DC Amp Fuse Size Selection Calculator (2025 Guide)
Correct fuse sizing is essential for the safe operation of any DC energy system, regardless of the application whether its your camper van or your shed. Choosing the wrong fuse can cause wires to overheat, or may result in damage to your equipment.
In this guide, you’ll learn exactly how to size a DC fuse properly, plus we will show you how to use our free DC Amp Fuse Size Calculator to get the correct ratings in seconds.
Disclaimer:
This calculator is for general information and educational purposes only and does not constitute professional electrical advice. Results are estimates only and may not be suitable for all systems or installations. Electrical systems can be hazardous so always follow manufacturer instructions, relevant UK regulations, and consult a qualified professional before installation or modification.
Why Fuse Size Matters in DC Systems
The role of a fuse is to protect your cables from overload and short circuit faults, thus a correctly sized fuse ensures the cable never carries more current than it can safely handle. It does that by melting or vaporising a strip of material (metal or non metal) inside to physically break the current flow through it.
How to Calculate the Right DC Fuse Size
Calculating the fuse size is simple, just use the equation below.
Fuse Size = Maximum Continuous Current × 1.25 (a 25% safety factor)
This 25% margin ensures the fuse can handle short bursts without blowing, but still protects the cable.
Additionally, keep in mind that when powering a device as the batteries discharge voltage will decrease which will result in higher current discharged. The inverter efficiency will also affect the amount of current discharged from the battery.
Calculating the correct fuse size isn’t as simple as entering numbers into a calculator. That’s why all our Energy Solutions Kits come with fuses, cables and protection devices sized specifically for the components in your kit.
Example:
If your inverter draws 80A continuously the recommended fuse size would likely be 100A since 80A × 1.25 = 100A
DC Amp Fuse Size Selection Calculator
You can use this calculator to determine the correct fuse size for any DC circuit by entering the maximum continuous current flowing through the circuit or a device is expected to draw. Do not use our calculator to choose your fuse size if a range is already stated in the manufacturer manual.
Here’s a list of common applications:
- Battery to Inverter Fuse: Use the inverter’s maximum DC current (see its datasheet).
- Battery to DC Distribution Fuse: Use when a battery feeds multiple DC loads through a distribution block e.g a Blue Sea fuse box.
- MPPT Charge Controller Output Fuse: Use the controller’s maximum rated output current.
- Any General DC Circuit: Any circuit where you know the maximum current and need to size a fuse to protect the cable.
Please do not use this calculator to determine a fuse size for the following:
- PV Input to MPPT: PV array current is calculated differently (Isc × 1.25).
- AC Circuits: It only works for DC electrical systems.
- For Cable Selection: Use our cable calculator instead
- BMS-Limited Batteries: Instead size the fuse so it doesn’t exceed the BMS maximum discharge current, not just the circuit demand
Typical DC Fuse Sizes for Common Systems
| System Type | Continuous Current | Recommended Fuse (Check Manual) |
|---|---|---|
| 12V Fridge | 5A | 10A |
| Small Solar Charge Controller | 15A | 20A |
| 1000W Inverter (12V) | 90A | 120A |
| 2000W Inverter (12V) | 170A | 225A |
| 3000W Inverter (24V) | 140A | 180A |
What Type of Fuse Should You Use?
For DC systems, the most common options are:
- £5 MIDI Fuses: Great for inverters and high-current loads.
- £7 MEGA Fuses: Compact, reliable, ideal for DC distribution.
- £3.35 Inline Blade Fuses: Used in automotive applications and for low-current electronics, and appliances.
For standard 230V UK Plug Fuses check our £3 3 Amp, 5 Amp 13 Amp Fuses Set For Appliances
Where Should the Fuse Be Installed?
Ensure the fuse is installed as close to the power source as possible to comply with best practices and safety guidelines:
- To ensure the fuse protects the entire cable run
- To size the fuse based on the cable rating or device draw, whichever is lower
If in doubt it’s worth obtaining a copy of the IET BS7671 wiring regulations, they’re the go to standard for every electrician.
Final Tips for Safe DC Fuse Selection
- Always size the fuse with the aim to protect the cable from overheating
- Add a 25% margin above the maximum continuous current
- Choose reputable fuse brands
- For large systems (100A+), use Midi or MEGA fuses
- Never oversize the fuse “to stop it blowing” that’s unsafe
For all things cable related visit our Complete Cable Selection Guide
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