I want to replace a standard 15A breaker with a GFCI breaker. There are 10-15 circuits coming into the breaker box in conduit, not in NM-B cables.

How do I determine which neutral (of the 10-15) goes with the specific circuit I'm looking to change to GFCI?

  • 1
    Do you have the same number of neutrals in the conduit as hots? – NoSparksPlease Dec 10 '20 at 4:46

First and most importantly:


Then you should trace the HOT wire from the breaker back to where it enters the panel.

Once you have found that you should be able to identify its NEUTRAL and GROUND wires.

Trace those back to where they connect to a bus bar.

  • If you feel you need to turn the main off to do this it is best to turn the individual breakers off so when re energizing you are not dumping the entire load on the system. Question, with the power off how do you identify the correct black or breaker? – Ed Beal Dec 9 '20 at 20:29
  • Just as a heads up. Please note that if there are intermixed neutrals within your circuit(from other circuits), which may be present in your circuit breaker wiring, the GFCI will trip.. – binaryOps20 Dec 10 '20 at 0:21
  • I believe that this technique works well, @EdBeal, for residential wiring that is done with NM-B cables. It may be more difficult to do this with individual wires in conduit. – FreeMan Dec 10 '20 at 15:21
  • @freeman I have the same example but also conduit and as it turns out the op is talking about conduit. – Ed Beal Dec 10 '20 at 15:27
  • I see that, @EdBeal - I didn't read your answer until after I'd typed my comment. Also, I just edited that detail into the OP to make it clear for future readers. – FreeMan Dec 10 '20 at 15:32

This is easy turn the breaker for that circuit off verify the receptacle is dead. Now trace the black wire on that breaker to the cable it enters the box on the white is that neutral. If you have conduit the neutral may be white or gray and if more than 1 neutral in the pipe they are supposed to be grouped if not grouped I tie the hot and neutral together with a wire nut at the receptacle, then go back to the panel and find find the neutral that is connected to that breaker (depending on other loads sometimes you have to lift the neutral from the buss but not often) I then group those wires with tape or a tywrap/ zip tie and they never get mixed up again.

  • There are multiple (3-4) neutrals coming out of the same conduit entering the breaker box. how would I identify which of those it is? – bkdraft62 Dec 9 '20 at 21:13
  • Go to the receptacle that you turned off wire nut that receptacle hot and neutral together or jumper the receptacle. Now trace all 4 of the hots to there breaker and turn them off now lift the neutrals one at a time and check for a short between the hot and neutral when you find the short you created that is the correct neutral. Tag them together with tape of a zip tie. Remove your short and you can put the GFCI in. Note you are turning off the hots so when you lift the neutral you won’t get shocked or Frap your meter – Ed Beal Dec 9 '20 at 21:39
  • All you have to do is turn off everything in the house that consumes power that would be a similar load, then put a 100 watt bulb on the outlet, turn it on, then use your clamp on ammeter to measure the current at the panel on the black, then use the clamp on ammeter to clamp on to likely neutral candidates until you find one that is carrying the same exact current measurement - that is most likely the one. Turn off the master breaker, disconnect that neutral, turn the master back on and see if your 100 watt bulb is now off. – Ted Mittelstaedt Dec 9 '20 at 22:02
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    I never turn off the main and don’t guess on most likely. Most home owners don’t have clamp meters some have non contact testers and less have continuity testers or ohm meters. If you want to know a way to make your method not a guess I use a old Christmas light flasher or goes in the docket behind the light bulb so you get a Pulsating value on your clamp but again most home owners don’t have clamps. – Ed Beal Dec 10 '20 at 0:41
  • thank you for the detailed and quick response! – bkdraft62 Dec 10 '20 at 14:25

You're in conduit.

It's a Code violation for the installer to have failed to identify which neutrals go with which hots.

How to match hots to neutrals

What I do is (before installing the GFCI) identify all the hots coming out of that conduit. Follow them back to their breakers and identify them. Then for each circuit: turn off its breaker and run around and see what lost power. Identify 1 load or receptacle that is on each hot wire.

We need 1 active, turned-on load on each hot - lamp, heater, even a night light will be fine. The load must be turned on.

Now, turn off all those breakers... and lift all those neutrals off the neutral bar. They will be hot, they can nail you - if you're used to neutrals being "safe", forget that - they're insulated for a reason and you're about to meet it.

Now, turn on 1 hot breaker. Run your voltage tester down each neutral wire, if you have a 2-wire tester measure between neutral wire and panel chassis. Do not stop at the first neutral that lights up the tester. Test every neutral.

Turn that breaker off and repeat with the next breaker. Only 1 breaker on at a time!!!

The reason the neutral is "hot" is because the power is trying to return to source. It's coming from the breaker, going through the load that you turned on, and trying to return via the neutral wire, and it can't because you disconnected it. So hot voltage is just sitting there.

Any hot that lights up 2 neutrals is a serious problem. It means there is a crossed neutral there. That itself is an urgent problem, and if it's the GFCI's hot, you'll need to fix it before the GFCI will work.

If a neutral lights up for 2 hots, that is most likely a multi-wire branch circuit, which is legal, but you will not be putting that GFCI breaker on that. MWBCs require a 2-pole GFCI breaker. Also, any multi-wire branch circuit, GFCI or not, should be landed on a 2-pole (NOT TANDEM) breaker whose handles are tied. It's usually acceptable to field-fit a factory approved handle tie between the breakers.

Mark them, then fit an appropriate GFCI breaker

Once you have identified all the hots and neutrals, use electrical tape, ty-wraps or other method to group all the neutrals with their partner hot(s). This is the identification task which the installer should have done.

You can then put the neutrals back on the neutral bar, and turn all the breakers back on.

If you identify a "monogamous pair" of 1 hot and 1 neutral, you can put a 1-pole GFCI breaker on that.

2 hots + 1 neutral can use a 2-pole GFCI breaker.

3 hots + 1 neutral is bad unless you are in a 3-phase panel.


You may not be able to use a single pole GFCI breaker if multiple opposing or out of phase hots share a neutral. The code calls this a Multiwire Branch Circuit and now requires that these breakers to be handle tied and the wires including the neutral be grouped or identified. For many years handle ties and identification were not required. If you install a single pole breaker on a MWBC then as soon as current is introduced an alien leg the GFCI will trip. No damage will be done, but sometimes returning a used electrical item can be difficult.

Your best bet is to get a clamp-on amperage tester. You can get one for $15 at Harbor Freight or Amazon. Set the meter to 20+ amps and look for current that matches on the hot and neutral. Ideally you should find identical numbers, but often numbers will be a few percentage points off, this is usually a result of tester quality or proximity to other live circuits. If a MWBC exists you may need to turn off the other breaker to make the current match if there is any load on the other breaker. It's a good idea to try to match all the circuits contained in the same conduit, and group the matches with a band of tape.

If multiple circuits have similar numbers then it may be useful to add load if possible. A 100w light bulb may suffice, but will add less than 1 amp. You may want to use a bigger load like a portable heater. To calculate expected amps from the added load divide watts by 120, so a 600 watt heater should use about 5 amps, loads are rarely precise.

***The most important thing is when you go to make the breaker termination is to turn of at least all the breakers that feed wires into the same conduit and verify no current is on the white. Really it is best to turn off the whole panel to replace the breaker. If you have made some mistake and miss-identified pairs or there is some cross wiring you can get a nasty shock from a white wire when you disconnect it from the bus bar.

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