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I need to change the cord on an electric dryer from 4-prong to 3-prong. My questions concern 1) what to do with the ground when moving to the 3-prong dryer and 2) how to test whether the installation is working as it should using a multi-meter.

The dryer is a Bosch WTMC3300US/01. The relevant section of the manual is here: Manual page describing 3-prong hookup

The ground strap that the documentation mentions is not present nor is it available. I have fashioned a connection from 10 gauge copper wire. The new installation looks like this:

Proposed 3-prong setup

My research on the wisdom of this connection is all over the place, and I'm very interested in not screwing up. (One electrician I asked told me to connect the middle lead of the pigtail to the ground screw and leave the neutral empty. The most read post on this site says not to connect the ground and neutral. The manual says to use the ground strap, presumably connecting the neutral and ground.)

This is the wiring diagram for the dryer:

dryer wiring diagram

This is the original 4-prong configuration:

enter image description here

Thanks for your input.

EDIT: This is the inside of the current dryer receptacle, which is connected to a THQL breaker in the junction box (picture of that in the comments):

Inside of current dryer receptacle

techxorcist
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    What make and model is your breaker box? (That's the biggest determiner as to whether the other solution to this issue, namely using a GFCI protected NEMA 14-30 with a "No Equipment Ground" label as per 406.3(D)(2)(c), is practical for you) – ThreePhaseEel May 27 '20 at 23:07
  • Thanks for your question. The breaker box is a GE PowerMark Plus Load Center with the following information on the label:
    • Type 1 Indoor Enclosure
    • Front Cat. No.: TLM1212C

    Image posted here: https://photos.app.goo.gl/sEUKpRYGDo2WaKxL7

     – techxorcist May 27 '20 at 23:21
  • First, your picture didn't appear to post. Second, is the breaker for the dryer circuit a full-sized two-pole breaker (2" wide, THQL) or a half-sized two-pole breaker (1" wide, THQP)? – ThreePhaseEel May 27 '20 at 23:22
  • @ThreePaseEel would a THQP NOT perform adequately for this use? Would a THQL perform better? – Jim Stewart May 28 '20 at 00:58
  • @JimStewart The THQP is a double-stuff. GE has a unique approach where half-width breakers can straddle 2 spaces. – Harper - Reinstate Monica May 28 '20 at 01:43
  • @JimStewart -- there's just not enough room in the half-width THQP frame to squeeze in two breaker mechanisms and a GFCI electronics board :P – ThreePhaseEel May 28 '20 at 04:43
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    Can you post a photo of the inside of your dryer receptacle box please? – ThreePhaseEel May 28 '20 at 04:43
  • I have installed a 50-A 2-pole THQP in my GE panel to connect my whole house surge protector so I'm familiar with how they straddle two spaces (and learned they do NOT fit in one space), but I was wondering how the std THQP perform in comparison to the std THQL (not the GFCI version). I have a THQL 30-A GFCI that I have no yet installed to protect my 3-wire dryer connection. I cannot face putting in a new NEMA 14-30R and changing the cord to a 4-wire so I plan to put the jumper in the 4-wire position when I install the GFCI receptacle. – Jim Stewart May 28 '20 at 13:16
  • The breaker appears to be a THQL: https://photos.app.goo.gl/QSBLKmqmBEDfK4AX6 (Sorry about the incorrect link earlier.) – techxorcist May 28 '20 at 15:08
  • So you have a ground wire in the cable. You can install a 4-wire receptacle NEMA 14-30R and use your original cord. You could check the continuity of the bare copper gnd before you connect it. You could use a VOM between this bare copper and the gnd of the washer receptacle close by. – Jim Stewart May 28 '20 at 19:31

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So here's the deal with that 3-prong 4-prong thing.

If you hang around any time on this forum, you find hear about loose connections. "My circuit lost power". Oftentimes, it's "my circuit lost power, but the hot AND neutral are both reading as hot". That's because the loose wire is neutral. (And that's why neutral wires have insulation). These happen literally every week here.

Now, imagine someone didn't have a safety ground and decided to bootleg ground off of neutral. What happens when that neutral wire gets loose? It becomes hot, of course, but so does everything touching the ground.

Super deadly.

Now, in the 60s when grounding came in, appliance makers lobbied for an exception. They feared that a forced rewiring would interfere with appliance sales. "What about ground?" They got an exception cut in the rules that dryers and ranges could wire the chassis to neutral - in other words, bootlegging ground.

And it's exactly the same risk. Neutral wire gets loose, the chassis of the machine WILL energize at line voltage.

So even though 3-prong outlets are leeegal, we like to see them avoided at all costs.

I think ThreePhaseEel is going to come in with a few ways to do that, but the first thing you can do is open up the 3-prong receptacle and see if there isn't really a ground wire (or metal conduit back to the panel) back there. Often, the ground is present, but the builder changed it to 3-prong because the buyer said they had a 3-prong dryer.

Harper - Reinstate Monica
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Just change the receptacle out

Since you have all four wires (black, red, white, bare) there, there's nothing stopping you from changing that bad old NEMA 10-30R out for a nice, safe NEMA 14-30R. You can then put the 4-prong cord back on the dryer, remove the field-fabricated replacement bonding strap, button things back up, turn the breaker on, plug the dryer in, and enjoy your dryer, without having to worry about an open neutral causing your dryer to zap you!

ThreePhaseEel
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    Thank you. The NEMA 14-30R is installed and the dryer is running now. I appreciate the questions and advice on doing it right. – techxorcist May 29 '20 at 21:11
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What you have done is correct but whether it is safe is another thing. If I had to do this at gunpoint, I would have used a Stranded piece of wire with eye connectors at each end. That would give a much more solid connection at both ends without a wire wrapped around a screw over a eye termination.

To test the connections with a multimeter I'd go from the middle terminal to each end terminal and look for 120V, then from the two end terminals and look for 240V.

You should really do this according to current code and leave the four wire plug and run your existing feed into a new box and add a four prong receptacle.

JACK
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