Conflicting two ways of measure Fuse

Hey guys. You know what they says, the more you read - the more you "confused".

Fuse protect wire. Some folks recommends fuse based on wire size. eg: 250A to 2/0 AWG. 400A to 4/0 AWG

The other opinion would be based on inverter able to draw. eg: 150A fuse to SPF5000ES /48x0.85

If fuse is too low for inverter, fuse blows.
If fuse is too high for wire, insulation melts, then wire shorts.

So size the fuse at least big enough for inverter, and size wire at least big enough for fuse.

Use efficiency of your inverter at full load (0.85?)
Use lowest battery voltage (48V? for my lead-acid, I use 42V)
Add another 25% to avoid nuisance blowing, and because wire connections and ambient temperature contribute to fuse temperature.
And my contribution, add 12% for what I call "ripple factor"; current drawn from battery carries rectified sine wave, causing additional heating.

5000W / 48V / 0.85 efficiency x 1.25 margin x 1.12 ripple = 171A, use 175A fuse.

You can use a larger fuse if wire is larger.
There are fast blow, slow blow, and "semiconductor" very fast fuses. Perhaps the fuse will save your electronics.
But fast fuses will blow just due to capacitor inrush, so consider pre-charge.

I just got some 200A semiconductor fuses. Even though I use AGM battery, I'm going to set up precharge.

Hedges said:

5000W / 48V / 0.85 efficiency x 1.25 margin x 1.12 ripple = 171A, use 175A fuse.

You can use a larger fuse if wire is larger.

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Thanks. Yes initially I planned to use 175A rate for whole setup. Then saw some folks mentioned on fuse according wire size.
I wondered if I should use larger fuse(250A) on larger wiring(4/0) or stick with 175A on 4/0. Same 5000W inverter, 200A BMS

- I also planned to use 160A breaker(25ka) to "protect" the fuse besides as on/off switch.

- At this point I'm not sure if I connect two 16cells battery to busbar separately(requires 2 fuses each) or diagonal connection where I saves 1 fuse?

I fuse each battery individually prior to bus bar then fuse inverter cables also, fuses are pretty cheap insurance

Wire Size vs Fuse.
1) The wire must be able to support the Amperage going through it. Your AIO @ 48V/5000W can handle 10,000W surge. As hedges pointed out, 175A Support is needed but for the wire, it is better to upsize a bit for comfort elbow room and to reduce line loss.
2) Fuses / Breakers have to be sized to support the Amerage demand of the AIO & the surge potential. A High Quality Fuse/Breaker can handle such surges.. Therefore a 175 to 200A breaker or fuse is acceptable.
3) Each Battery Pack should be fused or have a Breaker that matches what the BMS can output.

BTW: We use terms like 12V/24V/48V as a legacy nod to Lead Acid and as a "general" voltage value for the level of the equipment. 48V is 3.000V per cell which is 0% SOC is staying within the Working Voltage Range (which is 3.000-3.400 Volts per cell). A 48V system is ACTUALLY 51.2V or 3.200Vpc which is LFP Nominal Voltage.

Here is a Wire Chart from SouthWire related to their Very Fine Wire welding cable which most of us use for battery & inverter cables. Note the AMPACITY handling for the wire sizes. 10,000W @ 51.2V uncorrected = 195A (always safest to use the larger values - Anti-Murphy's-Law rule. Your wire shoudl be at least 2 AWG (190A) but 1 AWG (220A) would add a good buffer and keep line loss down.

Steve_S said:

Therefore a 175 to 200A breaker or fuse is acceptable.

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How about 160A breaker and 175A fuse, so breaker will trip before fuse open. Will this work?

Each Battery Pack should be fused or have a Breaker that matches what the BMS can output.

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JK BMS 200A but inverter on 170A range.. Can you check this Bussmann Fuse?

Your wire shoudl be at least 2 AWG (190A) but 1 AWG (220A) would add a good buffer and keep line loss down.

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I've already have 2/0 AWG for the inverter and ordering 4/0 AWG for batteries.

solitarysecluded said:

How about 160A breaker and 175A fuse, so breaker will trip before fuse open. Will this work?

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Attempting "coordination", study the trip curves carefully.
The shape will differ, and maybe under some conditions fuse would blow first. (That's what you want for such a high current that breaker would be damaged.)

I'm starting to like magnetic-hydraulic breakers, think they can trip faster for moderate overloads, better to protect motors.

I considered class T fuse in addition to my 200A thermal-magnetic main breaker, and came up with 400A fuse.

solitarysecluded said:

How about 160A breaker and 175A fuse, so breaker will trip before fuse open. Will this work?


JK BMS 200A but inverter on 170A range.. Can you check this Bussmann Fuse?


I've already have 2/0 AWG for the inverter and ordering 4/0 AWG for batteries.

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I hate to say it but you are over complicating and this is an area where "Keep It Sweet & Simple" is more often than not the correct solution. Over wiring is not bad in general, because the longer the run the more derating occurs so it helps to limit line loss, but getting too carried away has other side effects. IE My "run" length from battery terminals through to Inverter is 14'/4.2m so I have immediate experience you can say... Went from 2/0 to 4/0 when I updated my system a ways back.

No worries Steve, I agree with you. I'm all for simplicity and safety.

I went for 4/0 now so less headache on inverter upgrade next time. I'll use 175A as advised by you and Hedges. Thanks guys!

For knowledge purpose, if we run two inverters and two battery racks - in event one inverter failure, the other active inverter's amp will be divided half to both battery racks?

If yes, I guess only the fuse near inverter is working while both fuses near battery racks become "temporary unusable" or "too large"?

