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British vs American Fuses

When I first got my TR4, I was warned against using the standard barrel fuses you can buy in a local parts store (AGC, often made by the Bussman company--sometimes labeled as "Buss"). It turns out that while they look like they will fit the fuse holders in British cars, they are actually slightly different sizes. More importantly, the ratings systems used are vastly different. One member of the Triumphs mailing list sent me this description which helped me understand what the differences were. He has graciously allowed me to reprint it here.

Perhaps it is time to revisit this topic, since it's an important one.

Like so many other things about our Triumphs, the British did things just a little bit different than the Americans, including what the ratings marked on fuses mean. The British Standard that Triumph used rated fuses by the current that was guaranteed to make them blow instantly while the American (and now international) standard rates fuses by the current that they will carry forever. Fuses work on heat, so a small overload may take a long time to build up enough heat to melt the fusible element and blow the fuse. How warm the fuse was to begin with plays a large role too. So the carry rating winds up being quite a bit lower than the blow rating... the difference is roughly 2 to 1!

So, what this means to you is that, if you elect to use common American standard fuses (Bussman is one maker, there are others) in your Triumph, you need to use a much lower rated fuse than what your Triumph documentation says to use. There have been cases reported where a restored Triumph has caught fire from a short, because someone used American fuses with the Triumph recommended rating marked on them... the short did not blow the fuse but instead set the wiring harness on fire!

Here is a conversion chart, lifted from a long-ago Bussman fuse catalog:

Bussman Conversion Chart

English Type    American Standard Replacement
   50 amp          AGC 30
   35 amp          AGC 25
   30 amp          AGC 20
   25 amp          AGC 15
   20 amp          AGC 10
   10 amp          AGC  7 1/2
    5 amp          AGC  3

Personally, I feel even this chart results in fuses rather larger than needed, so for example I use an AGC 20 to replace the original "35 amp" fuse in my TR3A. My conversion chart would go more like :

Randall's Recommendations

English Type    American Standard Replacement
   50 amp          AGC 25
   35 amp          AGC 20
   30 amp          AGC 15
   25 amp          AGC 15
   20 amp          AGC 10
   10 amp          AGC  5
    5 amp          AGC  3

Tim Buja and Glenn Merrell put on a good visual demonstration of the difference in fuse ratings at VTR this year, where they first put in a "25 amp" Lucas-equivalent fuse and applied enough load to blow it. Then they installed a "25 amp" American fuse and applied the same load. The fuse did not blow, but after just a minute or so the wire started visibly leaking smoke...

By the way, proper fuses with the correct British markings are available from the usual suspects (Moss, The Roadster Factory, Victoria British, etc.). The British Standard fuses are also a slightly different physical size than American standard fuses, so it's probably best to buy the proper British fuses. American fuses fit in most British fuse holders, but not all of them.

Randall Young
October 17, 2005