Is my apartment's wiring safe? Arc-Fault breaker flips intermittently
September 19, 2015 10:47 PM   Subscribe

My apartment has an Arc-Fault Circuit Breaker in the electrical panel that trips intermittently. While investigating troubleshooting steps, I discovered that I have have two circuits that share a common neutral wire. These two circuits, however, are independently switched at the panel. I have a two-gang light switch that appears to bridge the two circuits. Is this safe? Here's my basic circuit diagram. The "Bed/Bath Closet" light is controlled by the Bedroom breaker, not the bathroom breaker. But I was able to measure 220V across the two Bed/Bath switches that are controlled by independent circuit breakers.

The apartment was gutted and remodeled in early 2009, and I believe all wiring is new. It looks like the wiring is in conduit, and all the electrical boxes for switches and receptacles are metal. I learned that the 2008 NEC electrical code requires AFCI breakers in most "living areas" so I presume that's why it's installed.

This breaker has flipped intermittently. I don't think the load is the problem. Cursory research says that I should be able to run 1920W constantly (80% of 2400W) with much higher spikes possible for motors starting. The maximum wattage of everything connected to the circuit is about ~1900W, but it's never all drawing power, and the breaker has flipped multiple times with very little power drawn. The breaker never trips immediately. Sometimes it takes several minutes or an hour to trip again, other times it won't trip for weeks.

The circuit diagram shows the two-gang electrical box that has switches for both the Bedroom circuit and the Bathroom circuit. Using a volt meter, I was able to measure 220V across the two switches (e.g. measuring across switch contacts B-F in my diagram) which are controlled by two separate breakers. However, the breakers in my electrical panel for these two circuits are not switched together. Based on my research, this is not up to code and could expose anything plugged in to dangerous voltage in certain conditions.

My burning questions:

1. Is it safe to leave these circuits on?
2. Is it safe to turn these circuits off independently? (or for the AFCI breaker to trip independently)?
3. Is this up to code?
4. Is this definitely why my AFCI breaker trips intermittently?
5. What should I do about it? Is this something the property owner needs to fix?
posted by reeddavid to Home & Garden (4 answers total)
So B and F are switched in the same box? If so, I definitely prefer to have those on tied breakers, but when the inspector looked at my re-wiring which did that, he asked why I had the tied breakers.

Are you running motors on the AFCI circuit, or just lights? The brushes on motors definitely arc, our AFCI breakers seem to do okay with our Miele vacuum cleaner, but if I bring the shop vac, sawzall, or other power tools in to the house, I have to run an extension cord from a non-AFCI circuit. My impression is that nuisance trips is a part of having them.
posted by straw at 1:25 AM on September 20, 2015

It is perfectly normal for the voltage to be 220-240 Volts when measuring between two 120 Volt hot legs that are on opposing sides of 240 Volt two-phase service.

Your sketch of the wiring connections in your apartment appears to omit the neutral wires and only shows the hot wires. Without more information we have to guess at what is going on. It would be helpful if you'd include both the hot and neutral wires in your circuit diagram. Have you verified that the bedroom switch, the closet switch in the bathroom, and the closet light are wired in a standard three-way switch setup? I will assume they are.

Taking your diagram at face value, I assume power is supplied to the closet light from the bedroom circuit via the bedroom switch, and the closet switch in the bathroom is not connected in any way to the bathroom circuit. If this is the case you don't have a shared neutral. You see 240 volts because the bedroom circuit is on a different phase than the bathroom circuit. If that is the case your AFCI breaker is not giving you nuisance trips due to a shared neutral. Look elsewhere for the source of the problem, one possibility is a defective AFCI breaker.
posted by RichardP at 6:55 AM on September 20, 2015 [1 favorite]

Seems all right if I'm interpreting your diagram right. If the bath/kitchen circuits are served by a single two pole breaker or two singles tied together then ya, you might have a shared neutral but it isn't shared with the closet light (or shouldn't be anyways; you would have to trace the wires to be sure).
posted by Mitheral at 9:15 AM on September 20, 2015

And now looking at your Electrical panel image I see the bath/kitchen breakers are not tied. They are just two separate breakers in a tandem package. So your bath and kitchen should not be sharing a neutral.
posted by Mitheral at 9:18 AM on September 20, 2015

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