Melted Neutral Mystery

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brycenesbitt

Senior Member
Location
United States
I was asked to inspect because the CFL's blew up. The HO reported the lights had been flickering for days.

Sure enough, the meter showed 240 volts on the nearest receptacle. The open neutral was easy to find: it was the one with the melted insulation and that sort of brightly colored oxidized copper color. It snapped on touch, before I could take any measurements.

But here's the part I am confused about. This was a multiwire branch circuit with a common neutral. The two breakers were untied and on the same phase. That could account for the melted wire, especially since the neutrals in the panel were all shoddily installed and loose. 15 amp + 15 amp is 30 amp onto a 14 gauge wire.

---

But one of those breakers was tripped.
So where could the 240 volts have come from? What was the circuit path?

--

Background:
House was built 1935. The problem feed left the subpanel on bx cable, and disappeared into a closet, reappearing on the knob & tube running all the interior lights plus most of the outlets. The HO reports the closet used to hold a fuse box. But if so, could there have been only two fuses? Would four or six not have been more common?

The K&T had been hacked to death throughout the house: extension cord quality wires hand wrapped onto the k&t, a section of k&t wire removed and run through an empty section of bx during a remodel, dangling chopped circuits, failed vinyl electrical tape patches everywhere, and a particularly nice added ground wire thoughtfully connected to a 3 food section of copper water pipe dangling in mid air.

The original K&T, of course, was in perfect condition. Though I did find an 1935 error: one missing tube, and a period "hack" to provide a replacement with wire sheathing.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Through a load connected to the other phase.

But that load would have to be connected on the load side of the break?
Which would be another code violation.
One of the problems working with K&T is that there may be only one neutral "web" instead of individual branch home runs.
Makes installing GFCI breakers a lot of fun!
 

kwired

Electron manager
Location
NE Nebraska
But since the other phase's hot breaker was tripped, how could any current from that phase pass through the common neutral?
A circuit from a third breaker is tied into the neutral for this circuit somewhere.

Turn off the breakers you have located, check voltage again - to a remote known grounded point. You probably will read 120 volts from the neutral in question to ground - it is fed from whatever is still a part of the messed up situation - circuit tracer may help with sorting things out.
 

growler

Senior Member
Location
Atlanta,GA
I was asked to inspect because the CFL's blew up. The HO reported the lights had been flickering for days.

Sure enough, the meter showed 240 volts on the nearest receptacle. The open neutral was easy to find: it was the one with the melted insulation and that sort of brightly colored oxidized copper color. It snapped on touch, before I could take any measurements.

But here's the part I am confused about. This was a multiwire branch circuit with a common neutral. The two breakers were untied and on the same phase. That could account for the melted wire, especially since the neutrals in the panel were all shoddily installed and loose. 15 amp + 15 amp is 30 amp onto a 14 gauge wire.

---

But one of those breakers was tripped.
So where could the 240 volts have come from? What was the circuit path?

--

Background:
House was built 1935. The problem feed left the subpanel on bx cable, and disappeared into a closet, reappearing on the knob & tube running all the interior lights plus most of the outlets. The HO reports the closet used to hold a fuse box. But if so, could there have been only two fuses? Would four or six not have been more common?

The K&T had been hacked to death throughout the house: extension cord quality wires hand wrapped onto the k&t, a section of k&t wire removed and run through an empty section of bx during a remodel, dangling chopped circuits, failed vinyl electrical tape patches everywhere, and a particularly nice added ground wire thoughtfully connected to a 3 food section of copper water pipe dangling in mid air.

The original K&T, of course, was in perfect condition. Though I did find an 1935 error: one missing tube, and a period "hack" to provide a replacement with wire sheathing.

Sounds like this house is ready for a complete rewire.
 

brycenesbitt

Senior Member
Location
United States
I agree. K&T's gotta go!

