voltage on neutral line

[arclightzero]

We have a 60Hz 3 phase wye bus with around 5-8 vac running through it at around 30 amps/phase. Since neutral lines don't run trhough breakers, this poses a problem when working on equipemt that is supposedly deenergized. I am not sure where this voltage is coming from, and I assumed that the voltage should be shunted to ground through the bonding at the transformer. Anybody have any ideas as to where I could begin looking for a problem?

 

[dereckbc]

Re: voltage on neutral line

Start at the transformer and work your way through the system. Sounds like a poor connection. You should'nt read any more than 3-to-4 volts between N-G at the most distant location under full load. So if you do not have any or little load, you should not be reading voltage.

 

[scott thompson]

Re: voltage on neutral line

5-8 VAC sounds kind of normal - especially if you are finding this like 50 feet or more away from the main service / SDS Transformer.

One way to check on this - being that I Assume (yep, I said it!!!) the circuitry in question is the 30 amp circuit(s) would be to determine the voltage drop on the circuit(s).
Measure the L-C, L-L and L-L-L Voltage at the point of origin (main service, or Step-Down Transformer if used), then measure the L-C, L-L and L-L-L Voltage at the point where you find the 5-8 VAC C-G reading.

Chances are the "Missing" Voltage will be the C-G Voltage.

FYI:
L-C: Line to Common Grounded Conductor,
L-L: Line to Line,
L-L-L: Line to Line to Line - AKA 3?,
C-G: Common Grounded Conductor to Ground (equipment grounding conductor).

Reply with your findings!

Scott

 

[Ed MacLaren]

Re: voltage on neutral line

this poses a problem when working on equipemt that is supposedly deenergized.

Does that mean the 5-8 volts is present when the service/feeder/branch circuit in question is shut down?

wye bus with around 5-8 vac running through it

Between what two points is the 5-8 volts being measured?

Ed

 

[arclightzero]

Re: voltage on neutral line

Does that mean the 5-8 volts is present when the service/feeder/branch circuit in question is shut down?

I cannot secure the branch at the service and isolate my shop from the transformer as it would shut down other areas. However, we are the only shop on the transformer that has this problem, so I assume that the problem isn't at the transformer.

Between what two points is the 5-8 volts being measured?

Sorry, I forgot to specify... Measuring neutral to both safety ground and the bus ground. Same measurement both places.

 

[dereckbc]

Re: voltage on neutral line

When you turn a breaker off, there should not be any voltage between the neutral and safety ground, assuming this is from the main panel and not a sub-panel. If the breaker is from a sub-panel then 5-to-8 volts is still too high IMO (more like 1-to-2 volts for feeder drop).

What you are seeing is a voltage drop developed on the neutral conductor from from the transformer to the load device. The voltage drop is proportional to the load current or I*R.

My advice is too start at the transformer and verify the MBJ and work your way down stream. It could be your neutral is undersized, a bad connection, or just an extremely long run developing a large voltage drop along the way.

You would like too limit the total voltage drop to 5% or less for the circuit, which means you would measure 114 volts L-N, 3 volts N-G under full load. But there is no code requirement for 5%.

 

[arclightzero]

Re: voltage on neutral line

Originally posted by dereckbc:

It could be your neutral is undersized, a bad connection, or just an extremely long run developing a large voltage drop along the way.

I have found that our neutral lines are the same size as the 3 phase conductors, and as opposed to other areas that are fed from transformers that are within a short distance from the shops, our shop is fed from a transformer that is several hundred feet from us.

 

[dereckbc]

Re: voltage on neutral line

Is there a sub-panel between you and the transformer?

 

[arclightzero]

Re: voltage on neutral line

Originally posted by dereckbc:
Is there a sub-panel between you and the transformer?

There is a panel at the transformer but that is it. The lines come straight into our shop from the transformer and go straight into our bus

 

[dereckbc]

Re: voltage on neutral line

"Straight to your bus". Is that a sub-panel bus? If it is, measure the voltage between N-G at your sub-panel bus. That should be the voltage drop incurred on the feeder. Turn it off and should be 0.