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diy solar

300A fuse caught fire?

Bluedog225 said:
It’s a big deal.

As you probably know, the fuse needs to blow before the wire catches fire.

This chart explains it all. Fuse has to be lower than wire ampacity.

Click to expand...
This confuses me. First, 2/0 welding cable can handle 300 amps. Second, I thought the whole idea of an oversized fuse was to not have nuisance blows but to have protection in case a major inrush happens from a short. At that point, even a large fuse is going to blow before it damages the wires.
 
Tulex said:
This confuses me. First, 2/0 welding cable can handle 300 amps. Second, I thought the whole idea of an oversized fuse was to not have nuisance blows but to have protection in case a major inrush happens from a short.
Click to expand...

This also confused me for a long time too.

You can’t simply say, “2/0 can handle 300A”. It can, but only IF the insulation jacket is rated for a temperature of 105C and it’s a short run of cable. If you use cheaper 75C cable, it’s only good for 175A.

The other part is you don’t fuse to avoid “nuisance blows”, you fuse for less than the amp rating of your cable. If you use 105C 2/0 copper cable, use a 300A fuse, if 75C, use a 150A fuse. Be sure to check the exact specs of the copper cable you intend to use before sizing the fuse.

Now if you do get get nuisance blows, then your inverter is sized too large for your cable and fuse. Either get a smaller inverter or size up cabling and fuse. Sizing up cabling could mean going to a thicker gauge or simply higher temp rating insulation.

At that point, even a large fuse is going to blow before it damages the wires.
Click to expand...

Maybe? But that’sa guessing game. You don’t want to guess that a “large fuse” is going to protect you. Ya gotta do the math properly. The ONLY thing the fuse is designed to protect is the cable it’s fused on. It’s to prevent fires, nothing else. If you place a 300A fuse on cable that’s rated for 150A (but you think is rated for 300A), you’re literally playing with fire.

Quality wire has the temperature rating in Celsius written on the insulation.
 
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My 2/0 cable is rated from -50C to +105C. The manufacturer of my 2/0 cable, Windy Nation, states it is rated for 325 amps and temp rated from -50C to +105C. My total draw on my batteries through my 24 volt 3000w inverter is about 2800 watts if everything is turned on all together. Was thinking that a 175A or 200A would be fine and would blow way before the wiring insultation got hot enought to melt.
 
UGT said:
My 2/0 cable is rated from -50C to +105C. The manufacturer of my 2/0 cable, Windy Nation, states it is rated for 325 amps and temp rated from -50C to +105C. My total draw on my batteries through my 24 volt 3000w inverter is about 2800 watts if everything is turned on all together. Was thinking that a 175A or 200A would be fine and would blow way before the wiring insultation got hot enought to melt.
Click to expand...
Yeah that would be fine.

3000/24/.85= 147A. So your 175 or 200A fuse would be dandy. Assuming your cable runs are short, like 15 feet or less.
 
UGT said:
My 2/0 cable is rated from -50C to +105C. The manufacturer of my 2/0 cable, Windy Nation, states it is rated for 325 amps and temp rated from -50C to +105C.
Click to expand...
Sure, but at 325 amps the cable will be 105c
That means everything the cable touches will also be exposed to 105c. That would likely melt any plastic fuse holder and likely the inverter side connection too.
 
UGT said:
My 2/0 cable is rated from -50C to +105C. The manufacturer of my 2/0 cable, Windy Nation, states it is rated for 325 amps and temp rated from -50C to +105C. My total draw on my batteries through my 24 volt 3000w inverter is about 2800 watts if everything is turned on all together. Was thinking that a 175A or 200A would be fine and would blow way before the wiring insultation got hot enought to melt.
Click to expand...

Buy both a 150 and 175 amp fuse.

If the 150 blows for no apparent reason at max load, replace it with the 175.
 
Tulex said:
This confuses me. First, 2/0 welding cable can handle 300 amps. Second, I thought the whole idea of an oversized fuse was to not have nuisance blows but to have protection in case a major inrush happens from a short. At that point, even a large fuse is going to blow before it damages the wires.
Click to expand...
I work at it the following ways. I do this because I want to mitigate any fire risk that may occur. It’s great to ‘protect the wires’ but protecting against fire is the goal.

First, I say it is incorrect to merely fuse for the wire jacket rating. Second, in high-load 12V-48V applications, I don’t want to see any heat build anywhere. Third, our systems are essentially different than grid power which has in practical terms “unlimited” amp delivery resources.

