Why does my 240 V dryer affect these voltage measurements?

Question

Why would my 240 V dryer outlet show the proper voltage when nothing is plugged in, but when it’s plugged in, all 240 V go to one leg and 3 V on the other leg?

We had an old dryer that just stopped working last week. No electronics worked on the dryer. We bought a used dryer with a three-pronged cord and plugged it in. It also did not work. We had an appliance guy come out and tested continuity which appeared to check out. He said it was most likely an issue with the outlet or the breaker.

We tested the outlet and when nothing is plugged in, the hot-hot test at 240 V, hot-neutral both test at 120 V. However, when I plug in the dryer and test the cord, hot-hot still test at 240 V, one leg hot-neutral test at 3 V and the other hot-neutral test at 240 V.

I had an electrician check out the dryer and the connections (through text and my sending him pictures) and he says the issue is with the dryer. “The circuit board is throwing a short”. Here’s what I’ve done:

1. I exchanged the old three-pronged outlet for a new three-pronged outlet
2. Tested the breaker (no issues)
3. The electrician had me test the plug whip itself (see pictures) and based on the readings here he determined that the issue is with the dryer.

Everything I’m reading and watching points to an issue with a disconnected or loose neutral…which is in fact the case. The outlet has four wires: two hots, one white neutral, and one green ground.

The two hots are connected to the hot legs, the green ground is connected to the neutral spot, and the neutral is loose in the back of the box. Why is that? Is that causing the issue? And if so, why now?

Answer 1

3-wire dryer connections are dangerous

Especially when neutral fails.

As you measured, starting the machine pulls neutral from the middle to the voltage of one hot leg (because neutral is disconnected somewhere, and the 120V loads inside the dryer are pulling it to the phase). The middle is also ground. So neutral is being energized at 120V to ground. The trouble is, on a 3-wire dryer connection the chassis is hardwired to the neutral wire. (which is a very stupid and dangerous idea, but this was Code prior to 1996, and it still applies to older installations like yours.)

Since neutral is connected to chassis of dryer, this failure energizes the chassis of the dryer. You are very lucky you haven't been killed. Don't push your luck.

Convert to 4-wire

Since you actually have 4 wires in your dryer wiring in the walls, you should convert this to a 4-wire connection ASAP. This will eliminate the shock hazard of a lost neutral.

It is important, when changing to a 4-wire dryer cord, to follow the wiring instructions in the dryer's manual. The strap which connects dryer chassis to neutral must be removed.

The mess-up with the ground is also dangerous. Obsolete it.

It is bad to have an old 3-prong NEMA 10-30 receptacle wired with ground on the 3rd pin. That is not ground, it is neutral. I suspect that your outlet has been miswired this way for quite some time, it finally broke, and this is what caused the dryer to stop working. In this setup, it could not only electrify the dryer, it could electrify grounds around your site! This should be corrected at once.

But this will just happen if you follow the above advice and convert to a 4-wire receptacle and cord.

Otherwise, correct the wiring so the 3-wire plug is hot-hot-neutral. This type of connection does not connect ground, which is why it's bad.

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Edit: Regardless, review the breaker panel end of all 4 wires to make sure they are on the correct bus. Actually, on the neutral and ground, remove them altogether from the bus, inspect, clean and reattach. Use a torque driver to set the correct screw torque (this is where it usually goes wrong).