Cable size calculator

How the cable size calculator works

It sizes copper cable to BS 7671:2018+A4:2026. First it works out the design current and picks a fuse. Then it finds the smallest standard size that carries the fuse rating with 10 % to spare, after the corrections for heat, bundling and insulation.

Next it checks voltage drop over the full run, out and back. In a 12 V van it uses the BS EN 1648-2 formula and limits. On an Orion XS output it follows Victron's own cable table, as BS EN 1648-2 allows where the maker sets the limit. For other Victron kit, it also reads the maker's cable table and then uses whichever answer is larger.

How it picks the battery fuse

The fuse protects the cable, so it is sized to BS 7671 Table 4D1A, not to the device. On a MultiPlus 12/3000 with 95 mm² cable that means 250 A as standard: 250 A plus 10 % is 275 A, under the cable's 284 A clipped rating. Victron's MultiPlus-II manual (section 4.3) lists a 400 A DC fuse for this model. We show that for reference only, because a 400 A fuse is above what the cable is rated to carry, so it would not protect the cable. The inverter draws about 222 A at full continuous power, so 250 A doesn't blow in normal use.

A Lynx Distributor has four MEGA fuse ways. Most circuits go straight onto a MEGA in one of those ways, and the cable is sized so that MEGA protects it. The smallest MEGA in Victron's fuse datasheet is 40 A (80 V range). The small plastic SmartSolar charge controllers (75/10, 75/15, 100/15 and 100/20) have 6 mm² power terminals, and Victron's manuals cap their battery fuse at 20 to 30 A, so a MEGA is too big for them. For these, and only these, a dummy MEGA fuse (a solid link) goes in a free Lynx way. A short cable, 3 m at most, runs from that way to a fuse block fitted right next to the Lynx, and a blade fuse in Victron's range then protects the 6 mm² cable to the controller (a MIDI 58 V fuse in a MIDI holder on 48 V systems). BS 7671 allows that short unprotected run where the fuse at its far end protects it (Regulations 433.2.2 and 434.2.1). If all four ways are already in use, there is no free way for a dummy, so the calculator asks you to talk to us. Every other circuit that goes straight onto a MEGA, including larger controllers such as the 100/30 and 100/50, gets at least 10 mm² cable, often 16 mm² or more depending on how it is run, the current and the length, and the MEGA is sized to protect that cable. The 100/30 and 100/50 terminals take cable up to 16 mm² (Victron datasheet).

Empty Lynx ways only need a dummy fuse if something powers the Distributor's fuse LEDs. Victron's Lynx Distributor manual says the LEDs don't work without a Lynx Smart BMS or Lynx Shunt VE.Can, and most of our builds use neither, so we leave unused ways empty. If your system has one, Victron says to fit a dummy fuse in every unused way so the red warning LED doesn't light. Tell the calculator under Advanced and it adds them to your parts list. The Lynx Distributor itself is an optional add-on below the parts list, so it's only listed if you tick it.

We never use a short-circuit-only fuse as the cable's overload protection. Littelfuse rates MEGA fuses from 300 A to 500 A, and MIDI fuses from 150 A to 200 A, for short-circuit protection only (MEGA 298 and MIDI 498 datasheets). So above 250 A, the calculator picks a Class T in its own holder instead of a MEGA. Victron rates one Lynx Distributor at 1,000 A for all its fuses together (Lynx Distributor manual 5.2.2), and the calculator checks that total if you enter your other fuses.

The battery fuse also has to break a lithium short circuit, which we take as Ah × 50. On 12 V we allow an MRBF up to 200 Ah per battery as a stated, cautionary exception, with an MRBF on every battery in a terminal fuse block. The block is never bolted straight to an M8 battery post: it comes loose in your kit and mounts on a 3/8 in stud or busbar next to the battery, close to its positive terminal (Blue Sea 5191 datasheet), fed by a short cable of 3 m at most, run so it is short-circuit and earth-fault proof (BS 7671 Regulation 434.2.1). Above 200 Ah per battery it's a Class T, fitted on the board with the supplied battery tails connected to it, within 1000 mm of the battery (BS EN 1648-2). On 24 V and 48 V banks we fit a Class T on every battery.

Breaking capacities come from the fuse maker's datasheet: Eaton Bussmann rates the MRBF at 10,000 A at 14 V d.c., 5,000 A at 32 V d.c. and 2,000 A at 58 V d.c. (Marine Rated Battery Fuse series datasheet). Class T fuses break 20,000 A. Our lithium leisure battery fuse sizing guide explains it in full.

What it covers

• Battery to Lynx, including the fault current your battery bank can deliver.
• Inverter and inverter/charger feeds for the Victron range.
• Orion XS DC-DC chargers, input and output.
• Solar panels to the MPPT, and the MPPT to the battery side.
• 12, 24 and 48 V loads from a fuse block.
• Shore inlets and 230 V final circuits.

When the cable size calculator says talk to us

Some builds need a design check, not a number. You'll see "Talk to us" when the fault current is too high for the fuse, a run is longer than the maker allows, or the load is outside what one circuit should carry. In those cases we don't show a cable size at all.

A Borderline result works on paper but leaves little to spare, so going up a size now is usually the cheaper fix.

Fit it properly

Crimp lugs with a hex crimper and adhesive-lined heat shrink. Torque each connection to the maker's figure, then mark it. Above all, the tool gives guidance only. It can't see your build, so have the design and the finished job checked by a qualified electrician.

Want a second pair of eyes? Book a free call, or read our campervan and cabin electrical guides. The current BS 7671 edition is published by the IET.

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