough per foot that nothing will get dangerously hot, regardless of the length.

But on a longer run, this can add up to significant voltage drop - the appliance may not get as much voltage as it wants, which can result in poor performance.

You can compensate for that by using larger wire. Here is a calculator to help you work that out. This is rarely needed in within-the-house wiring; it would have to be a pretty big house.

The next step is to figure out if your devices can work with that amount of power. I generally assume that previous people who worked on a circuit had a "method to their madness" and didn't do random stupid things. You may find your appliances are able to work with circuits of that ampacity - check the documentation and/or with the manufacturer.

For instance it's very common to have ranges which actually work fine on 40A, but are supplied with a 50A plug for reasons discussed above.

Or this may be a replacement appliance, and the original one did work with that ampacity. In which case you'll be able to find other appliances which will work. This is likely cheaper than pulling new cable.

Or you may be able to adapt your use of appliances so you can stay within circuit limits. This may or may not be legal. For instance if your heat pump has a multi-stage auxiliary heat system, you may be able to disable one of the stages.

  • This is a great answer! I had already figured out most of it from the other answer but this is very well put together and all in one place. Also pointing out that once the circuit is safe, being able to replace the appliance cheaper than the wire is a very good point.
    – kinar
    Commented Nov 26, 2016 at 15:12

There are two ways to determine wire gauge:

With calipers

You can use a pair of calipers to measure the diameter of the wire and look it up in a table -- this is best for wires that are unmarked or uninsulated. Turn the circuit off first though, just to be on the safe side!

By the labeling

Most (insulated) wires and cables have the wire gauge, voltage rating, insulation type, and other identifying information printed on the insulation/jacket. You should be able to read it right off the jacket or insulation, although it may take a bit of a Twister game (and a flashlight) to get it off wires that are inside a wall.

  • Ok, so I'm gonna assume the electrician did this when he determined that the breakers were oversized. I pulled the range away from the wall and on the back it says "when flexible cord is used, it must be 40 ampere". Is that referring to the actual appliance plug? Or the romex in the wall? Or both? The recipticle it is plugged into says 50A.
    – kinar
    Commented Nov 23, 2016 at 5:06
  • @kinar -- that's referring to the cord between the appliance and the wall. Commented Nov 23, 2016 at 5:08
  • ok so how can I determine if the wire in the wall/ceiling (assuming 30A) needs to be replaced? I mean, it makes sense to me that if the plug requires 40A, and the breaker is 40A, that the logical way I got into this situation is that a previous owner replaced the range and the breaker but not the wire. But I'd like to verify if possible before tearing apart my ceiling so the electrician can run new wire.
    – kinar
    Commented Nov 23, 2016 at 5:23
  • @kinar -- if it's 10AWG (i.e. 30A) wire, then it needs replaced. Commented Nov 23, 2016 at 12:31
  • 1
    @kinar -- Speedy's answer in that thread is the correct one -- a cooktop or single oven by itself usually requires a 30A circuit, while a full range is 40A minimum. Commented Nov 24, 2016 at 0:55

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