In a message dated 97-09-14 21:43:01 EDT, Teacher122@aol.com writes:
> The 15 amp would be the safe choice. However, it may well blow too.
> The experts(whomever they may be) say that you should never put in a fuse
> of
> a higher rating.
Tom:
I'm not an expert, but I think if I shuffle my feet and talk fast enough, I
can fill in till an expert gets here, without anyone finding out.
Yes, you are right, it is not a good idea, in general, to use a fuse of a
higher rating. However, going from a 17 amp fuse to a 20 amp will not cause a
problem. Contrary to what most people think, the purpose of a fuse is not to
protect the devices - lights, radios, etc, - fed by the fuse, rather it is
to prevent a fire from wires which have become overheated from overcurrent.
Every wire has a recommend maximum current rating, which is based strickly on
temperature rise in the wire, and its effect on the insulation (voltage drop
concerns are covered by selecting the proper size wire). As you can imagine,
these ratings are conservative.
Not too long ago, I had the main feed wire burn in two on one of my LBCs,
while about 100 miles from home. I jumpered from the battery with a 16 gauge
wire, and drove the rest of the way home, with the lights on, with no
problem. To be on the safe side, I held the wire in my hand so I would know
if it got too hot. I figured I might get just a few miles down the road to
the next exit where I could pull in to a service station to make a better
repair. The wire got warm, but never got hot, so I drove the rest of the way
home, holding the wire just to be sure. That 16 gauge wire was carrying ALL
the current in the car.
I feel quite confident that moving from a 17 to a 20 amp fuse will cause no
problems. Usually, when the experts say don't use a fuse of a higher rating,
they are talking about going from, say, a 15 amp fuse to a 30 amp. I still
think it is a good idea to use the proper size fuse, never-the-less, but I
certainly wouldn't do without my car in a case like this, while waiting to
get the right size fuse.
Dan Masters, retired Electrical Engineer,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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