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wire insulation discoloration


Jesse
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The white insulation on all of the neutrals in the subpanel has discoloration - brown - near the contact point at the neutral bar. Looks like overheating. Weird to see on every neutral, though. Other than one breaker double-tapped, no other issues. All suspect wires are same color and size - likely same spool of product.

Any ideas?

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It could be from a lightening (sp?) strike. There's a semi-famous photo of discolored neutrals that's been floating around HI sites for years now; some say too many per terminal, others say lightening.

Some kind of unbalanced load is the only other thing that comes to mind, but that's getting too techy for me. Wait & see what Jim K. can offer.

Brian G.

Techy Trekkie [:-alien]

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Originally posted by Jesse

The white insulation on all of the neutrals in the subpanel has discoloration - brown - near the contact point at the neutral bar. Looks like overheating. Weird to see on every neutral, though. Other than one breaker double-tapped, no other issues. All suspect wires are same color and size - likely same spool of product.

Any ideas?

A picture or two might be useful...

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Originally posted by Jesse

The white insulation on all of the neutrals in the subpanel has discoloration - brown - near the contact point at the neutral bar. Looks like overheating. Weird to see on every neutral, though. Other than one breaker double-tapped, no other issues. All suspect wires are same color and size - likely same spool of product.

Any ideas?

I can think of three possibilities right off:

1. The electrician forgot to tighten the neutral lugs. I've seen this twice. Both times, the wires were toasted to various degrees -- likely in relation to the loads that they carried. That is, they were all toasted, but not all equally toasted.

2. One of the neutral lugs was loose and the heat build-up from that connection spread outward in both directions toasting the neighboring wires. I've seen this several times. Each time, the degree of toasting decreases with the distance from the loose lug.

3. The connection between the busbar and its crossover bar was loose. Heat radiated outward, toasting the neutral wires.

I can't think of any situation where the neutral connections would become toasted without something being loose.

- Jim Katen, Oregon

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A while back, Mr. Doug Hansen actually covered this scenario at a seminar. He used an actual electrical panel as a prop to demonstrate.

The cause was an incorrectly wired multi-wire circuit.

I can't quite remember; forgive my attempt at an explanation and also my apologies for my lack of knowledge. . . I didn't quite understand it back then, either. I believe it was something to do with the breakers feeding the multi-wire from the same leg rather than alternating legs. The voltage potential back to the neutral was too high.

Jim, does this make sense? And, thank you for cleaning my butchered explanation.

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Originally posted by randynavarro

A while back, Mr. Doug Hansen actually covered this scenario at a seminar. He used an actual electrical panel as a prop to demonstrate.

The cause was an incorrectly wired multi-wire circuit.

I can't quite remember; forgive my attempt at an explanation and also my apologies for my lack of knowledge. . . I didn't quite understand it back then, either. I believe it was something to do with the breakers feeding the multi-wire from the same leg rather than alternating legs. The voltage potential back to the neutral was too high.

Jim, does this make sense? And, thank you for cleaning my butchered explanation.

A multi-wire circuit uses two hot lines and a single neutral. To properly install such a circuit, you connect each of the hot lines to a different leg of the panel. This way, current from one line cancels current from the other. If there were an equal load on each line, there would be no current on the neutral. If there were unequal loads, the neutral would only carry the difference.

Now if someone were to connect each side of a multi-wire circuit to the *same* leg, current on the neutral could double. This would cause it to become warm. However I think that the entire neutral wire would heat up, not just the busbar. I don't believe that busbar connections have enough resistance to become hot spots like that -- unless, of course, the connection were loose.

- Jim Katen, Oregon

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My bad. I mis-read the original dilemma and saw that all the neutrals were discolored, not just one.

Thank you Jim. I still need help 'cuz I still can't grasp the concept of how the loads on each line can cancel each other.

A multi-wire has a red and black *hot* wire: 120v goes out the black, 120v goes out the red; all the voltage returns on one neutral.

I can't get how connecting the red and black wires to alternating *legs* would change the voltage.

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Originally posted by randynavarro

. . . A multi-wire has a red and black *hot* wire: 120v goes out the black, 120v goes out the red; all the voltage returns on one neutral.

I can't get how connecting the red and black wires to alternating *legs* would change the voltage.

In the transformer at the street, there's a secondary coil. This is where the electricity for the house originates.

Remember that the "legs" in the service panel, orginate at the two ends of that coil . The "neutral" is a wire that's connected to the center of that coil.

Electricity is induced into the coil. The electricity changes direction 60 times a second. So when one end of the coil is pushing electricity, the other end is pulling. Then, 1/60th of a second later, they change and the first end pulls while the second end pushes.

Now let's go into the house. If you connect one leg to a contact on a light bulb and connect the other leg to the other contact on the light bulb, the light bulb will experience 240 volts and it will burn very brightly. In the transformer, you've got the whole length of the coil pushing and pulling electricity through that light bulb. This is how we get 240-volt electricity.

