Wire gauge in the 12 V system: voltage drop, ampacity, fusing

In a 12 V system, ten times as much current flows for the same power as at 120 V. That's why undersized wires are the most common problem here: flickering lights, inverters shutting down, weak charging. This article explains the two criteria for the wire gauge and the role of the fuse.

Criterion 1: Voltage drop

Every wire has a resistance, and voltage drops across it: ΔU = I × R. For DC the wire length counts twice (out and back). Copper conducts with κ ≈ 56 m/(Ω·mm²) at 68 °F (20 °C).

ΔU [V] = 2 × L [m] × I [A] ÷ (56 × A [mm²])

16 ft (5 m), 20 A, 10 AWG (≈ 5.3 mm²): 2 × 5 × 20 ÷ (56 × 5.3) = 0.67 V ≈ 5.6% of 12 V

Common recommendation in vehicles: at most 3% drop for load circuits, 1–2% for charging circuits (solar, DC-DC charger) so the charge voltage arrives at the battery. That's practice, not a code – but a proven one (ABYC E-11 uses the same 3% / 10% thresholds for boats).

Voltage drop calculator

Criterion 2: Ampacity

Regardless of voltage drop, a wire must not heat up excessively. The permissible ampacity depends on the wire type (insulation temperature rating), installation method (free air, bundled, in conduit), ambient temperature (engine bay!) and bundling. Wire manufacturers' tables and standards (ABYC E-11 for boats, NEC for buildings, SAE J1128 wire types) provide the values. The larger of the two gauges – from voltage drop and ampacity – must be chosen.

Ampacity values are deliberately not listed here as a table: they depend too much on wire type and installation. Manufacturer data and standards are authoritative; when in doubt ask a professional.

The fuse protects the wire

The fuse sits as close to the battery as possible – the wire behind it is protected, the wire before it is not.
The fuse's rating follows the ampacity of the wire, not the load.
For high currents (inverter, DC-DC charger) fuse types such as MIDI, MEGA, ANL or Class T fuses are common; automotive blade fuses end at about 30–40 A.
Every branch from the battery gets its own fuse.

Typical currents in an RV

Load / source	Current at 12 V	Note
LED lights, USB	0.5–3 A	16–14 AWG usually sufficient
Compressor fridge	4–6 A	sensitive to low voltage
Water pump	3–6 A	short running times
DC-DC charger 30 / 50 A	30 / 50 A	fuse input and output
Inverter 1,000 W	≈ 90–100 A	4–2 AWG depending on length
Inverter 2,000 W	≈ 180–200 A	2 AWG to 1/0 AWG and up

Wire gauge calculator – calculates the minimum gauge for voltage drop

Practical tips

Use fine-stranded wire (marine grade or SAE automotive) – solid household wire breaks under vibration.
Crimp lugs with a proper crimping tool; soldered joints fail under vibration.
Check contact resistance: warm terminals and fuse holders are a warning sign.
Make negative wires as thick as positive wires.
Convert metric figures: 21 mm² ≈ 4 AWG, 34 mm² ≈ 2 AWG, 54 mm² ≈ 1/0 AWG.

Sources

ABYC E-11 (AC and DC electrical systems on boats), SAE J1128 (low-voltage primary cable), ASTM B258 (AWG cross-sections), NEC (National Electrical Code)

Published 09/19/2026.