How to calculate the fuse rating needed?

[warrior]

I have a orange beacon on my roof came on the Landy,

I've quickly wire it on to a battery and it works, well needs a bulb in one side,

I'm goin to re wire all the little bit on my Landy so all the switches are in a nice neat line as they are all different and all over the place,

My light bar has some inline fuses in the battery box but my rear work light don't appear to be fused,

What fuse should I use for the 2 rear work lights, ( don't know the wattage of them)

And what fuse should be on my orange beacon? That say 55w inside it has 2 h1 bulbs and 2 little motors,

Thanks

 

[The Mad Hat Man]

W=VI
55=I/12
I=55/12
I=4.5A

5A fuse

 

[livers]

W=VI
55=I/12
I=55/12
I=4.5A

5A fuse

and just to show the working out,,,,,,,oodle,oodle,oodle,oodle,oodle,
*grabs a figure out of thin air*
"5A fuse"

 

[baggyjim]

made total sense to me but i am an engineer

 

[SASBOB]

10amp if there are 2x55w bulbs as stated

 

[baggyjim]

ok again for dummmies
watts / volts gives amps

so 55 watts / 12 volts = 4.5833 amps so a 5 amp fuse to be used

 

[The Mad Hat Man]

10amp if there are 2x55w bulbs as stated

oops- yup - H1 bulb is 55W - if 2 then double the total Wattage, so double the fuse rating. Any idea what current (or wattage) the motors are?

 

[warrior]

Not sure on the rating of the motors couldn't find anything on the unit, and if I need anything eles then its watts/voltage or red to red black to black and blue to bit.

 

[The Mad Hat Man]

yup - Amps = watts/voltage - yu can add a bit to that, but dont double it or anything silly.

 

[ricc]

the motors will be a fairly low wattage to start off stick a ten amp fuse in.... if it blows stick a fifteen in....

 

[SASBOB]

the motors will be a fairly low wattage to start off stick a ten amp fuse in.... if it blows stick a fifteen in....

Thats good advice, number of cars i have to rewire cos people thought that was a good idea

 

[skint-as-usual]

What fuse should I use for the 2 rear work lights, ( don't know the wattage of them)

Undo the covers and look at the ratings on the bulbs themselves, this will answer all.

If they are, say, 50W Each then total draw (if both on one switch and one fuse) will be 100W.
so, as before Watts = Volts * Amps
so 100 = 12 * unknown amps
unknown amps = 100 / 12
unknown amps about 8.33, so a 10 amp fuse ??

 

[Tronicus Maximus]

if you really want to exactly how much it pulls without a finger in the wind you could just hook it off a 12V battery with a ammeter in the circuit (or clamp meter if you got the coin!) and take a reading.

But i REALLY just go with the rating these guys are stating: the maths is correct.

 

[oxides]

Where does this 12V thing come from? The alternator is going to be kicking out 13 to 14 volts so the current will be higher.

 

[skint-as-usual]

Where does this 12V thing come from? The alternator is going to be kicking out 13 to 14 volts so the current will be higher.

Ok, on a running alternator there may be, what 14.4 volts (from memory topps.)

using: unknown amps = 100 / 12

then unknown amps is smaller ??? as we are unknown amps = 100 / 14.4.

 

[chunkymunky]

Ok i have similar wiring confusion.

Thermistatic rad fan. Obviously 12v. Relay has 30A fuse in it. But surely it isnt gonna need 30A cable? Cable from ignition is fused to 10A.

Question. What rated cable to use?
What fuse in lead from battery?

 

[SASBOB]

Battery to Relay 30a
Relay to Fan 30a
Ign to relay, whatever, a relay only needs 500ma to switch.

 

[chunkymunky]

Bugger. And i used 8amp figuring split 4 ways. No drama.

Just confused as wires from fan are bout same size as 8amp.

Think i will give manufacturer a call.

 

[SASBOB]

It`s nothing to do with the size of the cable, that doesn`t dictate the load it can handle. It`s the thickness and number of strands that determines the load handling.

I fit car security and some immobiliser circuit wires are similar in size to speaker cable but can still handle a starter cut.

 

[chunkymunky]

Hmm. Instructions lacking big style.