Spaziou Cable Lab · DC electrics

12 V cable size calculator: voltage drop, losses and standard cable sizes

Calculate a minimum copper cable cross-section for voltage drop using the one-way length. The tool doubles the distance internally for positive and negative conductors, then rounds up to the next standard size.

Spaziou rule: the voltage-drop result is only an electrical minimum. Also meet the cross-section required by current-carrying capacity, temperature, cable grouping and, above all, the equipment manual.

Calculator

From consumption to cable.

Quick table · 12 V · copper · 2.5%

One-way distance versus current.

This is a table of voltage drop, not a universal current-carrying capacity table. Values are already rounded up to the next standard cable size.

Current1 m one-way3 m one-way5 m one-way
10 A1.5 mm²4 mm²6 mm²
20 A2.5 mm²10 mm²16 mm²
40 A6 mm²16 mm²25 mm²
80 A10 mm²35 mm²50 mm²
100 A16 mm²35 mm²70 mm²
Do not use this table to choose an “exact fuse”. A 100 Ah battery does not determine the fuse rating. The fuse must protect the conductor and be coordinated with maximum current and the equipment manual. Place it as close to the source or battery as practical for the system design and documentation.

Real cases

Why a cable that “seems thick” may still be insufficient.

1,000 W inverterAt 12 V and 90% efficiency, current is about 92.6 A. Voltage drop is only the first check; the inverter manual may require a much larger cable cross-section.
Refrigerator that turns offThe compressor starts, the voltage drops in the cable/connectors and the low voltage protection can disconnect the refrigerator.
MPPT → batteryUse the controller's maximum output current. This cable run is sized differently from panel → MPPT.
Panel → MPPTEnter PV-array current (Imp/Isc according to the chosen design criterion); do not automatically divide panel watts by 12 V.

Fuses

The correct order is cable → equipment → protection.

Protection must interrupt overcurrent before the conductor becomes unsafe and also meet the equipment manufacturer's specification. High-current systems may use MIDI, MEGA, Class-T or other fuse types depending on voltage, breaking capacity and system design.

Why the manual takes priority: For its Inverter Smart 12/1600, Victron specifies 70 mm² cable for runs up to 5 m and a 250 A external fuse. Do not apply that pairing to another inverter: it demonstrates why “1,600 W = X mm²” is not a universal rule.

Purchase after calculation

Cable and tool with exact ASIN.

JatilEr battery cable 16 mm² 50 cm M8JatilEr · 16 mm² · 50 cm · M8Red/black copper cable pair with M8 terminals. Only a suitable option if your calculation and the manual permit 16 mm² and each one-way run fits within 50 cm.

View on Amazon →

DIHOOL battery cable 25 mm² M8 50 cmDIHOOL · 25 mm² · 50 cm · M8Exact 25 mm²/4 AWG, 50 cm variant. Use only if your calculation, current-carrying capacity and manual permit 25 mm².

View on Amazon →

DIHOOL battery cable 35 mm² with M8 terminals for camperDIHOOL · 35 mm² · 50 cm · M835 mm²/2 AWG variant with M8 terminals. Check length, connection size and equipment requirements before buying.

View on Amazon →

HX-50B crimping tool for cables from 6 to 50 mm²HX-50B · crimping tool 6–50 mm²Hand tool for battery cable lugs in typical camper cable sizes. Check that crimping die and lug match the exact cable.

View on Amazon →

VEXUNGA MEGA fuse holder with fuse 125 AVEXUNGA · MEGA/ANM fuse holder · 125 AMEGA/ANM format reference with exact ASIN. Do not default to 125 A: the fuse rating must be based on the conductor, load and equipment manual.

View on Amazon →

Do not buy “the thickest cable” without checking terminals and equipment. Cross-section, length, flexibility, insulation, M8/M10 connection and operating temperature must suit your actual circuit.
Inverter and protection → Electrical guide 12 V → Electrical planner →
Technical references · reviewed 24/09/2026 Victron Energy · Wiring Unlimited Victron · Inverter Smart cable and fuse recommendations Dometic · low voltage alert and wiring losses

The formula uses approximate copper resistivity of 0.0175 Ω·mm²/m at a reference temperature. It does not automatically model heating, grouping, insulation, terminals or added resistance from fuses, shunts and connections.