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Original Post (Thread Starter)
by Hugh Jörgen
Hugh Jörgen
New harness and Pazon installed.
Replaced the 12V coils with 6V PVL's in series.
Got it back together enough to kick it and to my surprise it started right up.
I ran right out and bought a lottery ticket.

In doing a quick function check, rear brake, light; front brake, no light.
Wiring diagram shows a single white wire feed to brake switches from ignition switch.
Apparently the front brake switch had been fed from the same white as the 12V coils.
That white now powers the Pazon.

Is it bad form to tap off the Pazon power feed, or better to run a loose wire back to the ignition?

Liked Replies
by quinten
Originally Posted by Stuart
Quinten, Quinten ... facepalm

You've been posting about electrics on this Forum for years and years ... Sometimes I believe you've finally begun to grasp some of the basics, then you burst my bubble posting things like:-

Originally Posted by quinten
the white wire in a normal harness is rated to 20 amps ,and fused at 20 amps ( 17.5amps in old money )
facepalm Cobblers ... the original single fuse might be 17.5A continuous but there isn't any way on God's green earth any of the wires are rated for that ... Have you really never, ever bothered to look up Lucas wire current ratings or their modern equivalents? shocked

Original 1970 Commando coils and brake switch/lamp wires were 14 strands, Lucas rated for 7.5A

Afaik, the closest to 17.5A rating you might find on non-electric-start Commandos are 28-strand wires to the battery terminals? Originally rated by Lucas for 15A ...

Originally Posted by quinten
comiing through the ign. switch , which may have its own current limitations . (15 amps )
laughing Please do feel free actually to demonstrate 15A though the standard Lucas ignition switch ...


Golly whiz stuart , I wasn't aware I needed to meet your approval .
do you have some special expertise ? are you a moderator , i dont think so .
I'm not a bit surprised at this personal attack
Youve been slipping back into your old ways of late
you know
the one that previously got a new banded from britbike

you do not understand the fundamentals of a wire gauge capacity .
they are not found on a simple chart , where the ampacity is de-rated for the less informed .

the resistance of a wire run is it's cross sectional area ... and its length .

Short pieces of any gauge wire can handle higher currents ... than the same cross section in a longer length .
the loads have to be calculated taking into account the length of the wire
and not an amp. number from a generic chart .

lucas was sizing harness wires towards a maximum ampacity ,
in consideration of the short lengths of the wire runs .
a penny saved was a penny earned .
They knew what they were doing when they sized the wires .
Providing just enough wire gauge to do the job ( at an acceptable voltage loss )
They would have had their slide rules out and double-checking what they could get away with .
( nowadays there are online voltage drop calculator )

you dont fuse a wire at 17.5 amps unless It can do the job
Otherwise there's no point to adding the fuse at all .
( in fact every point in the harness must hold upwards of 40 amps... for the 1/10 for second a total short will draw )
During this time the wire loss will climb to 10 ~15 %

You can pull 20 amps through 3ft of 14 awg gauge wire at 14 volts
with acceptable voltage drop under 3%

14 ga ...voltage drop of 0.37 volts or 2.66 % loss

16 ga ... voltage drop of 0.48 V or 3.44% loss

18 ga ... voltage drop is 0.77 V or 5.47% loss

You and I probably wouldn't build a custom harness with these losses
but lucas would .

( I'll let someone else figure out converting awg to swg ) ... or counting the strands and diameter .

my statment stands as written , a stock harness will pull those loads ,
It may have to ... if a 20 amp fuse is fitted and blows .
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