OT Electrical Conundrum

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wrote:

Consider for example a duplex receptacle wired so that the top and bottom outlets are on opposite legs of the 240V service. Now plug something into each outlet and turn it on. If the neutral is no longer grounded, you've just completed a 240V circuit through that 120V duplex receptacle and the devices plugged into it. In effect, removing the neutral turns it from two loads in parallel on two 120V circuits, to two loads in series on one 240V circuit.

Your assumption is exactly correct. That's why it's so dangerous to lose the neutral in a multiwire circuit: because the entire circuit downstream of the failure goes to the highest potential available.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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snipped-for-privacy@milmac.com (Doug Miller) writes:

Ah yes, thank you. Didn't consider the effect of the load.
scott
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wrote:

Oddly, if there's no load, there's no problem. But the moment there's a load, there's a BIG problem -- it's basically the same as losing the neutral on the feed from the power company, except that it affects only two circuits, instead of all of them.
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Doug Miller (alphageek at milmac dot com)
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<...snipped...>

Quick example, suppose a shared neutral as you describe has say a cell phone charger that draws 20 watts on one circuit, and a 1500 watt heater on the other. Turn them both on and cut the neutral, now all of a sudden you are dropping almost the entire 240 volts through the cell phone charger since the resistance of the heater is so much less in comparison.
You can play with the E=IR etc. to calculate the actual voltage drop on each load, but the effect will be that the lower-wattage appliance will be exposed to a way-too-high voltage.
--
Make it as simple as possible, but no simpler.

Larry Wasserman - Baltimore Maryland - snipped-for-privacy@charm.net
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DonkeyHody wrote:

You've already located one potential fire hazard so in all probability there are others lurking. Even if you don't replace devices or change to pigtails I would certainly pull each device and change them from push wire to the screw terminals.
Pete C.
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No argument there...

.. but I'll give you one here. I'm pretty close to 100% certain that (a) the National Electrical Code requires no such thing, and (b) even the thirty-nine cent made-in-Mexico receptacles at Home Depot are UL-listed (and therefore Code-approved) for feed-through connections.
If you have information to the contrary, I'd like to see it.
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Doug Miller (alphageek at milmac dot com)
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On Fri, 05 Jan 2007 12:15:59 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

Nothing contrary actually, just that almost all electricians I know do it this way. Reason fewer call backs.
Mark (sixoneeight) = 618
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(sixoneeight)@hotmail wrote:

Yes, I know that. I agreed that it was best practice. I was disputing the claim that "most building codes require it."
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Doug Miller (alphageek at milmac dot com)
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DonkeyHody wrote:
<snip>
> Since the receptacle was old and tired and I already had it out of the > box, I replaced it and two other elderly and infirm receptacles on the > same circuit. Everything works.
> What was wrong?
My guess is that the old receptacles were of the "strip & stuff" residential grade.
Basically, you strip the insulation of the solid conductor wire, then stuff the bare wire in the hole in the back of the receptacle.
The wire is retained in the receptacle by using the Chinese finger puzzle technique on a spring. Basically, a one way clamp.
If the receptacle carries enough load for a long enough period of time, the spring gets tired.
Replacing the receptacles with a better grade that has side wired screw terminals should solve the problem.
One other possibility I really don't want to think about.
What is the age of the home?
Any chance we you have aluminum wiring?
Lew
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