I’ll start with the third consideration, amp capacity. I’ll use my currently in place arrangement as an example.
  • A 2000W inverter at nominal 12VDC presents a potential ~167A load (at steady maximum delivery, not surge). I use 2/0 cables to the busbar. Do I fuse that at 200A? 325A as windyNation publishes? No. I would fuse that at 175A using a maxi fuse, Class T, or a properly rated known brand name DC breaker. (I wouldn’t probably spend on a Class T here due to cost, and the chance of nuisance blows. I just utilize a regular fuse because the lithium batteries each have Class T installed so the kA problem is solved.) I normally fuse at just over the maximum demand of the device because anything much over that may never blow if there’s a fault that can cause a fire that doesn’t exceed the load of the inverter. I just looked and I currently have 150A ANL fuse ahead of the 2000W inverter. It’s never blown even when I maxed things out in testing (inverter starts the stupid light-duty craftsman tablesaw! Never expected that).
  • My cheap QZRELB inverter has a “soft-start” feature which probably solves the surge nuisance blow problem.
  • Related to above, a 2000W 120VAC output inverter ‘can’ support a 120VAC 16.6A load. Using 12ga romex is safe in that range as it is rated at 20A. Or is it?! A 20A breaker will never trip at a 18A fault… it should only see 16.6A in normal use, and even a 24A ‘surge’ will not immediately trip a 20A breaker. So I would and do use a 15A 120VAC main breaker. (There are chinesium 16A but I don’t think you’ll find regular US 120VAC panel breakers at 16A). I also have the branch circuits 15A or 10A breakers- I want them to trip in an event, not test their resilience. They never trip; 800W coffeemaker is a 7A load. (I used 10ga romex from inverter to breaker box btw. Some 12ga and some 14ga branch circuits.)
  • So that’s overkill but it should not have a fire event.
The Second Point:
  • I oversize inverter feed cables (and as mentioned fuse for the load). The SCC output cables are slightly oversized. As is the panel cabling. But the inverter cables are substantially bigger and never get hot.
  • The cost difference to oversize these short cables is often a lot by percentage while the difference in actual dollars is generally only a pizza or two different.
  • It’s an insurance policy you only have to pay once. Cheap insurance.
First point was, “it is incorrect to merely fuse for the (printed) wire jacket rating”
  • The welding cable from windyNation mentioned above is what I use. Its maximum amp rating is for free-air welding use. But cables strung across the shop floor to your work is a bit different than what you have with them cable-clamped to your wall.
  • I use the ABYC chart. Blue Sea kindly publishes it online. I know it’s conservative and takes into account many factors we’d normally have to calculate and design for. But ABYC has plenty of safe headroom. 2/0 for example is listed as 200A… And 200A at 12VDC, nominal, is good for up to 2400W inverters. (I won’t get into the how/why of 3000W/250A)
  • But again, I would fuse at or maybe 15% above the max load the inverter could present, but never more than the ABYC chart for the cable. I just want the fuse to blow if something outside my installation design happens.
 
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400bird said:
Sure, but at 325 amps the cable will be 105c
That means everything the cable touches will also be exposed to 105c. That would likely melt any plastic fuse holder and likely the inverter side connection too.
Click to expand...
I read that differently but am certainly open for discussion. At an ambient temp of 105c that wire is rated to carry 325 amps, not the temp the wire gets up to. My normal commisioning procedure is to take systems up to thier rated ampacity and leave them there for at least 30 minutes. Other than a loose conection or a faultly crimp I rarely see temps more than ambient plus 10F and never more than ambient plus 20F.

Here is some good reading.
 
Tulex said:
I think it's more about something going grossly wrong. With DIY batteries, it seems to me it's the most likely place something fails.
Click to expand...

Even with proper fusing, poor connections can overheat, melt the insulation jacket, smoke and possibly lead to fire.
 
OzSolar said:
I read that differently but am certainly open for discussion. At an ambient temp of 105c that wire is rated to carry 325 amps, not the temp the wire gets up to.
Click to expand...
It's not. If the temp rating was ambient temp, why would the wire be able to carry more current at higher temps? The copper is fine at 105c, if anything resistance increases with temps. The insulation melting/breakdown/fire temp is the issue and why silicone (with it's high temp resistance) gets a higher currents rating. The copper will get warmer with more current and the silicone can handle it.
OzSolar said:
Here is some good reading.
Click to expand...
Maybe I missed it, but I didn't see anything in that link one way or another.
You'll need to point out the details if you think they're backing up your theory.
 
UGT said:
Wasn't sure about the 300A fuse all along. I am now trying to decide on going with 175A or 200A. I am fortunate to have been able to shut down the system in time. From the posts above am convinced that the way I had the stack with the nut in between the two lugs was the problem. Shudda known better but now I do. My system is mostly used for backup in case the grid goes down here in Austin. With everything that is hooked up to my inhouse transfer switch I would pull abut 2800 watts and that is if everything runs at the same time. So thinking that the 175A would be enuff without experiencing blown fuses. I have a couple of 175A fuses that I had left over from my camper van solar system. Checked the fuse holder in the van I didn't make the same mistake with it. Thanks for the comments.
Click to expand...
Always good to be open to learning, so points to you instead of defending it. Glad you were safe and there to catch it.
 
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