# What size fuse for 100ah battery?

Unlike circuit breakers, if a fuse blows you’ll have to replace it. And since replacing parts every other day is the last you need, you’ll have to size your fuse in a way that it only blows if your system is at risk.

However, if the fuse is oversized, it will fail to protect your system when needed.

So, how do you size a fuse properly?

Well, simply put, the amperage of the fuse that you need for your 100Ah battery has to be a little higher than the maximum amount of current (in Amps) that you’re planning on pulling from the battery and equal to or lower than the amount of current that your wires can safely conduct.

Let’s see what this means.

## What size fuse for a 100ah battery?

The size of the fuse that you need for your 100Ah battery must be 25% higher than the Amps you’re planning on drawing at battery voltage (12 Volts for example), and lower than or equal to the ampacity of the wire you’re using for your battery.

Generally, a 12V-100Ah battery is designed to output up to 100 amps at 12 volts, if your appliances do not require more than 100 amps (@12Volts), and you’re using a 1/0 AWG pure copper wire, a good fuse size for this amount of current and wire size would be 110A or 125A.

However, you might not need that big of a wire or fuse if your appliances require less than 100Amps at battery voltage.

To properly size your fuse, you’ll first need to determine the maximum amount of current that you want to be able to draw at battery voltage. You can do that by using one of the following formulas.

If you’re using an inverter:

Maximum Amp Draw = (Continuous Power Rating of the inverter (Watts) ÷ Lowest Battery Voltage (Volts)) ÷ 0.85

Related:

What size inverter do I need for a 100Ah battery

What size fuse between battery and inverter?

If you’re not using an inverter:

Maximum Amp Draw = Maximum Power Usage (Watts) ÷ Lowest Battery Voltage (Volts)

Once this value is determined, according to the National Electrical Code (NEC), the fuse size (in Amps) needs to be 25% higher than the value:

Fuse Size (Amps) = Maximum Amp Draw  x 1.25

Since fuses have specific standard ratings (15, 20, 25, 30, etc…), the fuse size you determine with these formulas will likely not match these standard fuse ratings.

But all you have to do is find the next larger fuse size, with the condition that it is not higher than the ampacity of the wire (more on this in the next section).

The standard fuse sizes (in Amps) that you might easily find are 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250.

To understand this better, let us look at a couple of examples.

Example 1:

For this example, let us say you’re trying to run 4 appliances on your 100Ah battery:

• A refrigerator that is rated at 200 Watts of power
• A TV that is rated at 60 Watts of power
• A fan that is rated at 50 Watts of power
• A light bulb that is rated at 70 Watts of power

Let’s also assume that we’re not using an inverter.

The refrigerator is obviously going to be left on 100% of the time. But at some point, you’d want to be able to run all of these appliances at once. This means that your maximum power usage in such a case would be:

Maximum Power Usage (Watts) = Power Usage of the refrigerator (Watts) + Power Usage of the TV (Watts) + Power Usage of the fan (Watts) + Power Usage of the light bulb (Watts)

Maximum Power Usage (Watts) = 200 Watts + 60 Watts + 50 Watts + 70 Watts

Maximum Power Usage (Watts) = 380 Watts

So, at some point, your total power usage is going to be 380 Watts.

To determine the wattage (power usage in Watts) of each of your appliances, please check their electrical specifications (at the side, back, or bottom of the appliance).
If you can’t find the power usage of the appliance in Watts, simply multiply the Amperage (Amps) of the appliance by its Voltage (Volts) to determine the Wattage (Watts) of the appliance (Watts = Amps x Volts).

Now, let us assume that the 100Ah battery is rated at 12 Volts. In general, the lowest operating voltage of a 12V battery is 10 Volts. If you’re using a 24V – 100Ah battery, the lowest voltage is going to be 20 Volts.

Using our formula, the Maximum Amp draw @ 10 Volts is:

Maximum Amp Draw (Amps) = Maximum Power Usage (Watts) ÷ Lowest Battery Voltage (Volts)

Maximum Amp Draw (Amps) = 380 Watts ÷ 10 Volts

Maximum Amp Draw (Amps) = 38 Amps

According to our calculations, the fuse must be rated at around 25% more than 31.66 Amps:

Fuse Size (Amps) = Maximum Amp Draw (Amps) x 1.25

Fuse Size (Amps) = 38 Amps x 1.25

Fuse Size (Amps) = 47.5 Amps

The next larger fuse size would be a 50 Amp fuse.

However, again, the amp rating of the fuse should be lower than the amp rating of the wire you’re using. More on that below, but first, let’s look at another example.

Example 2:

For this example, let’s say we’re trying to run the same appliances, with the same battery (12V-100Ah), but this time we’re using a 500W inverter to run them.

