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.

 

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.

Your fuse’s job is to protect the wiring. Suppose that there is a short circuit close to the battery due to damaged cable insulation.

 

Your fuse’s job is to protect the wiring. Suppose that there is a short circuit close to the battery due to damaged cable insulation.

I'm saying you can't get a fault current of 1,000A exceeding the 297 2A fuse's rating. The DCR is too high for that, just for the fuse element alone.

Even if you did get say 2,000A fault current for many millseconds, so what?
It is relevant and very important for a mains fuse- but this is a low voltage application.
At 12-32V there is no sustained arc going to happen. The fuse is plastic body. The element simply melts.

 

I'm saying you can't get a fault current of 1,000A exceeding the 297 2A fuse's rating. The DCR is too high for that, just for the fuse element alone.

Even if you did get say 2,000A fault current for many millseconds, so what?

It strikes an arc and catches fire.

It is relevant and very important for a mains fuse- but this is a low voltage application.
At 12-32V there is no sustained arc going to happen. The fuse is plastic body. The element simply melts.

Don’t you believe it! If the fault current exceeds the maximum breaking capacity it strikes a sustaining arc, which melts the fuse, body and fuse holder. I think that exceeding the breaking capacity is unlikely, but I would err on the side of safety and use a fuse that can break 5kA.

Its a DC application, which is a far more dangerous situation that AC, because the voltage doesn’t return to zero 100 times a second.

 

Thank you very much for discussions.
I think a 5A fuse is a better choice for this setup. The motor may draw more current for a short time when it starts, so a 3A fuse could blow too easily.

I would also install the fuse as close to the battery as possible. This helps protect the wire if there is a short circuit.

A standard blade fuse should work fine for a small 12V DC project like this. If you can measure the motor's starting current, it will make choosing the right fuse much easier.

 

No one mentioned a soft start with PWM. 2 A is very easy to deal with. You can make a simple soft starter for example, by charging a capacitor with a current source and faster than that, applying this voltage to change the duty cycle of the PWM. Even a 555 can do that. I'm pretty sure any general purpose op amp can do that too.
But that's only if your application has enough space for this extra circuit, cost, and maybe other things.

If you want to protect passively, you can use a PPTC fuse. It won't blow, just will get hot. We used that for 800 mA motors in a product for years and it was never a problem (and neither a single motor got shorted, so IDK if it really works in practice).

 

No one mentioned a soft start with PWM. 2 A is very easy to deal with. You can make a simple soft starter for example, by charging a capacitor with a current source and faster than that, applying this voltage to change the duty cycle of the PWM. Even a 555 can do that. I'm pretty sure any general purpose op amp can do that too.
But that's only if your application has enough space for this extra circuit, cost, and maybe other things.

If you want to protect passively, you can use a PPTC fuse. It won't blow, just will get hot. We used that for 800 mA motors in a product for years and it was never a problem (and neither a single motor got shorted, so IDK if it really works in practice).

Again, you need to watch the breaking capacity.A pptc fuse will catch fire if the fault current exceeds its breaking capacity.

 

Again, you need to watch the breaking capacity.A pptc fuse will catch fire if the fault current exceeds its breaking capacity.

True. This is the case for anything in series with the fault. But 2 A and 12 Vdc PPTC fuse isn't a big deal.
https://www.mouser.com/en/ProductDetail/Bourns/MF-RHT200-2?qs=GvnsgvzI0l/q6lBlkiMtjg==
This one is rated for 2 A and maximum 16 V. If I got to understand it well, it's the only thing you need to know, because in a short circuit, all the voltage is on the PPTC. It heats and holds on until the hold on current is reached, then it cools down and rearms.

 

Just remembered I ignored the breaking capacity, but look, it can handle 100 A, and it has 110 mOhms, which with some more resistance from the source, cables... gets to be at least 0.5 ohms. If it's too insecure to put it, just put 2 in series, so then you have 220 mOhms and it will break the circuit always.