Generator Backfeed and Transfer Switch on a Rural Service
A rural home has a 200 A service and a 12 kW portable generator connected through a manual transfer switch installed by the homeowner. The transfer switch is a 60 A two-pole unit mounted beside the panel. The homeowner ran a 60 A feeder from the transfer switch to a 60 A breaker in the main panel using three conductors plus a ground, and bonded the generator neutral to the frame at the generator. The transfer switch switches the two ungrounded conductors and the neutral. The main breaker is not opened during generator operation. The homeowner reports that when the generator runs, the lights flicker and a GFCI receptacle in the bathroom trips, and that the generator's own breaker trips when the well pump starts.
1. Identify the hazards in this arrangement and explain the flicker and GFCI trip.
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Several hazards. First, the main breaker is not opened, so the generator can backfeed the utility service. The transfer switch must isolate the utility, either by switching all ungrounded conductors with a positive interlock or by requiring the main to be opened. If the transfer switch switches the neutral as well, that is acceptable only if it is a switched-neutral transfer switch rated for the purpose; otherwise the neutral must remain solid and the generator neutral must not be bonded at the generator when the transfer switch is a separately derived system. Second, bonding the generator neutral to the frame while the transfer switch also switches neutral can create a parallel neutral path and a ground loop. The flicker suggests the generator is overloaded or the neutral is floating or shared. The GFCI trip suggests neutral current is returning on the grounding path, which a GFCI detects as a current imbalance. The well pump starting trips the generator breaker because the starting current exceeds the generator's surge capacity.
2. What must be corrected, and how do you verify the transfer switch isolates the utility?
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Correct the system by: confirming the transfer switch is listed for the application and switches all ungrounded conductors with a positive mechanical interlock so the utility and generator cannot both be connected; determining whether the system is separately derived (transfer switch switches neutral) or not (neutral solid), and bonding the generator neutral accordingly; removing the generator frame neutral bond if the transfer switch switches neutral and the generator is not a separately derived source; and sizing the generator for the well pump's locked-rotor or starting current, or adding a soft starter. Verify isolation by opening the main, placing the transfer switch in generator position, and measuring for voltage on the utility side of the transfer switch (should be zero), then reversing to utility and confirming no voltage on the generator side. Confirm the GFCI holds by measuring neutral current on the grounding conductor under load, which should be near zero.