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We have two separate buildings, a house and a garage/workshop, about 20 feet apart, sharing the same electrical service. On the service entrance, there's a Siemens whole-house surge protector. The service enters the workshop where it splits up in a large junction box. One part goes to the load center breaker box for the garage/workshop, which has a 100A main breaker. The other part goes through PVC conduit under ground to the load center breaker box for the house, which has a 200A main breaker. The grounding system for the house/garage/workshop combo includes nine 10-foot copper-clad ground rods connected to each other (in a loop around the buildings) by 300 feet of buried 1/0 AWG bare copper wire, and connected to the service entrance with 1/0 AWG bare copper wire.

(It's perhaps worth noting here that the electrical inspector, when passing our electrical for final, told me it was one of the best-wired and best-grounded houses he had ever seen. There's no shoddy work on which to place blame for this weird issue.)

The house's load center is primarily filled with combination AFCI+GFCI circuit breakers. The water heater in the house is a gas tankless and hasn't given us any problems.

The garage/workshop is primarily GFCI breakers (no AFCI or AFCI+GFCI breakers). There's a small mud room in the workshop serviced by a small 56-amp 240-volt (no neutral, just two legs and a ground) electric tankless water heater on a 60-amp circuit.

Sometimes, and not every time mind you, or even most of the time, but sometimes, when I turn on the sink or shower in the workshop mudroom (thus activating the electric tankless water heater), two combo AFCI+GFCI breakers in the house (a separate building, remember) trip. It's always the same two breakers: The circuit for the server/equipment rack in the basement and the circuit for the bedroom receptacles. It's never any other circuits in the house that trip, and it's never any of the circuits in the garage that trip.

It might be worth noting that the circuit for the server/equipment rack has a very large power conditioner / surge protector / battery backup system on it, and the bedroom circuit has surge-protection-built-in receptacles. However, these aren't the only circuits in the house (or even the garage) with surge protection. The (AFCI+GFCI) circuit for the entertainment center in the living room has a surge-protection-built-in receptacle and a battery backup but never trips, and a (GFCI) circuit in the garage/workshop building has a surge protector / battery backup plugged into it and never trips.

I'm at a loss to explain how turning on a tankless water heater in one building can trip (the same two) AFCI+GFCI breakers in another building. Anyone have any thoughts?

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    one of the best-wired and best-grounded houses he had ever seen. There's no shoddy work You might want to allow for some flex on that, trust me. Nobody's perfect and there's always another page in Code... and overbuilding, while nice, is orthogonal to that. For instance notice the water heater is 56A not 60A? why and what does that mean for breakering? Hint: 125% – Harper Jul 30 '18 at 17:53
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    @EdBeal: The electric tankless has only three connections: two hots and a ground. It does not have a neutral connection anywhere in the system. All of its components run on 240V; nothing is 120V (it's the same way with my 240V air compressor, which has no room for a neutral). Its instruction book also clearly says that it only needs a ground and two hots. – Nick Williams Jul 30 '18 at 19:38
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    @JimStewart: The latest electrical code requires many areas of the household to now be protected by either a combo AFCI/GFCI breaker or by an AFCI breaker and GFCI receptacle: The laundry room, bathrooms, and kitchen are all required to have this protection now. The code writers are considering requiring this protection for all areas of the house. I made the decision to use AFCI / GFCI breakers everywhere inside after considerable research and considering the various protections these different devices offer. – Nick Williams Jul 30 '18 at 19:42
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    @Harper: The installation instructions for the tankless electric water heater were very clear: 1) Use a 60A breaker, 2) Use 4 AWG 60ºC wire or 6 AWG 75ºC wire for the two hots, 3) Provide no neutral, 4) Provide a ground. It's a listed product, so code requires it to be installed strictly according to the instructions. – Nick Williams Jul 30 '18 at 19:44
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    Nick I was providing an example of things I have found since you did not provide a model or brand. Also note a very heavy load such as this can create a voltage droop that can cause your device to trip as another possibility. – Ed Beal Jul 30 '18 at 20:49

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