How to Calculate Fuse Size for Solar System

This article will discuss how to properly size your DC fuses in your DIY solar panel system.

Why and When to Fuse?

Every wire in your solar system is at risk of melting and burning. That’s why a fuse protects your wire and not your appliances.

This is very important to know: a fuse protects the wire.

Knowing that we need to select a fuse for our chosen wire. Don’t know how to select the right wire size? Read my article about calculating wire size for solar systems.

We can divide the solar system into two parts:

  • Wires from the solar panels (Solar wires)
  • The rest of the system

How to Calculate Fuses for Solar Wires

All the wires up to the charge controller are considered solar wires. These fall into a different category because we need to consider voltage drop.

This means the wire will be much bigger than required to minimize wire losses.

Let’s use an example of a 50ft wire that delivers 48V at 10A to the charge controller. To deliver 10A, we can use a 14AWG or 2.5mm² wire. But this is without voltage drop considered. In order to reduce the voltage drop to 3% we need to select a 10AWG or 6mm².

Voltage drop calculation

10AWG can carry 40A. This is 4 times the amount that is really needed. Therefore our range will be bigger.

The lowest fuse we can use is:

10A*1.56=15.6A

The highest fuse size we can use is 40A because the maximum current for a 10AWG wire at 90° insulation temperature is 40Amps.

We need to choose a fuse that is between 15.6A and 40A. I recommend a 20A or 30A fuse.

You might have noticed I used 1.56 as a safety factor. These are two safety factors together because of outside conditions.

1.25 x 1.25 = 1.56

How to Calculate Fuses for DC Wiring

Wires from the charge controller to battery

The current in these wires will be limited to the maximum output of the charge controller. Let’s say your charge controller has a maximum current of 40Amps, then you need to select a wire based on 40Amps.

40A x 1.25= 50A

A wire that can carry 50A is 8AWG or 10mm². The maximum current through that wire is 55Amps at 90° insulation temperature.

The lowest fuse we can use is 50A.

The highest fuse we can use is 55A. This is the maximum current through the wire.

Since there is no 55Amps fuse, we will use a 50Amp fuse.

Wires from the battery to the inverter

Assume we have a 12V battery and a 1000W inverter. The maximum current the inverter can draw is:

1000W/12V=83A

83A*1.25=104A

We need to find a wire that can carry 104 amps.

This will be a 3AWG wire at 90° insulation. This wire can carry a maximum of 110A.

The minimum fuse size is 104A.

The maximum fuse size is 110A.

Normally we have to size the fuse in between these two values. As an exception, I would use a 100A fuse.

You need to increase the wire size or the insulation temperature if you want to size your system with the 4A extra.

This will increase the wire from a 3AWG to a 2AWG (35mm²).

A 2AWG wire can carry a current of 130A at 90°C insulation.

Now we still keep the minimum fuse size of 104A, but the maximum fuse size increases to 130A.

A fuse that fits in between is 125A.

Conclusion

Choosing a fuse for your DC solar system is not that difficult.

  • Minimum fuse size is based on the load.
  • Maximum fuse size is based on the current capacity of the wire.

You then need to select a fuse in between these two.

Here is a list of popular AMI/MIDI fuses for 12 and 24V systems:

  • 30A
  • 40A
  • 50A
  • 60A
  • 70A
  • 80A
  • 100A
  • 125A
  • 150A
  • 175A
  • 200A

Here is a list of AMG/MEGA fuses (max 32V DC):

  • 80A
  • 100A
  • 125A
  • 150A
  • 175A
  • 200A
  • 225A
  • 250A
  • 300A
  • 350A
  • 400A
  • 450A
  • 500A

MEGA fuses for 58V systems:

  • 125A
  • 200A
  • 250A
  • 300A

List of ANL fuses (max 32V DC):

  • 35A
  • 40A
  • 60A
  • 80A
  • 100A
  • 130A
  • 150A
  • 175A
  • 200A
  • 250A
  • 300A
  • 350A
  • 400A
  • 500A
  • 600A
  • 750A

List of CNN fuses (looks like ANL fuse) for max 80V DC

  • 10A
  • 35A
  • 40A
  • 50A
  • 60A
  • 80A
  • 90A
  • 100A
  • 125A
  • 150A
  • 175A
  • 200A
  • 225A
  • 250A
  • 275A
  • 300A
  • 325A
  • 350A
  • 400A
  • 500A
  • 600A
  • 700A
  • 800A

NH000 fuses

These are fuses used in Europe for high-voltage AC and DC. I used them extensively when I was working at the chemical company BASF. They are my preferred choice for lithium battery fuses because they are widely available here in Europe.

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