Wire and Fuse Calculator

What are you sizing a cable 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.
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.

Battery to device, one direction. The calculator doubles it for the 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.