Three-wire dryer outlet -- how can it be safe?

[Tman]

Took a look at the schematic today for the dryer (240v). I've always wondered how they serve up 240 V with 120V components (e.g. timer motor) on only three wires.

You see, I always thought that the grounded conductor should never be current-carrying, in the theory that if the ground feed should break, then the metal chassis of the appliance does not get energized.

Well looks like that "3rd" wire, is a neutral, and judging by the dryer schematic, is both used as a chassis ground and a current-carrying conductor for the 120V items on the dryer -- such as timer and drum motor.

OK -- am I totally wrong about the code and theory, or is this unsafe? Seems to me that dryer would get real "hot" should their be a fault in the neutral conductor to the breaker box.

I know I'm missing something here as this is SOP as far as I am aware. Someone please inform me? T

[Buerste]

Google: "Split Phase" In a nutshell, the two hots are 180 degrees out of phase with each other. The potential between either phase and ground/neutral is 120v. The potential between the two hots is 240v. A single phase of Alternating current looks like a sine wave on an oscilloscope with the wave swinging up and down. With split phase, it looks like 2 waves on the scope. As one wave is bottoming out the other is topping out. They are like mirror images of each other. If you measure the peak of either wave to ground, it is

120v, if you measure the top peak of one wave to the bottom of the other, it is 240v. There, clear as mud! Any 240v device will run on just the two hots, any 120v device will run on either hot and ground/neutral.

[Bob]

At one time the National Electrical Code (USA) allowed 120/240 volt ranges and dryers to have a shared grounding and neutral conductor if it was being fed directly from the service equipment. Because of the potential danger with an open neutral, the Code now requires four wires.

[Don Young]

This used to be permitted on residential dryers and ranges on the assumption that the robust connections would be properly made by a qualified installer and very unlikely to come loose. This was in the time before 120V grounding receptacles and small appliances were not grounded. There were (are?) some dryers with a center tapped heating element to provide the 120V for the timer. Not a very good design.

Don Young

[zxcvbob]

It's theoretically not as safe as having separate neutral and ground wires. But the unbalanced current is very small, the conductor is very large, and a 240/120V 3-wire circuit is only allowed when connected to the main panel (not a subpanel), where the ground and neutral are bonded to the same buss.

If the neutral wire were to inexplicably come loose, I *think* the timer relay would open and remove power to the drum motor. So the 120V on the chassis would be current limited by the series resistance of the timer. (I could be way wrong on this last point)

Bob

[Stormin Mormon]

Oddly enough, the third wire is not a neutral for the main

220 volt heater element. Since the current flows through the black wires, or black and red. But doesn't need a neutral.

The only time the third wire behaves as a neutral, if there is a 110 volt timer.

It's a bit strange, but seems to have worked for all these years.

[RBM]

The motors are generally 120 volt as well

[RBM]

It was acceptable when installed as others have described, it's no longer acceptable in NEW installations, but since there aren't piles of electrocuted housewives across the country, existing installations are still acceptable

[Tman]

My house was built in 2000, fwiw. Also, BTW there is a non-trivial 120v load through that neutral -- not just the timer motor, but also both main motors (mine is a combined washer / dryer stacked unit)... T

[Gary H]

Actually, the peak is 170V (RMS * 1.414) but you don't need reality to get in the way here :-)

340V. Shut up, reality :-)

[Wayne Whitney]

The term you want here is "grounding conductor", since "grounded conductor" is another (better) term for what is commonly called the neutral.

As others have mentioned, this arrangement was once allowed but is no longer allowed in new installations. Existing installations are grandfathered, so if you buy a new electric dryer, you should have a choice of cords: a 3-wire cord (with a bond on the machine between the chassis and the neutral) or a 4-wire cord (where the bond on the machine should be removed).

I'm not sure of the reason for the original rule allowing a 3-wire circuit, although I have heard that it was a conservation measure during World War II. If so, it was judged that the copper savings was more valuable than the increased risk in the event the neutral conductor was compromised.

Cheers, Wayne

[DerbyDad03]

re: "since there aren't piles of electrocuted _housewives_ across the country"

A rather sexist comment right there...unless you are implying that women are more sucsebtible to electrocution than men.

[Harry K]

Yeah. It took me some time to figure out how a shop was getting 240 on only two wires. Used to be common wiring back when.

Harry K

[RBM]

You are quite correct, it was not I that suggested it was only the timer motor. Clearly there have been few incidents as a result of the wiring method or the NEC would have demanded it be changed years ago

[RBM]

re: "since there aren't piles of electrocuted _housewives_ across the country"

A rather sexist comment right there...unless you are implying that women are more sucsebtible to electrocution than men.

Not at all, my reference merely dates back to the mid century when it was typically women that used these machines almost exclusively

[bud--]

I believe the NEC switch to 4 wires for dryer circuits was about 1996. I would think your house has one. (You may have to look in the box behind the dryer outlet to find out, or in the service panel.)

Any recent dryer would have an isolated neutral circuit that comes out to a connection terminal. The terminal is then jumpered to the dryer "ground" (or a jumper is available). For old houses where the dryer circuit is 3-wire combined neutral-ground, the jumper is left. For newer houses (which should include yours), the jumper is removed and a 4 wire cord is used.

IMHO manufacturers should make dryers and ranges that operate on 240V (do not need a neutral). It would eliminate the problems and confusion. Couldn't cost that much more.

[Tman]

I didn't read it as such at all. I'm sure a lot of men get electrocuted every day just as well --- while slaving away bringing home the bacon, so that wifey can have a nice hot meal on the table when he comes home!

T

[Van Chocstraw]

The third wire is neutral and the return path for the '2 hots'. Always was that way. Neutral can also be ground since it connects to the same place as a forth 'green ground' wire on new installs at the panel. Green is also a return path for the hot wires IF there is a short to the frame.

[Tman]

Well i looked closer and learned something today...

My house, built circa 2000 does in fact have a 4x (2H, 1N, 1G) receptacle (sp?).

The dryer, cheapest Kenmore unit bought in 2000 also and delivered by the dryer guy, has a 3 wire plug -- current carrying neutral-- jumpered to the chassis ground at the dryer end of it.

It's just that the 3-prong cord plugs into the 4-x outlet, leaving the central ground unused (and, until now, unnoticed).

Oh well, just one more reason to replace the clunker....

T

[hallerb]

yeah I look and wonder but things keep changing over time. in most cases its safer and more energy efficent