adithorp
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| posted on 15/11/07 at 01:11 PM |
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calc's for fuse ratings?
I should know this but its lost some where in the bachwoods of my memory. Once you've worked out the load on a circuit, how much do you add to
it for the fuse rating?
adrian
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craig1410
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| posted on 15/11/07 at 01:28 PM |
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Hi,
I'm not sure that a simple "margin" above normal loading is sufficient. Depending on the type of load you may have surges, usually
at power on which can blow a fuse even though the fuse is rated well above the continuous load. Normally inductive loads (eg. motors) produce surges
on power up but other loads such as headlight bulbs can also produce surges.
The most important thing in my view is that your wiring at every branch is capable of handling the maximum continuous load which it can experience.
This is defined by the rating of each fuse in each branch summed together as branches are combined. Beyond that, all you need are fuses which
don't blow under normal circustances and you can probably make an educated guess by adding a small overhead onto the expected rating. If the
fuse blows regularly then try one a little bit bigger but never exceed the rating of any of your wiring or you may get smoke and flames!
Also, remember that the voltage is typically 14V when operating not 12V so bear this in mind when measuring or calculating current requirements for
each load.
Hope this helps,
Craig.
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02GF74
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| posted on 15/11/07 at 01:58 PM |
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^^^ wot he says.
filament lamps work by heating up a metal wire whose resistance increases with temperature. So when you first switch on a lamp, more current flows
through it than when it is up to operating temperature - this is why you will find your domestic light bulbs are more likely to blow when you turn on
the lights.
The current thourgh a simple inductive load is constant, the voltage induced across is will vary according to rate of change of current and
inductance. Can't remember much of electrical motor theory now but think this is still the case.
Anyway a simple rule of thumb is to ensure the fuse current rating is less than that of the loom's!! it is preferable for the fuse to melt
rather than the loom getting really hot and insulation burning. 
So rule of thumb 2 - work out power of load to determine current - then double that for the fuse.... or simply take the series land rover approach and
use 35 A for everything as long as you meet the wire rating rule above.
just thought: another way is take a look at fuse ratings as used in haynes manuals.
[Edited on 15/11/07 by 02GF74]
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r1_pete
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| posted on 15/11/07 at 02:14 PM |
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Amperes law states that Amps = Watts/Volts, so a 60 watt bulb at 12 volts draws 5 amps. i.e Amps = 60/12. Use in conjunction with the above
you'll be ok.
[Edited on 15/11/07 by r1_pete]
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Macbeast
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| posted on 15/11/07 at 07:03 PM |
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As a rule of thumb, probably add 50% to the calculated load current.
To some extent you are limited by the available fuse ratings - you can't normally get an 11.5 amp fuse. For a circuit drawing up to 6A I would
use a 10A fuse, up to 10A I would use 15A fuse, anything more than that I would try to split down to separate circuits.
[Edited on 15/11/07 by Macbeast]
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tks
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| posted on 15/11/07 at 07:30 PM |
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then to add another rule of thumb.
for every mm2 you can transport 6Amps..
(atleast) thats what they told me.
Tks
The above comments are always meant to be from the above persons perspective.
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omega 24 v6
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| posted on 15/11/07 at 07:58 PM |
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cables and fuse should be constantly loaded only to no mre than 80% of their rated amperages as a rule of thumb.
If it looks wrong it probably is wrong.
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adithorp
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| posted on 15/11/07 at 08:24 PM |
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Cheers guys. Two numbers stuck in my head; Load+50% and loadx2. So I'll split the difference and pick the nearest fuse.
adrian
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chriscook
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| posted on 17/11/07 at 11:07 AM |
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I started by putting 5A fuses in all positions when I first switched the electrics on. If one blew when I turned that circuit on (ie the lights) I
decided if it should blow a 5A fuse under normal circumstances and then increased the value of that particular fuse if it was ok. Most stayed at 5A.
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craig1410
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| posted on 17/11/07 at 01:58 PM |
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quote:
Originally posted by chriscook
I started by putting 5A fuses in all positions when I first switched the electrics on. If one blew when I turned that circuit on (ie the lights) I
decided if it should blow a 5A fuse under normal circumstances and then increased the value of that particular fuse if it was ok. Most stayed at 5A.
Fair enough but did you do this with the engine running and therefore the electrics running at 14V and not just 12V?
I powered up each circuit one at a time using an ammeter in place of the fuse and using a 0-18V, 0-10A power supply as the source instead of a
battery. That way I could set the power supply to 14V and ramp up the current until it reached normal operating level. I then just picked a fuse which
was the next standard size up from that level with a minimum 50% overhead to avoid thermal fatigue on the fuse.
The main advantage of using a power supply instead of a battery is to avoid any huge currents flowing if a mistake has been made. Battery's can
flow 300A+ which can melt your loom before you even notice that something is wrong.
Cheers,
Craig.
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chriscook
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| posted on 17/11/07 at 04:25 PM |
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quote:
Originally posted by craig1410
Fair enough but did you do this with the engine running and therefore the electrics running at 14V and not just 12V?
Well its been running like that for a good few thousand miles so far.....
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