Cable sizing and fuses
How cable sizing, fuses and voltage drop work — and why the right cable area is decisive for safety, performance and an efficient off-grid system.
The strength of your system
An off-grid system is never stronger than its cables and fuses.
You can have the best battery on the market, the most powerful inverter and the most expensive solar panels — but if the cable sizing is wrong, the system can suffer:
Voltage drop
Overheating
Poor performance
Fault codes
In the worst case, a fire risk
In this guide we cover:
How cable sizing works
Why fuses are vital
How to do the maths
Common mistakes
Practical examples for 12V, 24V and 230V systems.
What is cable sizing?
Cable sizing is about choosing the right cable area for the current that has to be carried.
When current flows through a cable, resistance appears.
The thinner the cable and the longer it is, the greater the resistance.
That leads to:
Heat
Energy losses
Voltage drop
Worse performance
In low-voltage systems such as 12V this matters even more, because the current is often very high.
Why it matters even more in a 12V system
In a 230V system, high power needs relatively little current.
In a 12V system the current becomes very high instead.
1000W in a 230V system is about 4.3 A, but in a 12V system it is about 83 A.
That is why powerful 12V inverters often demand:
Very thick cable.
Short cable runs.
Heavy-duty fuses.
How to calculate current (amps)
In a DC system, use this formula:
I = P / U I = current (A) P = power (W) U = voltage (V)
Example
A 2000W inverter in a 12V system:
I = 2000 / 12 ≈ 167 A
The system can therefore draw roughly 167 A continuously at the inverter's full load.
At peak power the current can go higher still, usually about double for a short moment.
What is voltage drop?
When current travels through the cable, some of the voltage is lost.
This is called voltage drop.
Too much of it can cause:
The inverter to shut down.
The fridge to work poorly.
Chargers to lose output.
Unnecessary heat.
Worse battery performance.
Recommended voltage drop
Follow these figures and your system will work well and reliably.
Critical loads: maximum 3% voltage drop.
Other loads: maximum 5% voltage drop.
High-current equipment: as little voltage drop as possible.
In an off-grid system people usually aim for:
Maximum 2–3% for an inverter.
3–5% for other loads.
What decides the cable size?
Four things decide which cable area you need:
Current (A)
More current needs thicker cable.Cable length
A longer cable means more resistance.
Note: always count there and back.
For example: 1 metre to the inverter = 2 metres of cable in total.Permitted voltage drop
A lower permitted drop needs thicker cable.Installation & environment
Temperature, bundling and ventilation also affect how much the cable can carry.
Example: a 2000W inverter in a 12V system
Assumptions:
Power: 2000W
System voltage: 12V
Cable length: 1 metre one way
Maximum voltage drop: 3%
The current works out at roughly:
I = 2000 / 12 ≈ 167 A
This normally calls for:
Around 50–70 mm² cable
Depending on cable type and installation
A longer run needs thicker cable.
Why short cables matter
In high-current installations, a short cable is almost always better than a thicker one.
That is why people usually place:
The inverter close to the battery.
The main fuse close to the battery.
The DC/DC charger close to the battery bank.
A shorter cable gives you:
Lower losses.
Less heat.
Better efficiency across the system.
What does a fuse do?
A fuse's primary job is to protect the cable, not the appliance.
This is a common misunderstanding.
If a short circuit occurs, the fuse must blow before the cable gets dangerously hot. Without a fuse, cables can become extremely hot in seconds.
Where should the fuse go?
The main fuse should sit as close to the battery as possible.
Preferably within 15–30 cm.
The reason is that the whole cable has to be protected.
If the cable shorts out before the fuse, there is no protection at all.
Common fuse types
MIDI — used in smaller DC systems.
ANL — common for inverters.
MEGA — higher currents.
Class T — very high currents and lithium systems.
How do you pick the right fuse?
The fuse must:
Be lower than the cable's maximum rating.
But higher than the normal operating current.
Example:
The cable handles 250 A.
Normal operation is 170 A.
A 200 A or 225 A fuse would then be a sensible choice.
Common mistakes
Cable that is too thin gives you:
Heat
Voltage drop
Power losses
Cable that is too long gives you:
Large losses
Worse efficiency
A fuse that is too large lets the cable overheat before the fuse blows.
Having no fuse near the battery is a serious safety risk.
Avoid cheap cable lugs — a poor connection creates heat and resistance.
12V vs 24V — why many move up in voltage
As the power goes up, 12V quickly becomes extremely current-hungry.
Compare 3000W in amps and you get:
12V system: about 250 A
24V system: about 125 A
That is why larger off-grid systems often use 24V or 48V.
The advantages of higher voltage:
Thinner cable.
Less heat.
Better efficiency.
An easier installation.
Kraftur's recommendations
For the best performance:
Keep high-current cables short.
Use quality cable.
Fuse close to the battery.
When in doubt, go slightly oversized.
Keep voltage drop to a minimum.
Use the right cable lugs and crimping tool.
A stable power system always starts with the right basic installation.
Tools from Kraftur
Do use our tool to size your cable and fuse — you will find it here: Kraftur — Tools — Cable sizing.