Stumped on Class T fuse:

[Saltyoldcruiser]

I'm about to do the final wiring on my 4S 280 AH battery with a 200 amp JK bms. I have double 2 awg wires from my negative on my battery to my BMS and will be using double awg2 wires to a class T fuse between the positive and the main bus bar. My heaviest draw will be a 2000 watt inverter. What size class T fuse should I be looking for?

 

[time2roll]

Parallel #2... I would have 110a Class-T on each wire.

 

[Ampster]

I would have 110a Class-T on each wire.

And I prefer the fuses between the battery and the wire, not at the bus bar.

 

[Saltyoldcruiser]

And I prefer the fuses between the battery and the wire, not at the bus bar

 

[Saltyoldcruiser]

Yes, The fuse will be before the busbar. I was thinking on one 225 amp fuse at the termination of the dual wires. That would make things a lot simpler for me. Thoughts on that? I'm thinking of the dual #2s as a single 1(0). Am I on the wrong track?

 

[Saltyoldcruiser]

Yes, The fuse will be before the busbar. I was thinking on one 225 amp fuse at the termination of the dual wires. That would make things a lot simpler for me. Thoughts on that? I'm thinking of the dual #2s as a single 1(0). Am I on the wrong track?

I'm using the dual #2s for the flexibility more than anything, and also because my bms has 2 terminals to spread the load.

 

[Ampster]

I'm thinking of the dual #2s as a single 1(0). Am I on the wrong track?

If one of the #2 wires fail the 225 Amp fuse won't protect the other #2 wire. That is the logic for separate fuses for each wire since the fuses are to protect the wires.

 

[time2roll]

How about go single 2/0 copper?

https://www.customcableusa.com/products/custom-battery-cable-marine-grade-by-the-foot

 

[Saltyoldcruiser]

How about go single 2/0 copper?

https://www.customcableusa.com/products/custom-battery-cable-marine-grade-by-the-foot

The issue is that my bms has 2 terminals for the negative lead from the battery and the bms terminals are M6. I'm reluctant to put one 2/0 cable on a m6 terminal. That, and there's the flexibility of the 2/0 wire. I think I would be fine with 1/0 wire with only 200 amps continuous but that's still a lot to ask of a m6 terminal screw.

 

[time2roll]

I modified an extra cell bus bar and placed across the two BMS terminals to minimize electrical stress on the circuit board to feed full power to the single terminal.

 

[Symbioquine]

Maybe this is relevant: https://diysolarforum.com/threads/s...-cable-arrangement-thoughts.60483/post-747718

 

[Saltyoldcruiser]

I modified an extra cell bus bar and placed across the two BMS terminals to minimize electrical stress on the circuit board to feed full power to the single terminal.

I did think about that but I'm trying to compact this circuitry into a rather small area. Every component takes up real estate that is kind of scarce. It might just come to that.

 

[Saltyoldcruiser]

I did think about that but I'm trying to compact this circuitry into a rather small area. Every component takes up real estate that is kind of scarce. It might just come to that.

I did think about that but I'm trying to compact this circuitry into a rather small area. Every component takes up real estate that is kind of scarce. It might just come to that.

But you'd still have 2 wires from the busbar to the bms, correct?

 

[JK14]

The issue is that my bms has 2 terminals for the negative lead from the battery and the bms terminals are M6. I'm reluctant to put one 2/0 cable on a m6 terminal. That, and there's the flexibility of the 2/0 wire. I think I would be fine with 1/0 wire with only 200 amps continuous but that's still a lot to ask of a m6 terminal screw.

Sorry if I am confused here, but the BMS and the dual 2AWG wires are all on the negative side right? From the main neg battery post to BMS to negative bus bar - and no fuses are needed.
The T-class fuse is best close to the main positive battery terminal then leading to the positive bus bar. There is no clear advantage in my mind to using a pair of 2AWG wires vs a single 2/0 wire (which has the same total cross sectional area) on the positive side.
Am I missing something? I don't see the reason the pair of BMS terminals should affect the wire choice to the fuse.

Of note: ABYC states that using pairs of parallel wires is acceptable to lower the voltage drop (not a big deal here with short wires) but that each individual wire should be able to handle the whole load. So that's one vote for a single wire, but may be less critical when not on a boat.

