Tony writes...
>I guess I will try to run short lengths of wires at high amperages to see
>whether the wire smokes or the fuse blows. I bet that the smoke limit
>for wiring is well known.
It is, but as Dan points out you need to consider many factors.
The following is from Belden company's technical library:
"The maximum continuous current rating for a electric cable is limited
by conductor size, number of conductors contained within the cable,
maximum temperature rating of the cable, and environmental conditions
such as ambient temperature and air flow."
Since there are so many factors here it gets really hairy to determine
the "smoke limit".
Belden has a chart that will allow you to run some calculations based
on an ambient temperature of 25C. Since I can't draw the chart
here, lemme do some calculations based on a single wire gage so you
can see what's involved. This is for copper wire only.
If we choose a SINGLE #18 awg wire insulated with a common vinyl
<60C. max temp> we can run about 15.2 Amps RMS before we smoke it.
But if that wire is in a harness with 2 other similar wires the rating
drops to 9.5 Amps. If there were 5 wires in the harness the rating is
only 7.6 amps. 10 wires and we can only pass 6.6 amps.
See what's happening here? The wires are all getting hot and their
ability to dissipate their heat is being compromised by the wire<s>
next to them.
Again....these calculations assume that ambient stays at 25C. and that
all the other conductors in the harness are the same gage and are
running at full capacity. Not the real world for sure.
Now...certain PVC insulations can run as hot at 105C. Some Teflon
coated wires can operate well over 200C but at a cost of 8 to ten
times that of PVC, I'm guessing Triumph did *not* use them. :-)
So now you build a wiring harness where we mix conductor sizes and
numbers all over the place as the wires break out to various
terminations. And as Dan pointed out, sections are running near the
hot engine, which gets hotter in Texas than in New England.
An then you toss a ballast wire into the mix for even more heat.
<only some cars> Things are getting complicated. :-(
What kinda' insulation did Triumph use on it's wires? Don't know.
If you knew, could you use Belden's chart to calculate the capacity?
Probably not as Belden's chart uses the gage of the wire as it's size
reference. British wiring uses a system of fixed diameter strands to
size the wire. Dan can give details on this.
Even if you knew all this, you'd almost have to strip the harness
apart to see what wires were physically grouped with others at all
points to determine the real capacity.
> I.E. 5 amp wire carrys 5 amps with no
>warming; when run at 10 amps continuous = hot but no melt;
>15 amps = insulation melt under worst case scenarios.
>Maybe the designers were using a different load limit when picking
>the fuses. Maybe they decided to let the smallest wires get close
>to melting before the fuse blows....
>
>-Tony
You can see that it not just as easy as calculating for the "smallest"
wire. :-)
Cheers!
Tom O'Malley in Southbridge Massachusetts
'74, '77 Spits
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