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SvCable calculator
The charging circuit
Calculate the Required Cable Cross-Section for the Charging Circuit
To ensure safe and efficient charging, we use a maximum voltage drop of 0.1 V as our baseline.
Be sure to account for both the positive and negative cables, and enter the alternator’s current output.
For example: 2.5 m cable (one-way distance) and a 75 A alternator → enter 5 m and 75 A.
The result is 67.5 mm² → always select the next larger standard size: 70 mm².
*If the required cable area becomes too large, you can use two cables of the same size in parallel.
Example: 8 m cable and 135 A gives 194.4 mm² → use 2 × 120 mm² = 240 mm².
Formula for calculating the cable cross-section: 0.018 × m × A / 0.1 = cable area in mm²
Cable dimensioning in the charging circuit
Choose your voltage drop
Calculate Cable Cross-Section Based on a Custom Voltage Drop
Use this method when you calculate the required cable cross-sectional area based on a user-defined voltage drop, such as bow thrusters, fridges or electric toilets.
Enter the total cable length (both positive and negative conductors), the current draw (A), and the acceptable voltage drop (V).
*If the required cable area is high, you can use two cables of the same size in parallel.
Example: 17 m cable and 750 A with 1.0 V allowable voltage drop gives 229.5 mm² → use 2 × 120 mm² = 240 mm².
Formula for calculating the cable cross-section: 0.018 × m × A / tolerated voltage drop = cable area in
mm²
Cable dimensioning on calculated voltage drop
Control of cable area
Here you can check whether your existing cables have sufficient gauge.
Simply enter the cable length (both positive and negative conductors), cable cross-sectional area, and current – and you will immediately see the resulting voltage drop.
Recommended maximum voltage drop:
Charging circuits: 0.1 V
Electrical loads (lights, navigation lamps, etc.): 0.5 V
How to calculate the voltage drop: 0.018 × m × A / cable area in mm² = voltage drop in volts (V)
(Where m is the total cable length – positive plus negative – and A is the current in amperes.)
When you want to know if your cables are sufficient, you can get the answer here.
Kontroll av kabeldimensionering
How do you convert Watts to Amps?
When you look at the specification for an electrical product, it usually lists how many watts (W) it consumes. But to determine how much current (amperes, A) it draws from your electrical system—such as a 12 V system in a boat or RV—you need to use a simple formula:
W / V = A
Example:
You have a 15 W lamp connected to a 12 V system:
15 W / 12 V = 1.25 A
This means the lamp draws 1.25 amperes when it is on.
In the same way, you can calculate the current draw for refrigerators, chargers, water pumps, and any other electrical loads operating on 12 V, 24 V, or other DC voltages.
Watt / Volt = Ampere
Ström (A) från Effekt (W)
How to calculate it
In some cases—especially with high-power equipment such as bow thrusters—the power rating is given in horsepower (hk) instead of watts. To calculate how much current (amperes, A) such an electrical load draws from your system, you must first convert horsepower to watts.
You do it like this:
hk × 735.5 / V = A
Example:
You have a 7 hk bow thruster in a 12 V system:
7 × 735.5 / 12 = 429 A
Conversion factor:
1 hk = 735.5 W
(Technically 735.49875 W, but 735.5 is fully sufficient for practical calculations.)
In some product specifications, the motor output is listed in hp (the American unit for horsepower).
You can use the same formula, but it’s a good idea to add a safety margin because real-world losses, startup surges, and motor inefficiency often increase the actual current draw. A common rule of thumb is to add 1 A per hp.