Checking for proper size wire

Here is some interesting information on wire diameters including the insulation… (since we don’t disconnect wires to check the bare conductor)

Newer 10 gauge wire measures around 3.6 mm including the insulation.

Older 12 gauge type TW wire measures around 4 mm including the insulation.

Be careful, a 30 amp breaker with nice fat wire in an older home could very well be type TW 12 gauge wire!

Note: the newer 12 gauge wire is around 3 mm including the insulation.

Bruce, are those dimensions good for all manufacturers of NM cable?

Where did you buy your caliper?

It would be nice if the manufacturers would help on this to be sure on the others.

Plastic Calipers are available at the online inspection tool vendors and northern tool has them too.

How is checking the ‘size’ of a wire within a visual inspection?

Are we now EE’s? And if you do not know what a EE is then you most certainly must not be inspecting electrical systems.

How is NOT checking the proper wire sizes at breakers OK for a normal home inspection? If we see a #14 on a 20-amp breaker, you are saying we shouldn’t report it? I guess we should also ignore a #14 on a 60-amp breaker according to you!

Mike…I believe an EE is something you constantly put in your mouth with these types of comments.

A 14awg on a 60amp breaker is not allowed but a 12awg on a 60amp breaker IS.

If I piss you off that is because you do not understand the code.

You made my point.


My first post was not clear.

I meant that you should not measure wires not look at their awg.

Can we play fair again?

](*,) I’m sure you can come up with some exotic situation where a #12 is Ok on a 60-amp breaker…just to prove your “superiority” over us lowly home inspectors, but this is a home inspectors electrical forum and it is generally understood that, unless otherwise stated, we are talking about NORMAL residential panels and circuits. And yes, I understand the code very well for those.

But you are correct on one issue…you do “piss me off” with your constant inference that HIs should do little more than stand in the middle of a room and stare at the receptacles…and you have been doing so for a few years now. It’s a very minor irritation though as, usually, I “consider the source” and ignore you. I think I’ll go back to that. Have a nice day.


I’ll give you that I am an ***. That said I took the time to become certified to make the comments that I do.

My posts are to keep inspectors from having their *** sued off.