Thread: Undersized wiring

I have noticed many times that homes around here built 35 years ago or more often have 10-guage wiring going to 40-amp breakers for the range, dryer, etc. I have always written them up but most of these were approved and did appear to be original equipment. Was it ever code to have this size wire on a 40-amp breaker?? Did it change or has it always been that 10-guage is too small for a 40-amp circuit? I am talking solid copper wiring and not just for condensing units although I call those out as well.

Similar Threads:

• Transformer & Low Volt Wiring in Main Panel
• From the old board, Abandon Wiring
• Pool Light Wiring
• Protection of wiring in the attic
• From the old board, Knob and Tube Wiring

Originally Posted by Frank Bombardiere

I have noticed many times that homes around here built 35 years ago or more often have 10-guage wiring going to 40-amp breakers for the range, dryer, etc. I have always written them up but most of these were approved and did appear to be original equipment. Was it ever code to have this size wire on a 40-amp breaker?? Did it change or has it always been that 10-guage is too small for a 40-amp circuit? I am talking solid copper wiring and not just for condensing units although I call those out as well.

Well, 35 years ago they may have had 30 amp breakers for those? The 40 amp breakers appear to be original?

No, 40 amps was not allowed, even back then.

I can't tell if the breakers were original or not, but I have seen it so many times over the years that I have started wondering if it was once code or if they just allowed it as there are approval stickers on most of them and the panel has had no added circuits or signs of anything being changed. Thanks Jerry

Three of the last five homes I inspected, all 70's and 80's vintage, had dryer circuits comprised of 40A breakers and #10 copper wire. I called for further evaluation because a 40A breaker requires #8 wire, but I also thought this was too much of a coincidence so I contacted the ESA (Electrical Safety Authority).
They advised that, over some period in the 70's and 80's, it was acceptable to the ESA to have a 40A breaker on #10 wire for this application. The reason - some very powerful dryers were being produced over that period causing nuisance tripping of 30A breakers.
Dryers now must consume 24A or less and therefore only 30A breakers are allowed for the #10 wire used today.
It still leaves me a little confused about what I should be writing when next I find a 40A breaker on #10 wire in an older house.

Originally Posted by John Allingham

They advised that, over some period in the 70's and 80's, it was acceptable to the ESA to have a 40A breaker on #10 wire for this application. The reason - some very powerful dryers were being produced over that period causing nuisance tripping of 30A breakers.

John,

Let me understand this: The electrical authority, those in charge of electrical safety, actually allowed the use of 40 amp breakers on 30 amp wire instead of requiring 40 amp wire to match the 40 amp breaker for those larger clothes dryers?

I guess if you are the electrical authority you can make up your own rules, regardless how dumb they are.

Originally Posted by Jerry Peck

John,

Let me understand this: The electrical authority, those in charge of electrical safety, actually allowed the use of 40 amp breakers on 30 amp wire instead of requiring 40 amp wire to match the 40 amp breaker for those larger clothes dryers?

I guess if you are the electrical authority you can make up your own rules, regardless how dumb they are.

My sentiments exactly.

I'm assuming that the problem was tripping on start-up if you only had a 30A breaker. At start-up you have the highest motor draw and the highest draw from the cold element. Using a 40A breaker solved the problem.
To me it's not much different than using a 40A breaker on #10 wire in an A/C circuit which is OK if allowed by the data plate.

Originally Posted by John Allingham

I'm assuming that the problem was tripping on start-up if you only had a 30A breaker. At start-up you have the highest motor draw and the highest draw from the cold element. Using a 40A breaker solved the problem.
To me it's not much different than using a 40A breaker on #10 wire in an A/C circuit which is OK if allowed by the data plate.

It's a lot different.

The a/c compressor motor has a HUGE start up draw which last for some time as the compressor starts rotating and moving.

With a clothes dryer, the motor is a small motor and has a small start up current which does not last long, and the element cold start up is not as large and not for as long either, so neither should be a problem for a 30 amp breaker.

In you previous post you said "The reason - some very powerful dryers were being produced over that period causing nuisance tripping of 30A breakers", and that would be because they were really drawing too much to begin with on a 30 amp breaker, meaning those should have had a 40 amp circuit to start with. As you said "Dryers now must consume 24A or less", that's not a problem now.

With the a/c compressor, the start up current is much higher for much longer, which trips the 30 amp breaker but not the 40 amp breaker, then likely settles down to much less than 30 amp draw while running.