tracing wires

[puyehue]

[i wrote into another forum and my post was lost and have not heard from admin yet, sorry for cross posting, but need some advice]:

i have a 3" conduit with 7 wires (6- 2/0 with one gnd). the run is 250' with one pull box in the middle of the run. 3 wires service a barn, the other 3 were pulled to service another building, but were never used. i am considering pulling these unused wires to use for another project. but i need to know how to trace the neutral wire. i could use an amp meter and tick tracer for the 2 hots, but not sure best way identify the neutral? the wires are not labeled in the pull box - they come up a sweep and are a tight horseshoe back down the new sweep...

thanks for your thoughts

 

[Dennis Alwon]

Can you put an amp meter around the wires?

 

[puyehue]

yes, that will fit....
thanks

 

[gar]

110411-1636 EDT

I assume the wires are powered and that the neutral is earthed somewhere.

Put a screwdriver in the ground (earth) near the point where you want to test for the neutral. Measure the voltage between the screwdriver and each wire. The neutral should be pretty close to 0 V. The hots should be whatever their voltage is, and floating wires are between hot and neutral.

If the unused wires are not floating, meaning connected to something, then it may be harder to separate them from neutral with a voltmeter. In that case an unbalanced current load and current measurement may be an easier method.

.

 

[puyehue]

gar:

yes, 2 wires powered, 1 neutral earthed somewhere.
3 unused are floating.

i would like to test and pull wire from the pull box in the middle of the run. wires are whole, one length, no slices, all insulated in this box. is there a way to use voltmeter on these wires? (without exposing copper)

what is the best way to do an unbalanced current load test?

 

[n1ist]

Put a single load at the barn, hot to neutral, and then use a clamp-on ammeter at the pull box, one lead at a time, to find the two wires with current flow. Then switch the load to neutral and the other hot and repeat the measurements. This will give two wires that had current flowing one time, and one that had current flowing both times. The latter is the neutral.
/mike

 

[gar]

110411-1928 EDT

The suggestion by n1ist is a good solution.

A non-contact voltage method would work like this:

I used a 1.5" x 2.0" copper plate connected to one AC volts input of a Fluke 27, and the other input connected to ground. You need a high input impedance meter for this measurement. The longer the plate the higher the voltage, but this does not change the relative voltages. What this is is a capacitive voltage probe.

With the plate on the neutral edge of a Romex cable the reading was about 0.5 V and on the hot edge about 1.5 V. Plenty of difference to identify the hot from neutral. With different cables you will get different voltages. With conductors that can be separated you can get greater differentiation. Flat against the flat side of the Romex the voltage is about midway between the two edge measurements as would be expected.

Although I thought a narrower strip would improve the relative difference it did not seem to. My experiment was with a 6" length of #14 wire. These were very uncontrolled experiments.

Actually I had a bent copper plate that made it easy to clip onto the plate without getting the test lead alligator clip, test lead, and my hand close to the Romex. Thus, a more controlled capacitive field between the Romex and the copper plate.

.