30 amp dryer plug is hooked to a 40 amp double breaker, is this ok?

Question

I just bought a home and needed to buy a new washer and dryer. The new dryers need a 30 amp plug, currently has 50 amp plug. I replaced the plug but noticed it was ran to a double pull 40 breaker, is this ok or do I need to replace breaker?

Answer 1

The breaker is primarily in place to protect the wiring. So first thing you should evaluate is the size (gauge) of wiring on this circuit. For a summary of normal copper wire usage:

1. Breaker 50A Wire Size 6 AWG
2. Breaker 40A Wire Size 8 AWG
3. Breaker 30A Wire Size 10 AWG
4. Breaker 20A Wire Size 12 AWG

It is also recommended practice on dedicated circuits like this (circuit breaker via single branch wire run to single outlet) that the outlet be also sized to the wire and breaker.

Now with that said there is no real problem with changing your outlet to a 30A style and changing the breaker to a 30A two pole breaker as long as the wire is 10AWG or larger size. There would be a strong recommendation that you make sure to use a 4-prong type 30A outlet, as opposed to the 3-prong type, so that you get the safety ground connection all the way to the appliance.

Answer 2

As said in the previous answer, the existing wiring is probably 8 gauge; as long as it's 10 gauge, it's adequate. The receptacle should accept the wire either way - the terminals in the receptacles usually accept up to #4.

However it's actually more than a recommended practice in this case, it's in the code - NEC 210.21(B)(1) -

the breaker can't be 40 amp, it has to be swapped for a 30 amp.

Most dryers can operate with either a three wire cord with no ground, or with a four wire cord with a ground wire. If the existing receptacle did have a ground wire, you have to replace it with a four wire grounded type - simple.

Likewise if there's a ground available in the box, you'll replace it with a grounded type, even if the existing 50A isn't the grounded type.

If your existing receptacle is three wire / ungrounded, and there is no ground available in the existing box, the code does have provisions [see 250.130(C)] when replacing an ungrounded receptacle: you can use a new ungrounded receptacle, or run a ground wire to part of the existing grounding system and use a grounded receptacle. However since in this case you're changing the type of receptacle, not replacing with the same type, that may be a stretch. The safest bet would be to run a new 10/3 to the panel and connect it to the new 30A two pole breaker.