Current will split somewhat unevenly due to any differences in wire resistance, and different points on V/SoC curve of batteries.
That is more of a problem if both inverters are drawing maximum current, and a single battery can't supply 100% due to BMS limitations or knee of curve.

All fuses will effectively protect the wires downstream of them, in the event there is a short somewhere. They aren't "too large."

I couldn't find T-class at 150A-225A range. Out of stock and pending orders for many months.

So I guess I can put T-class 250A on 4/0 wire to each battery.. right?

As for inverters to busbar on 2/0 wire, I intend to put this 175A fuse.. it is doable?

Going from memory without looking it up again, those wire sizes seem correct.

Almost any oversize fuse should still protect against catastrophic shorts.
I rely on breaker in inverter for overload protection.

For "Semiconductor" fuses, I definitely recommend pre-charge of inverter capacitors before completing circuit.
I plan to do that, even though I use AGM.

I have come across another type JJS. I think the only difference is vol rating. JJS has 600V while semicon 500V.

The one out of stock at 160Vdc. Lot people still using 500V - 600V fuse, I guess voltage rating not matter much?

I can sympathize all around. Another characteristic about wire is the amps vs length. A 1" length of wire can handle insane amounts of current compared to 1 foot. 1 foot can handle significantly more than 10 feet without getting hot. I use a chart - https://www.powerstream.com/Wire_Size.htm - that shows 'chassis amps' (short lengths like 1 foot, 5 feet) vs 'transmission amps' (longer lengths like 50-100-200 ft) + wire specs (temp rating) + hands-on over several years as my guide. This length issue adds a bit of fuzziness sometimes and it's worth keeping in mind as you read suggestions. As a practical matter, if a wire is 'a little warm' to the touch then OK but hot is not OK and needs larger wiring

I agree with all above and to repeat, just in case it helps at all....
Fuse/breaker wise I go with "Protect the Wire"! Obviously you want wire size big enough to handle the current needed a load - but keep in mind these are 2 different concepts. So perhaps figure out the wire size you need for the inverter (load) and *then* size the fuse/breaker to protect that wire be it the minimum size or larger size - just keep the concepts separate and you'll be good. You can always have wire / fuse larger than you need but still, fuse to protect the wire. The inverter (or any respectable load) will be designed to take care of itself - you don't need a fuse/breaker to protect the load! You do sometimes use a breaker (instead of a fuse) as a practical way to de-energize a load so you can work on it but it's not about protecting the load.

Most of us are using 48V batteries, which reach maybe 60V or 65V max.
The higher voltage would only matter for high voltage batteries or PV strings, and PV strings don't need the massive AIC rating.

AC and DC voltage ratings of the fuses differ. This class T one is 300 Vrms AC, but only 125VDC for higher current fuses.

www.littelfuse.com/media?resourcetype=datasheets&itemid=78225034-cd5b-4006-8871-38e7378ac8f3&filename=jlln-fuse-datasheet

Hedges said:

Most of us are using 48V batteries, which reach maybe 60V or 65V max.
The higher voltage would only matter for high voltage batteries or PV strings, and PV strings don't need the massive AIC rating.

AC and DC voltage ratings of the fuses differ. This class T one is 300 Vrms AC, but only 125VDC for higher current fuses.

www.littelfuse.com/media?resourcetype=datasheets&itemid=78225034-cd5b-4006-8871-38e7378ac8f3&filename=jlln-fuse-datasheet

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Right. In that case semiconductor 500vdc mentioned above is not suitable..?

Just noticed Bussmann lowest is at 160vdc. LittlelFuse harder to source.. is this why people ended up with all breakers? lol

Nothing wrong with using higher voltage rated. May be a different body size, like 600Vrms class T vs. 300Vrms.

You can find these fuses used or old stock on eBay.

Hedges said:

Nothing wrong with using higher voltage rated.

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Can we say as long as amp is within protective range of inverter and wire size, higher voltage doesn't matter ya?

For knowledge purpose, does anyone install fuse as close as next to BMS, inside box perhaps?
Safe to do so or "too near for comfort" arc risk?

solitarysecluded said:

For knowledge purpose, does anyone install fuse as close as next to BMS, inside box perhaps?
Safe to do so or "too near for comfort" arc risk?

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Well, to meet 'code' in my jurisdiction.... metal boxes and conduit (similar to home wiring) is required. So in my case - yes, all breakers are in boxes. Even in my DIY trailer I use boxes. Its a common safety technique, regardless of code, to protect against drops, pokes, collisions, spills, etc and to contain arcs on connection points.

As a dramatic example, I had a but-joint arc thru and if it hadn't been in a metal box I could very well have had a fire.

solitarysecluded said:

I couldn't find T-class at 150A-225A range. Out of stock and pending orders for many months.

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PKYS has a variety of Bluesea of class T fuses in stock: https://shop.pkys.com/Class-T-Fuses_c_113.html

OffGridInTheCity said:

Well, to meet 'code' in my jurisdiction.... metal boxes and conduit (similar to home wiring) is required. So in my case - yes, all breakers are in boxes. Even in my DIY trailer I use boxes. Its a common safety technique, regardless of code, to protect against drops, pokes, collisions, spills, etc and to contain arcs on connection points.

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Make sense. Thanks for sharing.

I was wondering how most people do it, pros cons versus inside/outside battery rack.

solitarysecluded said:

I couldn't find T-class at 150A-225A range. Out of stock and pending orders for many months.

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Mouser Class T in stock Littelfuse >100A
some of these are pretty expensive, others are not