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Ohmforge Wire Gauge, Voltage Drop & Harness Calculator

Wire size from current, length and voltage drop; AWG ↔ mm²; harness cut list, BOM and netlist check.

V

Three-phase: line-to-line voltage.

A
More options
m

Distance from the source to the load; the return conductor is added automatically.

%
°C

Resistance rises about 0.4 % per °C; use the operating temperature for a conservative result.

For example 12 AWG, 4/0, 2.5 mm² or 0.75mm2.

One wire per line: id, from, to, length, gauge, colour (comma, semicolon or tab). Ends as REF.PIN (BT1.+, F1.1) to check them against a schematic; ends without a dot are splices or studs.

mm
%

Paste a .ohmforge.json project; the harness is checked against its nets.

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Try an example: 12 V, 20 A over 5 m (van or boat) · Solar: 24 V, 15 A, 12 m, 2 % drop at 60 °C · AWG ↔ mm²: what is 2.5 mm² in AWG? · Harness of a 12 V lamp circuit, checked against its schematic

More about this tool

How it works

Pick the conductor for a DC, single-phase or three-phase run from the current, the cable length and the voltage drop you allow, with copper or aluminium at the conductor temperature — results in AWG and in metric mm², with drop, power loss and a fuse suggestion as guidance. Convert between AWG and mm² with the ASTM B258 formula, or turn a harness wire list into a cut list with allowances, a bill of materials per gauge and colour, CSV exports and a continuity check against an Ohmforge schematic.

  • Resistance R = ρ(T)·L/A with ρ(T) = ρ20·(1 + α20·(T − 20 °C)). Copper: ρ20 = 1/58 Ω·mm²/m (IEC 60028 annealed copper, 100 % IACS), α20 = 0.00393 /K. Aluminium: ρ20 = 0.028264 Ω·mm²/m (61 % IACS, IEC 60889), α20 = 0.00403 /K.
  • Voltage drop ΔV = k·I·ρ·L/A with k = 2 for a feed and return wire (DC, single-phase), 1 for a chassis return (chassis resistance ignored) and √3 for a balanced three-phase line. The smallest conductor is A = k·ρ·L·I/ΔVmax; the tool picks the smallest AWG and the smallest IEC 60228 size at or above it and reports drop and loss (I²R in all current-carrying conductors).
  • AWG diameter d = 0.127 mm × 92^((36 − n)/39) (ASTM B258), with n = −3 … 0 for 4/0 … 1/0; area = πd²/4 of a solid conductor (stranded wire of the same gauge has about the same copper area).
  • Fuse guidance: the next standard rating at or above 125 % of the continuous load current. The fuse must also be at or below the current rating (ampacity) of the chosen wire, which depends on insulation, bundling, ambient temperature and the applicable standard; ampacity is deliberately not computed here.
  • Harness: cut length = routed length × (1 + slack %) + 2 × allowance per end. The bill of materials groups the cut lengths by gauge and colour. The continuity check joins all harness wires (splices included) and compares every group with the schematic's nets: a group spanning two nets is a short, a net spread over several groups is open, and a pin of a part the harness touches that has no wire is missing.

Electrical safety: this is a design aid, not a substitute for the wiring rules that apply to you (for example IEC 60364/VDE 0100, NEC/NFPA 70, ISO 10133 or ABYC E-11 for boats, vehicle manufacturer rules). Wire current ratings (ampacity) and fuse selection must be checked against those rules and the cable maker's data. Mains installations must be done by a qualified electrician.

Sources

Part of Ohmforge

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Related tools

Choose wire gauge from voltage drop and build a harness cut list

Size a cable from current, length and allowed voltage drop in AWG and mm², convert between the two, and turn a wire list into a cut list, BOM and continuity check.

What it is for

In a van, boat, solar system or robot, the question is rarely "which wire carries the current" alone. Long low-voltage runs lose volts first, so the wire has to be sized for an acceptable voltage drop. This tool covers three jobs:

  • Wire size from current and voltage drop finds the smallest AWG and metric size for a run, with drop, loss and a fuse suggestion.
  • AWG ↔ mm² table and conversion converts a size and lists all AWG sizes with diameter, area and resistance.
  • Harness cut list, BOM and continuity check turns a wire list into lengths to cut, a bill of materials per gauge and colour, CSV files, and an optional check against an Ohmforge schematic.

It deliberately does not compute ampacity, the current a wire may carry. That depends on insulation, bundling, ambient temperature and your local wiring rules. The fuse suggestion is guidance only.

Worked example

A 12 V fridge compressor draws 30 A, 4 m away from the battery, and you allow 3 % drop. Wire under load gets warm, so you enter a conductor temperature of 60 °C.

The tool needs at least 13.3 mm²: 6 AWG (exactly 3 % drop) or 16 mm² (2.49 % drop, 0.299 V). The nearby-sizes table shows why 10 mm² is not enough: 3.99 % drop and 14.4 W lost in the cable. The fuse guidance is 40 A, the next standard blade or bolt-down rating above 125 % of 30 A. You still have to check that the wire is rated for 40 A where you route it.

In harness mode, the example 12 V lamp circuit has four wires and 6.8 m of routed length. With 15 mm allowance per end and 10 % slack you cut 7.6 m. If wire W4 is wrongly routed from the lamp back to BT1.+, the check reports that the harness joins BT1.+, F1.1 and LP1.2, which belong to separate nets, and that BT1.- is not wired.

How to read the result

In sizing mode, Smallest cross-section is the exact area the drop limit requires; AWG and Metric are the first standard sizes at or above it. Each row of Nearby sizes shows drop, percent and the power lost in all current-carrying conductors.

In harness mode, Cut length = routed length × (1 + slack) + 2 × allowance. The Bill of materials sums cut lengths per gauge and colour. A short means one harness group touches two schematic nets. Open means a net is split. Missing means a pin of a part the harness touches has no wire.

Tips and common mistakes

  • Enter the one-way length. The return wire is added for you; choose Feed wire only, return through the chassis for a chassis ground.
  • Use the operating temperature: copper resistance rises about 0.4 % per °C.
  • AWG and metric sizes never line up exactly. When you substitute, take the larger area.
  • Write wire ends as REF.PIN (BT1.+, F1.1) to use the continuity check. Ends without a dot are splices or studs that join wires.
  • Check the fuse against the cable datasheet or the wiring standard you work to (for example ISO 10133 or ABYC E-11 for boats).

FAQ

Why does the tool not show the maximum current of a wire?

Ampacity depends on installation conditions and the standard you follow, so a single number would be misleading.

Can I use it for 230 V mains circuits?

You can calculate the voltage drop, but mains installations must follow local rules and be done by a qualified electrician.

Where does the AWG table come from?

From the ASTM B258 definition, d = 0.127 mm × 92^((36 − n)/39).

Can it build the wire list from my schematic?

Yes, in the Ohmforge workspace: Instruments → Wiring harness… → Build from this schematic.

Guide last updated:

For scripts and AI agents

  • JSON API: POST https://ohmforgewiregauge.usefulweb.app/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://ohmforgewiregauge.usefulweb.app/mcp · tool name ohmforge_wire_gauge
Example request
curl -s -X POST https://ohmforgewiregauge.usefulweb.app/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"mode":"size","voltage":12,"current":20,"lengthM":5,"circuit":"dc","maxDropPercent":3,"temperatureC":20,"material":"copper","fuseSeries":"blade"}'
Feedback · ideas & bug reports welcome