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Inflight load return wire size

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stuart(at)stuarthutchison
Guest

Posted: Mon Jun 13, 2011 12:17 am Post subject: Inflight load return wire size

G'day Bob,



I have an all-metal aircraft without much room for large diameter wires,

especially two.   I recall reading that it's a good idea to have power and

return lines of the same size for small systems, but is it really necessary

to have a return wire to the battery of the same size as the starter wire?

In other words, I have # 2 AWG CCA fatwire from the battery (down aft) to

the starter contactor at the firewall, but I would like to use a # 6 CCA

fatwire for inflight currents?  I was thinking of grounding the engine to

the airframe, but using an # 6 CCA fatwire for return to the battery

(insulated from the airframe - to which normal inflight loads [lights,

avionics etc] would be connected).  Is this idea OK?



Cheers, Stu

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benboatright(at)gmail.com
Guest

Posted: Mon Jun 13, 2011 3:57 am Post subject: Inflight load return wire size

There was a very good article in one of the aviation magazines recently that discussed grounding. The article pointed out that your ground wire should be as large as (or larger) than your positive wire. 



Keeping the size to those rules should minimize and/or eliminate any ground feedback looping.

  

Ben

J. Ben Boatright

-----

benboatright(at)gmail.com (benboatright(at)gmail.com)

-----

Member, AOPA and EAA

 



On Mon, Jun 13, 2011 at 4:13 AM, Stuart Hutchison <stuart(at)stuarthutchison.com.au (stuart(at)stuarthutchison.com.au)> wrote:

[quote]  --> AeroElectric-List message posted by: "Stuart Hutchison" <stuart(at)stuarthutchison.com.au (stuart(at)stuarthutchison.com.au)>

 

 G'day Bob,

 

 I have an all-metal aircraft without much room for large diameter wires,

 especially two.   I recall reading that it's a good idea to have power and

 return lines of the same size for small systems, but is it really necessary

 to have a return wire to the battery of the same size as the starter wire?

 In other words, I have # 2 AWG CCA fatwire from the battery (down aft) to

 the starter contactor at the firewall, but I would like to use a # 6 CCA

 fatwire for inflight currents?  I was thinking of grounding the engine to

 the airframe, but using an # 6 CCA fatwire for return to the battery

 (insulated from the airframe - to which normal inflight loads [lights,

 avionics etc] would be connected).  Is this idea OK?

 

 Cheers, Stu

 

 

 

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 http://forums.matronics.com

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 le, List Admin.

 ="_blank">http://www.matronics.com/contribution

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mrspudandcompany(at)veriz
Guest

Posted: Mon Jun 13, 2011 4:48 am Post subject: Inflight load return wire size

I have an all-metal aircraft without much room for large diameter wires,

especially two.   I recall reading that it's a good idea to have power and

return lines of the same size for small systems, but is it really necessary

to have a return wire to the battery of the same size as the starter wire?

Cheers, Stu



		I own a "tin" T/C aircraft (Cessna Cardinal) and it has the

battery in the rear.  The negative terminal is connected directly to the

airframe with a short cable.  There are no other cables going to the

negative side of the battery.  This seems to work well for Cessna, in that

they use the whole airframe as ground reference.  When you use this method

always be sure that you have a substantial ground strap from the engine to

the airframe.



		Roger

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nuckolls.bob(at)aeroelect
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Posted: Mon Jun 13, 2011 6:19 am Post subject: Inflight load return wire size

At 04:13 AM 6/13/2011, you wrote:

 	  Quote: 	
		  

<stuart(at)stuarthutchison.com.au>



G'day Bob,



I have an all-metal aircraft without much room for large diameter wires,

especially two.   I recall reading that it's a good idea to have power and

return lines of the same size for small systems, but is it really necessary

to have a return wire to the battery of the same size as the starter wire?

