Parallel fuses. Very dumb or forgiveable dumb - Page 1
Forum section: electronics / projects

author: cybermaus
author_rank: Frequent Contributor
timestamp: May 19, 2025, 12:22:09 pm
content: So I had a need to fuse some 12V MPPT solar at 30 AmpsFortunately, I very convenient had some spare spots on a fuse bank.Less fortunately, they were normal size automotive blades, and those are not ideal for 30 Amps. One should not use them above 20 Amp, go to Maxi size instead.And indeed, a test run with the battery empty and full sun so full 30Amp for 2 hours meant they got a tad too warm.So I had the .... "briliant" ... idea of putting 2x 20 Amp fuses in parallel.Also inspired because my own camper, an A-listed brand German Westfalia, did the same from factory with the battery-2-battery charger. Monkey see, monkey do.ConsiderationsPro:- heat will spead over twice the area- If one fuse is slightly off balance, it will get warmer, increase resistance, and thus throttle back in respect to the other (auto-leveling)- If one goes, the other goes near instantly- Variance means it probably is more similar to 2x15 rather then 2x20, so same 30 amps.- Having two actually averages out fuse variance a little, the nominal value is higher (40 Amp), but a single 30 Amp might actually go higher then the averaged pair of 20 Amps(Cable is 6mm2 so enough anyway)Con:- It just feels wrong.Anyway, now the pair stays at 45 celcius, where before on a single 30 it went to 105 celcius (2 hours of 30Amp load)

author: BradC
author_rank: Super Contributor
timestamp: May 19, 2025, 12:35:54 pm
content: I've used paralleled blade fuse holders between batteries on an APC UPS, and also in a 24V power chair. The APC's are 2 x 30A (max theoretical draw is 54A) and the power chair is 2 x 20A (max theoretical draw is ~35A). In both cases I've not had any issues. When I was setting them up a decent "short test" certainly blew both fuses quick smart.In both cases the connected cable far exceeds the fusing, so as far as protection goes "it'll do". Yeah, perhaps not ideal.

author: bostonman
author_rank: Super Contributor
timestamp: May 19, 2025, 01:50:14 pm
content: Your setup doesn't appear to have an issue, but the concern would be:QuoteYou could theoretically have one fuse seeing 19.xx A and the other seeing (30 - 19.xx) which would place the circuit close to the 20A max. You could measure it to confirm it's 10 each fuse, but, as I'm sure you're aware, resistance can change causing electricity to take the path of least resistance.

author: cybermaus
author_rank: Frequent Contributor
timestamp: May 19, 2025, 02:35:35 pm
content: Quote from: bostonman on May 19, 2025, 01:50:14 pmWell, a 20 Amp fuse will probably only blow instantly at 40 Amps, and take 30 Amp for possibly up to half an hour. Or something like that, would need to consult the actual sheets.What I do know without checking the sheet is they are not supposed to blow at 20 Amps, and take days at 21 Amp. So 30-19.xx is a bit overly precise.What I should be sure of is that the pair will blow before the wire melts and causes a fire. The fuse is there to protect the wire, not the MPPT device. (The MPPT device has an internal fuse for that)Having said that ... parallel fuses just feels wrong. Hence me fishing for validation.It just looks so nice. Otherwise I'd have to use a dangling in-wire maxi fuse. Probably would have better cooling though.

author: MarkL
author_rank: Unknown
timestamp: May 19, 2025, 03:01:14 pm
content: I've seen parallel fuses used by two reputable inverter manufacturers. One was Samlex in a 600W inverter which used two parallel standard-size blade fuses, and another was Xantrex in their ProSine 2.0 (2.0kW) charger/inverter which used two MEGA (bolt mount) fuses. I don't recall the exact current ratings in either unit, but both employed them on the positive battery inlet and they were both 12V units.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 19, 2025, 03:27:20 pm
content: The fuse elements themselves are probably fairly balanced (assuming identical fuses: definitely dont parallel mismatched fuses). That needs to be reasonably well controlled in manufacturing to get the right trip point. The way you can have a problem is if the wiring or contact resistance is mismatched and causes current hogging by one path. Keeping the current paths balanced and low resistance will help current sharing.Overall this is not ideal but in your situation will probably work.I don't think it's important here, but the physically larger fuse will have a higher voltage and current interrupt rating, and paralleling them doesn't increase that.

