View single post by Mark Rosenbaum
 Posted: 07-18-2006 02:38 pm
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
What fuse size does the stereo manufacturer recommend?  Use a fuse of that size.  (You'll also need to determine whether they are talking about the must-carry rating or the must-blow rating of the fuse).

Lacking the manufacturer's recommendation, you need to know how many watts of sound, total for all channels, the stereo delivers.  That, and the stereo's efficiency, will determine the current requirement.  Assuming 50% efficiency is a good place to start: while a fuse may occasionally blow if the volume is turned up high, you are also reasonably well protected against wiring and equipment faults.

Example: 120 watt stereo.  Divide by efficiency of 50% or 0.5 = 240 watts.  240 watts = 12 volts x 20 amps.  You would start with a fuse having a must-carry rating of 20 amps.

The stock JH fuses are rated 35 amps.  IIRC this is the must-blow rating, which, for Lucas fuses, means a must-carry rating of 18 amps.  Cylindrical fuses of that sort have at least two age-related failure modes, so it's a good idea to replace all of them every 20 years or so.