 

[Saltyoldcruiser]

Sorry if I am confused here, but the BMS and the dual 2AWG wires are all on the negative side right? From the main neg battery post to BMS to negative bus bar - and no fuses are needed.
The T-class fuse is best close to the main positive battery terminal then leading to the positive bus bar. There is no clear advantage in my mind to using a pair of 2AWG wires vs a single 2/0 wire (which has the same total cross sectional area) on the positive side.
Am I missing something? I don't see the reason the pair of BMS terminals should affect the wire choice to the fuse.

Of note: ABYC states that using pairs of parallel wires is acceptable to lower the voltage drop (not a big deal here with short wires) but that each individual wire should be able to handle the whole load. So that's one vote for a single wire, but may be less critical when not on a boat.

Yeah, this has been something I've stressed over to be honest. I do need to use a paired wire on the negative side because the bms has two terminals and there's not enough room for a single 2/0 terminal on either one. it's designed for paired wires. I do realize that it's not optimal but I'm going to run a pair of awg 2 wires from the bms to the negative terminal where both wires will terminate in a single lug. I've checked the lug and it crimps the pair of awg 2 wires nicely with a Temco hydraulic crimper. I could indeed run a single 2/0 wire from the positive to my class T Blues seas fuse block. The only reason I'm using a pair of awg 2 positive wires is flexibility. There are some tight bends that I'll never get the 2/0 to make. As you stated the cross section of the 2 awg pair is the the equivalent of 2/0. It might actually be a tad bigger than 2/0 so the ampacity is not the issue. I guess that's why we take the precaution of a class T fuse. There are some physical limitations to what I can and can't do and this is the compromise I've somewhat reluctantly accepted. Another point is that while this is a 200 amp bms with capability for a 350 amp surge, I'll be setting it up for a max 175 amps and dialing back that surge to 200. It will never see 200 amps. I don't like running my electronics at near their rated capacity so I bought oversize and will limit the output to something more like what a single 2 awg wire could handle for a brief load.

 

[JK14]

Got it! That all makes sense and seems totally reasonable.
Of note, the Overkill Solar 100A BMSs sometimes comes with 2 pair of 8AWG already soldered to the board - seems silly to remove those and add a little bus bar.
With quality crimps, well torqued connections and relatively short wire runs, you should be good to go!

 

[rhino]

You could also use a single stud terminal that can be mounted and have both of the BMS cables attached to that and single cable the rest of the way. Other option is to use a butt splice where you can insert both of the 7 AWG cables from the BMS into one end and larger gauge on other end.

 

[Saltyoldcruiser]

Got it! That all makes sense and seems totally reasonable.
Of note, the Overkill Solar 100A BMSs sometimes comes with 2 pair of 8AWG already soldered to the board - seems silly to remove those and add a little bus bar.
With quality crimps, well torqued connections and relatively short wire runs, you should be good to go!

 

[Saltyoldcruiser]

Got it! That all makes sense and seems totally reasonable.
Of note, the Overkill Solar 100A BMSs sometimes comes with 2 pair of 8AWG already soldered to the board - seems silly to remove those and add a little bus bar.
With quality crimps, well torqued connections and relatively short wire runs, you should be good to go!

Yeah, I just did a test crimp with a pair of 2 awg wires in a rugged 2/0 lug. The crimp came out very nice. I did cut to check for gaps and it's rock solid. A true gas tight crimp. If there were a cable failure the crimps would be the most likely place and I'm feeling really good about that.

 

[Saltyoldcruiser]

You could also use a single stud terminal that can be mounted and have both of the BMS cables attached to that and single cable the rest of the way. Other option is to use a butt splice where you can insert both of the 7 AWG cables from the BMS into one end and larger gauge on other end.

Yeah, the issue seems to be that best practices would not use 2 cables. Short of using a fabricated 2 into one bussbar on the BMS I'm locked into branching to 2 wires at some point. I did a test crimp with a pair of 2 awg wires into a 2/0 lug and it looks fine. I cut it and there are no gaps whatsoever. I'll go with a single 2/0 positive cable to my class T fuse and a double 2 awg cable with a single lug on the battery side and 2 lugs for the bms side. The negative cable will be less than a foot long.