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Posted by David Sherman [72.47.9.228] on Sunday, December 05, 2010 at 20:25:00 :
In Reply to: Re: Cowl light question posted by Jim (Maine) [74.65.136.227] on Sunday, December 05, 2010 at 19:37:32 :
Yes, if they have 1157 bulbs, you might have accidentally hooked up the bright (brake) filament rather than the dim (marker) filament. The bright one draws about 2 amps, and the dim one half an amp.
A pair of standard 85 watt headlights will draw 14 amps at 12 volts. That plus 4 amps markers wired the the "bright" filament would give you 18 amps and it wouldn't take much else to push it over the limit.
However, a fuse will take a slight overload for quite a long time without blowing. If the fuse blew immediately, I'd suspect a short circuit. One way to find a short circuit is to keep putting in higher and higher amperage fuses until the short starts sparking or the wire starts burning. It works in houses too. I don't recommend that. The better way is to first look in the new stuff you added on for anything obvious, then disconnect all the new stuff and see if the fuse holds, then re-connect things temporarily one at a time until it blows.
Ideally, you will calculate the maximum drain on every circuit and pick a fuse value such that the maximum expected current will be no more than 80% of the fuse rating. Ideally, you will also make sure ALL wire downstream of any given fuse is heeavy enough to carry a short circuit current safely until the fuse blows. For 20 amps, that means at least 14 gauge in automotive work. Many accessories come with tiny little #18 or #20 leads, which can carry the normal load of the device, but will burn up if the device shorts out inside, before a 20 amp fuse blows.
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