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MG MGB Technical - Fuse Size

Anyone have a list of what size fuses should be used if adding in-line fuses for things like, wipers, heater, lights etc on a 67 B?

I am sure there must be a formula for fuse size VS application.
Bruce Cunha

Bruce - Fuses should be rated to the wire size that is being used so that a short at the far end of the wire doesn't fry the wire and all those in the same run with it. I have some information on that and will send it to you as soon as I can dig it out. Dave
David DuBois

Dave. If you should dig it out, please be so kind as to put it in some understandable form and we will add it to your section of the website. This is something I have wondered about also, but have not yet had to ask (behind in everything, of late). Thus, a short write up and the applicable information would be of value to many. Thanks.

Les
Les Bengtson

Use the smallest fuse that won't blow regularly. Nothing bigger than a 25 amp rating certainly, and if adding a new box with multi fuses, it depends on how many circuits you are running and the associated running load. If you have all your lights, and a set of driving lamps together, a 15 amp rating ought to hold, (depending on wire size) and the heater fan and the wipers could be fused alone each with a 10 amp fuse for protection, and that's the only big draws you have. The MGA used to go with only 2 fuses of 25 or 30 amp rating...only 2 circuits for the whole car.
Wayne Hardy

Hi all.

If you can't find a 'list' figure you can calculate an acceptable rating without difficulty if you know the normal power rating of the item being powered.
Just allow 1A for each 12W of power consumption (12V systems only !).
Round up the figure to the next highest fuse value if necessary. Don't forget that lights often come in pairs so the value needs to be doubled.

If in doubt, go for the next larger fuse value if the circuit is a critical one, ie one that could cause a dangerous situation if it blew without warning.

Some items (notably motors and lights) draw a significant surge current when they are first switched on so may need a higher than expected value.

If you own a multimeter you could always measure the actual current IF you are confident that the current is within the operating range of the meter. Simply make a little adapter (with an inline fuse) to allow the meter to be plugged in where the fuse normally goes.
A spare dashboard ammeter could be used for very approximate measurements.
A meter with a DC current clamp can be very useful for measuring higher currents.

Light bulbs sometimes take excessive current when they 'blow' due to a shorter-than-usual piece of the broken filament providing a current path.

Fuse characteristics are quite complex because the simple, inexpensive 'bit of wire that melts' is really quite a crude device. Factors include their ability to withstand a brief surge current without blowing and their ability to blow within an acceptable time when the actual current is only slightly higher than the rated current.

Don
Don

Go to British Wiring's web site at;
http://www.britishwiring.com/Comp.htm
Click on Wire (Bulk)and look at page 2. The table gives cable size in strands with carrying capacity in amps and general use for each size. Below the table is a British strand wire to US gauge conversion.

Don made some good suggestions about how to determine fuse sizes. This article chart for the maximum size fuse fuse to use with each wire gauge. It also has formulas for calculating the size fuse to protect a circuit. Don's method is easier.
http://www.bcae1.com/wire.htm

You can also use a 10 amp battery charger to get a close indication of the surge current in heater motors and other high current items.

Clifton
Clifton Gordon

In my post above the second paragraph, second sentence should read. "This article has a chart for the maximum size fuse to use with each wire gauge size."

Clifton
Clifton Gordon

As always, the knowledge of this group is outstanding.

Most appreciate all the advice.

And for Wayne

That is all the 67 BGT has, two fuses that cover everything. I am fiddling with putting a two speed wiper in the 67, and although it is an MGB motor, I want to have a little extra protection prior to flipping the switch.
Bruce Cunha

This thread was discussed on 27/01/2006

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