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SvCable calculator
The charging circuit
Here you calculate which cable cross-sectional area is needed in the charging circuit.
To ensure safe and efficient charging, we use a maximum voltage drop of 0.1 V as our baseline.
Include both the positive and negative cable and specify the alternator 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 → choose the nearest larger size: 70 mm².
*If the required cable area is large, you can use two cables of the same dimension.
For example: 8 m cable and 135 A gives 194.3999 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
Here you calculate the cable cross-sectional area based on a user-selected voltage drop, for example for a bow thruster, refrigerator, or toilet.
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.
*If the required cable area is large, you can use two cables of the same dimension.
For example: 17 m cable and 750 A with a tolerated voltage drop of 1.0 V 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
Cable cross‑section check
Here you check whether your existing cables have sufficient cross‑section.
Simply enter the cable length (both positive and negative conductors), cable cross-sectional area, and current – and you will immediately see what voltage drop you have.
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 and 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 consumption for refrigerators, chargers, water pumps, and everything else that runs on 12V, 24V, or other voltages.
Watt / Volt = Ampere
Ström (A) från Effekt (W)
This is how you calculate
In some cases, especially with more powerful equipment such as bow thrusters, the power is specified in horsepower (hp) instead of watts. To calculate how much current (amperes) such a load draws from your electrical 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).
In some specifications, the motor power is given in imperial hp (the American designation for horsepower). In that case, you can use the same formula, but it is advisable to add a safety margin, since real losses, starting currents, and inefficiencies often make the actual current slightly higher. A rule of thumb is to add add 1 A per hp.
So in the example above, but with a 7 hp bow thruster:
429 A + 7 A = 436 A