Tripping breaker(a)

[mdhills]

I’ve got an interesting puzzle and I’d be curious if others have heard of anything like this:

- I’m seeing multiple circuits trip breakers when loaded if a recently-added circuit is on

- these circuits are on a recently added sub panel, and largely (all?) AFCI
- these had been working with spot checks until this past week, when a new circuit was added for a gas tankless water heater
- the breaker for the tankless does not trip
- the other breakers do not trip if the breaker to the tankless is turned off

Any guesses?

Matt

[Roly]

I’ve got an interesting puzzle and I’d be curious if others have heard of anything like this:

- I’m seeing multiple circuits trip breakers when loaded if a recently-added circuit is on

- these circuits are on a recently added sub panel, and largely (all?) AFCI
- these had been working with spot checks until this past week, when a new circuit was added for a gas tankless water heater
- the breaker for the tankless does not trip
- the other breakers do not trip if the breaker to the tankless is turned off

Any guesses?

Matt

First look to see if the neutral and ground wire from the new tankless are connected to the correct bus.   Neutral and grounds are not the same.  Was the sub panel installed by a regular contractor? If not check to see ....
Is there a separate bus in the sub panel for grounds and another bus for neutrals ?    Has a bonding screw in the neutral bus been removed (could also be a link) ?   
 Sounds like neutral current from tankless is on ground bus.   Roly

[mdhills]

Thanks Rolly,
How does putting current on ground cause the other breakers to trip?

(This was work by contractor, so they will fix, but I’d found the behavior interesting)

Matt

[Roly]

Thanks Rolly,
  How does putting current on ground cause the other breakers to trip?

(This was work by contractor, so they will fix, but I’d found the behavior interesting)

Matt

Not seeing it the other arcfault breakes are probably combo  breakers which are also gfci.     The neutral current must match the current in the phases, it cannot be more or less or it will trip. Not sure without being there I would think the tankless is getting the neutral thru the ground bus and basically tieing the neutral and ground bus together so there is a parallel path.   Roly

[crokett™]

Another thing I would check in the sub panel is if the neutral and ground busses are bonded. They should not be. They should only be bonded at the main panel.

[Roly]

I’ve got an interesting puzzle and I’d be curious if others have heard of anything like this:

- I’m seeing multiple circuits trip breakers when loaded if a recently-added circuit is on

- these circuits are on a recently added sub panel, and largely (all?) AFCI
- these had been working with spot checks until this past week, when a new circuit was added for a gas tankless water heater
- the breaker for the tankless does not trip
- the other breakers do not trip if the breaker to the tankless is turned off

Any guesses?

Matt

Please post what the results were.    Roly

[Snipe Hunter]

Another thing I would check in the sub panel is if the neutral and ground busses are bonded.  They should not be.  They should only be bonded at the main panel.

+1
Treat the sub panel as if it were an appliance. Separated ground and neutral bus in the sub panel.

[mdhills]

Thanks for the suggestions.

As of today, the cause remains mysterious:
Confirmed the subpanel jumper between ground/neutral was removed
Confirmed the water heater circuit+device had the ground and neutrals electrically isolated.
Confirmed the pigtails appear to be to correct buses (AFCI neutrals to the breakers, and then pigtail to neutral bus, etc.)

Some more load checks:
When plugging the water heater into the water heater circuit, but leaving power off, resulted in trips on at least 2 other AFCI circuits (kitchen hood and a porch outlet temporarily used with air fryer) when those were operated.
When powering the water heater from the air-fryer outlet, the kitchen hood did not trip. 
Unplugging the water heater and loading that circuit with the air fryer did not result in a trip when the kitchen hood was turned on.
I did not see a trip on the microwave circuit.

Would any of these make a difference:
  - the water heater circuit is a standard breaker, with GFCI outlet
  - (the other circuits that I've seen tripping are AFCI-protected -- one to a kitchen vent and another to an exterior outet )
  - the water heater is on a different hot leg than the circuits that were tripping

I'm expecting the electrical guy to come tomorrow, so will let folks know what is learned.

Matt

[Roly]

Thanks for the suggestions.

As of today, the cause remains mysterious:
Confirmed the subpanel jumper between ground/neutral was removed
Confirmed the water heater circuit+device had the ground and neutrals electrically isolated.
Confirmed the pigtails appear to be to correct buses (AFCI neutrals to the breakers, and then pigtail to neutral bus, etc.)

Some more load checks:
When plugging the water heater into the water heater circuit, but leaving power off, resulted in trips on at least 2 other AFCI circuits (kitchen hood and a porch outlet temporarily used with air fryer) when those were operated.
When powering the water heater from the air-fryer outlet, the kitchen hood did not trip. 
Unplugging the water heater and loading that circuit with the air fryer did not result in a trip when the kitchen hood was turned on.
I did not see a trip on the microwave circuit.

Would any of these make a difference:
  - the water heater circuit is a standard breaker, with GFCI outlet
  - (the other circuits that I've seen tripping are AFCI-protected -- one to a kitchen vent and another to an exterior outet )
  - the water heater is on a different hot leg than the circuits that were tripping

I'm expecting the electrical guy to come tomorrow, so will let folks know what is learned.

Matt

After thinking about it having the neutral bus grounded in the sub panel would not cause the breakers to trip.    You have something else going on with the neutrals, probably mixed up with the circuits they feed or they are grounded again.   Please let us know what the issue was. Roly

[Roly]

I was reading some gas tankless heaters can induce a signal in the wires on the arc fault breakers ans cause a trip.   Try moving the water heater breaker and wires away from the arc fault  breakers and see if that works.  Roly