Loose or broken Neutral

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Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
I've got a troubleshooting situation where it seems to me that I have a loose or broken neutral. A single phase sub-panel (of a 3 phase, 208/120 Y system) reads 123 & 116V L-N with no load (main breaker off). When the loads are powered up the 116 volt leg reads around 106V and the other leg goes up slightly to 126V. The voltage reading at the MDP is 123 & 118V. The loads are dynamic so it is unbalanced- sometimes severely (I haven't yet taken a current measurement as the ammeter I had with me didn't work). The distance from the MDP to the sub panel is around 300-400 feet I would guess. My question is: Could a severely unbalanced load be responsible for the voltage anomalies? That was one theory thrown out today but my guess is a loose or broken neutral between the MDP and the sub panel. Any thoughts?
 
Sierrasparky

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
From what you tell us it sounds like a bad neutral. By the way what was the voltage at the MDP when the load was applied. Did it fluctuate like at the sub?
 
kwired

kwired

Electron manager
Location
NE Nebraska
Electron_Sam78 said:
I've got a troubleshooting situation where it seems to me that I have a loose or broken neutral. A single phase sub-panel (of a 3 phase, 208/120 Y system) reads 123 & 116V L-N with no load (main breaker off). When the loads are powered up the 116 volt leg reads around 106V and the other leg goes up slightly to 126V. The voltage reading at the MDP is 123 & 118V. The loads are dynamic so it is unbalanced- sometimes severely (I haven't yet taken a current measurement as the ammeter I had with me didn't work). The distance from the MDP to the sub panel is around 300-400 feet I would guess. My question is: Could a severely unbalanced load be responsible for the voltage anomalies? That was one theory thrown out today but my guess is a loose or broken neutral between the MDP and the sub panel. Any thoughts?
Click to expand...

Are your feeder conductors heavily loaded for their size and are they sized for voltage drop? Resistance of the long conductors will cause symptoms like this.

Neutral has less voltage applied across it so resistance of neutral will block more percentge of current than the higer voltage of line to line. Same thing happens on a shorter length of conductors if you throw in some added resistance to the neutral because of a bad connection. Your voltage readings are still in a tolerable range. I once got called to a country club that had voltage issues in a maintenance shed. My first thought was bad neutral all the classic signs were there. Load one line voltage will go down and other line voltage goes up. We traced that line for faults and only spots we found were very questionable - dug them up and they were fine. Whoever installed that line ran a #2 aluminum feeder nearly 2000 feet. I expected voltage drop problems but the raised voltage on the other line threw me off. But it should be expected to do that.

If you want to test your neutral for open circuit, disconnect grounding electrodes bonding jumpers etc if necessary to ensure that current must flow through your tested path and then put a load on it. If the load operates the neutral is not open. If you want to find a high resistance connection put a heavy load on it and the bad connection should heat up. If both of these tests pass you may just have voltage drop issues.
 
Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
OK some troubleshooting info updates. The readings in the OP were taken during the hottest part of the day with the heaviest loads on the system. (This is an RV park system BTW) This morning I took readings:

Voltage at MDP with no loads applied 124 and 119V - so only a drop of about 1 V on each leg when under load.

Voltage at sub panel with no load applied 123V and 117V and with load on 124, 27.5a and 116V, 48.4a, Neutral load wass 36.8a. Disconnected GEC and panel bond and found 3 volts between N and GEC.

I will check all this again today when the loads are at their peak.
 
Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
Update: I disconnected one leg from each of the three branch circuit breakers in the sub panel in order to keep load on only one leg at a time. Here's what I got:

No load
A 124V B 120V

Load on B only
A 126V B 111V, 76a

Load on A only
A 116V, 72a B 122V

So it seems that I have a neutral problem between the sub and the MDP
 
kwired

kwired

Electron manager
Location
NE Nebraska
Electron_Sam78 said:
Update: I disconnected one leg from each of the three branch circuit breakers in the sub panel in order to keep load on only one leg at a time. Here's what I got:

No load
A 124V B 120V

Load on B only
A 126V B 111V, 76a

Load on A only
A 116V, 72a B 122V

So it seems that I have a neutral problem between the sub and the MDP
Click to expand...

Depending on size and type of conductors and the fact you said they were 300-400 feet long makes me think your readings could be expected.

If you put a load on the neutral you will raise the voltage on it in relation to the other ungrounded conductor. Added resistance because of long length will increase that voltage. The more balanced the load is between the ungrounded conductors the less current on the neutral and more voltage balance also.
 
Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
So you're answer to my original question, "Could a severely unbalanced load be responsible for the voltage anomalies?" is "yes".

I wondered about that. The last situation where I actually had an underground break in the neutral the voltage swing was quite a bit unlike this is - around 2 volts up and 8-9 volts down.

Based on that I decided to try and balance the load by measuring the current and repositioning the breakers so the current was better distributed between the legs. I did that and now the voltage is 118V on A and 116V on B with about 40 and 60 amps respectively. I have been checking the voltage readings throughout the day and so far it looks good.
 
zappy

zappy

Senior Member
Location
CA.
Electron_Sam78 said:
I've got a troubleshooting situation where it seems to me that I have a loose or broken neutral. A single phase sub-panel (of a 3 phase, 208/120 Y system) reads 123 & 116V L-N with no load (main breaker off). When the loads are powered up the 116 volt leg reads around 106V and the other leg goes up slightly to 126V. The voltage reading at the MDP is 123 & 118V. The loads are dynamic so it is unbalanced- sometimes severely (I haven't yet taken a current measurement as the ammeter I had with me didn't work). The distance from the MDP to the sub panel is around 300-400 feet I would guess. My question is: Could a severely unbalanced load be responsible for the voltage anomalies? That was one theory thrown out today but my guess is a loose or broken neutral between the MDP and the sub panel. Any thoughts?
Click to expand...

Why did the customer call you? Where they having issues with lights flickering or something?
 
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