Ampacity

["Mike Nakamura" <mike_nakamura@domain.elided>]

Sorry everyone, I wrote too soon on wire run with 12 gauge.  I actually
used 10 gauge when I wired the Montreal.  Since Montreal batteries are
in the rear, I took off positive power at the alternator.  I fused this
take off close to the alternator and used multiple wire ties to hold it
firmly in place.  The most dangerous electrical circuit on that car was
from alternator to main fuse panel.  Not sure if a fusible link was in
that run, but I didn't see one close to the alternator.  Then the hidden
and long battery to engine compartment runs, hope they are in good
shape!

Back to you car.

Depending upon how you wired the system there may be a weak link or two.

Possible weaknesses:

1. If you ran a common 12 gauge wire from relay to ALL 4 high beam
bulbs, this one wire will be marginal unless the distance is under three
feet, not possible in a car.  If you ran four wires for high beams, one
to each filament (from the relay) this is a non-issue.  It requires a
relay with four #87 terminals (or other common connection point).  It's
probable that you ran two wires from the relay to the 4 filaments (one
for H4, 75W and one for H-1, 100W).  At all 12 gauge, the H-1 circuit
carries a maximum of ~15 amps (200 watts / 13.5 volts); this is good for
11 feet with 2% loss.

2.  More likely is an undersize wire from the positive source (battery
or alternator take off) to the relay.  This wire will carry all 26 amps.
Since it is for high beams and they will probably not be active for long
continuous periods, you are probably looking at a 5% voltage loss rather
than the desired 2% loss (if run long periods, loss goes up as wire
heats).

http://www.poweriseverything.com/impt-info/v-a-tablespage.html

http://www.homestead-specialties.com/hs_wiring.html

I have seen applications where 65, 75, 85 and 100w bulbs were run on
"stock" wiring.  The long term effects are dangerous, but many car
owners get away with it.

I guess that your 12 gauge upgrade, no matter which exact routing you
used will present no problems.  If you can run 10 or 8 from positive to
high beam relay that would be good, however while the 12 causes slightly
greater voltage loss, I suspect the heat generated is not enough to
damage the wire. 

As always, YMMV. 

Mike Nakamura
Fall City WA USA
mike_nakamura@domain.elided
91 Spider Veloce
94 164 LS
02 Dakota
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