🔌 Choose the right cable – avoid voltage drop in your 12V system
Incorrect cable sizing is one of the most common causes of problems in boats and off-grid installations.
Symptoms can include:
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equipment shutting down
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flickering lights
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diesel heater faults
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batteries seeming weak
Often, the problem isn't the equipment — it's the cables.
⚡ Why cable thickness matters
When current flows through a cable, resistance occurs.
If the cable is too thin:
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voltage drops
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equipment receives less current
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efficiency decreases
The longer the cable and the higher the current, the more important correct sizing becomes.
📏 What happens when cables are undersized?
Typical problems:
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cables getting hotter
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reduced performance
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increased energy loss
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unstable operation
Many people experience, for example, diesel heater faults even though the battery is fine — because the voltage drops during startup.
🧮 Use the cable calculator here
To make things easy, I've created a cable calculator.
It helps you find the right cable size based on:
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cable length
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current draw
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voltage
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permissible voltage drop
👉 Use the cable calculator here:
https://sejlstroem.dk/pages/kabelberegner
🚤 Common installation mistakes I see
The most common ones:
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undersized cables “because they were already there”
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long cable runs without upsizing
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cheap cable lugs with poor connections
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failure to account for starting current
Small changes can make a big difference.
🔧 Good rules of thumb
✔ choose a cable that's slightly too large rather than too small
✔ keep cable runs as short as possible
✔ use quality cable designed for marine use
✔ ensure good connections
❓ Frequently asked questions
Can cables that are too thick cause damage?
No — they typically result in lower losses and more stable operation.
Does voltage drop matter at 12V?
Yes, a lot. Small voltage differences matter more at low voltages.
Can I use the same cable as in a house installation?
Marine cables are often better at withstanding moisture and vibration.
🤔 Not sure?
If you're unsure which cable size to choose, feel free to send me:
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the equipment you're using
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cable length
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battery voltage
I'll be happy to help you find the right solution.
⭐ Why correct cable sizing matters
✔ stable operation
✔ fewer faults
✔ better use of battery capacity
✔ longer equipment life