Fuse

[jpc6000]

Witch fuse I best uses

When I have Battery 30 Amp do I use 30 Amp fuse? Or 40 Amp fuse?

With battery 17s and motor 4300 watt, do I use 70 A or 80 Amp fuse?

 

[fechter]

The fuse should be rated slightly higher than the maximum expected current draw. For a 30A system, a 40A fuse would be good.
The fuse is there to blow in the event of a short circuit. You don't want it to blow under normal conditions.

My Sur-ron is about 4500 W and uses a 100A circuit breaker.

 

[E-HP]

Witch fuse I best uses

When I have Battery 30 Amp do I use 30 Amp fuse? Or 40 Amp fuse?

With battery 17s and motor 4300 watt, do I use 70 A or 80 Amp fuse?

I'm assuming the fuse if for short circuit protection. What is your battery pack configuration? Even a cheap Chinese generic 18650 cell can output 50A when short circuited, so that times the number of parallel groups would be the value to stay below so the fuse blows.
Usually if you're pulling 70A, it's for a good reason, where you don't want a fuse to blow (e.g. making a left when there's oncoming traffic, etc.). I'd set the fuse to be well above what your system could normally pull, while staying well below the short circuit current to ensure that it only blows under a short.

 

[amberwolf]

And remember that fuses don't just flat out blow at a specific current.

You have to check the manufacturer datasheet for the specific fuse you are using to see the chart showing current vs time to blow. (and if there is no datasheet, don't use that fuse; you can't know how it will behave, or if it will protect your system or not).

So if you had a 40A fuse, but there are parts of your system in series with that fuse that will actually be damaged if current exceeds say, 50A for even a few seconds, but the fuse is rated to sustain 50A for 1000 seconds before it blows, your system will probably already be on fire long before the fuse protects you.

Not usually a problem, but it can be, dependign on exactly how you designed and built your system.