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Story time: Last week my furnace failed. The blower motor run capacitor went toast, so the motor wouldn't spin. The furnace is on a multi-wire branch circuit and shares its neutral with the basement. When the furnace motor failed, other devices in the basement went up in smoke (literally). A catalytic cap on the control board of my brand new water heater exploded and a ceramic cap in one of my computer UPSes also exploded.

The question: I want to understand, on a very technical level, how exactly the blower motor was able to damage other devices that were solely connected over the neutral wire. My research has pointed to things like "imbalanced load" and "dips or surges", which of course is correct but completely devoid of the electrical detail my nerdy brain is craving.

So

  • What happens electrically at the motor terminals when the motor run cap fails and the motor stalls?
  • How does this affect the 120v AC between the hot and neutral on the furnace's branch of the circuit?
  • How does this affect the alternate phase 120v AC between the hot and neutral on the basement's branch?
  • Does the phase difference between the two branches of the circuit play a role in this failure?
  • Why did existing protections (such as fuses on the hot and neutral power wires inside the water heater, or built-in surge protection on the UPS) not prevent the damage?
  • Other than running new copper for a dedicated furnace breaker (which I'll probably do anyway), how can I protect my home and devices from damage from these kinds of failures in the future?

I'm not an EE, but I am a hobbyist and enthusiast and it's the technical details of how the motor failure cascaded across the neutral wire that I'm confused by. I thought about putting this on DIY SE but since it's the electrical technobabble I'm interested in I decided this was the best place.


Because someone does not seem to understand, I will try to make this abundantly clear. This is NOT a question about doing my own electrical. I am NOT asking for advice on how to wire my house. I am NOT asking about building code or furnace ratings. This is NOT a DIY question.

This IS a question about how the design of a multi-wire branch circuit allowed a (failed run capacitor on an induction motor) failure on one leg to propagate over the neutral wire to damage equipment on the other leg. Or, generally, how could any electrical failure on one circuit affect another circuit over a shared common? Does the phase difference between the two circuits play a role in how a failure could propagate? If so, how? This IS an electrical engineering question.

According to my understanding of electrical circuit design, this kind of failure propagation should not be possible. Which means that either the circuit is incorrectly wired or that my understanding is flawed. I assume the latter because the circuit has passed multiple city inspections over the years and has passed wiring tests before and after this failure. I am here to improve my understanding of how electricity works.

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  • \$\begingroup\$ This seems like much to blame on the motor failure. Is there any chance of a lightning strike or power surge (from outside the home)? Are there any hints of a service interruption (flashing/reset clocks)? Do you have an autostart generator and auto-transfer switch? \$\endgroup\$ Commented Jun 2, 2023 at 21:21
  • \$\begingroup\$ Do you know the order of the failure? I'm thinking motor cap fail -> overload -> breaker trip -> other device damage? \$\endgroup\$
    – user199402
    Commented Jun 2, 2023 at 21:46
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    \$\begingroup\$ Looks like your wiring is done not properly and neutral not grounded. Phase voltage appeared on neutral wire . \$\endgroup\$
    – user263983
    Commented Jun 2, 2023 at 22:29
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    \$\begingroup\$ this question may be more suitable for diy.stackexchange.com/questions \$\endgroup\$
    – jsotola
    Commented Jun 2, 2023 at 22:52
  • \$\begingroup\$ No lightning strikes. No breaker trips. No blown fuses. The furnace failed first, standard "buzzing" associated with a failed run cap. @jsotola I specifically said that this question is about the ELECTRICAL technical details. DIY SE is for DIYers to fix things. The furnace is fixed. I want to know how the failure occurred in the first place, because I agree with Chris that this is a bit much to blame on a motor failure, but the failed motor is the root cause. \$\endgroup\$
    – Thomas
    Commented Jun 4, 2023 at 23:50

1 Answer 1

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The issue can be explained if you have a failed neutral connection. In some locales, the electricity is split 120 VAC | 0 (neutral) | 120 VAC, with the two 120 VAC 180° out of phase. All three wires go to the outside power transformer. That means there is 240 VAC between the two "hot" wires, and 120 VAC between either and neutral. This allows both 120 VAC appliances (e.g., lamp or radio) and 240 VAC appliances (e.g., electric dryer or stove) to run from the same entrance panel.

If the neutral wire breaks, there is still 240 VAC across the outside wires, but 120 VAC appliances are effectively paired in series across the 240 VAC. For example, if you have just two appliances on, one in each half of the circuit, and one is a refrigerator drawing 5 amperes (600 W) and the other is a lamp drawing 0.1 A (12 W), about 235 VAC would be across the lamp, and 5 VAC across the fridge. poof! Lamp is torched.

Not only is that a fire hazard, but the neutral wire in the house now has 235 VAC on it. This means the shell of some two-wire appliances can be a serious shock hazard.

The issue was not the motor, but the missing connection between the house neutral wiring and the center-tap of the outside power transformer. Consider it luck that the house did not burn down nor was someone electrocuted. Fuses did not blow because the circuit current rating was not exceeded. The appliance voltage rating was exceeded because of defective house wiring, or defective wiring between the outside power transformer and the house entrance panel.

This is not a DIY project. Get a certified electrician, who will contact the electric company, to fix this ASAP.

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  • \$\begingroup\$ Thanks for the response. Just to reiterate, this question is NOT about the repair, it's about how the design of a multi-wire branch circuit could have lead to this failure in the first place. That said, the branch circuit passed inspection when the furnace was installed in 2016 and again last year during a water heater replacement. Before and after the failure, the shared neutral was connected to the grounded bus bar in the breaker panel. \$\endgroup\$
    – Thomas
    Commented Jun 5, 2023 at 0:05
  • \$\begingroup\$ During the failure there was no water in or around the furnace that could have caused a short. It has been raining pretty consistently for the last few weeks so humidity has been higher. Hypothetically, could damp or degraded insulation on the romex have caused the problem? With every endpoint disconnected (all outlets unplugged, all light switches off, furnace hot disconnected and capped) I am getting about 10v of phantom power, presumably due to self-capacitance over a pretty long circuit. Would that possibly contribute to the failure? \$\endgroup\$
    – Thomas
    Commented Jun 5, 2023 at 0:07

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