I have no appetite for recommending replacement of the knob & tube: the payback is just too small. This is especially true for the overhead lights: the plaster is intact everywhere, which means nobody's messed with the original wiring outside the basement. If rats ate the K&T insulation, that causes no problem, and with new efficient bulbs the overhead light circuits are way under-amped. I've looked for but not found any credible reports of fires caused by undisturbed K&T (but if you know of some, do post).

The modern wiring is a complete mess with every sort of horror.

Pictured is the burned common neutral, and the panel it burned in:

Burned Shared Neutral 240VAC USA.jpg

20738617435_3e5d778a58_b.jpg

Question: could that neutral have overheated slowly over time, simply because it was not tight at the panel?
 

growler

Senior Member
Location
Atlanta,GA
Question: could that neutral have overheated slowly over time, simply because it was not tight at the panel?

Sure, you will find lots of burned neutral conductors just because of loose connections. They can even get hot enough to burn the wires that are close to them.
 

growler

Senior Member
Location
Atlanta,GA
House was built 1935.

I have no appetite for recommending replacement of the knob & tube: the payback is just too small. This is especially true for the overhead lights: the plaster is intact everywhere, which means nobody's messed with the original wiring outside the basement. If rats ate the K&T insulation, that causes no problem.


What, no knob and tube in the attic?

Has anyone checked with the insurance carrier to see how they feel about it?
 

brycenesbitt

Senior Member
Location
United States
What, no knob and tube in the attic?
Has anyone checked with the insurance carrier to see how they feel about it?

Yes, there is K&T in the attic for all the overhead lights and certain top floor outlets. That's untouched and in good condition. That's two circuits, center neutral, 12 gauge trunk and 14 gauge branch wiring.

The house is in an area with majority K&T, the California exception to the insulation over K&T rule, and a local ordinance requiring certified electrical inspection of the K&T. Insurance is readily available in this area for K&T homes.

The modern upgrades on the other hand are a complete train wreck. As far as I'm concerned the pictured subpanel is a complete and immediate loss: the neutral screws are too damaged. A different new modern subpanel is professionally wired, but nothing is labeled, and it feeds the 14 gauge ground level knob & tube with a 20 amp breaker. One sub-panel ties neutral to ground by hapenstance, and has no other apparent ground (it's possible the intent was for the feeder bx cable to be the ground but that connection was interrupted at the main panel end). Screws and cover plates are missing all over the place. Extension cords inside and out are hardwired in, and all the CFL's and florescent fixtures were blown up by the overvoltage. The wet areas have no GFCI's, the washer outlet has no ground, there is random chopped wire sticking out here and there, and every modern box I opened up had some tangle of crap that made me want to inspect with the power off and a flashlight.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Yes, there is K&T in the attic for all the overhead lights and certain top floor outlets. That's untouched and in good condition. That's two circuits, center neutral, 12 gauge trunk and 14 gauge branch wiring.

The house is in an area with majority K&T, the California exception to the insulation over K&T rule, and a local ordinance requiring certified electrical inspection of the K&T. Insurance is readily available in this area for K&T homes.

The modern upgrades on the other hand are a complete train wreck. As far as I'm concerned the pictured subpanel is a complete and immediate loss: the neutral screws are too damaged. A different new modern subpanel is professionally wired, but nothing is labeled, and it feeds the 14 gauge ground level knob & tube with a 20 amp breaker. One sub-panel ties neutral to ground by hapenstance, and has no other apparent ground (it's possible the intent was for the feeder bx cable to be the ground but that connection was interrupted at the main panel end). Screws and cover plates are missing all over the place. Extension cords inside and out are hardwired in, and all the CFL's and florescent fixtures were blown up by the overvoltage. The wet areas have no GFCI's, the washer outlet has no ground, there is random chopped wire sticking out here and there, and every modern box I opened up had some tangle of crap that made me want to inspect with the power off and a flashlight.
I agree with you that the mere presence of K&T is no reason to wave your hands in the air wildly and scream, "Rewire! Rewire! This place, and all who inhabit, are doomed to a fiery death!

