Neutral Backfeed

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notoriousdln

Member
Location
Myrtle Beach, SC
Recentley I wired a little office buildout and had no power going into the offices, yet was getting a 29 volt backfeed on the neutral of the lighting circuit. I realized this because I was wiring a lay in fixture and got zapped from the neutral to the grid. The offices were wired with 8 single phase circuits sharing 3 neutrals. EX: A-B-C with a shared neutral; A-B-C with a shared neutral; B-C with a shared neutral. The sets of 3 circuits were on 3 pole breakers and the other 2 circuits were on a 2 pole breaker per the 2008 NEC. All circuits were pulled in a 1" EMT to a Main J-Box. Let me remind you I had NO POWER onto ANY of the circuits. I have verified as well that all neutrals were made up to corresponding circuit neutrals. Could one the the Neutrals really picked up 29 volts just in the panel somewhere? I am puzzled as to why I got shocked!!. And just so ya'll know, when I did turn power on to all circuits, everything works as it should. Any Ideas??
 
cadpoint

cadpoint

Senior Member
Location
Durham, NC
I can only think if you can pick up ghost voltage in a three way switch, why not a three phase wire system...

U're system is not balanced...
 
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notoriousdln

Member
Location
Myrtle Beach, SC
the panel I landed all of the circuits in was a panel I installed for some other offices in the same building, this particular little build out was an extra on the project. after I got shocked I took apart the neutral in a switch box and tested it to ground, thats where i saw the voltage, no voltage to the hot leg.
 
erickench

erickench

Senior Member
Location
Brooklyn, NY
If there is a voltage at the neutral then it means that the voltages are not balanced. The B-C circuit may be causing it. Your drawing current from two phases instead of three like the other circuits.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
erickench said:
If there is a voltage at the neutral then it means that the voltages are not balanced. The B-C circuit may be causing it. Your drawing current from two phases instead of three like the other circuits.
Click to expand...

If he has 29 volts from neutral to ground it suggests a lot more then simple current imbalance.

notoriousdln does this building have a 480 volt service?
 
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DBrede

Member
if the 29 volts is present with the circuit breakers in the "off" position then i think it's objectionable neutral current, either a main bonding jumper missing at the service or an improper bonding jumper at the sub-panel feeding these circuits
 
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notoriousdln

Member
Location
Myrtle Beach, SC
the building has a 480 service and has the proper bonding jumper. We used a delta to wye transformer to step it down. In my subpanel I do not have the neutral and ground bonded. In the other offices fed out of that panel I do not have the backfeed, thats what is throwing me off.
 
erickench

erickench

Senior Member
Location
Brooklyn, NY
If the neutral and ground are not bonded then I am not surprised that you are getting a voltage reading from neutral to ground. NEC 250.2(A)(1) state's that an electrical system that is grounded and connected to earth will have a stabilized voltage during normal operation.
 
kwired

kwired

Electron manager
Location
NE Nebraska
notoriousdln said:
the panel I landed all of the circuits in was a panel I installed for some other offices in the same building, this particular little build out was an extra on the project. after I got shocked I took apart the neutral in a switch box and tested it to ground, thats where i saw the voltage, no voltage to the hot leg.
Click to expand...

What exactly was the ground that you tested to? Equipment grounding conductor run with other circuit conductors, metal raceway, building steel or metal piping system - if any of these did you verify how well it was bonded to the grounded service conductor? Or did you test to a driven test electrode?

Next thing to find out was your neutral conductor or the grounding source you mentioned the portion that was at system ground potential.

If the place has separately derived systems in it there may be bonding issues with the separately derived systems.
 
kwired

kwired

Electron manager
Location
NE Nebraska
notoriousdln said:
the building has a 480 service and has the proper bonding jumper. We used a delta to wye transformer to step it down. In my subpanel I do not have the neutral and ground bonded. In the other offices fed out of that panel I do not have the backfeed, thats what is throwing me off.
Click to expand...

Is there a main bonding jumper for the separately derived system and where is it located? Is there only one? It should be in the transformer or at the first disconnect but not both. It should also have a grounding electrode conductor that is bonded to the service grounding electrode system
 
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notoriousdln

Member
Location
Myrtle Beach, SC
the 29 volts i had was tested to the metal box, which is grounded. the main disconnect for the entire building itself has the neutral and ground bonded. my subpanel does not have them bonded. my subpanel is fed through a breaker in the main panel, through a transformer, and to a main breaker "subpanel" which is where the back feed comes from.
 
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notoriousdln

Member
Location
Myrtle Beach, SC
you would be correct. the transformer is properley grounded to the main grounding conductor as well as building steel. the neutral is created in the transformer as its delta/wye. you wouldnt bond them there would you?
 
erickench

erickench

Senior Member
Location
Brooklyn, NY
Would help if we had a diagram. If the subpanel is fed directly from the transformer then it is separately derived. And if there is no system bonding jumper then the system is ungrounded and the neutral voltage is therefore unstable. Perhaps you would like an explanation of how the phasors(vectors) are added up?
 
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