Hello, All....Seeking some suggestions. Last week, I connected a hot tub for a guy who had it given to him by a neighbor. It had supposedly had nearly all electrical parts replaced at some time and looked to be in good shape.

My installation consisted of installing a new 100A sub-panel, reusing the 2P50 amp GFI breaker in it, reusing the same piece of cable, same disconnect, everything. The only material (aside from the sub panel) that I supplied was some PVC and liquid-tight conduit between the disconnect and the tub itself. Connections were typical: L1, L2, N and G on very good-looking lugs.

Now the guy says that the GFI breaker is tripping. I've checked it with a Wiggy and can force the GFI to trip every time. I can also run the tub fine, but when the heat comes on, the breaker trips on it's own. I've measured for resistance from both legs of the heating element to ground and they test clean, as in no resistance to ground. There is obviously plenty of control wiring that uses the neutral, but again, nothing seems to be out of order.

The guy is a friend, so he's not blaming me for the installation part. The problem is that we really don't know for sure if the tub was in good working order, though the former owner says it was. There's no doubt that the reused breaker could be faulty, but it holds just fine until the heat kicks in. I've seen this dozens of times before.

Is there something that any of you can think of that I might be missing?

Check the current on the equipment grounding conductor, and also the bonding conductor from the equalpotential grid to the busbar for the bonding of the pumping equipment in the tub compartment. I have seen a few lately that show current at levels just below trip thresholds for the gfi breakers. Then the sporatic tripping starts with the slightest bump of current imbalance.

Sure ... the heater is bad.

To really check for current leaks, you need to test with a megger; your continuity tester will not show the small leaks. Heck, I've had breakers 'instant' trip on circuits that showed no ground with a continuity tester ... but failed the megger test very clearly.

I agree. Disconnect the wires from the heater, and megger the heater element to the grounded casing.

An ordinary ohmmeter doesn't apply enough voltage to find high resistance leakage.

I agree with "Reno" and "NJ", the heater is bad and you need a megger to test for leakage. If you don't have a megger you could try disconnecting the leads at the heater and then run the tub, if the GFI doesn't trip it would almost guarantee the element was bad.

Personally, I think that the guy who gave away the hot tub knew that he had a bad heating element. If that's not it, the tub having been drained and moved could have caused any potential heating element issue to hurry itself along.

I like the idea of disconnecting it and showing this guy that it cycles fine with the heating element disconnected. Doh! I sat here thinking of all kinds of "techno" ways to show the guy that it is the element, but in reality, the simplest way would be more obvious.

I don't think that I've ever seen this problem where it wasn't the element anyway.

Thanks very much to everyone for the ideas so far.

The easiest way to prove it is replace the element. If it works you are done. I know they do go bad and short internally to the case. I have replaced a few in my spa for the same problem.

That's the exact reason Germans don't like the concept of RCDs - it seems some old electric range elements have the very same issue ad once connected to a protected supply will trip the RCD. The stories of such incidents magnified as such stories usually do and now many people believe it's impossible to connect ranges to an RCD. In fact it's simple - if the element trips the RCD it's faulty and has to be replaced. Period. Yet people are known to even remove the ground connection of the range (or bypassing the RCD)...

I've been seeing a lot of these tubs being given away lately; it seems that the 'fashion' has passed.

With the increased use of second-hand tubs, it seems to me that many will already have problems ... or, even, be damaged in transit. I strongly suggest everyone bone up on their troubleshooting skills.

For those wishing to get fancy .... it's quite reasonable to get significant heat from the sun. That is, you pump the water through a coil of 5/8 irrigation tubing atop the gazebo, enclosed in a safety glass faced case. I've seen these coils used to heat full size pools; they should be even more effective with a smaller tub.
The fun would come in designing the controls

If you doubt the statement above, just scan the "Free" section of craigslist.

The controls are not really that tricky but you will need a valve actuator and they can get pricey.
Then all you really need is a thermostat that picks a relay controlling the actuator that directs water to your solar collector.
I am doing something similar with my pool, no fancy electronics, just a few relays and some switches.
I got real lucky and found a guy selling some Pentair valve actuators cheap on Ebay.
The real trick is keeping the heat in the spa. You need to insulate it and keep it covered, whether you solar heat it or pay the PoCo.