Fusing of individual battery strings.

Synchro

Searching is not working for me - worse since the upgrade.
Three strings of 8 Rolls 6v feeding a 175A breaker in a MMP.
Battery interconnects and inverter wiring is 2/0.
I want to put a class T fuse between each positive end and the battery bus bar.
175A to match the breaker, or 200A because it is a fuse?
Or am I completely off base?

BB.

Search is not going to work well for a few hours--The index is being rebuilt after this afternoon's major forum software upgrade (I have no idea how the "new search" will function).

Personally, I have very good luck using Google and their "site:" tag--Just add the address of the website you want to limit your searches to. For example:

• site:solar-electric.com battery fusing

More or less, for any NEC/UL/NRTL Listed fuses and breakers... You define the maximum current you want to pass continuously, and then multiply by 1.25x (or 1/0.8).

Or look at it as we want a maximum of 80% of rated branch circuit (wiring/breakers/fuses) of current. If the average (minutes to hours) of current exceeds 80% of rated current for fuse/breaker, the over current protection device may trip. Running at 100% of rated current, the device will trip in minutes (or even an hour).

For surge current, there are thermal/slow blow devices vs fast(er) action. Motors and such can use "slow blow" to allow higher surge current (seconds) to bring the motor/load up to speed.

Breakers and fuses "protect" wiring, they do not protect loads. So, if you have 2/O wiring, you can pass around:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

145 to 195 amps in Copper Cable in conduit (look at National Electric Code for details on fill factor, ambient temperatures, insulation type, etc.).

Or using:

http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp

As high as 280-330 Amps

Note that the NEC is much more conservative and may be required if you have building permit requirements in your area.

Also note that for DC Battery Bus applications, we need to keep voltage drop to a minimum... Around 0.5 volts max @ 12 volts, 1.0 volts for 24 volt, and 2.0 volts for 48 volt systems... I.e.:

12 volt inverter running battery down to 11.5 volts with 10.5 inverter cufoff--And a 2x surge current of ~2x rated current (0.5 volt drop during normal operation, 1.0 volt drop maximum during high surge current).

Any, back to fuses/breakers, for example running a 1,2000 watt inverter:

1,200 Watts * 1/0.85 inverter eff * 1/10.5 volt batter cutoff * 1.25 NEC derating = 168 Amp minimum Branch Circuit/Fuse/Breaker rating to run inverter at full load

Note--For battery banks with 1 to 2 parallel strings, I would suggest battery string wiring+fusing be rated to Inverter DC input requirement (as above). For 3 or more parallel strings, I would suggest a mininum of 50% of inverter input requirement (note, I personally suggest 1 string as ideal, 2 strings if you want a "backup" string, and 3 strings maximum to keep maintenance and fusing/breaker costs lower--Current sharting between batteries can be difficult with paralleled strings--plus other issues). You can parallel more strings--But we should talk about it first. Normally, I would suggest larger AH batteries (and you can go larger AH batteries at 6/4/2 volt batteries/cells too--To keep the individual battery blocks from getting too heavy).

-Bill

Synchro

But if I use 200A class T per string? (Cost Effectiveness).

BB.

It would appear to be OK--A bit more than NEC would allow... But these are not in conduit--So wires getting air circulation/cooling.

-Bill