What voltage drop should I allow in a 12V circuit?

For most 12V applications, a voltage drop of 3% (0.36V) is the accepted standard. This leaves at least 11.64V at the load, which is sufficient for most automotive and marine devices. For sensitive electronics, lighting, and audio equipment, a tighter 2% drop (0.24V) is recommended. For basic non-critical loads such as heaters or winches, up to 5% (0.60V) is sometimes acceptable. The voltage drop limit you choose directly affects the required wire size: a tighter limit requires thicker wire.

Why do 12V systems need thicker wire than 240V systems?

Wire size is determined by current (amps), not voltage. At 12V, delivering the same power (watts) requires far higher current than at 240V. For example, a 120W load draws 10A at 12V but only 0.5A at 240V. Higher current causes more voltage drop across a given wire resistance and more heat generation, so thicker wire is needed. This is why 12V wiring in caravans, boats, and solar systems uses noticeably heavier cable than household wiring for the same power level.

Should I use AWG or mm² wire sizes in New Zealand?

New Zealand uses metric wire sizing (mm²) for most electrical work under AS/NZS 3000 (the Wiring Rules). However, AWG (American Wire Gauge) is commonly used for automotive, marine, and imported solar components. Both measurements describe the copper conductor cross-section area, just in different systems. This calculator shows both so you can match the specification of whichever standard applies to your components. Always verify conductor ratings with your supplier, as insulation type and temperature rating also affect the maximum current capacity.

12 Volt Wire Size Calculator

This calculator works out the minimum wire gauge for a 12 volt DC circuit, in both AWG and mm², so the cable you fit does not overheat or lose too much voltage over its run. Undersized wire is a common problem in caravans, boats, campervans and solar setups, where long cable runs and high current at low voltage cause far more voltage drop than the same power would at mains voltage. To use it, enter the current draw in amps and the one-way cable run length in metres, then pick an application type (or set a custom voltage drop percentage), the system voltage, an optional safety margin, and the ambient temperature. It returns the minimum AWG and mm² wire size, the actual voltage drop and voltage at the load for that size, a calculation breakdown, and a reference table comparing every standard AWG size at your amps and length. A verdict below the results tells you plainly whether the recommended wire is adequate, or whether you should shorten the run, raise the system voltage, or reduce the load. Remember to measure the real cable route rather than a straight-line distance, and size the negative return wire the same as the positive. These figures are indicative only; final wire selection should also account for current-carrying capacity, insulation rating, bundling with other cables and any wiring standards that apply to your installation.

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Updated June 2026 Based on Ohm's Law and standard copper resistivity (1.724 × 10−8 Ω·m). AWG to mm² conversions per ASTM B258.

1. Circuit Details

Current draw (amps)

One-way cable run length (metres)

Application type

Custom maximum voltage drop (%)

2. System Options

System voltage

Safety margin

Ambient temperature

Recommended Wire Size

Minimum AWG
-
Lower number = thicker wire

Minimum mm²
-
Cross-sectional area

Actual Voltage Drop

At recommended wire size

Voltage at Load

Nominal 12V system

Calculation Breakdown

System voltage-

Current draw-

One-way run length-

Total circuit length (x2)-

Max allowable voltage drop-

Required resistance (max)-

Required min cross-section-

Safety margin applied-

Recommended min cross-section-

Wire Performance at Selected Size

Recommended AWG-

Recommended mm²-

Wire resistance (total circuit)-

Voltage drop-

Voltage drop %-

Voltage at load-

Power lost in wire-

Temperature derating-

AWG Reference Table for 12V Systems

AWG	mm²	Max Amps (typical)	Resistance (mΩ/m)	Voltage Drop at 10A over 3m one-way	Suitable For

Result: Enter your circuit details above.

How to Size Wire for a 12V DC Circuit

Wire sizing for 12V DC systems is based on two key considerations: keeping voltage drop within acceptable limits, and ensuring the conductor can safely carry the current without overheating. This calculator focuses on voltage drop, which is the primary sizing factor in low-voltage DC systems over any significant cable run.

