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Old 12-07-2020, 07:56 PM   #1
tmw
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Scary find

The attached photo is the fuse that goes between my solar controller and the house battery from my 320 watt array. I was a little frightened to find it and I only found it because I noticed the controller screen was dark while winterizing. I thought it was a bad fuse or fuse holder. For fear of burning down the boat yard, I disconnected the solar array.



Last winter I changed the controller from a PMW to a MPPT. I was comparing the data logs to see the difference and noticed that I would commonly get 22-23 amps of charging current at peak times, mid summer. I'm guessing that the high current was enough to get it hot and slow cook the fuse and holder but never enough to blow the 25 amp fuse.



Either way, I'm changing it over to a circuit breaker. The math says I shouldn't get that much more power out of the panels. The wiring is either 8 or 10 gauge. I'll have to check but I'm wondering if I should just go to a 30 amp breaker with 10 gauge cable and leave it at that or change the cable to 8 gauge and go higher.





Thanks
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Old 12-07-2020, 08:42 PM   #2
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22 or 23 amps should not "slow cook" the fuse, even less its holder if properly sized.
If you are not sure about the size of the cable you need or the size of the breaker/fuse I would recommend you to check with an electrician.

L
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Old 12-07-2020, 09:03 PM   #3
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Thanks Lou,

I suppose it could have been just a bad connection in the fuse holder which is why I'm thinking a hard wired circuit breaker as a replacement. I have little doubt the current ever exceeded 25 amps because the fuse was still working and the data is recorded every 10 minutes and never got that high.



When I put it all together I was trying to decide between 8 or 10 gauge on the tray cable and I don't recall what I actually used but for sure not less than 10. 10 gauge is sized perfectly for 30 amp at the length needed according to ABYC charts. 8 is the overkill step up.

Thanks again
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Old 12-07-2020, 09:08 PM   #4
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I always go at least 1 size up in cables.
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Old 12-08-2020, 12:17 AM   #5
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Caveat: I'm not an electrician. I'm assuming you calculated the required gage correctly (10awg @25A gives 10' round trip circuit distance @3% voltage drop, so 5' max distance). 3% of 330W is about 10W across the 10' of wire - shouldn't be enough to cook anything anywhere.

It looks like the hot spot was at the fuse holder, maybe a loose connection that was generating the heat due to high resistance. If the fuse holder and fuse are spec'ed and connected properly that shouldn't happen & you shouldn't need to move to a breaker. Perhaps the holder used was not designed for high current? I'd make sure the problem isn't present anywhere else (alternator wiring, battery chargers) where you see more than a few amps.
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Old 12-08-2020, 12:25 AM   #6
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Quote:
Originally Posted by socalrider View Post
Caveat: I'm not an electrician. I'm assuming you calculated the required gage correctly (10awg @25A gives 10' round trip circuit distance @3% voltage drop, so 5' max distance). 3% of 330W is about 10W across the 10' of wire - shouldn't be enough to cook anything anywhere.

It looks like the hot spot was at the fuse holder, maybe a loose connection that was generating the heat due to high resistance. If the fuse holder and fuse are spec'ed and connected properly that shouldn't happen & you shouldn't need to move to a breaker. Perhaps the holder used was not designed for high current? I'd make sure the problem isn't present anywhere else (alternator wiring, battery chargers) where you see more than a few amps.


Most likely it was a defective fuse holder.
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Old 12-08-2020, 01:17 PM   #7
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A poor connection will definitely heat up especially when run for long periods of time. Those types of connectors "Faston" are notorious for this. A few years ago I had to investigate exactly this type of problem on a $1M piece of equipment at work. Same issue. A rather flimsy "faston" connector could not handle 13A continuous.

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Old 12-08-2020, 01:25 PM   #8
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I've had Faston connectors heat and fail at less than 10 amps. They really aren't very good for even medium current connections. A proper hermetic crimp and screw down connector are far better.
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Old 12-08-2020, 02:11 PM   #9
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Thanks. I agree about the festoon fuse holder and don't think I have them anywhere onboard. Usually if I find them I replace them mostly because I only want to carry one style fuse. For an inline applications like that, I normally only use an atc/ato holder like the one in the attachment. This one is rated at 30amp. The cable run from the controller to the battery is short, maybe 5 or 6 feet round trip. So my understanding is 10 gauge for 30amp circuit is still slightly overkill using he the 3% loss table and well within 10% loss.



It must have just been a poorly seated fuse connection. Either way, the fuse holder has to be replaced because it was the holder that lost the connection the fuse is surprisingly still intact.


Thanks to all.
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