I have a 480v panel that feeds a 125kva transformer that feeds a 120/208 panel. On the 120v receptacles I’m getting 10 volts neutral to ground. In the transformer neutral and ground are bonded to building steel and in the 120/208 panel the neutrals and grounds are separated like there supposed to be. With there being no grounding or bonding issues I’m not sure what else it could be
10volts neutral to ground
- Thread starter fireryan
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I'd take the same readings at the transformer.
ptonsparky
Tom
- Occupation
- EC - retired
You are probably measuring the VD of the neutral at the point of testing. That is high so you have reason to be concerned. What is the amp load of that circuit? What are the wire sizes?
Turn it off and test again.
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
Try checking the voltage with a low impedance or "Wiggy" type meter. It could be phantom voltage.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
If you can, measure the neutral current at the transformer and compare it to the line currents there to see if it's significant.
Are there be a lot of nonlinear loads on the 120V circuits that could increase the neutral current?
Most likely it's a neutral issue, but if there's any building steel in the vicinity of the receptacle you might check the voltage from receptacle ground to the building metal (just in case there's a problem with EGC connections, bootleg grounds, etc.).
Are there be a lot of nonlinear loads on the 120V circuits that could increase the neutral current?
Most likely it's a neutral issue, but if there's any building steel in the vicinity of the receptacle you might check the voltage from receptacle ground to the building metal (just in case there's a problem with EGC connections, bootleg grounds, etc.).
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
I suggest also measuring the hot-to-ground and the hot-to-neutral voltages at a receptacle. The hot-to-ground minus the hot-to-neutral should be about what you measure for neutral-to-ground, if the neutral voltage drop is caused by currents on the same phase. If not then there could be a voltage drop on the neutral conductor of a feeder that includes other phases. This is especially true if the hot-to-neutral is higher than the hot-to-ground voltage.
If you know that the neutrals are are not shared with other phases between the receptacle and panel then this possibility should not occur. Still worth measuring though.
oldsparky52
Senior Member
- Location
- Wilmington, NC USA
Did you take neutral to ground readings at the panel that feed these circuits?
I will be as soon as I can open the panel. The plant doesn’t allow you to open panels while energized
ptonsparky
Tom
- Occupation
- EC - retired
Do you get to turn them back on for testing once the cover is off or is it just a look see?
swptln
Member
- Location
- East Brookfield, MA
Sounds to me like your dealing with a third harmonics issue. If there are non-linear loads being supplied by multi-wire branch circuits, your going to see voltages over 1.5V between the neutral and eg conductors. Only way to fix it, is 200% neutrals and K-rated transformers on your distribution and dedicated neutrals for all branch circuits. Multi-wire branch circuits with shared neutrals are your worst enemy feeding non-linear loads. Basically what happens is, with non-linear loads your shared neutrals do not carry just unbalanced current, they carry full load and it does not cancel out the balanced load, it triples. Hence the term third harmonics. With linear loads, equal loads on the phase conductors cancel out on the neutral, they don't with non-linear loads.
So, in order to handle a 20A 120/208V multi-wire branch circuit, you would need a #6 CU neutral to properly handle the load of all three phases. If you run dedicated neutrals, the #12 conductors can handle it on the branch circuit side. It wont help the distribution side though, which is why you need K-rated transformers and 200% neutrals on the system.
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