Wire and Fuse Calculator

What are you sizing a cable and a fuse for?

Each one uses a different safety factor, so start here.

How much power?

Ah
Total of the whole bank. A bigger bank can dump more current into a short, so it rules out the weaker fuse types.
This is the normal running voltage of a lead-acid starter/alternator system — 13.8 V for 12 V, 27.6 V for 24 V. No need to change it.
Standard charge voltage for this system size — 14.2 V, 28.4 V or 56.8 V. No need to change it.
amps
The rated current of your DC-DC charger.
%
The DC side pulls more than the output current, because the charger itself isn't 100% efficient — 88% is a reasonable default.
The NEC method can recommend a larger wire, because it assumes a hotter conductor.
volts
From the panel's spec sheet — voltage at maximum power, not open-circuit.
String voltage is Vmp × panels in series.
amps
Also from the spec sheet. This, not the working current, is what sizes the cable.
Each parallel string adds its full Isc to the combined current.
watts
Continuous, not surge or peak.
%
90% is a safe assumption for most inverters.
amps

The cable itself

Cable type sets how hot it may get. Length decides voltage drop.

The temperature rating is really a proxy for how much current the cable can take — a cable rated higher can carry more current at the same size, which is why welding/marine and silicone cable often need a smaller size than normal building cable for the same job.
Battery to device, one direction. The calculator doubles it for the return path.
Two separate runs, each doubled for its own return path.

Which fuse or breaker?

Your result

Cable size
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Protection
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The working

What this assumes
  • Copper conductor. Aluminium needs a bigger size and is not covered here.
  • You choose how the cable is run, because it changes the answer a lot. Building wire in conduit uses NEC Table 310.16 and the same wire in open air uses Table 310.17 — 4 AWG is 95 A in conduit but 140 A loose. Both at 30°C ambient.
  • Welding and silicone cable are rated separately, using our own free-air figures — silicone's higher-temperature insulation lets it carry somewhat more at the same size. Neither has a conduit figure. Silicone is offered from 10 AWG up to 1/0 AWG; welding cable covers that same range and continues further, up to 4/0 AWG, for jobs silicone does not reach.
  • Ambient is assumed to be 30°C / 86°F. A hot attic or a sealed battery box runs warmer, and everything derates.
  • Building wire stops at 4 AWG. Bigger sizes exist in the NEC tables, but not in a fine-strand form worth recommending, so they are not offered. Above 4 AWG the answer is welding or silicone cable.
  • The 8 AWG welding figure of 55 A is lower than the rest of that table would suggest. It is kept as published, so 8 AWG welding cable is treated conservatively.
  • LiFePO4 only. Cells are taken as 2.75 V at cutoff, 3.2 V nominal, 3.65 V fully charged.
  • Current is worked out at the low-voltage cutoff, where it peaks. Voltage drop is measured against the nominal bus voltage, which is the figure everyone quotes percentages against.
  • Fuse voltage ratings, interrupt ratings and maximum bank capacity are our own current figures. A fuse used above its DC voltage rating, or on a bank bigger than it is rated for, may fail to break the arc.
  • Bank capacity only gates the Inverter / converter path. "Any other DC cable" assumes this run is downstream of a fuse block, busbar or panel that's already fused for the full battery fault current — so this fuse only has to protect its own branch, and isn't checked against the bank's Ah.
  • Resistance from NEC Chapter 9 Table 8, stranded uncoated copper at 75°C. Fine-stranded welding cable measures slightly lower, so the real drop is a touch better than shown.
  • A fuse protects the cable, so it is never above the cable's rating — that limit is absolute. It may sit a few percent below the calculated design current, because that figure carries a 25% margin built in to prevent nuisance tripping, not to protect anything.
  • A breaker is protection and a disconnect switch in one, and it resets after a fault. Check that yours has a published DC rating before trusting it on a battery.
What this assumes
  • Takes the larger of two checks, same as the other two paths: voltage drop, and ampacity (90°C free-air PV cable ratings — 30 A at 12 AWG, 40 A at 10 AWG, 55 A at 8 AWG, owner-supplied). If the design current is above 55 A, no offered PV cable can carry it at all, and the answer is to split into parallel strings feeding separate charge controllers.
  • Fixed 1.25 safety factor, applied to Isc, not adjustable — this matches NEC 690.8's requirement for PV source-circuit conductors.
  • Current is Isc × parallel strings × 1.25. Voltage is Vmp × panels in series, and the drop percentage is judged against that string voltage.
  • Copper or aluminium, at either a 20°C (EU-book) or 75°C (NEC) conductor temperature. The NEC method assumes a hotter conductor, so it recommends a larger cable for the same job.
  • PV cable is only offered in three sizes — 4 mm² (12 AWG), 6 mm² (10 AWG), and 10 mm² (8 AWG) — because that is what is commonly sold as PV cable. This calculator rounds up to the next one and flags it if the job needs more than 10 mm².
  • No fuse or combiner-box guidance here. Series fusing for parallel PV strings is a separate topic this calculator does not cover.
What this assumes
  • Two separate circuits, sized independently: starter/alternator battery to the charger's input, and the charger's output to the house battery. They usually run at different voltages and currents, so there's no reason they'd share one cable size.
  • Output current comes from the charger's own rating; input current is worked back from it through the charger's efficiency: Pout = Vhouse × Iout, Pin = Pout / efficiency, Iin = Pin / Vstart. Both currents then get the same fixed 1.25 safety factor used everywhere else in this tool.
  • Resistivity is fixed at copper, 20°C (ρ = 0.0171 Ω·mm²/m) — there's no material or temperature choice on this path, only the cable's insulation/temperature rating, which sets its ampacity table.
  • Takes the larger of two checks per circuit, same as the other paths: voltage drop, and ampacity from the selected 90°C / 105°C / 200°C table (owner-supplied). Each of the three tables is its own data set, not shared with the wire tables used elsewhere in this calculator.
  • AMI/MIDI is recommended over AMG/MEGA. MIDI's standard sizes go lower (M5 studs), so it usually fits under a smaller cable; MEGA's smallest size is 60 A (M6 studs), which can force a bigger cable even at modest currents. "Pick one for me" uses MIDI whenever it's eligible.
  • MRBF is offered as a fallback for 48 V house systems. Both AMI and MEGA cap at 32 V DC, so on a 48 V system (charges to 56.8 V) neither is eligible — MRBF (58 V) is the only one of the three that still works there.