Whether it is nobler to suffer the slings and arrows of.......
(Some guy called Bill Shakespeare)
Anyhow, back to electrics and how to calculate fuse ratings for the
spridget.
I have decided that the spridget has insufficient fusing of the loom. Fuses
are there to protect the wiring from burning out due to shorts. The best
method is to calculate the absolute max current in the section to be
protected and to set a fuse with a rating 'marginally' above that level.
For practical purposes, a few amps above (20-30%?) seems the best
recommendation. The UK spridgets have one 17 amp fuse to cover the entire
sidelight circuit. Four 5watt bulbs on the four corners, four 2 watt bulbs
on the dashboard, two numberplate lights at 10watts and then there is the
resistance caused by several yards of wiring. 38watts plus another 2-to-8
for the wiring. 40-to-46 watts on 12 volts means 3.5-to-4 amps maximum
current across the fuse. It is rated at 17 amps continuous and 35 amps
sudden surge. Think back. If you've ever seen a LBC loom burnt out, isn't
it always the red sidelight wiring or the white or brown wiring on the
ignition switch? One is underprotected and the other has no fuse at all.
Sad man that I am, I worked out the current drawn from each item on the car
(or substituted the official rating marked on the item where correct figures
would have required expensive kit)
Brake lights 47W + wires
Ignition wiring (standard) 20W unfused
Fuel pump 60W max
Sidelights (inc 4 panel lamps on UK cars)28W +wires
Horns 40W
Wiper motor 70W
Numberplate lights 10W
Handbrake warning/brake fail lamp 5W
Indicators 42W + wiring + a couple for the clicky thing
Hazard light 42W added to the figure for the indicators
Cigar lighter 100W!
Interior lamp/boot lamp 5W each
Heater blower 50W or more
Electric radiator fan 200w or more!
Radio 15W
Starter 800 - 3000W
With the boot open, the hazard lights on, the brake lights on, wipers,
heater, fuel pump running etc I think that the green wiring will max out at
18amps momentarily if the hazards flash at the exact same time as the fuel
pump ticks. I believe that a 15 amp fuse will cover this as it will reach
30amps momentarily before blowing. The purple wiring runs the horn, cigar
lighter, boot lamp, interior lamp and the feed for the 'flash' facility. On
my car (no 'lighter) that means 40W + 22W + 10W = 72W => 6amps max. A 5 -8
amp fuse should cover this (?)
Before I go any further with this, can anyone see if I'm going down 'the
wrong road'?
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