Al, not Tracy,
but I think what Tracy is saying that in the milliseconds
or so it takes the EC2 to calculate the proper coil
dwell time it is possible for a coil(s) to draw up to 9 amps (or perhaps
even 27 amps in your case). The
instantaneous
power in that case would be 27*12 = 324 watts IF
that current drain remained for as much as 1
second (actually probably less because when cranking you probably would
not have more than 10 volts). However, I believe the duration
of that surge is much shorter duration. IF I am correct then I
suspect your 16 gauge wire is more than adequate because of the very short
time interval in which this happens (I'm guessing that situation does not
last for even 100 milliseconds, Tracy??).
If that
is the case then the power (which is what we are really concerned
about) per second would be 1/10*27*12 = 32.4 watts average power
consumed during that time. The very conservative Power transmission
(bundled) for 16 gauge is 3.7 amps x 12 volts = 44.4 watts. Therefore, IF
the surge is l00 milliseconds duration or less then it would appear you
wiring is more than adquate. Now if the surge condition can
actually last for as long a second then its a different
story.
Ed
Ed;
You’ve got that
right, but I guess I wasn’t really clear on what my concern is. I’m
not necessarily worried about the #16 wire being seriously overheated by
the draws of the coils. I’m worried about being able to safely
protect that circuit against a different failure – like a short – and not
have a condition that will continue to blow the appropriately sized fuse,
or trip a breaker.
On the broader
view, I think a potential spike of close to 70 amps on my engine critical
bus is not acceptable; so I think we need to hear from Tracy before
carrying this any further, and see that we are not off the track
here. And I don’t want to be redesigning my electrical system at
this point.
In my
particular case; the engine critical leads are protected by fuses; for the
reasons John paraphrased from Bob Nuchols; but also for maximum
reliability. The power to my breaker buses goes through the master
switch solenoid. Go directly from the battery(ies) to the engine
critical bus and eliminate a couple of single point failure modes.
My search for data led me to conclude that MTBF for properly sized fuses
is greater than for CBs. I wouldn’t try to convince anyone that one
way is better than another; just that my logic lead me to this
approach. I would assert; however, that there should not be a
circuit in my airplane; unless it is one wire all by itself that could not
damage anything else if it melted, that is not suitably protected against
a possible short circuit. Not having protection in the engine
critical circuits, or having one breaker for all the circuits doesn’t
might that criteria. So there is a fuse for each circuit; pump,
leading ign, trailing ign, primary injectors and secondary
injectors.
Al
I agree,
Al. Always better to make decision like this based on real
information rather than spectulation or back-of-the-envelope
scratchings.
Ed