12 replies to this topic

[yorkshirechris]

Right, basically, is there an easy way of calculating a fuse size if I give the power and voltage?

I know the Power/Current/Voltage triangle.

Basically the thing I am connecting up is 10W in power. Obivously it's powered by 12V.

But when I use my triangle it gives me 0.8Amps as the power surely it must be more than this?! Don't the smallest Blade Fuses start at 5Amps?

I don't want to put in a massive fuse because then if anything goes wrong it will probably burn before the fuse blows.

Much appreciated...

Chris

[Bungle]

as long as you use 5 amp (or bigger) wire the a 5 amp fuse will be ok

the fuse is there to stop the wiring burning out when a fault happens, so as long as the wire is bigger than the fuse the fuse will blow

[MiNi FiZ]

i get the same

10 / 12 = 0.83A

or

12 / 10 = 1.2A

which way round do they go

[yorkshirechris]

as long as you use 5 amp (or bigger) wire the a 5 amp fuse will be ok

the fuse is there to stop the wiring burning out when a fault happens, so as long as the wire is bigger than the fuse the fuse will blow

Right ok.

Hmm I have no spare bays on the fusebox but conveniently already have a 5Amp bay, which just has my Radio powered from it.

So, if I add my new lets say "accessories" to that circuit, should I increase the fuse to 10Amps (i'll probably use 17-20amp wire to be on the safe side anyway)? Or do you think it will be ok with 5Amps?

I don't want to bugger it all up you see, and have something burning. I'm not sure on the rating of the actual accessories I'm using. So I don't want them blowing before the fuse if you get me. But I don't want the fuse too low to blow all the time.

[yorkshirechris]

The triangle is

P
IV

So if you want 'I' you cover it up, giving you P over V, or P/V so 5 divided by 12

---------------------------


So I have a 5Amp fuse for my radio but the actual head unit itself is fused to 15Amps. If I wire my new accessories up to the same power feed, should I increase the fuse from 5Amp to 10Amp to be on the safe side? I don't want to stick a massive fuse in you see.

[Dan]

NEVER change the rating of the original fuses. Fuses don't exist to protect equipment, they exist to protect the rest of the wiring from a failure in that equipment. If the fuse fails it's generally because the equipment it was running has faulted. In that case the fuse is there to prevent this fault from starting a fire in the rest of the wiring, not to protect the (theoretically already damaged) equipment. The wiring in the circuit you will be adding equipment to is designed to be protected by a 5 amp fuse, if you increase this to a 10 the wiring may no longer be protected and a fire may result.

by the way nothing runs on 12v while the engine is running, the alternator provides around 14.2v.

[Bungle]

Hmm I have no spare bays on the fusebox but conveniently already have a 5Amp bay, which just has my Radio powered from it.

add the load of the raido to the load of your new 10w load and if they are still under 5 amps keep the 5 amp fuse

[yorkshirechris]

Throwing a spanner in the works here...

The total power is now 40W. Taking Dan's figure of 14.2V I get the Current to around 2.81Amps.

I am using 8Amp wire for both the power feed and Earth.

I don't know the loading for the radio. With this in mind is it a good idea to piggy-back the new power feed from the radio fuse of 5Amps.... turn everything on and at worst the fuse will blow... and if it keeps blowing, stick a 10Amp in? An electrician colleague told me it is but I'd like your opinion first...

Is that the best plan? Quickly because I'm hoping to get it all done tonight!

Cheers. Edited due to typo's

Edited by yorkshirechris, 17 December 2007 - 04:04 PM.

[yorkshirechris]

By the way I aren't changing anything of the original fuses. It's an aftermarket radio and it was originally piggy-backed off the heater believe it or not. I took it to a mechanic and he gave me some new wire so I could run a new feed from the stater - fusebox - radio. And told me to stick in a 5Amp fuse to the box.

[yorkshirechris]

Right well I have to go out and I've realised that if I use a 10Amp fuse with 8Amp wire the wire will burn before the fuse pops... so I'm off to exchange this wire for something 10Amp or above... and if the 5Amp fuse works fine then it works fine, the wire will be even more heavy duty than required.

Cheers

[Bungle]

yep just make sure you wire current rating is higher than the fuse rating

it would be best to use a smaller fuse just before each load as well

[yorkshirechris]

yep just make sure you wire current rating is higher than the fuse rating

it would be best to use a smaller fuse just before each load as well

What so wire a power feed from my 10Amp fuse bay, in 17amp wire (which is what i just bought) and before I split off to each accessory put a 5Amp inline fuse in...?

I forgot to mention the power wire will go to a Switch first and then split into two. Shall I put a small fuse before the switch or after the switch just before the actual accesory....?

[Bungle]

put the smaller fuse right at the accesory