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.
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:
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:
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.
Choosing your type of fuse
There are two things we need to keep in mind when selecting the right size fuse. These are:
- Interrupting current capacity
The voltage of a system has a lot to de with selecting the right fuse. For example, a MIDI, ANL, or MEGA fuse should only be used up to 24V. If we want to use a higher voltage, we need to select other fuses.
The reason why voltage is important is because it will affect arcing. If a fuse is blown, the current still wants to flow across the terminals of the fuse. The higher the voltage, the more potential there is for arcing. Thus with lower voltage applications like 12V and 24V there is less risk of arcing. We use class-t fuses filled with sand to extinguish the arcing.
Apart from the voltage itself, we need to look at a fuse with a DC rating. DC acts differently than AC and is harder to extinguish. An AC arc goes through 0 volts 50 or 60 times a second. DC current does not. That’s why its harder to extinguish a DC arc.
Interrupting current capacity
Another factor we need to consider is the interrupting current capacity. This is the current that will flow when our battery is shorted. Lead-acid batteries will have a lover short current because the internal resistance is higher than lithium.
The short circuit current of a lithium battery can be 10 times (source) the capacity or more. If we have a 100Ah battery, the short circuit current can be 1000A.
If we have a bigger battery bank like the one I built, a 12V 280Ah will have an estimated interrupting current capacity (ICC) of 280Ah*10=2800A
An ANL fuse has a ICC of 6000A at 32VDC while a MIDI fuse has 2000A at 32VDC. It is always important to have the ICC sized as high as possible.
List of Available Fuses
Brands I trust and recommend:
- Blue Sea Systems
- Use AMI and MIDI fuses for 12 and 24V systems.
- They are suited for a maximum of 32VDC.
- ICC of 5000A @ 16V DC, and 2000A @ 32V DC.
MRBS (marine-rated battery fuses)
- Use these fuses for 12 and 24V systems.
- They are suited for a maximum of 58V. But I do not recommend using these with 48V lithium batteries.
- ICC of 10000A @ 14V DC, 5000A @ 32V DC, 2000A @ 58V DC.
- Use these fuses for 12V systems.
- They are suited for a maximum of 32V. I do not recommend using these with 24 and 48V lithium batteries.
- ICC of 2000A @ 32V DC
- Use these fuses for 12V and 24V systems.
- They are suited for a maximum of 32V. I do not recommend using these with 48V lithium batteries.
- ICC of 6000A @ 32V DC
- Use these fuses for 48V lithium systems.
- They are suited for a maximum of 125VDC. I recommend using these with 48V lithium batteries.
- ICC of 20,000A @ 125V DC
- Use these fuses for 48V lithium systems.
- They are suited for a maximum of 250VDC. I recommend using these with 48V lithium batteries.
- ICC of 120,000A @ 250V DC
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.
What size fuse for 100ah battery?
Using our previous calculation, we need a 125A fuse.
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.