No mate, some of us are educated! lol
CEE or not, mike summed it up better
hiya lads,does anyone know the correct formula for calculating what amp fuse is needed for a given circuit,ie indicators/lights ect. thanks,andy.
v=ir i would guess.
i'd go by whatever my BMW says as a start point
or i=p/v current=power over voltage
[Edited on 19/11/09 by blakep82]
usually rather less than the amps required to fry the wires but the formula is just
Amps = Watts / Volts
The total current load plus 30% works fine, if motors are involve ie Wiper or Fan you might need to rate the fuses at 50% over the steady state
current.
But the fuse is really intended to protect the wiring not the equipment at the other end.
For example 21 watt stop light bulbs each use just under 2 amps current when both are on say 4 amps.
If you wire them up with 8 amp wire you could safely use a 7.5 amp fuse rather than a 5 amp.
[Edited on 19/11/09 by britishtrident]
You can of course just rate the fuse at 20% less than the wire. If there's a short you'll get oodles of amps. How often have you blown a
fuse due to drawing too much amps but not shorting?!?!?!
The fuse is there to protect the loom not the thing being powered (which is usually fubar'd if the fuse is blowing!!).
The fuse protects both the cabling and the current using equipment the, bit attached to it, if you had a bit of kit rated at 20 amps and fed it using
50 amp wire with a 50 amp fuse if the kit faulted it would burn before the fuse blew.
Using above example you would use 20 amp wire with 20 amp fuse.
If its a motor then starting current is around 3-5 times running but is ony seen for a short period, in this case you would use a motor rated fuse
that has a slow blow overload charateristic but a fast blow short circuit characteristic, and a noramally sized cable.
Watts = Volts x Amps as Mr Whippy states unless you are on AC, then it gets more complicated, but we are on a DC car.
You would look for a fuse serving fixed equipment to go in around 5 seconds under fault so you look at the fault current flowing and make a selection
from fuse manufacturers curves to ensure the fuse blows in the selected time period.
Luckily the automotive engineers have sorted most of this out for us.
Rule of thumb fuse the kit and select a larger cable.
quote:
Originally posted by big-vee-twin
The fuse protects both the cabling and the current using equipment the, bit attached to it,
quote:
Originally posted by MikeRJ
quote:
Originally posted by big-vee-twin
The fuse protects both the cabling and the current using equipment the, bit attached to it,
It is primarily to protect the cable to prevent fire. Obviously by virtue of the fact that the cable has to be adequately rated for the load, the fuse value is also related to the load current.
Are any of you Chartered Electrical Engineers like me?
(omfg i cant believe he just said that)
No mate, some of us are educated! lol
CEE or not, mike summed it up better
Oh Dear! This is starting to sound like Pistonheads!
Not a chartered engineer but logically here's how it is in the real world.
1 the device/load needs to be supplied by a proper cable capable of carrying the correct current for the device ( in real terms the cable should
not be continually loaded beyond 80% of it's rated max current)
2 the device/load will be manufactured to run within certain current consuming parameters ( a given fact )
3 If the device/load goes tits up ( yes that is a shop floor techy phrase LOL) then it could overload the cable by demanding more current than it
usually would ( it's now outside it's parameters and most likely fubared)
4 when this happens the fuse blows therby protecting the cable from overload ( not the device/load as it's most likely already damaged beyond
protection)
Of course in a domestic/commercial install then an RCD ( or more likely an RCBO) protects the cable AS WELL AS the person using the device.
quote:
Originally posted by Mr Whippy
Amps = Watts / Volts