Overloded Neutrals

[rayalvarado]

We did a service call on a home 120/240 single phase service.We found all the #14 gauge and #12 gauge neutral wires attached to the neutral bar in the meter main combo panel to be burnt. The larger gauge wires #10,#8 were discolored as well. They got really hot . Nothing was out of phase. All connections in the Panel were tight. The main breaker was tight. The breakers attached to the buss bar were tight. The service side connections were tight. the Meter was tight. The grounds were tight Everything in the panel was correct and tight.
It appears that the utility company may have lost a neutral leg, or one of the two hot legs got energized on the same phase at the same time from the utility.
Has anyone else experienced this before?

 

[GoldDigger]

It appears that the utility company may have lost a neutral leg, or one of the two hot legs got energized on the same phase at the same time from the utility.
Has anyone else experienced this before?

Well, losing a POCO neutral would have reduced the current in the neutral wires.

And having both utility ungrounded wires connected to the same side of the transformer secondary would only have caused problems if all of the overheated neutrals were part of MWBCs. That is something you can check.

That particular situation would have equally overloaded all of the different MWBCs that were actually loaded to 1/2 their capacity or more.
And any 240V loads would just have stopped working.

If somehow L1 and L2 got connected to two different phases of a three-phase 208Y/120 instead of the expected 120/240 single phase three-wire, the neutral current could have been more than the current on either ungrounded conductor and 240V loads might have appeared to work normally. Heating appliances would just have heated more slowly. But again, only MWBC circuits should have been affected.

And, no I have not experienced it myself.

 

[James L]

....all the #14 gauge and #12 gauge neutral wires attached to the neutral bar in the meter main combo panel to be burnt. The larger gauge wires #10,#8 were discolored as well.


All connections in the Panel were tight. The main breaker was tight. The breakers attached to the buss bar were tight. The service side connections were tight. the Meter was tight. The grounds were tight Everything in the panel was correct and tight.

It's a little hard for me to tell from the picture, but I have seen this happen from all the screws being too tight.

Hercules thinks that tighter is better, to the point of smashing the conductors flat. That might explain why the larger wires aren't nearly as charred - harder to smash bigger wires with a screw driver.

I could be wrong, but it looks like some of the screw slots are damaged on the side where pressure was applied with a screw driver. I've seen wires tightened almost to the point of breaking

It's a little hard to tell from the pic.

 

[sndbodkin]

Because the heat originated at screw location. The problem is usually the connection ether too tight or to loose. The color of the wire down stream seem to be white and not affected. I would check the connection. Also an amp meter could monitor the current that is passing through each wire and the total on the main neutral conductor.

 

[RLyons]

From the picture I would guess the buss bar pictured looks like one of those screw into the panel ground bar kits possibly not bonded correctly...screws holding bar to panel not making a good neutral connection???

 

[ActionDave]

I've seen this happen and the problem was the screw that fastened the neutral lug to the buss. It was hard to spot at first because the neutral conductor was stiff enough to keep the lug from wiggling.

 

[rayalvarado]

Thanks Everyone. Gold Digger, what does MWBC stand for?

 

[readydave8]

I've seen this happen and the problem was the screw that fastened the neutral lug to the buss. It was hard to spot at first because the neutral conductor was stiff enough to keep the lug from wiggling.

Something similiar a few times with Siemens panels, 2 neutral bars (one each side of breakers), screw not tight enough on the strap that bonded one to the other.

At least twice have seen the bar itself cracked (both may have been CH, at least one was).

I doubt that tightening screws to the point of damaging each conductor would be likely to cause such uniform damage.

 

[kwired]

I've seen this happen and the problem was the screw that fastened the neutral lug to the buss. It was hard to spot at first because the neutral conductor was stiff enough to keep the lug from wiggling.

Makes most sense to me, that whole bar (or at least the portion pictured) had to be hot for some reason, and that kind of heating usually comes from high resistance in a connection, though near the center of the picture looks to be the place that it may have been the hottest, maybe a connection in that area is what failed and it had enough load it got hot enough to heat a large portion of the bar. What OP may not realize is this connection can get hot enough the thing is glowing red - you just seldom see it because you don't get a call until it has gotten to the point the circuit is open and doesn't work anymore. That much heat at one terminal can translate into high temp across terminals that were otherwise good to start with.

 

[Lectricbota]

Thanks Everyone. Gold Digger, what does MWBC stand for?

multiwire branch circuit

 

[Lectricbota]

I have seen this on a siemens panel that has a nuetral bar on both sides with an insulated bar across the bottom for joining them if you use both sides for nuetral connections.

DIY homeowner was doing some work in the panel and forgot to put that bar back.

