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12V & 24V wire and cable size calculator

Size copper wire and cable for 12 V or 24 V DC circuits in AWG or mm². Enter continuous current, one-way length and installation conditions to compare ampacity and voltage drop, then open the circuit in Crimp.

12 V / 24 V DC planning · reviewed Oct 21, 2026 · ABYC E-11 table basis

Worked schematic

12 V wire-size example with a 20 A load

A 12 V supply feeds a 20 A load through a terminal fuse, 5 m of positive cable and a separate 5 m return. The cable lengths and current match the worked calculation below.

Interactive Crimp schematic. Pan and zoom to follow the circuit. This fixed example illustrates the calculation; your form inputs are separate.

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Red = positive · dark = return. Crossings are not junctions. Read the assumptions before adapting a circuit.

One way means 5 m

The positive cable runs from the supply to the load. A separate cable returns to supply negative: 10 m of copper in the complete circuit.

Read both cable losses

The drawing reports each conductor's voltage drop. Add the outgoing and return losses to compare with the calculator's complete-loop result.

Fixed metric example: 20 A at 12 V, 3% total drop allowance, 105°C copper outside engine spaces at up to 30°C, two conductors and no extra bundling. Wire area comes from the calculator.

The illustrative Blue Sea 5175 30 A MRBF fuse in a 5191 terminal holder is above the 25 A continuous-current baseline and below the modeled cable ampacity. The expanded zero-length connection represents a terminal mount, not extra cable.

Verify the equipment's permitted fuse, whole-bank fault current, maximum voltage, holder installation and exact fuse before building. Fuse/contact resistance is outside the wire calculation; your calculator inputs do not select this fuse.

Open this example in Crimp →Use your own inputs ↓
Units

Use the circuit's design voltage, usually 12 or 24 V. Supported range: 1–60 V DC.

Maximum continuous operating current. This tool adds 25% ampacity headroom; it does not size motor-start or cranking circuits.

Measure from source to load along the cable route. We add an equal-length return: 5 m here means 10 m of circuit conductor.

Copper only. A 105°C jacket rating is not a permissible terminal temperature.

Use the worst section of the route. Engine-space calculation uses the existing conservative 0.82 ampacity multiplier.

Count current-carrying positive and return conductors. A bundle needs a documented factor; it is not inferred from count alone.

Wire size is unknown

Still needed

  • Circuit voltage (V)
  • Continuous current (A)
  • One-way length (m)
  • Voltage-drop allowance
  • Insulation rating
  • Temperature / engine space
  • Conductor routing

Fuse selection is separate. No fuse is selected, placed or verified by this wire result. Check the fuse and holder.

Worked calculation

Wire size for a 20 A load with a 5 m cable run

Synthetic example: 20 A continuous at 12 V, 5 m one way plus an equal return, 3% allowance, 105°C copper outside engine spaces at up to 30°C, up to two conductors.

Metric conductor

10 mm²

Voltage drop determines this size

Ampacity constraint, including 125% headroom
25.00 A → 1.5 mm²
Voltage-drop constraint
9.72 mm² minimum → 10 mm²
Selected conductor area
10.00 mm²
Installed ampacity after derating
80.00 A (factor 1.000)
Voltage drop, outgoing plus equal return
0.350 V / 2.92%

AWG conductor

AWG 6

Voltage drop determines this size

Ampacity constraint, including 125% headroom
25.00 A → AWG 14
Voltage-drop constraint
9.72 mm² minimum → AWG 6
Selected conductor area
13.30 mm²
Installed ampacity after derating
80.00 A (factor 1.000)
Voltage drop, outgoing plus equal return
0.263 V / 2.19%

Method and limits

Ampacity and voltage-drop calculation method

For this continuous DC branch, required ampacity is current × 1.25. Available ampacity is the table value × temperature factor × documented bundle factor. Voltage drop is 2 × current × one-way length × copper resistivity ÷ area; the model uses 0.0175 Ω·mm²/m at 25°C. It does not model hotter copper resistance, terminal loss or unequal/shared returns.

We compare the smallest table conductor meeting each constraint, then choose the larger. Both can require the same table step. The 1.5 mm² mechanical floor can decide small circuits. If no available size meets both constraints, the result stays outside the table; the largest wire is not presented as sufficient.

Metric mode uses Crimp’s existing table. AWG mode uses actual nominal AWG areas with conservative ampacity taken from the next smaller metric entry. This keeps the same 1.5 mm² floor, so AWG selections start at 14 AWG. It can oversize compared with a native AWG ampacity table. Changing units preserves physical length but can change the selected conductor. Existing diagram pages still use approximate AWG counterparts; 16 mm² and 6 AWG are different areas.

Lower insulation ratings, hotter routes, conduit and thermal insulation are outside this calculator. Do not choose a cooler condition to get a result. Manufacturer terminal ratings, minimum wire, surge behavior, protection, cable construction and your governing installation rules still need review. The pinned wire and total factor follow you into the editor; review both after edits.

ABYC E-11 is the implemented table basis. Marine wire gauge, RV wire size, campervan cable size and Australian dual-battery wording do not establish UK or AS/NZS standards coverage.

Sources

Wire sizing sources and installation conditions

National Bureau of Standards / NISTCopper Wire Tables, Handbook 100AWG diameter progression and nominal conductor areas; accessed 11 September 2026. AWG-mode ampacity uses a conservative lower bound from Crimp’s reviewed metric table.Blue Sea SystemsChoosing the correct wire size for a DC circuit105°C copper, round-trip length and circuit drop allowances. Read 11 September 2026. Crimp asks for one-way length and adds an equal return.Blue Sea SystemsCircuit Wizard: installation conditionsInsulation, engine space and additional derating affect selection. Bundle factors in this tool must come from your documented installation basis.CrimpSizing methodology and engine limitationsExisting metric conductor table, approximate AWG counterparts on older diagrams, 125% continuous headroom and copper-resistance model.Blue Sea Systems5191 battery-terminal MRBF holderDirect battery-terminal mounting and 30–300 A fuse range; checked 21 October 2026. The product photo shows a sample fuse, not the example's selected rating.Blue Sea Systems5175 MRBF 30 A fuseThe wire example's illustrative 30 A fuse. Its published voltage and interrupt ratings still need comparison with the actual bank and fault-current evidence.
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