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CB and AWG for Garmin Pitot Tube AOA

 
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mikeeasley(at)aol.com
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PostPosted: Sun Oct 29, 2017 4:18 am    Post subject: CB and AWG for Garmin Pitot Tube AOA Reply with quote

I’m replacing my standard heated pitot tube with the Garmin heated/unregulated pitot tube/AOA. My existing wiring is 15A CB, 14 AWG, 12V system. That’s based on the 100W pitot tube heater, so 8A. Garmin wants a 20A CB and 10 AWG based on the length of the run, p.61 of the G3X Install PDF. On p. 33, Garmin gives some amperages at different temperatures, 12A (at) -40C, 9.25A (at) 0C, 7.3A (at)50C, 5.85A (at) 100C and 4.36A (at) 175C. It’s labeled “Initial Current Draw vs Probe Temperature”.
So my question is do I really need to run 10 AWG wire and use a 20A CB with this pitot tube? It seems like the tube might pull more than 8A briefly during warmup when it’s really cold, but then it will be at or below 8A once it’s warm to the touch. Due to the voltage drop at 12A it might take a few more seconds to warm up. I’m thinking my existing wiring is adequate for the new pitot tube.
Thanks for the help.


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user9253



Joined: 28 Mar 2008
Posts: 1907
Location: Riley TWP Michigan

PostPosted: Sun Oct 29, 2017 8:36 am    Post subject: Re: CB and AWG for Garmin Pitot Tube AOA Reply with quote

I agree with you.

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nuckolls.bob(at)aeroelect
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PostPosted: Sun Oct 29, 2017 11:22 am    Post subject: CB and AWG for Garmin Pitot Tube AOA Reply with quote

Quote:
I’m replacing my standard heated pitot tube with the Garmin heated/unregulated pitot tube/AOA. My existing wiring is 15A CB, 14 AWG, 12V system. That’s based on the 100W pitot tube heater, so 8A. Garmin wants a 20A CB and 10 AWG based on the length of the run, p.61 of the G3X Install PDF. On p. 33, Garmin gives some amperages at different temperatures, 12A (at) -40C, 9.25A (at) 0C, 7.3A (at)50C, 5.85A (at) 100C and 4.36A (at) 175C. It’s labeled “Initial Current Draw vs Probe Temperature”.

So my question is do I really need to run 10 AWG wire and use a 20A CB with this pitot tube? It seems like the tube might pull more than 8A briefly during warmup when it’s really cold, but then it will be at or below 8A once it’s warm to the touch. Due to the voltage drop at 12A it might take a few more seconds to warm up. I’m thinking my existing wiring is adequate for the new pitot tube.

Thanks for the help.

Pitot tube heaters have a very strong, negative
temperature coefficient. See:

http://tinyurl.com/y78oc7z3

Furhter, the warm-up time constant can be pretty
slow . . . 20 seconds or more. Hence, breaker
and wiring sizes for pitot tubes might seem
overly conservative.

Having said that, the words you quoted from
the Garmin manual seem quite clear. A cold
start from -40C (a temperature I hope you
never have to fly in), the rated peak current
is 12A, well inside the operating enevelope
for your 15A/12AWG configuration.

I think your risks for leaving it as-is are
low. BTW, 8Amps at 12v is 100 watts . . . under
that steady state condtion, the tube will be
considerably hotter than 'warm to touch'.

Keep in mind that pitot tubes are supposed to
shed accumulated ice at altitude (sub zero
temps)while being 'forced air cooled' by
TAS. A pitot tube on a Beechjet cruising
41K feet will have a clear air surface temperature
of over 100C!


Bob . . .


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