Discovered 10/2 wired 3-prong receptacle for dryer. Is it ok to leave as is?

Question

We removed a drop ceiling in our 2nd bathroom and discovered that some previous homeowner/DIYer had run wiring for a new dryer through the drywall ceiling into the wall, and hid it with the drop ceiling - rather than drill through the top plate of the wall. We shut off the breaker, unhooked the receptacle, drilled, and ran it through the top plate. The wire is 10/2. I am attaching a photo of how it was wired. Ground is screwed into the metal box, and then into the neutral prong. When we reconnected it, we re-wired it exactly as it had been. Dryer came with the house when we moved in 2 years ago. We have never had any problems with it. Dryer is on a 30A breaker all by itself.

Is it ok to leave it as is?

Answer 1

Replace it with a homerun of 10/3 and a NEMA 14-30. It's the only way to be sure.

The original installer was a massive cheapskate with no understanding of the NEC -- even if the install was old enough to be grandfathered in under NEC 250.140 and friends, it still was a violation at the time of install as using an uninsulated wire for the neutral (grounded conductor) is only allowed when using type SE cable, not type NM as the original installer did:

50.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.

Exception: For existing branch-circuit installations only, where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met.

(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.

(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.

(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.

(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

So, replace this abomination with a fresh homerun of 10/3 and a NEMA 14-30 receptacle -- you'll have to change the power cord on your dryer to a four-prong one, but that's a small price to pay for never having to worry about getting bit by your dryer again.

Answer 2

This is an outdated wiring method where you're just bonding the frame of the dryer to the neutral/ground wire. As long as the wire was intact, you would have no problems. However, if for some reason a short occurs and there is an issue with this wire, the frame will remain energized = bad.

In modern code, for new work, it's required that you have separate ground and neutral wires.

If you're messing with it and can update it, then I would. Run the new 10/3 wire and provide a new receptacle.

If you plan to leave it, then there are exceptions in 250.140 (NEC 2014) that cover this case.

Answer 3

The old 3-prong NEMA 10 receptacles, and their associated wiring, are bad news. They do not provida ground, and bootleg ground using neutral. The problem is the same as any bootlegging: if neutral breaks, things which are supposed to be safety-grounded are now danger-hot.

This kills people, and it's unreported in the papers because it is called "miswiring" when it is in fact proper wiring that simply failed. Wires fail, and they shouldn't fail deadly, which is why it is no longer proper - and hasn't been for 40 years. It is grandfathered, allowed to continue in service where already installed, that as a result of appliance industry lobbying.

The last guy felt Very Strongly about saving $3.00 on buying 10/2 instead of 10/3, so he used the "old style" of wiring instead of the modern and proper NEMA 14 which has separate ground. I hope that frappucino tasted good.

Or what often happens is folks get a used dryer, and it already has the NEMA 10 plug, and so they just fit a NEMA 10 socket instead of a proper NEMA 14 plug. Like measles resurfacing after vaccines beat it, NEMA 10 just won't die.

What to do

Replace the wiring with 10/3 obviously.

Get a NEMA 14-30 plug and socket.

Remove the jumper on the dryer so neutral and ground are no longer connected at the dryer.

Answer 4

It is not okay to leave it as is. Replace it with 10/3 (black, red, white, plus ground) at the very least. Install a new grounded receptacle on top of that at the very best.