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CB and AWG for Garmin Pitot Tube AOA

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mikeeasley(at)aol.com
Guest

Posted: Sun Oct 29, 2017 4:18 am Post subject: CB and AWG for Garmin Pitot Tube AOA

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: 1927
Location: Riley TWP Michigan

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

I agree with you.

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Oct 29, 2017 11:22 am Post subject: CB and AWG for Garmin Pitot Tube AOA

 	  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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