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Discussion Starter #1
So my girl had a spa installed at her house on a 240v run off a house subpanel. For safety, a basic fused disconnect box was installed a few feet away from the spa.

To the specs of the spa itself, the run is supplied at the subpanel by a two-pole 30 amp breaker. The disconnect box contains 30-amp time-delay current limiting buss fuses. The disconnect box layout is pretty standard; the two hot legs come in, hit the tabbed pull-out disconnect, continue on to their respective fuses, and then continue on to the spa (via some power cord bolt-down on the side of the home).

I haven't been able to put my thumb on it, but after some inordinate amount of time (sometimes days, sometimes weeks) of casual spa operation and having it just idle heat and pump clean from time to time, we notice the pump operation ceases completely but basic control panel features remains. I believe this is indicative that the spa is not seeing the full 240v anymore.

Replacing the buss fuses in the disconnect box fixes it, albeit temporarily.

When it's funked up, taking a DMM to the disconnect box I see 120v from each hot leg to ground just as they come into the box, and 240v across as expected. Measuring from after the fuses though, I see maybe 110v out of each hot, and absolutely nothing register when I go across.

Is it possible that fuses like that would wear in some way that those hots would become out of phase and/or even a little bit resistant?

Anyone know of any quick workarounds that wouldn't compromise her safety? Would I be better off just replacing it with a proper full-blown GFCI panel?

Thanks for readin',

- Cloud
 

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Electrician
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Neither of the fuses ever blow? Then the fused disconnect is defective. One of the hot legs breaks contact and the spa gets only 120v from the other hot to neutral. The control panel runs on 120 and just happens to be on that leg. Wiggling the fuse holders when you replace the fuses is what reestablishes contact, for a while. The 110 you measure is one hot leg on, and the other being back fed thru the spa. Unplug the spa to get true readings. I don't recommend one GFI feeding another. If there is a GFI in the spa or at the end of the cord, a simple disconnect is correct. If not, the disconnect should have been GFI per code and for safety.
 

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What you are describing sounds like one of the two fuses is blowing, but not both. The controls run on 120V, which is supplied by only one hot leg. When only one fuse blows, the controls still receive power from the other leg (either directly or back-fed through the 240V load). The 240V load (motor and heater) gets no power at all. Did you test the fuses before replacing them? Are you sure the fuses are sized correctly for this hot tub? If so and they are still blowing, then there may be a defect in the tub.
 

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Licensed Electrician
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Is the power for the tub GFI protected in the main panel?
 
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Is this a problem that:
1. Suddenly happened and persists to this day?
2. Gradually got worse and persists to this day?
3. Comes and goes?

Check for loose connections. A test load consisting of two 100 watt incandescent lamps in sockets wired in series attached to a small board and with clip on wires can help while you are making both 120 volt and 240 volt measurements.

You also want to pull out the tabbed disconnect block with fuses to check the tabs for burn marks suggesting a loose connection there.
 
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As post #3 says above, one fuse is blowing and the other "backfeeding". If you were to disconnect the spa, you would get nothing on one leg.

Check the manufacturer's recommended circuit amperage and wire size for that circuit. A long wire run might need a larger gauge wire.

A "not so good" connection at the fuse can cause heat and thus the fuse to blow.

And also check amperage draw on both hots. Electric motors if old and worn can draw more amperage. In cooler weather the heating element would run longer.
 

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Discussion Starter #8 (Edited)
Ahhhh, the idea of the backfeeding hadn't even occurred to me, so the readings with the single fuse blown definitely make sense now. Now that I'm back at her place I'll check the spa specs and take a closer look at the subpanel. I'll probably just end up replacing that box with a true GFI load center if it turns out they left it completely unprotected.

If so, am I understanding correctly that I need to get a GFI that's at least my 30A load (50A and 60A seem pretty commonplace) and that'll provide the essential ground fault protection all the same?

Thanks all around guys.

Cheers,

- Cloud
 

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Before doing anything I would see how much amperage the spa is drawing and find out from the manufacturer how much amperage it *should* be drawing.

If it is drawing more amperage than it should, that could indicate a problem with the spa which requires service.

Amperage is checked with a clamp amp meter.

And I would check the wiring connections on the fuse which is blowing - if a poor or loose connection, that can create heat - blow the fuse.

See "Non-contact thermometer" about 3/4 the way down the following page...
http://www.pqmeterstore.com/crm_uploads/test_tools_and_troubleshooting_electric_motors.pdf

(Also "Clamp-on ammeter" about 1/3 the way down the above page.)
 
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