I'm working for a school system with 20+ buildings of various ages from 100 to 0 years. Many of the fuse and panel boards are not marked or incorrectly marked. We've been having trouble adding outlets and lights to circuits with shared neutrals in 120/240, 120/208, and 277/480 configurations. I'm looking for a method to determine the origin of a circuit and the other hot wires associated with the shared neutral. Are there any trick ways.
My current idea is to make a flashing load of several amps, connect it to the circut. Then go back to the expected panel board and look for wires that amprobe at the flashing rate. This should id the hot and neutral for the target circuit. Then I grub in the panel board for other hots associated with neutral. AssUme'ing no one's tapped a random neutral wire in a junction, I've found the hots and neutral.
There are cable clamp signal generators made specifically for this- you clamp them at one end and walk along with a trace probe which can track them through walls. We use them all the time for tracking data cables; I've never personally used one for power cables, but I've seen them, our one meter had clamp attachments for it and probes for finding leakage from broken underground cables too. There is some issue with bleed-over to other nearby circuits when it branches out, but the signal is always attenuated on the other cables, and strongest on the one you're tracking.
If you know where the cable end points are, your solution sounds easiest. In fact, I did the exact same thing last week- used a small microwave (manually) pulsed on and off to verify which circuit breaker was feeding the string. A 10A pulse is hard to mistake!
[This message has been edited by SteveFehr (edited 09-28-2006).]
A flashing load is made by getting one of those old-fashioned "buttons", and inserting it in the socket under the bulb. They were once common for Xmas displays- and are still available. They are limited to 60 amp loads.
The problem with using several at once is that there is no way to have them trip in time with each other.
Still, the 1/2 amp fluctuation is useful in finding circuits.
I have also found the usual circuit toners to be useful in finding the appropriate neutral in the panel.
Adding a large load, as with a space heater, might also help find the right wire.