ECN Forum Forums General Topics of the Trade General Discussion Area Multi-wire branch circuit voltage readings

New install in a retail store. Adjacent receptacles in a telcom room on a 12/3 MC homerun. If nothing is plugged in, you have 120 V to neutral on each circuit. If you plug in a security system transformer you have 200+ volts to neutral on the other circuit(which still has nothing plugged into it). If I unplug that and plug my 3 prong tester into the circuit I Read 176 volts to neutral on the other circuit. The condition came to light when the inspector plugged his 3 prong tester in to the second circuit with the transformer plugged into the first and saw two green lights and an orange. What am I missing here? Checked the wiring and found no problem---ckts are on different phases and share a neutral. 120 volts is derived from a transformer since this is a 480/277 service.

It really appears the neutral is not correctly wired.

I sure hope that your Neutral is coming from the bonded secondary center tap of that transformer.
Joe

I've seen very similar readings from loose or mis-wired neutrals also. That would be my first guess. Please let us know what you find out...

Went back and looked in the panel today. The guy who made it up had the neutral from the 12/3 spliced to a piece of white THHN because it would not quite reach the neutral strip on the bottom of the panel. I followed this THHN down, across the bottom of the panel, past the neutral strip, back up the other side of the panel where it was wirenutted to the neutral tail from a GFCI breaker. Apparantly he had intended to use this piece of THHN to lengthen both neutrals so they would reach the strip. All he forgot was to CUT IT IN THE MIDDLE AND ATTACH EACH ONE TO THE NEUTRAL STRIP!!! Keep your eye on your help and never assume anything.

[This message has been edited by poorboy (edited 10-24-2006).]

LOL!!!! I shouldn't laugh, though, sounds like something I'd do!

It's amazing how much better something works if it's wired up correctly!

Thanks for the update poorboy. I'll keep that information in my troubleshooting brain-file...