Can you tell me what size wire I need, or how many amps the solenoid
   for a
   Mazda starter draws?



   Sandy McNabb

   Renesis in a Glass Goose

   As near as I can tell the the resistance of the solonoid is
   about .8 ohm.

   So the current will be about 15 AMPs. At 12 volts. V/R = A
   http://www.powerstream.com/Wire_Size.htm

   Lookslike about 10 gage stranded wire.

   Paul Lamar


   10 gauge is way overkill for a solenoid, which is intermittently
   used.The chart is for NEC code, which is continuous loading,
   which relies on max bundle size for wiring. Hence 14 gauge
   for 15 amp breakers (lighting loads) and 12 gauge for wall
   sockets (20 Amp breakers).

   14 gauge would be sufficient for solenoid use.I would be
   surprised if Mazda used anything larger.

   Bob Knuckles, Aeroelectric, puts out a pretty good book
   on aircraft wiring that I would recommend.

   Tom W

   -----------------------------------------------

   Tom W,

   I learned how to use my multi-meter to measure amps. The starter
   solenoid draws 8.14 amps so I'm considering an 18 AWG wire with a
   10 amp fuse....or should I go higher?

   Sandy McNabb
   Renesis in a Glass Goose

   Sandy, what are you gaining by using smaller wire?

   What you lose is a tiny bit of power to the solenoid.

   Starters are around 3 HP so 190 amps so the solenoid
   current is minuscule by comparison.

   Paul Lamar


   Is this wire kust to activate the solonoid, or carry the current
   to the Starter motor ? Virg

   Just to activate the solonoid.

   Paul Lamar



     Then 18/16 Ga should be enough, Virg

  I don't think 18 is acceptable.

  10 gage is .1 inch in diameter and 16 gage is .05

  inch in diameter what are you buying by scrimping
  on wire size?

  12 gage is .08 inches in diameter.

  http://www.powerstream.com/Wire_Size.htm


  Paul Lamar



A couple of thoughts:

  - The starter relay/solenoid coil presents an inductive load on the
controlling switch.  An 8A inductive load will require a fairly good
sized switch if you wish for longevity.  One of the pushbutton starter
switches might be appropriate.

  - As I have mentioned before, when the switch controlling the coil is
opened, the magnetic field in through the coil collapses, trying to
maintain the current flow through the coil.  Since the switch has been
opened, the only thing that can happen is the voltage across the
switch contacts will rise.  On a small switch, this will cause
arcing/welding of the contacts.

  - It would be good practice to wire a diode across the solenoid
contacts.  Doing so will make life on the switch easier by eliminating
the uncontrolled rising voltage mentioned in the previous point.

  - Using a small relay to control the solenoid would allow a
microswitch to operate the system - instead of the big pushbutton.
Not a big deal either way.  Just depends on what switches you want on
your instrument panel.

  - There should be a continuous duty master relay between the starter
solenoid and the battery.  A car doesn't usually include something
like this - the battery is often wired directly to the starter
relay/solenoid.  A stuck starter relay on an airplane can be very
dangerous/expensive.  The battery relay can be used to shut down the
starter in such a situation.  Further, an annunciator light should be
used to indicate when power is applied to the starter motor.

  - As someone else mentioned, continuous duty temperature rise is one
concern when deciding on wire size.  The other consideration is how
loop voltage drop may impact circuit performance.  One can relatively
easily calculate voltage drop based on the length of the wire run, the
resistance per foot of the wire (ohms), and the current through the
loop.

18g wire is on the order of .00751ohm/foot...  A 20 foot round trip
(including 18g ground wire) would have 0.1502 ohm resistance.  V=IR =
8A * 0.15ohm = 1.2V dropped.  So, if the battery voltage drops to 10V
while cranking, there'll be 8.8V delivered to the solenoid.  Probably
plenty to keep it functioning, but it's worth thinking about.  The
size of the fat wire between the battery and the starter is the bigger
concern, having direct impact on how well the engine cranks.

  - If the battery is located close to the starter, using a small relay
(also close to the starter) can essentially eliminate the voltage drop
to the solenoid.  20g or even 22g wire would be plenty adequate to
control the relay.

  - Making every wire _big_ will eventually add measurable weight to
the airplane.  In an airplane with a complex electrical system, this
could become substantial.  A bit of design/analysis up front can help
eliminate unnecessary wiring weight.  The relevant concepts aren't
hard to figure out.

Here's a pretty good article that talks about wire size:

http://www.epanorama.net/documents/wiring/wire_resistance.html

I too recommend Robert Nuckolls and his Aeroelectric book and list as
a great resource for figuring out everything from electrical
architecture to figuring out wire size.  Subscribe to the forum he
participates in via http://www.matronics.com/subscribe/ and explore
his website for a variety of resources and interesting articles:
http://aeroelectric.com/


Regards,

Matt-


Sandy must really be confused by now!

I did find the wiring size article by Bob Nuckolls (I have a
heck of time remembering
how he spells his name!).

http://www.aeroelectric.com/articles/wiresize.pdf

For aircraft I try to stick with the 10 degree C temp rise,
and his recommendation is
14 gauge.

I've got some solenoids from Aircraft Spruce, and will
run the old HP power supply
to see when they close, and open, voltage wise tonight.

Tom