Fuse selection

[Shawn]

G'day all,

I remember that there is a formula for selecting the correct amperage fuse for electrical components (eg heater radio etc) I've had several goes at Googling if but haven't found anything useful

I'm looking at replacing my standard fusebox with one with multiple fuses.

I thought the formula had someting to so with the wattage of the component and the voltage of the system?

Does anybody know?

cheers,
s

[brixtonmorris]

is ohms law what your looking for?

current I = the voltage / resistance of circuit

this will give you min fuse value.

watt is a measure of power and involves time

[woo]

Shawn,

The simplest way to get amperage is to divide the Watts by the voltage.

So for instance a 21 watt stoplight = 21/12 which is one and three quarters amp. 60 watt headlamp bulb = 60/12 which is 5 amps.


Robin

[Shawn]

That's what I was looking for - thanks heaps!

s

[brixtonmorris]

woo, how do we find out the fuse rating for the wiper motor using wattage?

[bigginger]

Amps = watts /volts, if that's any help

[brixtonmorris]

how do you know the amperage of the wiper motor?

[bigginger]

The only way I know would be to know the wattage, and do the sum above, sorry.

[brixtonmorris]

back to Ohms law then

[bigginger]

Just out of general electrical interest, here's a couple of very bad drawings -


- of some stuff I still remember from 'O' level physics. V=volts, I=amps, R=resistance and W=Watts, so I=V/R, or W/V, V=IxR or W/I, R=V/I etc etc. If you want to remember them, my aide memoire has always been 'Wives' and 'Virgins', a bit naff, I know, but I still remember it after 25 years...

[brixtonmorris]

you dont know what the wattage or current is on any of these components.
heater radio, so on.
its ok on bulbs because its stated, but not on general electrical components.

Ohms law rules. you need a resistance meter though!

[bigginger]

No, it's a snag...
a

[woo]

how do you know the amperage of the wiper motor?

I did an exercise last year to see how much current a newly fitted rear screen heater drew. While I was at it I looked at a number of other components including my windscreen wiper.

This is what I did:

I disconnected the brown wire from the A terminal on the Control Box and connected a Digital Ammeter between them. This enabled be to measure the current being drawn from the Battery with the engine off.

1. Turn on the Ignition ------- In my case this drew 3.7 amps.
2. With ignition on start wipers ------- This drew a total of 8.7 amps.

Therefore I concluded that my wiper motor drew 5 amps.

I tested other electrical items to see what the ‘worst case’ current draw would be and whether I needed to fit an alternator. This is what I got on my car.

Heater Fan 1.5 Amps
Rear Screen Demister 4.7 Amps
Wipers 5.0 Amps
Indicators (one side) 4.9 Amps (this included additional rears I had fitted)
Sidelights 2.8 Amps
Side and Headlights 11.1 Amps

These figures were actual figures, not rated figures.

So the worst case for me would be around 27.2 Amps, which included indicators. So running in Rainy Cold weather could consume 22.2 Amps. This was within the scope of my Dynamo’s output.

As I said these are MY figures of metered amperage. You shouldn’t rely on them for your car. My Ammeter supported up to 15Amps.

Normal Moggy fuses are:

One 35 Amp fuse between A1 and A2 that supports the Interior light and the Hooter.

One 35 Amp fuse between A3 and A4 that supports the Wipers, Heater, Indicators, Brake Lights and Petrol Gauge.

So you can see that depending on what items you have switch on that are connected to the A4 fuse, you can have a LOT of head room between the current actually drawn and the 35 Amp fuse.

The Hooter(s) on the A2 fuse can draw a lot of current. I fitted a second Windtone horn and drew 16 Amps each time I hooted. But there’s still a long way to go to reach 35 Amps.

Robin

[Onne]

Are you saying, my car has a 35A fuse just for the horn? I haven't got an interior light, being a series II....  I have fitted a hefty horn though

[woo]

Onne,

Check the wiring coming off of your A2 fuse. On my 1962 there are only 3 purple wires. 2 wires for 2 horns and 1 wire for the interior light.

I've looked at the wiring diagrams in the BMC manual and the A2 appears to feed horn and interior light on all models up to the end of the 948. I don't think I have a diagram for the 1098, but i can't see why it should be any different.

Robin

[uclarge]

So u can infact use modern blade fuses o­n morris minors "Main Power" then??? how handy! charly

[Onne]

Onne,

Check the wiring coming off of your A2 fuse. On my 1962 there are only 3 purple wires. 2 wires for 2 horns and 1 wire for the interior light.

I've looked at the wiring diagrams in the BMC manual and the A2 appears to feed horn and interior light on all models up to the end of the 948. I don't think I have a diagram for the 1098, but i can't see why it should be any different.

Robin

I have only got 1 purple wire off my A2 fuse

[woo]

onne,

I bet that goes to the hooter.

[Onne]

you guessed right!

[Shawn]

Ok...so measuring the resistance of most items would be fairly straightforward with a regular multimeter?



I've always been ok with voltage readings, but the rest of the meter has been a bit of a mystery to me (the meter was made in Taiwan and the instructions have lost something in translation)

Which of these ranges would I be using to get a meaningful reading?

Ps - Don't worry, I'm not about to throw my rudimentary electrical skills at this aspect of my Mog just yet - the plan is to learn a bit more about the whole fusebox and electrical protection side first and then decide if it's a good idea.

s