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

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wrote:

100-ohm and 200-ohm resistor in parallel across 120V.
Current through the path of least resistance = 1.2A
Current through the 200-ohm resistor = 600mA
Since this (200-ohm resistor) is not the path of least resistance, practical people WILL NOT be electrocuted :-)
BTW, I did not calculate 1 / ((1 / R1) + (1 / R2))
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wrote:

We are the AC Borg Your "path of least resistance" is irrelevant You are irrelevant You will be electrocuted
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Harry L wrote:

I'll die right!
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Are you sure you mean POLARITY there? The polarity is reversing 120 times per second. That sounds like "phase".

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Buerste wrote:

LOL, OP here. I took a couple EE courses when working on my MS aerospace engineering and did an internship where I designed one of the lightest 400Hz DC-AC inverter for aerospace use. I get the split phase, peak / RMS business quite fine. I'm just not totally up to speed with what's code and what's not -- and this rig seemed to be amiss.
I guess in the name of being pedantic, I'll point out that the two hots probably are not exactly "180 degrees" out of phase and "mirror images" of each other; close but in real life situations the reactive loads will not be [perfectly] identical across both phases, making the out-of-phase, well just a little bit different than pi radians :)
T
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What you are calling two hots are comming from a center tapped transformer. There is only one phase. They can not be out of phase with each other by any ammount not counting a couple of inches of wire from the transfromer windings to the load.
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Ralph Mowery wrote:

Think of the extreme where one H-N has a purely resistive load, and the other H-N has a highly reactive (say inductive) load, and remember that both the transformer and the wires have an inherent inductance. The voltage peaks as measured at the load will not be exactly 180 deg out of phase. T
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On Sun, 18 Jan 2009 22:04:22 -0500, "Ralph Mowery"

It depends on your point of reference. The normal point of reference (for wiring on the transformer secondary) is in the MIDDLE (center-tapped coil). Points are measured from that. That is, 2 phases.
What may be confusing to some, is these 2 phases are not 2 of the 3 phases being supplied to the transformer.
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The wiring comming into a normal house in the US is single phase. Not two phase. True two phase power has the phases 90 deg out instead of the so called 180 deg .
Two phase powe can be made from a 3 phase circuit, but it requires more than a simple center tapped transformer the normal house has.
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On Mon, 19 Jan 2009 19:13:57 -0500, "Ralph Mowery"

The problem with that is that "phase" already means something. Something that applies equally well to your (I mean the 90-deg apart one) 2-phase system, 3-phase power, and the normal system used in houses. Measurements are made in reference to a common point (ground) which is in the middle of that (residential) transformer. Measuring the ends of the transformer with a dual-trace scope will show TWO PHASES. In this case, one leg will show 90 degrees (most positive) while the other leg will show 270 degrees (most negative) at the same time. That's two different phases.

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Technicall it's called split phase.
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Tman wrote:

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.
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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
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Tman wrote:

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
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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.
--
Christopher A. Young
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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
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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
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RBM wrote:

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
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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
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