Western Electrical Contractors Association, Inc.

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Notes from the Field, Episode 9
What's Wrong with This Picture?

By Jimmie Slemp, WECA Instructor and Lab Manager
Recently, a friend reported that they were having some electrical issues with their house.
Something was definitely up with the electrical:
- The lights in the bathroom were flickering
- The lamp that was plugged into a particular receptacle made sizzling sounds and had a burning smell and then soon stopped working
- The TV that was plugged into another outlet in the same room was unplugged, and then the lights in the bathroom quit?
- The TV was plugged into another receptacle in another room and the bathroom lights started working again?!
Fig. 1

I was starting to wonder if the house was haunted! I was definitely curious and had my electrician-hunches. I arranged to visit the house. 

Upon arrival I went straight to the receptacle that the lamp was plugged into and took a reading. 

The receptacle (120 volt outlet) read 243 volts! I immediately had my suspicions and proceed to check the panel to take a look. See Fig. 1

As I had suspected, the circuit was part of a "multiwire branch circuit" (aka 3-wire circuit.)

Note that the 15 amp breaker (which is shut off) has a black and red wire connected to it. 

This indicates that the circuits are sharing the neutral. So after shutting off the circuit, I traced back the conductors and found the neutral. See Fig. 2
Fig. 2

Just touching the neutral with my screwdriver I could tell that it was loose.  So I tightened up the connection and went back to test the circuit.  Once back at the receptacle the meter still read 243 volts ...what??? 
I was confident that I had found the problem. Now it was time to step back and think things over. 

Typically 3-wire circuits are used as the "home run" and at the first junction box they split into their respective 2-wire circuits.  So as I looked at the layout of the house, I went to the receptacle that was closest to the panel and opened it up.  There I found the 3-wire home run and also found that not all of the neutrals were tight in the wire nut. See Fig. 3
Fig. 3

After reconnecting the neutrals, I went back to the panel and energized the circuit.  Even as I went back to the problem area of the house I could see that the bathroom lights were back on. I checked the voltage at the receptacle and we were back to 120 volts.
While multiwire circuits may be cost effective, they can also be problematic if not installed properly or if someone works on them while energized.  

If the neutral is "lost" during the operation of the circuit, any imbalance in the circuit will cause the voltages to divide proportionately. This will cause some voltages to go up and some voltages to go down and basically the voltage will go crazy! See Fig. 4
Fig. 4