Using our formula, we can determine the maximum amp draw:

Maximum Amp Draw (Amps) = (Continuous Power Rating of the inverter (Watts) ÷ Lowest Battery Voltage (Volts)) ÷ 0.85

Maximum Amp Draw (Amps) = (500 Watts ÷ 10 Volts) ÷ 0.85

Maximum Amp Draw (Amps) = (50 Amps) ÷ 0.85

Maximum Amp Draw (Amps) = 58.8 Amps

According to our calculations, the fuse must be rated at around 25% more than 49 Amps:

Fuse Size (Amps) = Maximum Amp Draw (Amps) x 1.25

Fuse Size (Amps) = 58.8 Amps x 1.25

Fuse Size (Amps) = 73.5 Amps

The next larger fuse size would be an 80 Amp fuse.

In the next section, I discuss wire sizes and how you can use the Ampacity of your battery wires as a reference to size a fuse for your 100Ah battery.

## Sizing a fuse for a 100Ah battery based on wire size

If you know the ampacity of the wire used at the terminals of your battery, the fuse you use should have an amp rating equal to or lower than the ampacity of the wire.

For example, if you’re using an 8 AWG copper wire, and your amp draw at the lowest battery voltage does not exceed 30 Amps, a good fuse size for such a setup would be a 40A fuse.

If you don’t yet know the ampacity of the wire that you need to connect your battery to the load, feel free to use our battery-to-inverter wire size calculator.

Related: Solar charge controller to battery wire size calculator

The following table provides the ampacity of some pure copper wire sizes with different insulation temperature ratings:

However, make sure to use the formulas in the previous section to calculate the proper fuse size based on your maximum amp draw (@ the lowest battery voltage):

Fuse Size (Amps) = Maximum Amp Draw (Amps) x 1.25

If the fuse size you’ve determined using the previous formulas is greater than the ampacity of the wire you’re using, it means you’ll need a bigger wire.

For example, let’s assume that our maximum amp draw is 50 Amps, and let us also assume that we’re using a 6 AWG wire rated for 75°C/167°F, which is perfectly capable of carrying 65 Amps without exceeding the 75°C/167°F that its insulation is rated for.

Now, let’s use our formula to find the proper size fuse:

Fuse Size (Amps) = Maximum Amp Draw (Amps) x 1.25

Fuse Size (Amps) = 50 Amps x 1.25

Fuse Size (Amps) = 62.5A

The next larger fuse size is 70A. However, again, the ampacity of a 6 AWG wire rated for 75°C/167°F is only 65 Amps.

This means that if we use a 70A fuse with this wire, it will allow more than 65 Amps to go through the wire, which will in turn increase the temperature of the wire above the temperature it is rated for (75°C/167°F), and might cause the insulation to melt.

In this particular case, a simple solution would be to use a bigger wire. A 4 AWG wire would be a good fit.

Now that we’ve discussed the size of the fuse that you need for your 100Ah battery, we’ll need to discuss the type of fuse that you should use.

## Time-delayed vs fast-acting fuses: which one should you use for your 100Ah battery?

If you’re planning on running any appliance that has a motor in it, such as refrigerators, pumps, or air conditioners, please note that these appliances require an inrush current (Amps) when they first start.

This inrush current can be many multiples of the normal current these appliances use.

This spike in current is instantaneous and usually only lasts for a fraction of a second, and it does not pose a threat to your wires and devices. However, this inrush current can cause your fuse to blow if it is a fast-acting fuse.

In the case of these appliances, and to make sure the fuse doesn’t blow unnecessarily, use a time-delayed (slow-blow) fuse.

Or, you can use a circuit breaker instead. The sizing process is the same, but circuit breakers have a slower response time than fuses, and if it trips, you won’t have to replace it.

## What size fuse between the charge controller and the battery?

The amp rating of the fuse required between your charge controller and the battery should be the same as the amp rating of the charge controller. For example, if you’re using a 20A solar charge controller to charge your 100Ah battery, the fuse should be rated at 20 Amps.

In addition, the amp rating of the fuse should not exceed the ampacity of the wire used between the charge controller and the battery.

To size the wire that you need between the charge controller and the battery, please refer to this page: What size cable from the solar charge controller to the battery?

For example, if you’re going to use a 20A charge controller to charge your battery, the minimum wire gauge should be 12 AWG, which can conduct 20 amps of current.

## Related Topics:

What size Inverter for a 100Ah battery?

What size solar panel to charge a 100Ah battery?

How long will a 100Ah battery last?

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##### Younes Anas EL IDRISSI

Younes Anas EL IDRISSI is the founder of RenewableWise.com and the driving force behind it. As a former Electrical Engineer and an energy self-sufficiency enthusiast, Younes' mission is to leverage his expertise and experience to simplify the complexities of solar energy and make it easily understandable for anyone looking into DIY energy solutions. Learn more about Younes and the story of RenewableWise here.

### One comment

1. #### harry

Excellent and useful article. Thanks.