Fuse Size Calculator

Updated:

Aug 1, 2023

Wire Size

Error: Your wire is not rated for this amount of current. Use a thicker wire.

Error: Select a wire size

Current Flow in Amps

Error: Enter a valid current

Wire Temperature Rating (Optional)

Recommended fuse size:

Note: Based on your inputs, we recommend this fuze size for copper wire only. If you're using aluminum wire, use a thicker wire.

How to Use the Fuse Size Calculator

Select your wire size from the “Wire Size” options.
Enter the load current of the system in the “Current Flow in Amps” field.
Select the wire’s conductor temperature rating from the “Wire Temperature Rating” option. This step is optional. But if you do not specify a temperature rating, the calculator will use 75°C (167°F) as the value.

Note: the fuse sizing calculator assumes that your wire is installed at an ambient temperature of 30°C (86°F).

But if the installation is at a different temperature, check the National Electric Code (NEC) for the corresponding ampacity factor.

Why Electrical Systems Need Fuse

The primary purpose of having a fuse in electrical systems is to protect the wires. The secondary purpose is circuit protection.

Without a fuse, wires will overheat and get damaged when exposed to currents above their maximum current. Even worse, the wires could start a fire, destroying the whole electrical system.

It’s not enough to just get a fuse for an electrical circuit; you must get the correct fuse size. Fuse wire comes in various sizes. Of course, depending on the size of their wire, fuses can handle different current loads.

To ensure a fuse protects your wires, the fuse must break when the load current goes just above the maximum current of the cables. The fuse must not withstand more current than cables can withstand.

How to Calculate Fuse Size for Solar System

Use the Manufacturer’s Recommended Fuse Size

Manufacturers of various electrical systems may specify the ideal fuse size for their systems.

For instance, the charge controller manufacturer in a solar power system may recommend the ideal fuse size for the connection with the battery.

In the same solar power system, the inverter manufacturer may also have a recommended fuse size for the inverter to AC outlet connection.

The product’s pack, body, or manual/spec sheet may specify the recommended fuse size. It may be stated as the fuse, fusing, or circuit breaker.

Going with the manufacturer’s recommended fuse size is the best option. But whenever there isn’t one, you may use a fuse sizing calculator or calculate the size yourself.

Calculate the Fuse Size Yourself

How to Calculate Fuse Rating for Solar Wires

When choosing the wire size for solar wiring, we often opt for a larger wire (cables with smaller AWGs) to compensate for voltage drop. As a result, the range for the correct fuse might be wider.

So, for instance, if your solar panel delivers around 15 amps of current, you may be looking to use a 14 AWG copper wire because the max current of that wire size is around 25 amps.

But then, to minimize voltage drop, you could use a 12 AWG copper wire instead. A 12 AWG copper wire can tolerate a maximum of 30 amps at 90 degrees.

Now that we know we are using a 12 AWG copper wire, we know the wire’s max current – 30 amps at 90 degrees. So, the wire’s max current would be the maximum fuse current – 30A.

The minimum fusing current would be the load current multiplied by a safety factor (the safety factor can be no less than 1 and no more than 2 – typically 1.2 to 2).

In our case, we’ll use a safety factor of 1.25. So, our minimum fuse current would be:

\(15 * 1.25 = 18.75 A\)

Since there’s no 18.75A fuse, the minimum fuse current would be 20 A, and the max current would be 30 A. While we can use either a 20A or 30A fuse in this case, a 20A would be better because it is closer to the continuous current.

You can calculate fuse size for any wiring following the same steps above.

Why are Fuses Rated?

Fuses are rated because they can only handle a certain amount of current before they break circuits. In other words, fuse rating indicates the best use case for a fuse.

For instance, it would be unfit to use a 20A fuse in a circuit whose continuous current is 30A; the fuse would melt or break the circuit before you get any use out of it.

Conversely, using a 50A fuse in a 30A circuit with 10 AWG wiring will not protect the circuit. Here’s why:

The 10 AWG wiring cannot handle more than 40A, but the fuse can handle as much as 50A. So, if there’s ever an overcurrent of over 40A but not more than 50A, the fuse will not break the circuit.

Consequently, the wiring would heat up and may destroy other circuit components and start a fire.

Charles Noble

Here at Spheral Solar, I’m a hands-on solar power enthusiast, constantly exploring and experimenting with DIY solutions. I’m dedicated to sharing my discoveries, insights, and even the bumps I hit along the way, all in the pursuit of a more sustainable future.