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MG MGB Technical - British vs American fuses

Is there a difference between American and British fuses? Does a 30 amp American fuse have a heavier filament than a 30 amp British fuse? I have been told that is so, and that wiring can burn out as a result of that. True? Thanks. Curt

I think the only difference these days is the shipping label put on the box in Taiwan.
S Best

The 35 Amp Lucas fuse used extensively in British cars was designed to hold 17 Amps "forever", and to blow "instantly" at 35 Amps, with various time delays in between (like 5 minutes at 25 Amps, for example).

Sounds very unscientific (and it is), because fuse response depends upon so many other factors, such as ambient temperature, vibration, and the quality of contact between the fuse and its holder.

Of course, being the owner of a "two-fuse" midget, I suppose we should be thankful that they used any fuses at all <grin> - - Alec

Alec Darnall

If you compare a USA 30 amp fuse and a Lucas UK fuse I think you will notice the UK fuse is a bit shorter than the USA fuse. I prefer the UK 30 amp, 17 amp hold on my MGs, being just a bit shorter, they fit better in the TF and B fuse holder.
Dave Kalp

Check Barney Gaylord's Slow Blow Fuses article at
I think it will clear up the question.
Tony Shoviak


I've seen the article and have asked at every auto store in my area. No one carries the slow blow fuses. Can anyone identify a source for these?
Tom Balutis

A 'slow blow' fuse to me consists of a length of fuse wire connected to a spring with a blob of solder. With a high overcurrent the fusewire blows instantly, with a low overcurrent for a given period of time the solder melts to break the circuit. I've seen these in electronic circuits, usuyally those with a power-up surge. The fuse wire is designed to carry the surge, which is not on long enough to heat up the solder. But if more than the continuous current flows after that it will melt the solder quite quickly. Standard Lucas fuses are not slow blow in my book, just bog standard fusewire or fusible strip. They have a rated current of 17 amps because the connections will start to heat up on currents higher than that (on late model cars at least one of the fuses was replaced by a circuit breaker because of the high currents it was being asked to carry), but the current will have to go up to about 35 amps before it will blow. You can take these fuses slowly up towards 35 amps and they will start to glow, but because they are in air and not whatever is in bulbs the 'filament' will burn out fairly soon. But if you stop short of glowing they will carry much more than 17 amps for a long period. One site for fuses of this type quotes 4 hours for 110% over current, 1 hour for 135%, and 10 secs for 200%. Apart from the last one that is much longer than slow blow.

You need a fuse with a *rated* current of 17 amps, but unless you can get the proper item you will probably only see 15 amp or 20 amp, which should be OK. If you get one just marked with 35 amps that is almost certainly its rated current, which is too high.
Paul Hunt

I second Paul's observations - the Lucas "slow blow" fuse is nothing like the traditional ones, which have either a spring, a carbon resistor, a filament wrapped around an asbestos core, or a body full of alumina granules. It's just a length of fusible wire, with some specific properties, and some creative ratings.

Personally, I'm going to use common 20 Amp fuses for my everyday driving, and save those dear 35 Amp "paper slip" fuses for the rallies and shows (though I'm not sure the judges will pull the cover from the fuse holder, anyway).

Cheers, y'all - - Alec

Alec Darnall

No you won't find these fuses at Murry's or Autozone.I don't think Moss or Vicky British has them either.You might try checking EBay or some of the parts places in UK.
Tony Shoviak

Paul - "Whatever is in bulbs" is nothing -- or close to it. The filament glows white hot but is not consumed because there is no oxygen. That's why light bulbs make a distinctive "pop" sound when broken - the air rushes in to fill the vacuum.
David Breneman

You can get 35/17 amp. fuses from Gordon at the B Hive.

Clifton Gordon

I got some a while back from Jeff Zorn at LBCarCo.
R. L Carleen

Slow blow fuses are also available from:-

Nigel J S Steward

Got a Jag dealer nearby? Ask for Jag part number C-39572. IT's the correct 17/35 fuse complete with paper label (although I've also seen them without the label and 17/35 printed on the glass....)
Rob Edwards

Cool, does this mean the full set of original fuses we have in our 79 are worth something on Ebay? :-)
The Wiz

Yeah, and for concourse it doesn't mattter if they are blown ...
Paul Hunt

If you can bear to go to a Triumph site, you can buy the correct fuses here:
The Wiz

I ordered some fuses from Wedgeparts and was happy with the service, even down to the guy calling me to confirm my order.

The Jaguar fuses have the paper insert, there is also a line made by Unipart, they have the printing on the glass, they are essentially the same fuse but those wanting originality may want to check which fuses they are getting.
The Wiz

Can I replace the original glass type fuse with blade type ones? I need to rewire my B and was going to add relays and was also thinking of using the modern blade fuses all monuted ona enw base plate where the normal 4 fuse holder lives.

Simon Jansen

I have seen Bs rewired to incorporate a modern "power distribution centre" (fuse and relay box combined), it's really down to what you are capable of. Simple four fuse holders for accessory circuits are readily available at auto parts stores, one of those could be easily adapted for a B or you could go the whole hog and install a bunch of extra fuses and all your relays in one large box.
The Wiz

Dan Masters

Of course you can replace your cylindrical fuses with modern blade type ... but you have to replace the fuse holder as well! :o)

I did a rewire of a V8 conversion a while back and the owner asked me to convert to blades, and provide some extras. I found a 6-pole fuse block that exactly fitted the original 4-pole mounting points. I also installed two relays and another 4-way blade type fusebox for the headlights, which made a very noticeable difference to the brightness and speed of switch of te uprated lamps.

On my own cars I've fused the fuel pump and overdrive circuits with in-lne cylindrical fuses installed at the switch or bullet connectors as appropriate, so I didn't have to modify any existing wiring and it can be returned to standard very easily if required. I've also added relays and in-lines for the heated rear screen, fog (front and rear) and spot lights on the V8. These also made a noticeable improvement to effectiveness/brightness.
Paul Hunt

Paul, I am redoing the whole loom in the roadster and was planning on replacing the old fuse holder with a plate mounted in the engine bay with the modern blade type holder as well as relays.

Dan, that site looks interesting. But as usualy for me anything in the states is a paint to get hold of.
Simon Jansen

"anything in the states is a paint to get hold of"

Did you have a brush with some poor suppliers, or is it something you'd rather gloss over ...

Paul Hunt

Must rememebr to proof read when I type replies quickly!

It is odd though. Some things I can buy from here with a credit card and they'll ship from the states no problem. Amazon is a good example. They will ship books and DVDs within a week usually and have very good prices (often cheaper than buying here). Then other places you find what you want from their web site and try to make an order, get all the way through to the end then find they only ship to the US.

That's fair enough US companies only shipping within the states but I wish they'd make it obvious before I waste my time trying to navigate round their web sites!

Paint unfortunately I can't order. They won't ship that :)
Simon Jansen


Advance Auto-Wire will ship to New Zealand, but we're not yet set up to take credit cards. We will accept checks or money orders, made out in US dollars.

Surface mail to NZ (4-6 weeks) adds $22 to the cost. Air mail (4-6 days) adds $68. For 2-3 day express delivery, add $145
Dan Masters

Hi Dan, you really need credit card orders. I know you know that already :) Getting cheques or money orders made out in $US is surprisingly expensive here.

I have two old wiring looms here I am going to clean up and put together into one good one myself. A long time ago I worked as an electronics technician and once wired up an electric car so one little MG shouldn't be too hard!

Simon Jansen

This thread was discussed between 06/04/2005 and 19/04/2005

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