Our cable selection calculator is designed to take the guesswork out of choosing the right cable size for your DC electrical circuits. By simply entering a few key details—such as the voltage of your system, the load in amps, and the round-trip cable length—the calculator will determine the appropriate cable size in line with ISO 13297:2020 standards. This ensures that your wiring is safe, efficient, and capable of delivering the required voltage to your appliances without excessive drop. Whether you’re planning a new installation or checking existing circuits, this tool makes it easy to get it right the first time. Just fill in the inputs, and let the calculator do the rest!

Disclaimer:

These calculators provide estimated cable sizes for typical DC circuits based on ISO 13297:2020 recommendations and user inputs. They assume standard conditions for copper conductors in marine or general electrical installations. Actual requirements may vary depending on installation specifics, including temperature, bundling, and load variations. Users must consult a qualified electrician, marine surveyor, or engineer to confirm the suitability of the calculated cable size for their specific application. The Fit Out Pontoon accepts no liability for damage, issues, or non-compliance arising from the use of these tools.

Assumptions and Presumptions

Conductor Material:
Calculations assume copper conductors with a standard resistivity constant of 0.0164 Ω·m/mm², as per ISO 13297:2020.

Voltage Drop Limits:
A 3% voltage drop is applied for critical circuits (e.g., navigation lights, bilge pumps, or emergency lighting).
A 10% voltage drop is applied for non-critical circuits (e.g., interior lighting or general equipment).

Cable Length:
The input for length represents the one-way cable run. The calculators automatically double this to account for the round-trip length.

Cable Size Matching:
Calculated sizes are rounded up to the nearest available standard size. The following sizes are used: 0.75 mm², 1.00 mm², 1.50 mm², 2.00 mm², 2.50 mm², 3.00 mm², 4.00 mm², 4.50 mm², 6.00 mm², 8.50 mm², 10.00 mm², 16.00 mm², 25.00 mm², 40.00 mm², 50.00 mm², 60.00 mm², 70.00 mm², 95.00 mm², and 120.00 mm².

Lighting Circuits:
For lighting circuits, the total current is calculated as:
Total Current (A)
=
Number of Lights
×
Amps per Light
Total Current (A)=Number of Lights×Amps per Light

Safety Margin:
The calculators do not include a margin for unexpected current surges or future circuit upgrades. Users should account for these manually if required.

Ambient Temperature and Bundling:
High-temperature environments or bundled cables require additional derating factors. These calculators do not account for these conditions.

What’s Not Included
Voltage drop across connections or terminals is not factored into the calculations.
Derating for high ambient temperatures or bundled cables is not included.
Adjustments for special cable types (e.g., tinned copper, armoured cables) are not made.
Assumes DC systems only—AC systems require separate considerations.
The tools are designed for simple circuits and do not cover complex multi-circuit designs or fluctuating loads (e.g., dimmers).

What is Volt Drop?

Voltage drop is the reduction in electrical potential that happens when electricity travels through a conductor, such as a cable. Think of it like water flowing through a pipe; if the pipe is too narrow or too long, water pressure drops. Similarly, electrical resistance in a cable causes voltage to decrease over its length.

Why is it Critical for Canal Boats?

On canal boats, cable runs are often lengthy. When power flows from the battery to an appliance and back, it must travel both ways, doubling the effective cable length. Appliances like pumps, lights, or fridges need sufficient voltage to operate correctly. If the voltage drops too much, the appliance may not work as intended—or worse, it could overheat or cause a fire due to excessive resistance converting energy into heat.

Certain devices are especially sensitive to voltage drop. For example:

• Navigation lights and bilge pumps typically require a minimal drop to function safely and effectively.
• Critical systems, such as those affecting safety, may need voltage drops limited to just 3%, while non-essential systems can tolerate up to 10%.

How to Avoid Voltage Drop

The golden rule is simple: choose the right cable size! Larger cables have less resistance and reduce voltage drop. By planning your electrical system correctly from the outset, you can avoid costly and dangerous mistakes later.

Simplified Voltage Drop Formula

The ISO 13297:2020 standard provides a straightforward approach to calculating the minimum conductor size required to limit voltage drop:

S = (I × L × 0.0164) ÷ Vd

Where:

• S = Cable cross-sectional area in mm²
• I = Current in amperes (A)
• L = Round-trip cable length in metres (out and back)
• V_d = Allowable voltage drop in volts

For a typical 12V system:

• A 3% voltage drop (used for critical Equipment) is approximately 0.36V
• A 10% voltage drop (used for non-critical Equipment) is approximately 1.2V

Practical Example

Let’s size a cable for a 12V water pump drawing 5A, positioned 7.5m from the battery.

Step-by-Step Calculation:

1. 1. Determine the round-trip length: L = 7.5m × 2 = 15m.
   2. Choose the allowable voltage drop (e.g., 10%, so V_d = 1.2V).
   3. Apply the formula:

S = (5 × 15 × 0.0164) ÷ 1.2 = 1.025 mm²

Round up to the next available cable size—1.5mm².

Fuse Consideration

Always fuse to protect the smallest conductor. In this example, a 5A pump with a cable rated for 20A might use an 8A fuse.

Key Considerations

• Round-Trip Distance Matters: Always double the one-way cable length.
• Voltage Sensitivity: Use a 3% drop for critical systems and 10% for non-critical.
• Safety First: Always err on the side of larger cables when in doubt.

Standard Cable Sizes Used

Our calculators recommend these common cable sizes: 0.75mm², 1mm², 1.5mm², 2.5mm², 4mm², 6mm², 10mm², 16mm², 25mm², and beyond for heavier loads.

For a professional-grade electrical system designed for reliability and safety, consult qualified marine electricians or refer to ISO 13297:2020 for full details.