Fuse rating

[Andy Borris]

When specifying fuses, how do I calculate what the blow amps should be?

Thanks

Andy  

[yorkshire_spam]

max reasonable load + a bit?

P = IV (power = current * voltage)
So if you have 2x55 watt headlamps

110 = I * 12
or... I= 110/12 = 9.1A load, so call it a 15A fuse.

Does that make it worse or better?

[That Man]

The rating on the fuse is the max load it can run at continuously, the blowing load IIRC is twice this hence a 17amp fuse would blow at 34amps, just work out (I=W/V) or measure the loads and fuse to that current

[thebrookster]

Roughly, yes, However, there is a surprising amount of science in fuses, as I discovered not that long back!!

Fuses can basically be tailored to the job. As That Man says, blowing might be double constant rate, but you also have to factor in time. If say, it is a motor the fuse is protecting, then start-up currents can be huge, but you don't want the fuse to blow every time you start it!

Also, say you have a 30A fuse, if the blowing capacity is double, 60A, what happens if you pull 45A through it? Still too much current, suggesting something is wrong. Would you want to leave it like this permanently?

Andy, you could do to give us some more info really, i.e. what are you trying to protect? If it is headlights, your fuse rating (the one on the box) needs to be slightly higher than circuit power, however something like a wiper motor or a fuel pump may need a higher rating.

Cheers,

Phil

[junkuser]

Another thing to consider is the rating and condition of the conductors either side of the fuse, right back to where they complete the circuit to the power source.
High resistance joints and fracturing conductors can heat.

If you have an ammeter, that is also part of many of the circuits.

[piman]

Hello Andy,

one confusing factor is that the Lucas glass fuses are rated at blow current while most others are rated at load current with a blow rating much higher (x 2 comes to mind?) Generally they are not used to protect against an overload, but rather for earth fault protection so actual rating is not so critical.
As Phil mentions, fuse can be quite technical, with respect to industrial ones, but I don't think that automotive ones are as sophisticated, partly due to the low voltage in which they are used?

Alec

[TedTaylor]

This is the advice that I would give to my engineering students when I was teaching, and also did feature in an exam paper that I had to mark when I was an examiner.

Find the current that is flowing through the circuit, preferably by measuring with an ammeter which will give the true current and allows for any extra current requirements of cable, switches and connectors, or by calculation (in simple terms Amps=Watts/Volts) from the power requirements of the item of equipment being protected which will give the minimum current flowing through the circuit but not allow for any additional normal loads.

Use the fuse with a rating (in amps) that is the next of the standard sizes above this, but if this is for example less than 20% above it may blow prematurely in which case the next standard size above this may be required but monitor the fuse performance.

Ted

[Hogie]

Hi,
    there is still more to fuses than the above. Hit the link below and go to Fuses.
Don't have one fuse for both headlights. Have a smaller fuse for each.

http://www.wmbgbrit.com/maintenance_british_sportcar.htm

Roger

[Andy Borris]

I need fuses for the main and low beams (one each), horn, flasher, rad fan, ignition relay and the EFI pump.

Don't really have access to an ammeter (why is it ammeter and not ampmeter?) unless there is a way of using a digital multimeter to measure the amps?

However, I can calculate the amp draw for all these  and then apply Teds rule of thumb (and why is it a thumb and not a finger?)

Sorry, got my "going off at a tangent head" on today. (and why is it etc.!)

Ady

[Hogie]

Hi Andy,
               ammeter should be am'meter as it is a silent 'P' as in bath.
It is quite uncomfortable to say amp meter - much nicer ammeter.
I believe ALL digital meters do current.

Haven't a clue about thumbs.

Roger

[JohnD]

Because 'fingers' measure whisky.
And your thumb is about an inch from tip to first joint, so useful for rough measurement and construction.

But a silent 'p' as in bath?   Swimming bath perhaps, unless they add that dye!
Or is it like the Young Ones?  The others spelt Rick's name with a silent p.

But the fuse? 135% more than the normal current draw  or the next size up, unless that is much more than that.
John

[thebrookster]

Quite agree John, fingers are for whisky, though I prefer to measure mine by the hand ;) ;)

Roger - would you care to explain where the bath comes from? You've certainly lost me on that one, so I wouldn't mind going off on a slight tangent here :)

[Hogie]

Ammeter should have a 'P'.
But it makes it hard to pronounce.
So you make it a silent 'P' and replace it with an apostrophe.
Eventually the apostrophe goes for a burton.

I'm not good with slight tangents. I'm more into the double helix and disappearing up somewhere the sun don't shine.

Most baths that contain children have a silent P

Roger

[That Man]

A lot of multimeters can measure current, but you must be very careful when doing so, most only measure up to 10amps some 20amps, if you go over that you'll blow the internal fuse in the meter. It's also common to have to plug the lead(s) into different sockets on the meter specifically for measuring large currents (100ma+). get that wrong and you'll need a new multimeter. ask me how i know   ??)

To Measure current the meter must be in the circuit, so all the power is flowing through the meter, hence why extreme care must be taken.