TR2/3/3A Tr3 conFused

Gents, Was working on my 'windtones' and looked at my fuses. Can't believe a two fuse car but... the glove box guide calls for a 50 and 35 amp (Brit reprint), my old Haynes manual calls for two 25 amp for Tr2/3's, and my car had two 30 amp (until I blew one) and now has a 30 and 25 amp. What's correct? The horns sound great when both work.

Whatever you determine that you need for fuse rating, go by that. Don't, however, make the mistake of substituting an American fuse for a Lucas fuse. The blow rate is much different for the two. Generally speaking, an American fuse will blow at a higher rate than it's British counterpart, which could play havoc with your Lucas wiring. The correct fuses are available at most vendors, although some will sell you the same thing that you will find at NAPA. Try TRF or Moss (talk with them first).

Bill

There has got to be a conversion table of some sort. The two 30 amps that were installed didn't appear US as they were shorter, but that's a guess. There is an original lucas 50 amp in the spare slot on the fuse box (I'll keep there for originality). I was told by another Tr guy that a Brit 50 amp equated to a US 35 amp!? So I'm guessing the Haynes 25 amp call could be for US fuses?

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I was told by another Tr guy that a Brit 50 amp equated to a US 35 amp!?

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That's roughly correct, and a Lucas 35 amp could be roughly equated to a US 20 amp.

Amps are amps whichever side of the Atlantic you're on... the difference I think gets into rating a fuse based on the constant load it will handle vs rating it on the (higher) current that will immediately blow it.

I use 20 amp fuses for both sides of the TR3A fuse holder just because the larger fuse is way more than the draw of those horns. In fact, I have the horns and the electric fan on that righthand 20 amp fuse and have never blown it.

The fuses I use are the usual auto parts variety but I open them up and insert an original looking Lucas paper in there, then close them up again. Looks nice on the '3 because the fuse box is not covered and in a highly visible location.

P.S. If there is interest I will post instructions on how to open up the glass fuse and a link to printable 35 & 50 amp fuse papers.

The confusion arises because the US & UK use different fuse rating systems. The original UK fuses were actually marked "17A Continuous - 35 AMP Blow".

A fuse requires heat (power, watts) to melt the element. The heat applied to a fuse element follows the formula of I squared R (amps times amps times the fuse resistance). Since the fuse resistance is approximately constant it boils down to the speed of melting is proportional to amps times amps.

The 17/35 amp English rating says that the fuse will carry 17 amps continuously & eventually melt at 35 amps. If for example the 17/35 fuse would melt in one minute at 35 amps, with 70 amps applied it would have four times the heat applied & melt in 15 seconds. With 140 amps applied it would have 16 times the heat & melt in 3.75 seconds. All simple fuses roughly follow this rule.

The USA rating system gives the current required for the fuse to eventually melt, not continuously carry. Thus a 20 amp USA rated fuse with 35 amps applied will melt in about the same amount of time as the 17/35 English rated fuse. A UK fuse marked 50 amp is about equivalent to a USA 35 amp.

The above melt times are for example only & the actual times may be different.
D

Amazing, where do you guys learn this stuff? Guess my tin foil trick is old hat.

Ok, I can work with that - thanks greatly. Geo, electricity is kind of like magic to me so opening fuses and expecting them to work again might be a reach but I'd like to hear your technique. You obviously had a slow 'mechanic day' and needed a project - I paint the inside of my hubcaps on days like that.

prb51, now that class is out, which should solve not only your problem but lots of other folks too. After you have been messing with these cars a while, you will be sitting around one day and say "I wonder what would happen if I tried this", I've learned to fix a bunch of stuff doing this as I'm sure we all have. Right Geo? The last two things were the flasher unit and reverse light switch on the transmission, they don't cost much but you always need them right now. Wayne