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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?

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It can be a number of things... This sounds like an issue that I had last summer and it was due to some exposed wire hitting a metal box - I guess shorting the circuit. Also happened when I plugged something into one line. –  DMoore Apr 25 '13 at 5:40
4  
Was the 30 Amp breaker GFCI? –  Edwin Apr 25 '13 at 5:49
    
@Edwin: The 30A breaker is indeed GFCI. The outdoor breaker box isn't just outdoor, it's also for a swimming pool control system, so it really needs to be GFCI. –  Charles Burns Apr 25 '13 at 15:33

3 Answers 3

up vote 5 down vote accepted

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.

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Agreed, a neutral to ground short would be my guess. –  BMitch Apr 25 '13 at 11:25
    
The outdoor receptacles have weather covers, but not ones that protect when an item is plugged in (though nothing was plugged in since it last worked). I'll have to upgrade and repair/replace the unit. –  Charles Burns Apr 25 '13 at 17:03
    
I opened the outdoor box and found some corrosion where the breakers connect. I cleaned some off and was able to power a 100W light, but still not my pool pump. I'll have to get a real electrician to rebuild the system properly. It turns out that the box is labeled "Indoors". sigh. –  Charles Burns Apr 26 '13 at 5:11

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.

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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.

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Megger makes several types of instruments. Are you talking about a Megger multi-meter? –  Niall C. Dec 14 '13 at 23:55
    
Thanks. I ended up having an electrician rebuild the entire outdoor circuit breaker box from scratch with new equipment. It was more expensive than I wanted, but the previous setup seemed to have been done by the previous owner, not a qualified electrician. Since it's connected to a pool, I didn't want to take any chances. –  Charles Burns Dec 16 '13 at 15:47

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