>I am adding Mitchell electric oil temp and pressure gauge.
>Mitchell states these each pull .8 amps. I would prefer
>to protect both these gauges with one CB or in-line fuse.
>Is there not a rule of thumb to determine CB size for a
>particular load? I am assuming a 2 amp breaker would be
>adequate for this application (total of 1.6 amp load).
There's a popular mis-conception floating around suggesting
that breakers protect gizmos while in fact, a breaker or
fuse has the task of protecting only the wire which
branches off the distribution bus. The smallest practical
wire for airframe installations is 22AWG. Per the wire
table I published earlier this week, a 22AWG is good for
5amps in free air . . . (OOPS, just checked the table and
found I'd left out a couple of values. Here's the table
with the two missing numbers)
AWG Ohms/ 35C Rise 10C Rise Max Path
No. KFeet Amps Amps for .7 volt Loss
at 35C rating.
2 .156 100 54 45 Ft
4 .249 72 40 39 Ft
6 .395 54 30 32 Ft
8 .628 40 20 27 Ft
10 .999 30 15 23 Ft
12 1.59 20 12.5 22 Ft
14 2.53 15 10 18 Ft
16 4.01 12.5 7 14 Ft
18 6.39 10 5 11 Ft
20 10.2 7 4 10 Ft
22 16.1 5 3 8 Ft
So our 22AWG wire is good for 3 amps burried in
a wire bundle. In the suituation where we
intend to drive 1.6 amps of hardware, 22AWG
is adequate in any installation scenario. Further,
since the CONTINUOUS load is not over 3 amps,
we could burry the wire in a bundle and still
PROTECT it with a 5 amp breaker since any expected
overload would most likely be a hard fault. There's
no need to de-rate the wire for protection purposes
because the continuous duty load is so small.
In answer to the writer's question, any breaker
from 2 to 5 amps is okay in this situation. I'm
a little suspicious of the .8 amp figures for
power to operate those instruments; here's why.
.8 amps times 14 volts is over 11 watts. Since there's
no ENERGY coming out of this system other that to wiggle
some pointers on an insturment, ALL of this power comes
off as heat. Which gizmos in each system disipate all
or any part of this much energy? Certainly not the
instruments . . . anything more than about 2-3 watts
inside a plastic case would COOK the innards. How
about the transducers? Hmmmm . . . I suppose they
COULD stand that kind of dissipation, they're designed
to function in a warm environment. But when the
rest of the world routinely produces sensors that
operate with tens of milliamps, I'm having trouble
visualizing how the thing would be build to consume
10 times that amount of power.
I'd recommend rechecking the literature -or- contacting
the manufacturer -or- hooking them up on the bench
with a power supply and see what the real numbers
are. I SUSPECT they are considerably less.
This doesn't change the breaker sizing task but it
does afford more confidence in the system's ability
to survive . . . 11 watts is alot of snort to handle
in a task that's been seeing smaller and smaller
power requirements through the evolution of new
designs.
Regards,
Bob . . .
AeroElectric Connection
////
(o o)
| |
| Go ahead, make my day . . . |
| Show me where I'm wrong. |
72770.552@compuserve.com
http://www.aeroelectric.com
|