Wire Sizing Calculator

Proper wire sizing is essential for safe and efficient electrical systems. Undersized cables can overheat, waste power, and in worst cases start fires. This calculator helps you pick the correct conductor size by checking two things:

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1. Voltage Drop – ensuring the wire is large enough that voltage loss stays within limits (typically 3% for critical loads, 10% for non-critical).

2. Ampacity – ensuring the wire is rated to safely carry the current at its insulation/terminal temperature rating (60 °C, 75 °C, or 90 °C).

 

The calculator always recommends the larger of the two requirements.

 

For more detailed guidance on wiring for campervans and van conversions, check out this comprehensive guide.

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Continuous vs Short-Burst Loads

• Continuous (NEC): runs for 3 hours or more — must be sized to full ampacity.

• Short bursts (intermittent): seconds to a few minutes only (e.g., starter ≤ 5 s, winch/thruster 1–5 min).

• Rule of thumb: if a load can run longer than ~10–15 min, treat it as continuous and size to ampacity.

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Continuous (size to ampacity)

These are things that can run for hours at a time, so you always size wiring by ampacity (safety/heat), not just voltage drop:

• Inverters (especially if they power laptops, kettles, induction hobs, etc.)

• Battery chargers / DC-DC chargers

• Fridges (they cycle, but effectively they’re “on” most of the day)

• Diesel / gas heaters (the blower can run all night)

• Fans & ventilation

• Lighting

• Water pumps (short cycles, but many times per day → treat as continuous)

• Any permanently powered electronics (routers, trackers, alarms, etc.)

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Assumptions and Key Points

• Copper Conductors: Calculations assume standard copper wire, with resistivity 1.724 × 10⁻⁸ Ω·m at 20 °C.

• Voltage Drop Selection: You can choose either 3% or 10% allowable drop.

• Distance: Enter the one-way length; the tool automatically doubles it for the round-trip (out and back).

• Ampacity Tables: Based on common NEC/ABYC values for copper conductors at 60 °C, 75 °C, and 90 °C ratings.

• Cross-Sectional Area: Results are given in mm² with the nearest AWG and IEC metric equivalents.

 

Formulas Used

The primary formulas used in this calculator are:

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1. Calculate Resistivity at Operating Temperature:
ρ₂ = ρ₁ × (1 + α × (t₂ - t₁))

Where:

ρ₂ = Resistivity at temperature t₂
ρ₁ = Resistivity at reference temperature t₁ (20°C)
α = Temperature coefficient of resistivity for copper (0.00404 per °C)
t₂ = Operating temperature (°C)
t₁ = Reference temperature (20°C)

 

2. Calculate Cross-Sectional Area (A):

A = (I × ρ × 2 × L) / V

 

Where:

I = Load current (Amps)
ρ = Adjusted resistivity (Ω·m)
L = Distance (one-way) in meters
V = Voltage drop (Volts)

 

3. Convert Area from Square Meters to Square Millimeters:

A_mm² = A × 1,000,000

 

Why Correct Wire Sizing Matters

Proper wire sizing prevents excessive voltage drop, which can lead to inefficiency, overheating, or even electrical fires. This calculator ensures that the selected wire size can safely carry the current over the specified distance, considering the operating temperature and voltage drop.

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Why Both Checks Are Needed

• Voltage Drop protects performance (lights stay bright, electronics run properly).

• Ampacity protects safety (wires don’t overheat).

• Standards (NEC, ABYC, BS 7671) always require you to satisfy both. This tool does exactly that.