Is it possible to use the 240V wiring for a dryer to also run a compressor?

Question

We have a laundry room adjacent to the garage. The dryer uses 240V, I'm not sure what breaker (amperage wise) it uses, but I'm going to assume 50A (washer and dryer are near each other, they're probably on the same breaker).

My husband bought a compressor that requires 240V as well. It is located right across the wall from the laundry room. It is maybe 10 feet away from the dryer plug.

Would it be possible to reuse the wiring that was done for the dryer to run the compressor too? Of course the breaker would be replaced to a bigger value. Or is the wire gauge likely too high for this?

Running wiring from the panel (that is in the basement) to the garage is about $1000, so I'm just curious if there is a way to minimize the expense.

We aren't planning to do the wiring ourselves, but is it possible to even convince an electrician to do this? Or is this a terrible idea?

The house was built in 2002, so it probably has reasonable wire quality.

Answer 1

It depends on your existing dryer receptacle

First, any electric dryer circuit will be 30A. Dryers require two hots (for 240V) and a neutral.

Obsolete dryer circuits have a 3-pin plug connection (NEMA 10-30), which does not provide any safety ground. If anything goes wrong with the neutral connection, the dryer chassis will be electrified. This is known to be unsafe, but is grandfathered on the rationale that the sockets are rarely disturbed.

Modern dryer circuits require a 4-pin plug connection (NEMA 14-30) which provides safety ground.

Your compressor does not need neutral but does need safety ground. Neutral is not ground and they cannot be interchanged.

If the dryer is a 3-pin plug/socket

First, open up the dryer receptacle junction box and see if a proper ground wire (distinct from the neutral wire) is hiding back there. Sometimes, ground is present, but someone has downgraded the socket from NEMA 14 to NEMA 10 because the buyer complains that it deoesn't match their 3-prong dryer plug. (this is illegal; the correct course is to change the plug to 4-prong). Ground may exist either as a separate wire, or as a metal conduit pipe back to the panel. So if that's possible, simply change the socket.

Second, ground can be retrofitted. A #10 wire can be run via any achievable route back to the service panel the dryer breaker is in, or any other junction point which has a #10 wire back to the panel, or any part of the grounding electrode system (the copper wires going out to the ground rods).

• However this only works if the neutral in the cable is correct; it must be either white or a bare-wire weave which wraps around the other conductors. In the latter case, it must be carefully insulated so it does not touch real ground. In this case, the receptacle should then be changed to NEMA 14-30.

Third, the wires can be left groundless if a 2-pole GFCI breaker is used to feed the dryer. Here, the receptacle can be changed to 14-30 even though ground is disconnected. The receptacle must be labeled "GFCI Protected. No Equipment Ground."

In all cases where the socket is changed to NEMA 14-30, the dryer plug must also be changed to 14-30. When this is done, you must remove an internal strap on the dryer which connects neutral to ground.

You can plug the compressor into a NEMA 14-30 socket

If you do, you'd change the compressor plug from NEMA 6-30 to NEMA 14-30, but it will be fine.

Keep in mind these large 30A plugs and sockets are not rated for frequent plug/unplug, and will tend to break. That is why it's so important not to use an obsolete, dangerous ungrounded NEMA 10-30 circuit.

You can add a second socket to the circuit

Nothing in Code prohibits placing two or more 30A receptacles on a 30A circuit. This means the dryer and compressor could be plugged in at the same time. It goes without saying that using both at once will trip the breaker. Don't do that.

The second socket can be a NEMA 6-30. Note that if your dryer outlet is "GFCI Protected/No Equipment Ground", then the additional outlet(s) need the same mark.

Answer 2

You need a dedicated receptacle for a clothes dryer. As far as I have been able to find there is not a specific section that calls out clothes dryers like the sections that call out Electric Ranges, but as you Monkee-walk your way through the 800+ pages of the code you will find you need a dedicated circuit. Most significant called out is Sections 110.2/90.7 that requires equipment be installed as Listed. Listing (UL,ETL,CSA) is only valid when used as instructed in installation instructions and owners manuals. The instructions with every clothes dryer calls out a dedicated circuit.

The second way you cross into the dark side is the code requires a calculation for breaker and wire size by adding 100% of resistive loads and 125% of motor loads to determine circuit size, so you have a 24A dryer and a 24A compressor, you need a circuit rated for 60A, then the code only allows a 30 amp receptacle on 30A breakers, and 20A receptacles on 20A breakers.

Then lets look at the safety issues prevented by that code, if you had a dryer that actually uses 24A on a 60A breaker go short circuit and draw double the current it wouldn't trip! And based on the tripping time curve of a circuit breaker it could draw 4 times the designed current (96 amps)and NEMA specs show it could take 2 minutes to trip. A fire could start in either of those situations.

To clarify a couple of your incorrect assumptions. Your washer and dryer are on separate circuits, the washer is called out as a dedicated circuit. The smallest wire allowed for a 30A dryer is #10 AWG, and that minimum size wire is always used, you can never upsize the breaker.