25 replies to this topic

[Scoop77]

Discovered that all my issues went back to those nasty push fit fuse holders under the bonnet so I've ordered two new waterproof fuse boxes and heat shrink connectors but will therefore be fitting blade fuses. What's the opinion on the fuse ratings, 10 for 10 etc or do I go up or down?
Cheers,
Steve.

[bmcecosse]

Just about any fuse will blow in the event of a short circuit - so I would fit 30 amp fuses. Unless the circuit is just for a wireless - something like that, in which case 5 amp is enough.


<<< This is very bad, dangerous advice in the opinion of all the technical staff and this post has been reported to us by members who are electricians. Following this advice is very likely to result in a serious fire in your car's wiring. >>>

Edited by Dan, 24 January 2013 - 01:07 PM.
Dangerous advice.

[DannyTip]

Theres a good table on wikipedia for this: http://en.wikipedia....use_(automotive)

Blade fuses are roughly half as they are rated as continuous amps rather than blowing amps like glass fuses.

[Scoop77]

Two different schools of thought there.

Just about any fuse will blow in the event of a short circuit - so I would fit 30 amp fuses. Unless the circuit is just for a wireless - something like that, in which case 5 amp is enough.

Do you mean a radio when you say wireless?

[Scoop77]

Theres a good table on wikipedia for this: http://en.wikipedia....use_(automotive)

Blade fuses are roughly half as they are rated as continuous amps rather than blowing amps like glass fuses.

So I think you're saying if the glass fuse says say 15 put a 30 blade in?

[Barman]

Here...

[Scoop77]

Other way round then!

[Bungle]

Just about any fuse will blow in the event of a short circuit - so I would fit 30 amp fuses. Unless the circuit is just for a wireless - something like that, in which case 5 amp is enough.

whether the fuse blows or not depends on the cable size

if the cable has a lower rating than the fuse the wiring will melt first

[Scoop77]

Some great links on that thread as well barman ;)

[Barman]

Some great links on that thread as well barman ;)

Very welcome!

[KernowCooper]

Not much of the standard Mini Wiring loom with the exception on the feed to the ignition switch and alternator is bigger than 14gauge and some of the earlier looms had some 12gauge, a 30 amp fuse as said above could indeed melt the wiring on 12 and 14 gauge. Here is the recommended gauges of wire and amps

15 amp. =14 gauge
20 amp. =12 gauge
30 amp. =10 gauge

Picture below shows 10 gauge insulated wire and as you can see thats of a higher current capacity than most of the wiring to the glass type fuse holders

Attached Files

•  10 gauge.png   36.65K   3 downloads

Edited by KernowCooper, 23 January 2013 - 06:30 PM.

[Scoop77]

Yes the wire for the fuses is a lot thinner than that. Think I have the gist now.
Thanks all.

[dklawson]

The Wikipedia link above does not open for me but the old thread link above provides a good discussion of fuse ratings.

Lucas fuses are not common in the U.S. so we have had to deal with this for years (what off the shelf fuse rating to use in place of Lucas fuses). Our basic rule of thumb is to take the Lucas fuse rating and multiply by 0.62 then choose the closest (lower) rated fuse that is commonly available. There are also "conversion tables" posted on several U.S. message boards discussing British cars. The tables are slightly different than the 0.62 multiplier but it is generally a difference that clearly involves whether to "round up" or "round down" after multiplication.

I will ping Tiger and ask that he post his comments to this thread.

[tiger99]

Yes, a blade fuse should be around about 0.6 of the rating of an old Lucas fuse. Some people say 0.5 is close enough, but it is a bit low and you might get fuses blowing without proper cause. I would explain that Lucas fuses were very odd in that they were rated by the current which would blow them very quickly, whereas all other fuses everywhere, as far as I am aware, are rated by their continuous current carrying capability. So, although you can buy standard 1.25" glass fuses anywhere (less common now as most are a standard metric size in the EU), using a standard 1.25" fuse from a general electrical or electronic supplier such as Maplins in a Mini is dangerous because it will be rated for continuous current. 35A continuous in a circuit rated for maybe 20A will overheat the wiring.

Blade fuses are rated the same as any other non-Lucas fuse, for our purposes. So a 35A fuse goes down to the best approximation which is likely to be 20A (ideally 21A, but I don't think you can get them), and a 25A will need a 15A or 16A blade. No need to be too exact, an amp or two either way is usually acceptable.

Another thing to consider is that standard car fuses are "quick blow", so if you have something on the car with a high inrush current at switch-on, that keeps blowing fuses, like a radio with large internal capacitors, certain motors, or even headlights (NEVER have one fuse controlling both lights, for legal and safety reasons!), you can use "slow blow" or "anti surge" fuses which will not blow instantly on a brief overload, but will still protect the wiring fully. Your wiring will not be harmed by twice its rated current for a few tens of milliseconds, or even a second, but it will be harmed by twice its rated current continuously.

Selecting the correct fuse can really tax the brain of designers in some awkward cases, however in the majority of cars it is not all that difficult, because the systems are fairly simple. The original 35A Lucas fuse was deemed to be correct when the Mini was designed.

You can also get miniature circuit breakers which do the job of fuses but are resettable, and they will be rated in the same way as a blade fuse. I find them to be a very attractive proposition nowadays. They tend to be a bit sluggish in action so effectively are slow blow. If you use several of these to feed different circuits, always put a blade fuse of sufficient total capacity upstream as a precaution in case one fails to trip.

By the way, there are at least two sizes of blade fuses in common use in cars, in the smaller current ratings, as well as some much bigger ones. I bought a bargain pack of blades in a supermarket in Germany. They were car fuses, as they were among car parts and accessories, but it turned out that they were too small for my car.

[Old Bob]

A seriously interesting thread (and indeed many of the contained links are thought provoking) but being a simple man - or technologically challenged - can anyone offer recommendations for a basic set of fuses for the more usual circuits?

I am replacing all the wireing on my project and have a maximum available of 10 fused circuits, all on modern blade fuses and all in the cockpit for ease of replacement. So, should I be looking at Head and Rear (left and right on separate fuses), fuel pump, screen heater, 12 volt socket, spots, fog lights, reverse light - indeed, what else since I have no radio? And what would be safe and sensible rateings for these fuses.

Project K.I.S.S. - if only it was.........

Bob