At 04:57 PM 3/7/2003, James V. wrote:
>Well, that makes sense that higher wattage bulbs should have heavier guage
>wire.  But on Frank Condelli's how-to, it suggests that all that is
>necessary is a larger 15amp fuse (along w/ the relays) when going to 100w
>bulbs:

There are two considerations: safe carrying capacity of the wire and
voltage drop.  From Rob't. Bosch, the carrying capacity at 50C/122F and
30C/86F is:
1 mm^2 or ~17ga -- 13.5 /19 amps
1.5 / 15 -- 17 / 24 amps
2.5 / 13 -- 23 / 32 amps
4 / 11 -- 30 / 42 amps
6 / fat 10 -- 38 / 54 amps

The voltage drop in volts per amp per meter of wire at 20C/68F (it
increases by a factor of 1.04 for each 10C rise)  is:
1 mm^2 -- 0.0185
1.5 -- 0.0127
2.5 -- 0.00760
4 -- 0.00471
6 -- 0.00314

So if you're running four 100-watt bulbs, your nominal draw at 12.6v will
be I = P/E = 400/12.6 = 32 amps, so your voltage drop per meter at 20C is
1 mm^2 -- .592
1.5 -- .406
2.5 -- .243
4 -- .151
6 -- .101
plus about ten per cent in each case to allow for hot days and the wire
self-heating.

Bosch says that for >15 watt lamps, the maximum permissible total loss in
the lighting circuit from battery + to ground is 0.9 volts, with 0.5 of
that being permitted between the 30 (supply) terminal of the light
switch/relay and the lamp.

Figure about a 0.1 volt drop across each fuse, and considerably less across
(properly rated) relay terminals.

With that in hand...measure your wire runs and the current in each run, and
size your wire accordingly.

david

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"