Determine Correct Fuse Amps

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Mr_B
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Mon Jun 01, 2015 9:04 am

Morning all you clever people!

I've started formalising my loadshedding/backup power system. So far it consists of a 2500w pure sine wave inverter, with built-in charger, connected to a 102amp deep cycle battery. To date the system has been standing loose in my garage and is powering 2 dstv decoders, a LED TV and 1 or 2 LED lights permanently. This weekend I mounted the setup properly and started with some 12v exterior security lighting. I'm using quality solid core 2.5mm flat wiring for the 12v circuits, but still need to add some protection. What I'd like to ascertain is how do I calculate the correct "size/amps" for each circuits fuse? I'm going to mount one of those multi-fuse holders and want to run one circuit per fuse as far as possible. Any help from the boffins?

B :subscribed:

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Stef
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Mon Jun 01, 2015 9:48 am

General rule: fuse must protect the infrastructure, thus the fuse rating must be less than the current carrying capacity of the wire in order to protect it.

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ChrisF
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Mon Jun 01, 2015 12:12 pm

Bretton I prefer to rate the fuses closer to what the circuit is intended for.


A 5m length of the high power LED strips draws about 6A, thus a 10A fuse for this would be perfect.

If you are going for the 10W flood lights - work on 1A each and take the next size up fuse.


Your tv setup should draw about 20 to 25A, thus a 30A fuse should do the job.



I start off with a single 50A bolt on National Luna type fuse close to the battery

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Stef
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Mon Jun 01, 2015 12:33 pm

I also like that....a little "breathing room" over the normal current draw, and then wiring & switchgear a little more than that taking into account voltage drops.

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Mr_B
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Mon Jun 01, 2015 1:39 pm

Good call on the 50amp fuse Chris... will do!

If memory services me... Volts x Amps = Watts

So the circuit's... Amps = Watts / Volts = 10w / 12v = 0.833...

So a 1amp fuse should suffice? Or a 2amp for a little extra head room?

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WayneSchalk
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Mon Jun 01, 2015 4:01 pm

Start with small fuse, if it blows put in a bigger one and try again..... hahahahaha :silent: :silent: :silent:

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Stef
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Mon Jun 01, 2015 4:08 pm

Mr_B wrote:Good call on the 50amp fuse Chris... will do!

If memory services me... Volts x Amps = Watts

So the circuit's... Amps = Watts / Volts = 10w / 12v = 0.833...

So a 1amp fuse should suffice? Or a 2amp for a little extra head room?

Excluding losses ...might be closer to 900mA but you'll have to measure.

add another 5W and use a 2A fuse :mrgreen:

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Mon Jun 01, 2015 4:18 pm

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ChrisF
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Mon Jun 01, 2015 4:23 pm

I could not find smaller than 5A fuses . . . Which is perfectly safe for the wire size I used.

still would rather use 2A if I can get any

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Mon Jun 01, 2015 7:49 pm

ChrisF wrote:I could not find smaller than 5A fuses . . . Which is perfectly safe for the wire size I used.

still would rather use 2A if I can get any
http://www.mantech.co.za/Stock.aspx?Que ... fuse+2Aand


Assuming you are looking for blade fuses. Glass fuses rated at 2A are a dime a dozen.


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