Water heater works on generator, but not on shore power

[Cadence]

Just keep in mind, any voltage checks are no good unless under load.

[Dave_S]

Another vote for dodgy water heater.

Because....
On shore power the breaker can blow at the dock or on the boat so the over draw is beyond the boats breaker. It is possible the generator can not produce the power to trip the breakers so even though the heater is drawing enough to trip the breaker the generator can't produce the current so it pulls the voltage down. You would hear the generator struggle more.

[RedHeather]

Thanks for your leads!

The voltage seems fine, always around 230V (it's an EU boat). We've checked the shore power cord (which is high amperage, so shouldn't be a wire gauge issue), but will now check the connector on the boat for corrosion, and look at the water heater element. Given that the air conditioning, microwave and stick blender all work, and we've now been in a dry climate for months, perhaps it is an issue with the water heater's element. And we'll see where the gen set is grounded, out of curiosity.

Thanks again, and we'll let you know.

[RedHeather]

It could also be calcium build-up on the element, but the difference in behavior between shore power and generator is what puzzled us so thanks for the many angles on that.

[sailorchic34]

Originally Posted by RedHeather

Thanks for your leads!

The voltage seems fine, always around 230V (it's an EU boat). We've checked the shore power cord (which is high amperage, so shouldn't be a wire gauge issue), but will now check the connector on the boat for corrosion, and look at the water heater element..

Have you checked it with just the water heater on with shore power. Does it still trip or would it continue to run.

As it runs with the genny but trips on shore power with the same element and wire, it has to be a difference (drop) in voltage on shore power causing the breaker to trip.

However a water heater element is fairly inexpensive in the typical boat kw range. So I would replace that first. If the breaker still trips replace the breaker (it's cheap too). Breakers can go bad). It could trip on shore as the voltage may be slightly lower which causes a higher amp draw, possible anyway. Lastly if all else fails replace the shore cable. Though check that the dock power is adequate.

I had to cut 10 feet off my cable to get to good wire. I need to replace my 10 year old shore cable this winter.

[Alan Mighty]

Originally Posted by RedHeather

The voltage seems fine, always around 230V (it's an EU boat).

Given that your boat was wired to EU standards, you may find that the metal case of the water heater is earthed with a wire that connects eventually to the earth wire of your AC shore power cord. The metal case of the water heater will likely be covered by some non-metallic insulation (which will probably also be an electrical insulator).

If the metal case of water heater is earthed in that way, and if you have access to that earthing wire somewhere near the water heater, you could put a clamp-on ammeter on that earthing wire.

If AC is leaking into the water from a crack in the heater element, that earth wire will be carrying some AC. Note the potential for personal danger if you place yourself in the circuit to earth. Enuff said.

Note that the metal case of the water heater may not be earthed. Even more reason for not putting yourself in a circuit to earth.

The dockside circuit breaker more likely just meters the difference between AC Active and AC Neutral.

Please post a photo of the element of the water heater, if it proves to be the culprit.

[Alan Mighty]

Note also that what trips an Earth Leakage Circuit Breaker can be cumulative.

Meaning that there is a remote chance that more than one AC appliance, when added together, might be faulting and result in a trip of the ELCB.

Inspection of the element of the water heater will eliminate the biggest risk.

If, as you reported, the fault has been around for some time, I would expect that you will find obvious cracks and a clearly enlarged and expanded region of the element.

[Rorzech]

Maybe buy yourself a nice length of Cab-Tire #8 AWG extension cord cable and buy the proper ends ( high quality ) and put them together real good, then never have to worry about the problem being caused by your equipment again. It'll be such a nice cord you might need to lock it to the dock.

[mohave_steve]

Originally Posted by capt jgw

Another cause could voltage from the dock supply. If you're out near the end of the line you may not be getting full 110-120 volts. As volts go down, amps go up and that can trip a breaker. Easy to check, if a volt meter shows like 90 volts that would be your problem. Good luck getting the marina to fix it.

The heating element in the WH is a (nearly) purely resistive load.

Current draw: I=V/R

Resistance is constant. If voltage drops then so does current. On inductive loads (motors) things play a little differently.

[J Clark H356]

I have a 5 kW generator and use 1 30 Amp power cord at the dock (have 2 but just parallel the 1 30amp and manage power by not running HVAC heat pump, microwave and hot water heater at the same time on shore power or I trip my boat mainbreaker. On genset I have 41.6 amps at 5kw 120 volts so can run all three with no tripped breaker. Could you possibly have a similar situation? Might be your shore power just doesn't provide as many amps as your genset?

[CharlieJ]

Commenting on Post #15:

Unfortunately, many boats have their 120v AC green ground wire connected inadvertently to their battery - terminal. This happens because many boats have their AC green ground wire connected to the ships ground/ engine block... as is the negative wire from the starter batteries/ ship's battery bank.

This is not an unfortunate situation; it is required by the American Boat and Yacht Council Standards.

... it also could cause a serious electrolysis issue anything the gen is running and waterhearer breaker on.

Possible but extremely unlikely as it takes very high AC (> 100A/m^2) to induce stray current corrosion.

[capt jgw]

One of my commercial boats does the same thing. 30A shore connection, with the water heater and cabin heater on the dock breaker trips. 50A genset runs everything just fine

[jamhass]

Originally Posted by mohave_steve

The heating element in the WH is a (nearly) purely resistive load.

Current draw: I=V/R

Resistance is constant. If voltage drops then so does current. On inductive loads (motors) things play a little differently.

This is correct. ... but theres more to the story...

Reason breakers sometimes trip on low AC voltage is when an AC motor is trying to start. It draws high current on startup, but can't get up to speed on the low voltage. This causes the breaker to blow. It's not a high current draw due to low voltage per se, rather the extended duration on the high current trying to start.

[RedHeather]

Thanks, Alan and others! The boiler works again with a new element. Now to investigate why it didn't trip on the generator... Thanks for the insights!

[boatbod]

Originally Posted by RedHeather

Thanks, Alan and others! The boiler works again with a new element. Now to investigate why it didn't trip on the generator... Thanks for the insights!

I'd recommend checking your generator wiring to make sure you have (a) green ground wire connected to generator case (b) strap between neutral and ground connected at the generator wiring box.

If you're missing either one, the breaker would not trip when the failed heating element exposed line voltage to the grounded water heater.