My hot tub trips the GFCI. THe GFCI holds when I disconnect the wires running into the hot tub. I have disconnected EVERYTHING from the circuit board including the heater and the GFCI still trips.

If you have any clamp on amperage meters, you need to see what the amperage draw on the circuit is when it is running. This will tell you if something is wrong with the pump circuit. It could be anything from a bad capacitor to an impeller obstruction.

Still would go with Miswired.
if this is an auxiliary GFCI Spa Disconnect,
240 VAC, 50 Amp, then...
All of the following needs to be this way:

• 4 wires from MAIN Circuit panel, into GFCI disconnect box, 2 hots, 1 Neutral, 1 ground
  • 2 hots go onto the disconnects Power Bus Bar for breaker
  • 1 white Nuetral goes to Isolated nuetral bus bar
  • 1 green Ground goes to ground lug directly on the box
• 1 GFCI in the auxiliary disconnect box
  • The White Pigtail of the GFCI Breaker goes onto the Isolated Nuetral bus bar. There are now two wires there.
• 4 wires come into box from Spa. (Unless the spa is Specifically a 3 Wire Connection Spa!)
  • 2 Hot/Line wires will go into the L1 & L2 lugs of the breaker respectively.
  • 1 White Nuetral Wire will go into breaker as well at the "Load Nuetral lug, usually marked as 'LN'.
  • It will NOT go on the Isolated Nuetral bus bar.
  • Green Ground wire will be connected to ground lug, 2 wires are now connected there.
• Connections at spa will be respectively connected: the 2 hots on line 1 and line 2. White wire goes to Nuetralnd green ground will go to a Ground Lug, most likely to metal control box.

If it's not wired in this manner, it is wired WRONG.

The resistance in the unit may be due to inadequate electrical wiring and over heating of the GFCI, due you have a thermal camera, can you check the temperature before and after the unit trips. Most common cause is Length of run, motor does not use much electricity but heater will trip due to over heating, don't over think problem. Check for proper wire size and make sure the length of run from meter can handle load and demand, this may solve your problem.

Because it is a GFCI. Try making sure that the grounds are all good. Maybe go the extra step of putting in grounding rods into the ground and grounding to the unit. These are just guesses.

Leaking heat element.
2 GFCI on same circuit.
Rat-chewed wire. Inspect wiring.
Nail through wire.
Melted wire nut in junction box.
Undersized wire and breaker has been overheating breaker, and breaker has gone bad.
Shorted breaker. Move wires to different same-size breaker.
http://waterheatertimer.org/How-to-wire-GFCI.html#2-pole-gfci
http://waterheatertimer.org/Color-codewire.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

#1. DO NOT GET INTO TUB FULL OF WATER WITH A GROUND FAULT.
2. There is a short. You need to get multimeter and start checking each part for current running to ground wire.
3) Power is ON. Connect power without GFCI.
4) Tape tester leads to wood sticks to keep hands away from power.
5) Do not touch anything directly or you will become the ground rod.
6) Ground wire must be present, and bonded back to main panel box.
7) Do not sit or touch directly on cement or bare ground. Do not touch or lean into anything made of metal.
8) Body and parts must be on sheet of plywood, or similar non-conductive dry surface.
9) Test each part with spa, but also test electric line coming from main box.
10) GFCI will also trip with any fault detected on line. So the wire coming to spa may have the fault.
11) Add a new dedicated line from main box to spa.

http://waterheatertimer.org/How-to-wire-GFCI.html
http://waterheatertimer.org/See-inside-main-breaker-box.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Either you have a ground fault or the GFCI is toasted. Turn off the power to the GFCI, open the box the GFCI is in and disconnect the output wires, close up the box, re-energize, and try to reset it again. This will tell you which problem you have.

Try this...

You mentioned a 8-3 cable suppling power to the sub-panel. I am assuming you have a 8-3 WITH ground or 4 conductors, black, red, white AND ground.

In the sub-panel do not bond the neutral bar to the box or ground bar. If there is a green bonding screw on the neutral bar...remove it. If you have only one (1) neutral bar add another bar for your grounds and bond this bar to the box. Keep the neutrals isolated from the grounds.

I hope this has helped. If you need additional information please send me a follow-question.

gfci's are designed to trip if they receive voltage on there ground/neutral side, therefore my vote is for Smithbrother I would say there is probably a partial short somewhere in you system.

There can be a few different reason for a trip. 1. Amps exceed rating off GFCI ie 15 amp receptacle can only handle up to 15 amp anymore and it will trip. 2. miss wiring; neutral must be on white screws hot on black and ground, green also inlet power (line) must be on bottom screws, outlet or load must be on top 3, a short, whether it be neutral or hot touching something in the box. Frayed wire, wire next to side of interior of box or yes moisture. Also a short in an appliance aven a new one can cause a trip. Remember if the refrigerator is on this same circuit, which is not a good idea it can cause a trip. At start up the fridge can spike to near 15 amps and a voltage spike can cause a gfci to trip. There are 20amp GFCI's so that can be your last resort, if all else fails.

I’m happy to help further over the phone at https://www.6ya.com/expert/josh_87ce33216d7b2148