If you connect one leg to a contact on a light bulb and connect the neutral to the other leg, the light bulb will experience 120 volts and it will burn less brightly. In the transformer, you've now only got 1/2 of the coil pushing & pulling electricity through the light bulb. This is how we get 120-volt electricity.

Now let's take two of these 120-volt light bulb setups. Connect one to one leg and one to another leg. Each still has its own neutral. If we could see the electricity moving inside the neutral wires, we'd see that while the electricity in neutral one was moving one way, the electricity in neutral two was moving the other way. They'd change directions 60 times every second but always be opposing each other.

Now let's get rid of the two neutral wires in these two light bulb set-ups. Instead, we'll use a single wire. It connects to one side of each light bulb and it goes to the center point of the coil in the street. As the electricity is pushing on the first circuit, it's pulling on the second circuit. In the single neutral wire, the pushes and pulls cancel each other out and there's no voltage on the wire. This is a multi-wire circuit. (If you use a big light bulb and a little light bulb, the neutral will simply carry the difference in voltage.)

So here's a question to see if you've been listening. What happens if the neutral breaks or becomes disconnected?

- Jim Katen, Oregon

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Originally posted by randynavarro

Jim, great explanation. . . I think!!

Out the door right now. I'm such a picture guy--I'm going to draw that out on paper when I get back later today.

I may get this yet!!

OK, Picture Guy. Here's a drawing from Wiring Simplified (Richter & Schwan) that might help.

- Jim Katen, Oregon

Download Attachment: icon_paperclip.gif MW_Circuit_Diag.pdf

400.59 KB

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Originally posted by randynavarro

Most excellent!!

Now I understand. Had to use your verbal explanation with your drawing and my own bad sketches!

Thank you Jim. I trust I can return the favor someday!!

85mm, f1.8, Super-Multi-Coated Takumar lens in mint condition with both caps and case.

- Jim Katen, Oregon

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Originally posted by Brandon Chew

My money is on "the two 120 volt circuits now have 240 volts".

Yes. If the neutral breaks or becomes disconnected, the two 120-volt circuits now become one 240-volt circuit.

Taking this a bit further, what happens when the neutral breaks or becomes loose at the service entrance?

- Jim Katen, Oregon

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Originally posted by Jim Katen

Taking this a bit further, what happens when the neutral breaks or becomes loose at the service entrance?

- Jim Katen, Oregon

This situation is like the old fused neutral systems with a blown fuse on the neutral while there is still power to the two hot legs. Current flow will stop, and it will appear that the power is off to the entire house. But the entire system will still be energized, with that electricity waiting to follow a good path to ground.

The part of my answer that still has me scratching my head is "what role does the earth grounding system play in this situation?" Wouldn't the current continue to flow through the earth ground, making everything appear to be ok?

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Yes. If the neutral breaks or becomes disconnected, the two 120-volt circuits now become one 240-volt circuit.

So, using the transformer picture analogy, it it accurate to picture the neutral as a *gate*? Since the gate is gone, or the neutral is disconnected, the current now flows back and forth at 240v?

Taking this a bit further, what happens when the neutral breaks or becomes loose at the service entrance?

Boy, I'm gonna be stubborn: now the current stops?

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Originally posted by Brandon Chew

Originally posted by Jim Katen

Taking this a bit further, what happens when the neutral breaks or becomes loose at the service entrance?

- Jim Katen, Oregon

This situation is like the old fused neutral systems with a blown fuse on the neutral while there is still power to the two hot legs. Current flow will stop, and it will appear that the power is off to the entire house. But the entire system will still be energized, with that electricity waiting to follow a good path to ground.

That would be the case if you were only using circuits on one leg. All of the individual circuit neutrals are connected to each other at the neutral terminal bar. So if you turned on any 120-volt circuit from one leg and any other 120-volt circuit from another leg, they would join to form a 240-volt circuit.

The overall point is that a 120/240-volt service to a house is really just a large multi-wire circuit. The neutral carries the difference in between the two hot legs. If each leg happens to be carrying identical loads, the voltage in the neutral is 0. If you cut the neutral, the entire circuit reverts to 240-volts. This is why we don't fuse neutrals.

It gets even stranger when you consider that the loads in this situation are in series rather than in parallel. So there's voltage drop at each load, depending on its resistance. A loose neutral can cause all sorts of instability and fluctuations in voltage as its impedance increases & decreases.

The part of my answer that still has me scratching my head is "what role does the earth grounding system play in this situation?" Wouldn't the current continue to flow through the earth ground, making everything appear to be ok?

Even if it's a good grounding connection, the earth has too much resistance to replace the neutral. Part of the current will flow through the earth, but not all of it. You'll definitely see instability in the system's voltage.

- Jim Katen, Oregon

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