On the other hand what you are describing in terms of Modern Upgrades were not done by a competent electrician, so it's not an accurate comparison.
 

david

Senior Member
Location
Pennsylvania
Its been my experience homes with knob and tube always have upgrades with evidence of different period wiring methods in the basements or attics.

Homes with the original knob and tube have few circuits as compared to more modern uses.

I see bedroom window AC equipment two or three units on the same circuit. Fuse panels with over fusing and almost always evidence that a fuse socket has been over heated. or damage in some way

I have yet to see a home that has the original knob and tube that has not been updated in someway
 

kwired

Electron manager
Location
NE Nebraska
I have yet to see a home that has the original knob and tube that has not been updated in someway
Me neither, but you will find certain areas that have been untouched in most of them. Especially the K&T that is ran in between floors of a multi-floor home and the original ceiling is still intact. Getting rid of the cobbled on updates and leaving those original portions alone is not normally a problem in my book.
 

growler

Senior Member
Location
Atlanta,GA
If rats ate the K&T insulation, that causes no problem.

Me neither, but you will find certain areas that have been untouched in most of them. Especially the K&T that is ran in between floors of a multi-floor home and the original ceiling is still intact. Getting rid of the cobbled on updates and leaving those original portions alone is not normally a problem in my book.


How can you tell the condition of the insulation without a visual inspection? Megger won't do much good. If you open for inspection you may as well replace it.

If the insulation is brittle and falling off or rat chewed it may not be a fire hazard but it can still be a shock hazard. Those conductors are required to be insulated.

What about the condition of the conductors at the fixture junction boxes? Anywhere there was a ceiling mount type fixture there are normally conductors that are so brittle that the insulation is falling off. In those old fixtures people used 100 watt lamps and there was no insullation between the fixture and the junction box.

The problem I see with leaving any Knob & Tube wiring is that when a house is sold you are pretty much forced to disclose the fact that there is still old wiring in the house and that it has not been completely rewired.

I have never seen Knob and Tube wiring in a house that was 80 years old that was in good shape. It could be the heat here but even the areas between floors gets hot even in winter because heat rises to the ceiling.
 

kwired

Electron manager
Location
NE Nebraska
How can you tell the condition of the insulation without a visual inspection? Megger won't do much good. If you open for inspection you may as well replace it.

If the insulation is brittle and falling off or rat chewed it may not be a fire hazard but it can still be a shock hazard. Those conductors are required to be insulated.

What about the condition of the conductors at the fixture junction boxes? Anywhere there was a ceiling mount type fixture there are normally conductors that are so brittle that the insulation is falling off. In those old fixtures people used 100 watt lamps and there was no insullation between the fixture and the junction box.

The problem I see with leaving any Knob & Tube wiring is that when a house is sold you are pretty much forced to disclose the fact that there is still old wiring in the house and that it has not been completely rewired.

I have never seen Knob and Tube wiring in a house that was 80 years old that was in good shape. It could be the heat here but even the areas between floors gets hot even in winter because heat rises to the ceiling.

If insulation has fallen off the conductor and it is still suspended between porcelain knobs and tubes - how is it a shock hazard? At least until someone does expose it?

Heat commonly does cause problems at luminaire locations, and still does even with 90C conductors sometimes.

I was in GA for 13 weeks in 1986 at Ft. Benning for Army Basic Training. I don't remember it being that much hotter then it gets here though it seem like hottest place in the world at times, but that wasn't necessarily because of where I was at as much as what I was doing. I really doubt the usual weather conditions have much effect here, unless you have a location that get higher then 150 - 160 degrees F.
 

growler

Senior Member
Location
Atlanta,GA
If the insulation is brittle and falling off or rat chewed it may not be a fire hazard but it can still be a shock hazard. Those conductors are required to be insulated.

If insulation has fallen off the conductor and it is still suspended between porcelain knobs and tubes - how is it a shock hazard? At least until someone does expose it?

Explain why you think it's OK to have bare conductors anywhere in a house. There is no way that you could say the house is "safe".
 
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