Why does my GFCI circuit breaker trip with any small load, even after replacing the breaker?

Question

I have a 20A 240V GFCI breaker in my main breaker box going to a conduit to another breaker box outside. All worked well last summer. Now, when I put any load (weed-eater, light, etc.) on an outlet connected to the outside breaker box, the indoor breaker immediately trips.

I've tried replacing the indoor breaker with a 30A model (it's 10AWG wire) but it still trips under any load. I've also replaced the outdoor breakers.

I am confused. If this were a short circuit, the breaker would trip regardless of whether there was a load.

Does anyone have ideas on why this is happening and how I can fix it?

Answer 1

Since it's a Ground Fault Circuit Interrupter (GFCI) breaker, it's very likely that you have a ground fault.

GFCIs work by detecting an imbalance in current between the ungrounded (hot), and grounded (neutral) conductors. This is why you only see the trip when the circuit is under load. If there is 0 current flow, the current is balanced.

I'd start by inspecting the receptacle. Turn off the breaker, then open up the box where the receptacle is installed. Look for damage to the receptacle itself, or water/moisture in the box.

For outdoor installations you should always have a weatherproof box, and a cover that is weatherproof even when the receptacle is in use.

Answer 2

I had the same problem, in my case it was a regular 20A breaker that I had to replace with a GFCI breaker per local code, since this new circuit was for the basement.

I didn't realize that the GFCI breaker had to be wired different than the normal breaker. After reading tester's answer above, I also came across a wiring diagram showing the different wiring diagrams, and it all made sense.

A normal breaker is wired like so:

A GFCI breaker is wired slightly different:

Notice how the neutral wire from the circuit goes through the GFCI breaker as opposed to going directly to the bus bar. This is so the GFCI can monitor the current on both wires, and open the circuit if the need arises. If the circuit's neutral went to the bus bar, the GFCI would trip when a load is applied because the breaker does not see the current going to ground.

Answer 3

You have a ground fault. But it's a neutral-ground fault, not a hot-ground fault.

On a sleeping/inactive circuit, zero current comes down the hot wire, so zero current returns on the neutral. If some of it returns improperly via the ground instead of the neutral, that doesn't matter - "some of zero" is zero, and the GFCI won't detect an imbalance.

But plug in any load, and the scene changes. Now the standard load comes down the hot, but it returns via both the neutral and ground wire, in proportion to their conductance (1/resistance). Now enough current flows down the ground (and not the neutral) to produce a detectable imbalance between hot and neutral. Snap!

Look for bootlegged grounds in added work, receptacles shorting against metal boxes or their neutral screws touching a bare ground wire, or a faulty piece of equipment.

Answer 4

This can happen if there is a loose connection, a broken wire, broken insulation, etc.

The usual symptom of these types of faults is you can measure full voltage when there is no load, but devices that you plug in just don't work (or are slow or dim). This happens because the small gap in the conductor caused by the loose connection or the broken wire is very resistive at high loads. Broken insulation can cause the same problem because the live conductor may be exposed to an adjacent metal conduit, ground conductor, etc.

However, depending on the nature of the break, the added load of one of these faults could trip a breaker.

Since you say that this circuit works fine when running your pool pump, I would inspect all of the wiring specific to this outlet. The short/break could also be inside the outlet.

You also mentioned "another breaker box outside". Does the outlet you're using have a breaker in this breaker box? If it does, I would investigate that breaker as potentially faulty. It may be that whatever device you're plugging in along with the pool equipment is drawing more than 30 amps and tripping the upstream breaker because the outside breaker has failed.

My final thought is that your pool equipment might be faulty and drawing more current than normal. The added load of your power tools could be the straw that breaks the camel's back.

Answer 5

Your breaker is a GFCI. They operate when there is an imbalance between the current going out on the hot wire and returning on the neutral wire. Nothing to do with current amount. Excess current is controlled by the normal breakers in the main breaker box not GFCI's. Therefore the imbalance needs to be looked for where most likely the hot wire has a further path to leak current to ground and so carry more current than the neutral carries.

It is not much - 30 mA is the value used most to cause a GFCI to trip.

Look for condensation in the box. But going by you saying it trips when things are plugged in, it looks like it has to be something you are plugging in.

They all need testing, ideally with a Megger to see if there is any leakage to ground from either the hot to ground or the neutral to ground. You may be able to use a multimeter but it is nowhere near as good as a proper Megger that produces a voltage of 250.500/1000 volts.

Try the multimeter set to as high a resistance range as you can, say 2 Mohm. Connect to hot wire terminal of the plug top that is on the appliance There should be no reading. Do the same between neutral and ground- no reading.

Take out a an appliance that has a 3 pin socket and has 3 wires feeding it. This is to make sure it has a grounded metal part like its frame. Plug this in- it should not trip the GFCI. If it does not then you have a faulty- a leak to ground in your outside devices.

Answer 6

Most likely this indicates that you have a fault between ground and neutral somewhere in the outside wiring.

When a load is applied some of the current flows through this ground to neutral fault rather than back through the neutral conductor. This results in an imbalance tripping your GFCI.