The formula used is derived from Ohm's Law and the relationship between a wire's cross-sectional area and its resistance:

Voltage drop (V) = Current (A) × Wire resistance (Ω)

Wire resistance (Ω) = Resistivity of copper × Total circuit length / Cross-sectional area

Rearranging to find the minimum cross-sectional area:

Minimum area (mm²) = (Resistivity × 2 × One-way length × Current) / Maximum allowable voltage drop

The resistivity of annealed copper is 1.724 × 10−8 Ω·m (or 0.01724 mΩ·mm²/m). The total circuit length is doubled because current flows out through the positive wire and returns through the negative wire. This calculator then rounds up to the next standard AWG size and mm² size.

Worked Example

A 12V bilge pump draws 10 amps. The positive cable run from the fuse panel to the pump is 3 metres. You want no more than 2% voltage drop (sensitive electronics preset), with a 1.25× safety margin.

Maximum allowable voltage drop: 12V × 2% = 0.24V
Total circuit length (positive + negative return): 3m × 2 = 6m
Required maximum circuit resistance: 0.24V / 10A = 0.024 Ω
Minimum cross-section: ρ × L / R = 0.01724 Ω·mm²/m × 6m / 0.024 Ω = 4.31 mm²
With 1.25× safety margin: 4.31 × 1.25 = 5.39 mm²
AWG 10 (5.26 mm²) is just below 5.39 mm², so round up to AWG 8 (8.37 mm²)
Actual voltage drop with AWG 8: 0.01724 × 6 / 8.37 = 0.01236 Ω, so 10A × 0.01236 Ω = 0.124V (1.03%)
Voltage at load: 12 − 0.124 = 11.88V

These are the default inputs in the calculator above. The results shown (8 AWG / 8.37 mm², 0.124V drop, 11.88V at load) match this worked example.

AWG to mm² Conversion Reference

AWG	mm²	Diameter (mm)	Typical Max Current (free air)
20	0.52	0.81	11A
18	0.82	1.02	16A
16	1.31	1.29	22A
14	2.08	1.63	32A
12	3.31	2.05	41A
10	5.26	2.59	55A
8	8.37	3.26	73A
6	13.3	4.11	101A
4	21.2	5.19	135A
2	33.6	6.54	181A
1	42.4	7.35	211A
0 (1/0)	53.5	8.25	245A
00 (2/0)	67.4	9.27	283A
000 (3/0)	85.0	10.40	328A
0000 (4/0)	107	11.68	380A

12V Wire Sizing Tips

Always measure the actual cable run, not the straight-line distance. Cable often routes around obstacles, adding length.
The negative return wire must be the same size as the positive wire. Both contribute to total circuit resistance.
Use tinned copper wire for marine applications. Salt air causes plain copper to corrode inside the insulation where you cannot see it.
Fuse as close to the battery as possible, on the positive wire. The fuse protects the wire, not the device.
Bundled cables get hotter than single cables in free air. If your cable is bundled in a loom or conduit with other wires, apply a derating factor of 0.7 to 0.8.
Crimp terminals properly. A poor crimp connection adds resistance and heat at the joint, negating the benefit of correct wire sizing.

Related Calculators

Essential Calculators hub
Electrical Power Calculator (P = VI): find watts, volts, or amps from any two values.
Amp Hour Battery Life Calculator: how long a 12V battery will run a given load.
Appliance Running Cost Calculator: annual electricity cost for any appliance.

Sources and method: Ohm's Law (V = IR). Copper resistivity: 1.724 × 10−8 Ω·m (IEC 60228, International Electrotechnical Commission). AWG cross-sectional areas per ASTM B258 (Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires). Voltage drop recommendations per ABYC E-11 (American Boat and Yacht Council marine wiring standard) and automotive industry practice.

This calculator provides indicative wire size recommendations based on voltage drop limits only. Actual wire selection must also account for current-carrying capacity (ampacity), insulation temperature rating, installation environment, bundling with other cables, and applicable wiring standards. For fixed electrical installations in New Zealand buildings, wire sizing must comply with AS/NZS 3000 and be performed or checked by a licensed electrical worker.