Calculating amps on multiple relays and fuses

Thread Starter


Joined Apr 2, 2024
Hello everyone. I'm trying to create a wiring loom for a small automotive project and I'm getting confused on how to work out the correct amp ratings for wires.

I do apologise if this is a very basic question, but I'm only young and new to this world. I've been searching forums and Google most of the afternoon with no joy so decided to pluck up the courage to ask on here...

So here's my brick wall....

Assuming that fuses 2/3/4/5 are all rated at 20amp, then does this mean that Fuse 1 has to be able to carry 80amps? Fuse 1 would be the Maxi Fuse?


Joined Jan 27, 2019
Welcome to AAC.

It helps to think of why the fuses are there. A fuse is used to protect the wiring and prevent fires and other damage.

The maximum any one branch from your main feed will draw is 20A for the rated time of the fuse type you select. Fuses can be fast or slow blow, and slow blow fuses act over an extended period of time—just how long will be specified in the datasheet for the fuse.

So let’s assume the worst case scenario: all four branches are drawing 20A for a total of 80A. This means the wiring from the accessory fuse box will be required to handle that 80A, for a maximum of the fuses rated blow time. Are you using wire that can do that?

If not, you need either different wire or different fusing. You might decide to leave the 20A slow blow fuses in the auxiliary fuse box but use a fast blow 80A fuse for the main feed. This way, if a condition arises where all the branches are drawing maximum load at once, the main fuse will blow protecting the feed from overheating.

Or, you could use a slow blow fuse but reduce the ampacity. This might be good if you never reasonably expect the four loads to simultaneously draw the maximum current, and, if they do, it indicates a likely failure of some kind. Again, this is all about heat. The fuses are melted by heat, and they are there to protect against the damage caused by heat.


Check the datasheet(s) for the wire and make sure you know how much heat and for how long the wire’s insulation can handle. Then, select a fuse that fits that profile—that is, one that will blow if the heating starts to be to an extent that will cause damage to the wiring and the things around it.

One more consideration—inrush current. Inrush is when first applying power to a load that includes a large capacitance, or to a tungsten filament lamp, there is a demand for current that far exceeds the normal operating condition. If the loads you are powering have significant inrush current, fuses selected for normal running conditions may not be able to handle the initial application of power. This is where careful selection of a slow blow type is required.

Thread Starter


Joined Apr 2, 2024
Thank you so much for explaining this to me in simple terms. I really appreciate it.

when you ask " Are you using wire that can do that? " (Handle 80a) then clearly I need 80a wire, but I am looking at prewired relay boxes such as the one below:

In practice I only need four relays fused at 20a, 15a, 10a, and 5a. This prewired box seems to have two live inputs; one for each half I assume.

So if I use two top blocks 2 & 4 for the 15a and 10a, then two bottom blocks 1 & 3 for the 20a and 5a, this would give an equal 25a max load on each of the two feed wires. So I just run two 25a rated wires to the battery, both with 25a inline fuses.

That should be fine I think.

I'm just wondering how such a prewired relay block could handle 6 x 20a relays. Doesn't seem feasible/


Joined Aug 7, 2020
I'm just wondering how such a prewired relay block could handle 6 x 20a relays. Doesn't seem feasible/
it can, if it is impossible (or unlikely) for all the relays to be switched on at the same time. This is known as “diversity”.
Think of the main fuse on the supply to your house, and the sum of all the ratings of the breakers for sockets, lights, etc.