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Old 09-12-2008, 12:54 PM   #16
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Safety of old wiring


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Originally Posted by YerDugliness View Post
The old wiring was replaced in 1966, so it is possible that new outlet boxes were installed. I do know they covered all the old plaster & lathe walls with sheetrock at the time. If so, the GFCI outlets should fit, but I'll keep that in mind as I proceed. I'll probably try just installing the GFCI's in the current boxes first (the path of least resistance ), but I'm ready if I need to do some new wiring.

Would you mind explaining further your comment regarding the two hots sharing one neutral?

This house was rewired with 12/2 (no ground) "romex" (??--it is copper wire, appears to have two insulated leads, one black and the other white, with what I would describe as a tough, white rubberized plastic sheath covering both of the leads).

The house was rewired by a couple of handyman types, there were no electricians in the area back in those days. I'm certain it's pretty much straightforward as to the wiring, although the main house circuit is IMHO overloaded b/c it has both the kitchen and the living room on it. There are no switch controlled outlets or lights controlled by three-way switches--are those the circuits you mean when you mention "...two hots 'sharing' one neutral?

The house has a basement, so I do have access to most of the wiring, which was run in the ground floor joist bays. There is one, maybe two circuits that were run overhead on the main floor, but that section of the house has a dropped acoustic tile ceiling system so there ought to be access for them, too. I'm almost afraid of what I'll find when I get into the ceiling area....maybe no junction boxes, that sort of thing. My mom was well known for cutting corners--hence my original belief that I had a dilema with the GFCI outlets. She did do some things right, though--I was able to replace two ceiling fixtures with rather heavy ceiling fans without any support problems, the current ceiling boxes were firmly secured with 2 X 4 between the joists.....

Thanks for the help, and if you don't mind please expand on the two hots sharing a common issue.

Dugly
When I started into the electrical trade, grounded cable just became a requirement in my area. This was 1957!
The first home that I bought (new) in 1964 had grounded cable also.
I'm surprised that ungrounded cable was even available in 1966.
From what you say, if i were you, I would seriously consider a rewire job, rather than going the GFI route.
Access, without ripping walls apart, etc. seems to be available in your case!
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Old 09-12-2008, 12:59 PM   #17
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Safety of old wiring


A very basic definition of the multiwire branch circuit. Two or more wires on opposite hot legs sharing a neutral.

The typical circuit has 1 neutral for every hot conductor.

If the 2 hots wre from the same hot leg and shared a neutral the current on the neutral would be doubled. This could create a fire hazard due to overloading.
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Old 09-12-2008, 06:36 PM   #18
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Safety of old wiring


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If the 2 hots wre from the same hot leg and shared a neutral the current on the neutral would be doubled. This could create a fire hazard due to overloading.
Trying to wrap my mind around this one, bear with me.....

The window A/C unit is on a 30 amp double pole 240V breaker, it's hard-wired to the A/C unit with 10/2 w/ground. The two insulated wires, the black and white, are each connected to separate poles of the breaker, but the ground is connected to the same bus to which all the neutrals (and my grounds) connect. It seems to me I have two hots and one neutral--is this a dangerous situation?? This was a later addition, outside the house with conduit, so I suspect it was a real electrician that did that work.

Trying to figure out how this could happen with 120V--conduited individual wire systems, I guess...but pulling individual wires through conduit would give one the chance to pull the correct gauge neutral.....???

Thanks--let me know if I need to call an electrician regarding the A/C, please.....it's worked without any difficulty for 10 years, but you know how Murphy's Law works....

Dugly
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Old 09-12-2008, 07:02 PM   #19
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Safety of old wiring


Google multiwire branch circuit.



120V Two wire circuit. Power comes from one breaker to outlet via black wire and returns to panel on white wire.

120V Three wire circuits. Power comes from two breakers via black and red wire, returns to panel on single white wire.

They can "share" the neutral because it is alternating current and each leg is using it at different times, sixty times a second.



Your AC circuit is 240V. It doesn't apply. The ground is not a neutral.

Ground is for safety. It provides a path back to the panel.

Neutral carrys current "back to the panel".
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Old 09-12-2008, 08:03 PM   #20
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Safety of old wiring


Here we go again, in disagreement!
The case of the a/c unit NO current flows through the neutral if the load on each hot conductor is equal.

If the load on one conductor were 25 amps and 20 amps were to be drawn on the other then 5 amps will flow through the neutral.
This is why we can have a 3 conductor cable feeding a split receptacle. If a 1500 watt kettle is plugged into the upper outlet and a 1500 watt toaster is plugged into the lower one, no current will flow in the neutral at all.
Then if you unplug one, the remaining load current will then flow through the neutral.
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