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I've just installed a subpanel in my attached garage. The rough in has passed inspection, however my main panel has standard breakers and has never had fault finding interrupter breakers in it, and was installed in 1990's.

The subpanel has the bonding screw removed and the ground is terminated to the subpanel box and goes to the main panel box with the neutral connected to the subpanel neutral bar and goes to the main panel neutral bar. The 2 hot wires go to a double poled breaker on the main panel bus. The main panel does have its bonding screw installed as required.

The main panel has the house ground attached to the neutral bar and then there is a ground wire running form the natural gas pipe then it connects to the copper water pipe and proceeds to the main panel and connects to the panel box. The situation is when the arc fault breakers are installed in the subpanel and turned on everything is fine until a load is applied to the subpanel circuit and the breaker trips immediately.

The loads used at the time was a chop saw, garage lights, and plugging my iPhone into its charger on the circuit.

When trouble shooting I have also disconnected the neutral and ground lines feeding the subpanel and checked between the neutral and ground for any shorts. None of the grounds have any shorts to the neutral.

I've been reading up on what could cause this and have not found a concrete answer to this problem. What I have come up with are these 2-thoughts.

  1. On the circuits of the subpanel has one switch box that is fed by two separate supply circuits. One runs the garage lights and the other runs outside lighting and accent lights in the studio in the garage. Would having the grounds at this box from both circuits being joined together and including the switch box cause this kind of problem?
  2. Could a neutral short to ground in one of the circuits of the main panel case this problem? If so then how does one find the circuit with the short?

Now if I were to turn the main breaker off in the main panel, remove the incoming neutral wire from the neutral bar, remove the bonding screw, unplug everything that has a plug in the house and then with my meter see if the ground and neutral are isolated from each other or is there a neutral short to ground in one of the circuits keeping them connected. I know this doesn't tell me which one, or is there a very simple way of detecting a Neutral to Ground Short with in multiple circuits?

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  • Unless you have combination/dual function AFCI + GFCI they look for different things. GFCIs would care about neutral and ground being commingled. AFCIs care about things that look like arcs to their tiny computer brains. So if you have AFCIs tripping you may be chasing the wrong thing.
    – Ecnerwal
    Commented Nov 3, 2020 at 15:44
  • Not quite following the supply to the switch box 2 separate circuits it would be a code violation to have lights from the house fed from a second source if this is what you are saying.
    – Ed Beal
    Commented Nov 3, 2020 at 15:45
  • I think it's "one box, two separate switches, two circuits", not two circuits feeding one device, from what was written. If the neutrals are joined in that box, it's a problem. It's also a bit hazardous for "thinking you turned the circuit off" when there's more than one circuit in the box, and THAT might be a different code issue, but not a reason for tripping GFCIs.
    – Ecnerwal
    Commented Nov 3, 2020 at 15:58
  • Hi Ed & Ecnerwal, sorry for the confusion in regards to the light switch box. The two subpanel circuits are not combined and thier respective neutrals are not combined. 1-circuit along with with its neutral support a bank of 8-lights. The 2nd circuit along with with its neutral support different bank of 6-lights. The only wires in the box that are all connected are the bare ground wires. I understand that a neutral short to ground here would trip GFCI as mentioned, however I only have AFCI breakers in the subpanel at this time. So it sounds like as mentioned I'm chasing the wrong thing.
    – Jeff D.
    Commented Nov 3, 2020 at 17:36
  • Hi Ecnerwal, What you said about AFCI's only looking for arcs. So if I understand you correctly the subpanel AFCI’s tiny computer brains only monitor their respective circuit they are feeding, and if arcing was occurring on a circuit of the main panel with only standard breakers it would not detect that?
    – Jeff D.
    Commented Nov 3, 2020 at 17:50

1 Answer 1

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AFCI's do have GFCI-like characteristics, because most of them include a "mini-GFCI" for detecting hot-ground and neutral-ground arc faults. That means you must handle their neutrals like a GFCI. (certain GE breakers are an exception). However, an AFCI will only trip due to neutral or ground mishandling downline of the AFCI. You need not search for neutral/ground challenges on the panel side; even if you wrongly put the subpanel's neutral-ground bond back in, the AFCI wouldn't notice that.

It sounds like your subpanel's neutrals and grounds are wired correctly based on your description.

Neutral is not ground. All grounds should be connected together. Neutrals should only be connected when their partner hots (the ones in the same cable) are connected via wirenut, device, switch etc. When 2 circuits are in the same box, their neutrals must be kept carefully separated. "Gray" is an allowable color for neutral, so feel free to mark one circuit's neutrals with gray tape.

Next, you need to carefully review the instructions for the AFCI and assure you installed it correctly. Seriously.

Next, it is a matter of "divide and conquer". You have left several appliances plugged in and haven't tried unplugging them. You really ought to try that! Now try removing circuit hot and neutral from the AFCI device - if the AFCI still trips, that tells you quite a lot. Then, re-add parts of the circuit one segment at a time until it trips - that tells you what part of the circuit it is in.

Steady on; ask questions.

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    Occam’s Razor Principle works again...Thank you so much! When you said "Next, you need to carefully review the instructions for the AFCI and assure you installed it correctly. Seriously." It reminded me of picture I saw go by online with some white colour under under the black wire on an install and sure enough I had messed up the installation of the breaker. These breakers have a neutral screw below the hot connection for the circuits neutral wire, which in turn keeps the neutral circuits specific to its respective breaker supplying the circuit. Thank You! again Jeff D.
    – Jeff D.
    Commented Nov 3, 2020 at 22:30

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