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10 gauge wire to 40 amp breaker for dryer


Bill Kibbel
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I inspected a house today with 10 gauge wires running to 40 amp breakers for the dryer. The wiring is newer Romex wiring and the outlet for the dryer is rated at 50 amps. The panel was signed off by the city recently. I called my electrician buddy and he says this is wrong. I am actually questioning whether it is wrong since the only connection is at the dryer outlet-- any help would be appreciated.

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I just got a new twist to things.

I did not realize that even though the receptacle and wire are rated for 90 degrees that the panel may not be. I did not even realize (or remember) that panels had a temp. rating that is typically 75 degrees. I checked my own home panel and there is no temp. rating on it, but I was just assured by another electrician friend that they do have temp. ratings for the panels-- too bad I did not check the panel for a temp. rating.

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I think NM-B is rated at 90°C.

10 gauge cu wire, rated at 90°C, is capable of handling a max of 40 amps. There are "adjustment factors" that changes the allowable breaker size to 30 amps. I think the adjustment factors are due to things like cramming 2 or more wires through a hole in a top plate and then draft-stop material gets crammed in there too.

At least that's how it is in my brain this late at night.

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Originally posted by Brandon Whitmore

I inspected a house today with 10 gauge wires running to 40 amp breakers for the dryer. The wiring is newer Romex wiring and the outlet for the dryer is rated at 50 amps.

That sounds off from the get-go. I've never seen a 50 amp dryer outlet. My guess would be that someone used a range outlet in place of a dryer outlet for some reason (and the wrong power cord to match), but in the endless race for bigger and better everything I can't totally rule out a 50 amp dryer.

I'd be questioning whether the outlet is correct for the dryer, and if it isn't go for the normal 30 amp outlet with the normal 30 amp breaker feeding it.

Brian G.

Mega-Dryer Has Arrived? [?]

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The installation that Brandon describes is wrong because the 40-amp breaker is too large to protect the #10 wires.

When using the ampacity tables, remember that:

1. Any circuit under 100-amps uses the 60-degree column. Doesn't matter what kind of wire, terminals, panel, etc. If it's under 100-amps, use the 60-degree column. (110.14©)

2. If it's Romex, use the 60-degree column to determine ampacity. Even though the insulated conductors inside the romex are rated at 90-degrees, the cable assembly is only rated for 60-degrees. (334.80)

3. Overcurrent protection (different from ampacity) can't be more than 15 amps for #14 copper wire, 20 amps for #12 copper wire and 30 amps for #10 copper wire. (240.4(D))

Bottom line: that #10 wire is only supposed to carry 30 amps and it's supposed to be protected by a 30-amp breaker.

- Jim Katen, Oregon

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Originally posted by Neal Lewis

Any exceptions on the wire/breaker size for an A/C condenser unit?

There are no exceptions for an AC condenser unit. However, remember that the circuit breaker on one of those circuits is *not* the overcurrent protection so the breaker can be larger than you would expect.

The motor provides the overcurrent protection in this case.

- Jim Katen, Oregon

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  • 12 years later...
3 hours ago, Loma said:

I have an ac unit that has a max fuse or circuit breaker of 50amps connected with 10 gauge wire and a 40amp breaker. This is installed by the builder before moving in. Is this correct?

Depends on whether or not this 'AC unit' has a manufacturer data plate with the minimum circuit ampacity, maximum breaker size information on it. All AC appliances with more than one motor should have this data plate. Where present, compliance with it would be a correct installation.

Edited by Marc
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  • 3 months later...
17 hours ago, plummen said:

Why is a dryer connected to a 50a recept to begin with?

Was the inspection sticker actually for the dryer hookup,or was it hacked together after the inspection was done?

Every pre-1978 dryer receptacle in my area is wired with a 40-amp breaker, #8 cable, and a 50-amp range receptacle. (Except for those where they used larger aluminum wire.)  All I can figure is that pre-1978 dryers called for 40 amp breakers. It's standard practice to use a 50-amp receptacle with a 40-amp breaker. 

As a home inspector, when I run across these (at least a few times a week), I recommend swapping out the 40-amp breaker for a 30-amp breaker to comply with the requirements of 21st century dryers, and to swap out the 50-amp receptacle for a 30-amp receptacle, to match the breaker. (I don't generally recommend upgrading to 4 wires unless it looks like it would be really easy to do so.) 

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1 hour ago, Jim Katen said:

Every pre-1978 dryer receptacle in my area is wired with a 40-amp breaker, #8 cable, and a 50-amp range receptacle. (Except for those where they used larger aluminum wire.)  All I can figure is that pre-1978 dryers called for 40 amp breakers. It's standard practice to use a 50-amp receptacle with a 40-amp breaker. 

As a home inspector, when I run across these (at least a few times a week), I recommend swapping out the 40-amp breaker for a 30-amp breaker to comply with the requirements of 21st century dryers, and to swap out the 50-amp receptacle for a 30-amp receptacle, to match the breaker. (I don't generally recommend upgrading to 4 wires unless it looks like it would be really easy to do so.) 

Electric dryers and water heaters here are all 30 amps. First I've heard of 40, 50.

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