Fuse sizing question

some of you are on the lacroix boards facebook group. there was some crap about fuses. I’m afraid to ask further questions there… too many drama hounds.

but I was curious about 2 things.

1.) why overcommt the fuse?

The lacroix nazare and lonestar use a 70A fuse on the mains. the ESC is set to draw 90A

I understand this isn’t very overcommitted, and could run for days at 90A without burning the fuse.

My question though is why overcommit the fuse rating at all? wouldn’t a 100A fuse blow on a short basically as instantly as a 70A? what would be the rational for going with a 70A

2.) why run the mains fuse?

i’ve read in random forum post that DIY scene doesn’t like to run mains fuses ?
what is the zeitgeist there and what’s the rational?

  1. It’s better to look at the actual const/timing of the fuses than just the amp rating itself. A sloblow fuse is spec’d above the maximum expected const amperage draw of the application. The ESC is not going to draw 90A under most circumstances for any extended period of time.

  2. It’s a pretty safe and easy way to add short protection. The fuse that is spec’d on a Nazere is not going to blow under normal operation range.


Maybe because they are Canadian? IDK anything else that would constitute the use of a fuse. I apologise.

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can you expand on that a litlte? if I were just looking at the const/timing of these fuses how would I arrive at the 70A ?


I added a bit above.

Remember that 90A is a ceiling and amps are based on load; it’s rarely peaking above 3000w (60a) for most riders. 4500W constant draw is more than a Nazere will pull under normal operation range.


I definitely understand those bits. I’ve been up hills at speed and recorded my amp draws. and I’m heavy. i tend to overheat the ESC and throttle before I can burn full amps for 90s, which I think is minimum to pop the fuse.

but from the other perspective, i’m thinking a 100A fuse would do the same kinda protection? maybe blow microseconds later on a full short? so how does one decide 70A was right for this application? this is where my curiosity lies.


As I don’t have the scratch paper that Charles used to spec the power system on hand, I can only guess. But the idea behind specing mains power fuse is that it should be just high enough to never burn out under any normal operating conditions while burning open as quickly as possible when a short or runaway condition occurs.

You can calculate the difference in time between a 70 or a 100A fuse, and in this case it might not be a huge difference but when it comes to thermal runaway in lithium-ion every ms can count. “Overspecing” a mains fuse is the wrong way to approach the solution it serves.

Question- have there been any reported cases of someone burning out a Nazeres mains fuse under normal operation?

To add to this: I’m confident you could set your battery amps to 120 and still wouldn’t burn the fuse under normal operating circumstances, which is why I’m saying that 90A isn’t the only metric that was used to spec the fuse.



i’m reading that as… every ms does count. set it as tight as possible based on time to blow, to the highest operating condition you expect.

that makes a little more sense to me.

nope. :slight_smile: and I already thought about that. and looked at the specs on the fuse. I personally think it’d be impossible to blow the fuse at 90A pretty hard to blow it at 120, but maybe possible.

when DIY’ing i’d have to size my own fuse. thus my curiosity.


I’m just gonna say thanks again. I think this all clicks for me now. I love that feeling. :slight_smile:


I think for most of our purposes 100A is going to be a good ceiling. If I’m ever pushing more amps than that I’d quickly be looking at a higher voltage system.

Anecdotal experience, but I’ve run with a DieBieMS using an 80A fb1 sloblow with total system amps set at 150A for the last 15 months or so on LoveChild (95/75 motor/battery per ESC). I would say I have pushed that board to it’s limits across all types of terrain and I’m easily 112kg fully geared. An 80A fuse was definitely ‘to spec’ in this case.


Just and datasheet example for the time constants