Pool Heater Nuisance Tripping

I have a pool water tankless instant heater that has two elements, each fed by a 220v 40amp double breaker.
When I measure the current to each element it is around 35amps.
When I check the heater a couple of times a day, often one of the circuit breakers has tripped. One trips more than the other, but both have tripped.
I never see it happen, of course. And when I reset the breaker it stays set. But when I go back a few hours later it'll be tripped.
Are there special breakers for this type of heater? Perhaps one with a time delay?
Suggestions welcome.
Stephen
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On Mon, 25 Mar 2013 10:14:33 -0400, "The Streets"

Are these GFCI breakers (test button)?
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wrote:

No, these are not GFCI breakers.
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The maximum current draw for fix equipment is not supposed to exceed 80% of rating of the circuit. In your case the maximum is 32 amps per 40 amp circuit and you're a good 10% over that. The excessive draw over time likely has weakened the breakers. The easy fix is to replace both breakers. The correct fix is to upgrade the wiring to 50 amp circuits.
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Thanks for the suggestions. Upgrading the wiring is probably cost prohibitive due to the distance from the panel, etc.
I'll at least try replacing the breakers first.
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On Mon, 25 Mar 2013 11:48:32 -0400, "The Streets"

What size wire do you have now?
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Keep in mind that the circuit breaker might have been protecting the existing wiring. Upsizing it might let the wiring overheat. If you know some or all of the following and want some thoughts, let us know: Do you know what AWG the existing wiring is? What type? And how is it installed (is it all in the same room, and surface mount? Or does it go underground, either direct buried or in a conduit)? While we're at it, how far is it (wiring length) from the circuit breakers to the heaters?
j
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Main panel to sub-panel is #4 copper with a 100a breaker in the main panel. Fairly long run (50'+) mostly in gray conduit along side of building. Short run (<10') from sub-panel to heater. #8 copper.
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On Mon, 25 Mar 2013 14:23:49 -0400, "The Streets"

If this is wire in a raceway, it is probably THHN/THWN and you can bump the breaker up to 45 or 50a. (using the 75c column of 310.16)
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wrote:

Short run from sub-panel to heater is in flexible liquid tight.
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news:r-Sdne-

Apparently this whole setup was not done by a licensed electrician. The #4 CU is only rated for 95 amps, not 100 amps. Then the two 40 amp heater circuits were undersized and also not put on GFCI breakers. If it were me and my family, I would spend the money to have a licensed electrician check the whole system out. Fortunately I am one so I wouldn't have to foot such a bill. But in your case, a $100-$150 bill is well worth making sure people that use the pool are safe.
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wrote in message

Don't know who did the original install but was told it was done by a licensed electrician. All connections between the heater and pool equipment are PVC so well isolated from the pool.
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Hate to burst your bubble, but water conducts electricity. If one of those heating elements ceases to become watertight inside the tankless unit, you will have electricity going straight through the water and seeking a source of ground. Quite frankly this potentially could be happening now and I would've suggested it as the cause of the tripping breaker if only one breaker was tripping. Both heating elements would have to be compromised in order for both breakers to be tripping. The chances of it randomly happening to both at the same time are very remote.
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wrote in message

Actually, it's just the impurities in water that conduct electricity. But your point is well taken.
While I wasn't involved in the initial installation, I was involved two years ago when the owners changed to a different model heater. At that time the installation was reviewed by a licensed electrian. And he ran the new wire from the sub-panel to the heater (the new heater was in a slightly different location and the original wires wouldn't reach). He was fine with the 100amp breaker in the main panel and didn't say anything about changing the sub-panel breakers to GFCI models.
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wrote:

You mean 85a. You only use the 90c column for derating.
It is not uncommon that people use 310,16(B)(6) incorrectly and assume #4 is OK for 100a. That table is only for the main feeder into a residence. This should be on a #3 or #2,.
If the wire going to the heaters is heaters is #8 THHN/THWN he can still put in a 45 or 50a breaker but he is wasting some power heating up that feeder. If it had a 90a breaker it is a "hold your nose" legal installation. (using the round up rule from 85a) In the grand scheme of things it is not that dangerous, even with the 100a.
Unfortunately there is no rule requiring GFCI on a pool heater (only a spa heater) He should verify that the 8 gauge solid copper bonding wire is present tho. That bonding grid should also pick up the rebar in the pool shell, the ladders,, pump, underwater light and any other metal or electrical equipment for or within 5' of the pool
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