 

[mjf]

Thanks Everyone. Gold Digger, what does MWBC stand for?

Multi-wire branch circuit

 

[GoldDigger]

Two hot wires of opposite polarity with a single shared neutral. Also called an Edison circuit in some areas.

Sent from my XT1080 using Tapatalk

 

[jim dungar]

And having both utility ungrounded wires connected to the same side of the transformer secondary would only have caused problems if all of the overheated neutrals were part of MWBCs. That is something you can check.

The service entrance is effectively one large MWBC.
If the utility had the connections wrong, the individual circuits would be additive, not subtractive, at the neutral bar. This is also a problem when people try to turn a small 120/240V 3w panel into a 120V 2W one by jumpering the incoming lugs.

You are correct the 240V loads would quit working.

 

[don_resqcapt19]

The termination screws in many of those bars are very hard to turn, even before they are compressing the conductor, and even more so if you try to turn them after they have overheated. It is hard to tell for sure from the picture, but it appears to me, based on the amount of thread still showing on the termination screws, that they were never correctly tightened.

Replace the bar and re-terminate the conductors after cutting off the head damaged parts.

 

[kingpb]

There are torque requirements although how many actually follow them.

That would be a good survey to pose.

 

[jim dungar]

In particular it looks like the screw/conductor in the center of the picture was the source of the heating. It has what appears to be arcing marks between the screw and the bus. All of the rest of the damage appears to be from radiated heat, the conductors further away and below the center are not as damaged as those directly above it.

 

[mbrooke]

Is the neutral buss bar rated to be a neutral buss or is it a ground buss? I have seen neutral wires put on a buss only listed for grounds where this has happened.


Does this panel have any shared neutrals? Any tandems or quads? Any 240 volts? 240 across the phases?



I do see the neutrals coming in at an angle rather than straight through the hole, that might be a culprit. Also note that an overheated screw may appear and feel tight but prior may have been loose.

 

[mbrooke]

Well, losing a POCO neutral would have reduced the current in the neutral wires.

And having both utility ungrounded wires connected to the same side of the transformer secondary would only have caused problems if all of the overheated neutrals were part of MWBCs. That is something you can check.

That particular situation would have equally overloaded all of the different MWBCs that were actually loaded to 1/2 their capacity or more.
And any 240V loads would just have stopped working.

If somehow L1 and L2 got connected to two different phases of a three-phase 208Y/120 instead of the expected 120/240 single phase three-wire, the neutral current could have been more than the current on either ungrounded conductor and 240V loads might have appeared to work normally. Heating appliances would just have heated more slowly. But again, only MWBC circuits should have been affected.

And, no I have not experienced it myself.

Not in a normal application. Ruling out harmonics or severe wave form distortion its not possible. There are millions of apartment buildings with single phase panels fed from a 3 phase 120/208 grounded Y source. Most have shared neutral circuits coming from them.


If that panel was hooked up as 120/208Y then the neutral conductors would just see the maximum of 20 amps as the worst case scenario like in a 120/240 system. Worst case would be only 1 phase is loaded at 20 amps the other at zero. Neutral would see 20 amps. If both phases were pulling 20 amps the neutral would still see 20 amps while in a 120/240 it would see zero amps. Hence why in a 120/208 circuit the neutral is counted as a current carrying conductor when only 2 of the phases are present and not in a 120/240 system, however the neutral will never go above the phases.

 

[GoldDigger]

Not in a normal application. True. Ruling out harmonics or severe wave form distortion its not possible. False. There are millions of apartment buildings with single phase panels fed from a 3 phase 120/208 grounded Y source. Most have shared neutral circuits coming from them.


If that panel was hooked up as 120/208Y then the neutral conductors would just see the maximum of 20 amps as the worst case scenario like in a 120/240 system. Worst case would be only 1 phase is loaded at 20 amps the other at zero. Neutral would see 20 amps. If both phases were pulling 20 amps the neutral would still see 20 amps while in a 120/240 it would see zero amps. Hence why in a 120/208 circuit the neutral is counted as a current carrying conductor when only 2 of the phases are present and not in a 120/240 system, however the neutral will never go above the phases.

I beg to differ.
You are correct as long as you have only resistive loads or only loads with matched power factor/phase angle on the two phases. But even without harmonics or severe waveform distortion you could have a higher current on the neutral if you have a low power factor inductive load on only one of the phases. (Which of the two phases makes a difference, since one way will reduce the neutral current and the other way will increase it.)
This is rather unlikely situation, but it is certainly theoretically possible.
(An inductive low PF on one phase and a capacitive low PF on the other can produce the worst case of double neutral current.)