I'm currently putting together a small 12V DC project and would like some advice before I finish the wiring.


The setup uses a 12V sealed lead-acid battery, a DC motor, a rocker switch, and a few LED indicators. Under normal operation, the motor draws around 2A, but I expect the startup current to be noticeably higher for a brief moment. I want to add a fuse to protect the wiring and the battery, but I'm unsure how to choose the correct rating.


At first I thought a 3A fuse would be enough, but now I'm wondering if it might blow every time the motor starts. A 5A fuse seems like another option, although I'm not sure if that would provide enough protection in case of a fault.


The wire I'm using is 18 AWG, and the cable run is less than one meter. The project will only be used occasionally, so it won't be running continuously for long periods.


I also have a few questions:

• Should the fuse be selected based on the normal operating current or the startup current?
• Is a standard blade fuse suitable for this type of project, or would another type be better?
• Should the fuse be installed as close to the battery as possible?
• Do you normally leave some safety margin when selecting the fuse rating?

I'd like to do this properly instead of choosing a fuse by trial and error. If anyone has experience with similar 12V DC projects, I'd really appreciate your suggestions and the reasoning behind them.

 

Seek out a Slo-Blow fuse. These rely on a slow blow fuse rating which uses time delay usualy marked with a T, e.g. T3A etc, they are designed to take a brief brief electrical surge without blowing.

 

You want to consider the total current draw under worst-case, not normal, operating conditions. Using slow-blow fuses will help deal with the initial inrush current.

If, as you've indicated, your fuse is protecting the wiring and the battery, not the motor or anything else, then sizing it is based on the ampacity of the wiring and the capabilities of the battery. I believe 18 AWG is rates for about 10 A to 15 A, somewhere in there (look it up to be sure). If your battery can handle that until it's discharged, then you can stop there and use that. If you want to go tighter to try to protect the motor or other stuff, then you need to estimate what the worst case current draw will be under worst-case load conditions. If you don't have any way of doing that (including just loading the motor and measuring the current draw), then you might start with a 5 A slow-blow fuse. That should be more than enough to protect the wiring and the battery.

 

Cable length is important too. For 2m of cable, a short circuit would be about 34mΩ - so low that the fault current would be determined mainly by the battery series resistance - let’s say about 15mΩ. So your prospective fault current would be about 240A. If you fuse it for the cable rating (say about 10A), then a short circuit will blow it in the tens of ms.
The most important parameter is the breaking capacity. Can your fuse of choice break 240A DC safely? DC ratings are MUCH lower than AC ratings. If it can’t, your fuse will arc and your fuseholder will catch fire.

 

Fuse specifications, in an adequately designed system, depend very much on the application. If this design is for a product, you can usually get assistance from the fuse manufacturer's application group. (that is a trade secret, folks)
Really, there is a whole lotof science involved with the selection of fuses for some applications.

 

the motor draws around 2A

If the motor draws 2A running and approximately 4-6A for a few hundred milliseconds at startup, then 3A fuse is not enough. Start with a 5A fuse at least.

 

If you limit yourself to using low voltage (automotive) fuses, say mini-blades like Littelfuse 297 series, you can see a 2A part will carry 2A forever and 3A about 2 seconds for it to blow.
A fuse is always located first, at the power source/battery. First to prevent fire from the wiring burning up or the pump motor if it jams.

Don't use cheap fuses say from aliexpress, they are known to be junk that doesn't clear until way above their ratings.

 

If you limit yourself to using low voltage (automotive) fuses, say mini-blades like Littelfuse 297 series, you can see a 2A part will carry 2A forever and 3A about 2 seconds for it to blow.
A fuse is always located first, at the power source/battery. First to prevent fire from the wiring burning up or the pump motor if it jams.

Don't use cheap fuses say from aliexpress, they are known to be junk that doesn't clear until way above their ratings.

View attachment 369677

We don’t know the details of the battery, but mini blade fuses will only break 1kA. It doesn‘t take a very big VRLA battery to have a short circuit current bigger than that.

 

We don’t know the details of the battery, but mini blade fuses will only break 1kA. It doesn‘t take a very big VRLA battery to have a short circuit current bigger than that.

I don't think you can get 1kA due to the wiring, fuse and fuseholder's DCR added up.
For a 2A fuse it's 56mohm cold for the one example I gave, theoretically 214A.
By your reasoning, how is a car protected by its fuses? In a collision, wiring short-circuits happen and the fuses seem to work. Trailer light plug shorting out, fuses seem to be fine.

 

In the motor installations I have been involved in, typically Slo-Blo was used.
By the look of this application they should also serve the purpose.