Please help me make sure if I have understood this correcttly.
Using a completely different example.
2X12V=24V (AGM), 2000W (HF) inverter, (MRBF) fuse.
AGM chemistry typically does not require as much breaking capacity as LFP, so MRBF should be fine.
2000W/.85//24=98A /.8=122.5A ≈ 125A.
You have a double forward slash, I think that is just a typo
I guess that this "≈" stands for approximately equal.
Minimum wire/fuse: 6AWG/125A (wire rating 115A max)
Medium wire/fuse: 4AWG/150A (wire rating 150A max)
Maximum wire/fuse. 2AWG/175A (wire rating 205A max)
Assuming ABYC rules and pure copper wire with 105C insulation.
This table shows the max rates.
Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.
baymarinesupply.com
For the core of the DC system which is generally not in conduit, raceway or bundles I would say this.
6 awg can be fused as high as 100 amps.
4 awg can be fused as high as 150 amps.
2 awg can be fused as high as 200 amps.
In a 6ft run, would 4AWG wire and 150A MRBF fuse be the best option here?
We also have to consider voltage drop.
Usually the core components are close enough together that distance related voltage drop doesn't matter much.
But to be thorough, lets look at the full process.
Since your inverter has configurable low voltage disconnect we can use 24 volts as the low voltage cutoff.
2000 ac watts / .85 conversion factor / 24 volts low cutoff = 98.039215686 dc inverter amps
98.039215686 dc inverter amps / .8 fuse headroom = 122.549019608 fuse amps
So the lowest fuse size we could use is 125 amps and the lowest wire size that can accommodate that fuse is 4 awg.
If we plug 98.039215686 amps, 24 volts and 12 feet round trip into the calculator I linked, you get 1.5% voltage drop for marine-rated (105°C dry / 75°C wet) wire.
Up to 3% voltage drop is considered acceptable for the core of the system.
Or using same example above but instead with my 8x280AH LFP cells, 4AWG wire, 200A Class-T fuse and 200A BMS would make sense?
A 200 amps Class t fuse is too large for 4 awg wire.
Remember the max fuse size according to the table above is 150 amps.
For 2x AGM batteries in series an MRBF fuse should be fine.
I'm sorry if the level of detail is overwhelming for you but for the sake of completeness I have been thorough.
Also. Is it optional using single (50mm2) battery wire even if inverter manual says it necessary connecting wires i parallel?
*According to the recommended battery wire size, 2 battery wires, connected in parallel, are necessary
for NP2000-11, NP2500-21, NP2500-22, NP3000-22, and NP4000-22.
(25mm2/3AWG RNB60-10 DC-100A(2P in parallel)
Please show me this in context using a link to the documentation.
25mm2 ~= 4 awg which can be fused as high as 150 amps.
I avoid double cables for many reasons.
One reason is if one of the negative cables becomes detached the other will have to carry the full current which might melt the insulation, possibly leading to fire.
Some manufacturers including Victron seem to recommend this practice but I don't think it is a good idea.
Now for the controversial part.
Its my policy to use the largest fuse size that the wire can accept for wires in the core of the system.
There are some exceptions and complications like in-line passive devices but I consider that out of scope for this discussion.
I just don't see the danger in doing this.
If someone were to describe a plausible failure mode where the lower fuse size would make a difference then I would reassess my position.
The fuse protects the wire not the inverter.
If the branch circuit fuse blows it means either their is a short circuit between the branch fuse and the inverter or that the inverter has suffered an internal fault and is already dead, either way the branch fuse should blow or the inverter should stop passing current.
Most of the non-passive devices we use have their own built-in protection devices, many even have an internal fuse.
I could be wrong but I call it like I see it.
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@Will Prowse
