So I'm having some confusion and actually trouble finding specifics on how to calculate the wire size needed between my battery bank and the rest of my system. I've attached a pdf of my preliminary diagram, not for full critique of it (though by all means go ahead if desired ?), but to illustrate my frame of reference when thinking about it. It's very easy to find examples of what other people use, but I haven't found hard facts about how they arrive at these sizes. From the wire size, I'm assuming the fuse size would follow? I've seen many examples of people using 250A terminal fuse on the battery, but I don't understand the WHY. Thanks!
How to calculate wire size between Battery Bank and System
- Thread starter abefroman
- Start date
You are already on the right path, much ahead of many people if you are asking the how and the why 
The short answer, figure out the max current of your system, size the wire based on voltage drop and ampacity, size the fuse to protect the wire. The main battery fuse should generally be at least 1.25x of max current.
There are two factors in sizing the wire "ampacity" and "voltage drop"
Ampacity = how much current a conductor can safely carry (variables that affect this are wire diameter, insulation temperature rating, ambient temperature, type of metal, and other environmental factors like whether the conductor is bundled with other wires in conduit or on a hot roof etc)
Voltage drop = this affects the efficiency and potentially functionality of your system (variables that affect this are wire diameter, wire length, type of metal)
From a safety standpoint the former is by far the more important factor, both need to be considered when sizing wire, generally, people try to keep the round trip voltage drop of the whole circuit below 3%, maybe people aim for 1% or 2%.
A calculator like this one from blue sea systems can calculate both factors for you, if you fill in the proper variables (if you expect temperatures above 85*F click the 'in engine compartment' check box).
Here are two ampacity tables one from the ABYC (marine electrical code) and one from the NEC (national electrical code) they should be roughly the same given the same variables.
One thing to be aware of, many 'voltage drop calculators' will calculate voltage drop but do not account for ampacity. Make sure that you account for both calculating wire sizes.
Yes (or vice versa). The important thing being that: max current < Fuse rating < Wire rating
The short answer, figure out the max current of your system, size the wire based on voltage drop and ampacity, size the fuse to protect the wire. The main battery fuse should generally be at least 1.25x of max current.
There are two factors in sizing the wire "ampacity" and "voltage drop"
Ampacity = how much current a conductor can safely carry (variables that affect this are wire diameter, insulation temperature rating, ambient temperature, type of metal, and other environmental factors like whether the conductor is bundled with other wires in conduit or on a hot roof etc)
Voltage drop = this affects the efficiency and potentially functionality of your system (variables that affect this are wire diameter, wire length, type of metal)
From a safety standpoint the former is by far the more important factor, both need to be considered when sizing wire, generally, people try to keep the round trip voltage drop of the whole circuit below 3%, maybe people aim for 1% or 2%.
A calculator like this one from blue sea systems can calculate both factors for you, if you fill in the proper variables (if you expect temperatures above 85*F click the 'in engine compartment' check box).
Here are two ampacity tables one from the ABYC (marine electrical code) and one from the NEC (national electrical code) they should be roughly the same given the same variables.
One thing to be aware of, many 'voltage drop calculators' will calculate voltage drop but do not account for ampacity. Make sure that you account for both calculating wire sizes.
Not sure about this, it would be a properly sized fuse for a 12v system with a 2000W inverter and wire with an ampacity over 250A.
Thanks Dzl for such a detailed reply! So I've done some homework I've tallied up all the loads in my system (Column G), and came out with a total amperage of 105A highlighted in yellow at the bottom. Using the Blue Sea calculator with a wire run of 2ft (4ft roundtrip) between my battery bank and the bus bar it recommends 4AWG and 133A fuse. Interestingly from some cursory research I don't believe many fuses exist in that range, they generally jump from 125 to 150. My thought here was to bump the wire to 2AWG and use a 150A fuse. Am I totally off the rails, or does this sound like a good plan forward?
I've tallied up all the loads in my system (Column G), and came out with a total amperage of 105A highlighted in yellow at the bottom. Using the Blue Sea calculator with a wire run of 2ft (4ft roundtrip) between my battery bank and the bus bar it recommends 4AWG and 133A fuse. Interestingly from some cursory research I don't believe many fuses exist in that range, they generally jump from 125 to 150. My thought here was to bump the wire to 2AWG and use a 150A fuse. Am I totally off the rails, or does this sound like a good plan forward?
Good work, that is a nice clear spreadsheet, that will come in handy a lot as you design/build.Thanks Dzl for such a detailed reply! So I've done some homework
Seemss like you are on the right track.
No harm in oversizing, its a smart choice. It is normal to get a specific value and then round up to the next common fuse size (so long as the wire is sized up as well). And you can always size up a bit more to account for a bit of safety margin or room to grow in the future, but that's up to you, the blue sea calculator should be conservative enough if the inputs are accurate. 2AWG + 150A fuse sounds perfect.
This langauge "Using the Blue Sea calculator with a wire run of 2ft (4ft roundtrip)" gives me the impression you may have input one-way distance into the calculator. The Blue Sea calculator expects round trip distance to be entered. Just want to make sure that is correct.
One last thing I would point out, be careful to purchase wire of the same temperature rating that you use in the calculator. Most marine rated wire is 105*C, most home wiring is not (90*C or 75*C are common). This is something lots of people overlook but is important. Blue sea lets you choose the temperature rating but defaults to 105*C.
Awesome, thanks Dzl....
Do you have a preference on "type" of fuse here? Terminal block, ANL, mega, something else?
Great to hear. It was interesting in the Blue Sea recommendation that it indicated a 150A fuse with 4AWG would be too high, which partly led me to sizing the wire to 2AWG. But I do like the possibility of sizing something up if desired in the future, so gives so peace of mind there.
Do you have a preference on "type" of fuse here? Terminal block, ANL, mega, something else?
I was wondering how that would come across haha, but I used 4ft in the calculator, so should be good there.
Good callout, I'll make sure to do so. I used Ancor duplex wire for all my DC load wiring, so intending on sticking with higher quality wire here too.
We are talking about a main battery fuse for a lithium battery, correct?
If so, a 'Class T' fuse is the best choice, with MRBF and ANL being acceptable choices as well.
These are the three types I am aware of that are rated for 'main battery circuit protection' in the marine electrical code due to their ability to interrupt large amounts of current. Class T is the type that is specifically recommended for lithium batteries, and is most often the type inverter manufacturers will recommend if they make a recommendation.
Any of the three I think. I don't believe shorted lead acid batteries are capable of pushing the same magnitude of current as lifepo4 unless it is a very large battery bank (no idea how 'large' is defined), so any of the three are rated for lead acid banks IIRC. Blue sea sell all three types and they give a bit of guidance on there product pages.
They also have some useful explainer articles on the topic
Part 1: Choosing the correct wire size for a DC circuit
Part 2: Selecting a Fuse and Fuse Holder
Choosing Circuit Protection
Perfect...one last question if I may, should the wiring between the batteries in the battery bank match the wiring to the bus bar? When I bought the batteries from Renogy I also got 4AWG 12" cables to interconnect them. Should I replace them with 2AWG instead? The 4AWG would be the "weakest link" in the circuit right?
When in doubt always jump up a size in wiring.
The added cost is negligible in your system and the additional benefit is a more efficient system (less resistance, heat and wasted energy).
Good question, I'm not actually sure, but personally if I were unsure I would go with the 2AWG.