author: langwadt
author_rank: Super Contributor
timestamp: May 19, 2025, 03:29:36 pm
content: https://www.littelfuse.com/assetdocs/powr-speed-application-considerations?assetguid=a08de14c-7ab6-48bc-bf68-787ccefb4c21

author: bostonman
author_rank: Super Contributor
timestamp: May 19, 2025, 10:00:47 pm
content: QuoteI accidentally went too precise, you're correct. Without looking at fuse datasheets, and actually only having experience glancing at them without the need to analyze for a design, I expected the fuse to blow within thirty or so seconds of reaching (in this case) 20A; didn't realize it could be longer.As for my precise math, it was more to point out that one side "could" sit at near 20A, but again, you're correct.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 20, 2025, 12:28:22 pm
content: Fuses never ever blow "instantly". The quickest blowing fuses I know of are around 100ns, and you have to try very hard to blow it that quickly (Wikipedia indicates a 5kV 1uF capacitor, resulting in 1kA peak current). Easily measurable on an oscilloscope.https://en.wikipedia.org/wiki/Exploding-bridgewire_detonatorQuote from: bostonman on May 19, 2025, 10:00:47 pmAnd again, a 20A fuse can handle 20A continuously, without it blowing (maybe after 20 years, of old age, higher oxidation rates at elvated temperature, etc.).There is no precise math in fuses. It's more experience gained though trials, but there is materials science involved. And you can read the important conclusions of years of experience in a single datasheet. AlsoEEVblog 1677may be worth watching. He's blowing quite some fuses in that video, and even putting them in the freezer for some time has a significant influence. That's both surprising and sort of expected. Cryogenic "heat" treatment is a whole separate branch of materials science.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 20, 2025, 12:45:11 pm
content: I did a bit of reading and apparently paralleling fuses is quite common and mostly works as expected (I.e. it doubles the current) But there are some caveats. Fuses too close together heat each other, and may blow at a smaller current, balancing the restance of the fuses is recommended to be within 5% (That includes the paralleled wire sections) There are some more (also unxepected) changes, so I recommend you do some reading too. Just type in some magic words in a search engine.

author: Gyro
author_rank: Super Contributor
timestamp: May 20, 2025, 12:59:02 pm
content: Industrial fuses quite often use two paralleled fuse elements. Granted, they use the same contacts and they are maybe matched for element resistance. Eg...P.S. It is possible that this is done to reduce thermal mass of the fuse elements(?). The fuse shown above is an 'FF' semiconductor protection fuse.

author: bostonman
author_rank: Super Contributor
timestamp: May 20, 2025, 01:17:06 pm
content: Those videos were interesting.QuoteTrue. Maybe I shouldn't have used numbers to explain. My brain went off on "ideal" and was more focused on explaining my thoughts without considering the time/current curves.Placing fuses in parallel can come down to configuration too. Using "ideal" numbers for a moment: If for example someone had wire that was rated for 5A and the load was 10A, then they may decide to cheat and run two wires with a 5A fuse in each wire thus "parallel" fuses.This wouldn't be practical because one wire may see 6A and the other wire 4A. Assuming "ideal" numbers, the 6A would blow and the other wire would see the full 10A and blow.Using realistic numbers: one wire could sit at 6A for an extensive amount of time running the risk of the 5A rated wire (or board trace) getting very warm.The OP wasn't doing this, I understand, my example was just using a way of placing fuses in parallel that would be wrong.

author: Circlotron
author_rank: Super Contributor
timestamp: May 20, 2025, 01:36:34 pm
content: If you have for example 5 metres of wire leading up to the fuses, run separate identical half thickness wires to each fuses instead of one full thickness wire. This will help towards current sharing and maybe even save you some money on wire.

author: cybermaus
author_rank: Frequent Contributor
timestamp: May 20, 2025, 02:22:10 pm
content: Quote from: Gyro on May 20, 2025, 12:59:02 pmThose "fuses" seem to contain a diode element?Maybe helps with pre-heating the FF (double fast?) aspect of the fuse?)

author: Grandchuck
author_rank: Super Contributor
timestamp: May 20, 2025, 02:28:24 pm
content: Diode symbol means it is designed to product semiconductor circuits and will open at fault currents just over rated.

author: cybermaus
author_rank: Frequent Contributor
timestamp: May 20, 2025, 02:35:04 pm
content: Quote from: Grandchuck on May 20, 2025, 02:28:24 pmI did not know that, but it checks out:From a eaton manual:https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/bussmann-series-high-speed-fuses/bus-ele-an-10507-hsf.pdf