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Our new dryer has 3 prongs vs. 4. We are changing to the new 3 prong dryer outlet, but want to make sure we are grounding. The wires coming from the wall our white, red & black - the the copper wire. Do I need to ground it?? Thank you! Molly

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    What make and model is the "new" dryer? – ThreePhaseEel Jun 17 '17 at 22:28
  • Can someone add pictures? What do you mean four prong. Ive never seen this and have been born in the usa i have no idea what you are talking about. – JonH Jun 18 '17 at 13:51
  • @JonH -- the OP is referring to the distinction between NEMA 10-30 and NEMA 14-30 – ThreePhaseEel Jun 18 '17 at 14:56
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Switch the cord, not the socket

You have a proper NEMA 14-30 dryer receptacle with a ground wire, which is good. However, your "new" (old) dryer has a 3 prong (NEMA 10-30) cord. This can be rectified -- get a 4 prong (NEMA 14-30) dryer cord and install it on the dryer in place of the existing cord, making sure to remove the neutral-ground bonding jumper in the dryer.

  • What's the fourth prong for? Is this a North American thing that uses two phases of a 110V supply for higher power? – David Richerby Jun 18 '17 at 11:12
  • There is a prong each for ground, neutral and 2 lives – ratchet freak Jun 18 '17 at 11:25
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    @DavidRicherby yes, in the USA we center-tap our 240V supply. And call that neutral and bond it to ground. This is not entirely unsimilar to how the UK provisions construction site power (110 center tap grounded). They're not really two phases, it's the same phase. We call it split-phase. We do not bring 3-phase "wye" down local streets. – Harper Jun 18 '17 at 14:19
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    There should be a wiring diagram on the dryer, or in the manufacturer's installation instructions. Make sure you follow the instructions for wiring the new 4-prong cord. – Tester101 Jun 19 '17 at 12:42
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You are not grounding, and this can kill you

I have no idea why you think 3-prong sockets are new. They are obsolete and dangerous. They are "exactly what they look like": the 3-prong outlet omits the ground, and the 4-prong outlet adds the ground.

Any problem with the neutral wire will result in the dryer frame being energized at 120V. Touching that and anything grounded, like the washing machine right next to it, will shock you.

It is illegal to convert a 4-prong to 3-prong. The home improvement store will happily sell you one, but they are only legal to sell for like-kind replacement which is not what you are doing.

The appliance manufacturers got a loophole put in Code so they could continue to sell appliances to old houses wired with the 3-prong outlets. In this case they bootleg ground, which is as bad as it sounds. Why on God's green earth the appliance store let one go out with a 3-prong plug is beyond me.

When reconfiguring the dryer for a 4-prong cord, make sure to remove the neutral-ground bootleg jumper. Neutral should be separate from ground.

The official name for the 4-prong is NEMA 14-30. The official name for the hazardous 3-prong is NEMA 10-30. Not to be confused with the safe but useless-for-your-dryer NEMA 6-30, which does have ground but not neutral.

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  • I suspect they got it used (garage sale, from a friend, etal) – ThreePhaseEel Jun 17 '17 at 23:54
  • Who lives in an older house. Well, even "old" (Read 15 year old) dryers will be made to take 4-prong cords. Though you can always steal the cord off the outgoing dryer... – Harper Jun 18 '17 at 3:05
  • Yeah, that'd do it. We're stuck with NEMA 10 over here :/ (50's vintage house) – ThreePhaseEel Jun 18 '17 at 3:12
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    @Xen2050 and all the other reasons to keep neutral separate, of which there are many. Grounding is not a copper industry conspiracy, at least that's what they pay me to say! The fact that they're bonded at the panel doesn't make them the same thing. It's merely to peg neutral at a near-ground voltage, to reduce its hazard, and this is only done in one place. Google "why are ground and neutral separate in subpanel" for more on that. . – Harper Jun 18 '17 at 14:26
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    And really nothing says neutral and ground must be bonded at the same voltage. If your supply transformer had a tap that was 3v off center, you could bond your ground to that, and there'd be a 3V difference between neutral and ground. This would still serve every purpose of the neutral-ground bond. The point is, you need to bias neutral to something, otherwise it's biased to nothing and you'll have an isolated system and it will rattle/float, potentially at 2400V if the supply transformer has primary-secondary leakage. – Harper Jun 18 '17 at 14:35
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I was just reading this and was going to comment about the "good old days" syndrome, when something hit me. Usually a dryer receptacle only has three prongs. I almost got caught up in the same argument, but a dryer has no need for a neutral it runs off of 240V not 240/120V and a standard receptacle provides slots for both sides of the 240V circuit and one slot for the grounding wire, and you usually run a 10/2 w/ ground romex to the receptacle.

I don't know why someone had a neutral for a dryer unless it was some sort of a special order. So he ordered a new dryer with the right configuration for a standard dryer. I would install a matching receptacle and wire nut off the neutral in the box.

  • Most dryers in the US run the heater on 240V but the timer and drum motor on 120V, sadly. I wish you could get dryers that plugged into a NEMA 6! – ThreePhaseEel Jun 18 '17 at 14:57
  • I have seen small dryers that were 120v very rare in the U.S. The heating element is 240v on most and is the big power consumer at close to 5Kw on most. – Ed Beal Jun 18 '17 at 15:09
  • I'm only going with the information that was given. @molly says it is a new dryer. That tells me it came with the right correct configuration. So the real check would be a the nameplate as required by NEC article 422 Appliances. Some run off 240V and some run off 240/120V. That would be the real place to check. – Retired Master Electrician Jun 19 '17 at 12:56

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