In other words, I have # 2 AWG CCA fatwire from the battery (down aft) to

the starter contactor at the firewall, but I would like to use a # 6 CCA

fatwire for inflight currents?  I was thinking of grounding the engine to

the airframe, but using an # 6 CCA fatwire for return to the battery

(insulated from the airframe - to which normal inflight loads [lights,

avionics etc] would be connected).  Is this idea OK?


	


    I'm not getting a good mental image of your proposed ground

    system architecture. How long are the wires in question?

    If you take the approach illustrated in Figure Z-15 View -A-



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z15K1.pdf



    . . . you can consider the option of taking a same-size battery

    ground wire to the single-point, firewall ground -OR- grounding

    the battery locally



http://aeroelectric.com/Pictures/Grounding/Local_Battery_Grounds_1.jpg



    2AWG wire may be somewhat oversized too. Unless you have a

    really big engine or the battery is a long ways away from

    the starter, 4AWG wire might be adequate to the task.



    But I do suggest you review your grounding philosophy in

    light of suggestions offered in Z-15 and chapter 5 of

    the 'Connection.

   Bob . . .

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stuart(at)stuarthutchison
Guest

Posted: Mon Jun 13, 2011 8:00 pm Post subject: Inflight load return wire size

Thanks Bob,



Yes, I have an IO-540 with the starter about 10 feet ahead of the battery.

This configuration is needed A) due to space limitations and B) for weight

and balance.  I understand the need for a big cable to carry start current

and planned to ground this current via the airframe to a local bonding strap

next to the battery.  However, after start, normal inflight currents will

obviously feed via the same big cable to the hot pole of the starter

contactor, then to the busses and I would like to ground via a dedicated

return to the battery post that is only 6 AWG and is insulated from the

airframe.  I don't understand why we need an return cable the same size as

the starter wire when only 60-70A max is passing through this during flight?

Z-15 shows the engine bonding strap connected to a 2 AWG return all the way

to the battery, but I propose not to do this, since high currents are only

required during start and can be grounded via the airframe for that short

period.  I would have thought any noise generated during start would be

acceptable, because it is temporary?  Perhaps different sized power and

return wires cause noise regardless of whether they are both quite able to

carry the current .. I don't know?



Kind regards, Stu



--

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jun 14, 2011 4:40 am Post subject: Inflight load return wire size

At 11:54 PM 6/13/2011, you wrote:

 	  Quote: 	
		  

<stuart(at)stuarthutchison.com.au>



Thanks Bob,



Yes, I have an IO-540 with the starter about 10 feet ahead of the battery.

This configuration is needed A) due to space limitations and B) for weight

and balance.  I understand the need for a big cable to carry start current

and planned to ground this current via the airframe to a local bonding strap

next to the battery.  However, after start, normal inflight currents will

obviously feed via the same big cable to the hot pole of the starter

contactor, then to the busses and I would like to ground via a dedicated

return to the battery post that is only 6 AWG and is insulated from the

airframe.


	


   Ground what? If the engine is grounded to airframe (at

   the firewall ground bus?) and the battery is grounded

   to airframe locally, everything else goes to the firewall

   ground bus. You don't need any wire going back to the

   battery(-) for grounding other things.



 	  Quote: 	
		    I don't understand why we need an return cable the same size as

the starter wire when only 60-70A max is passing through this during flight?

Z-15 shows the engine bonding strap connected to a 2 AWG return all the way

to the battery,


	


    For a plastic airplane that. That pathway CAN be

    eliminated in metal airplanes as described above.



 	  Quote: 	
		    but I propose not to do this, since high currents are only

required during start and can be grounded via the airframe for that short

period.  I would have thought any noise generated during start would be

acceptable, because it is temporary?  Perhaps different sized power and

return wires cause noise regardless of whether they are both quite able to

carry the current .. I don't know?


	


   This is not a noise issue, only cranking performance.

   Do Z-15 with local ground for the battery, single point

   firewall ground for everything else (with exceptions

   for remotely grounded, non-victims noted) and you're

   done.

   Bob . . .

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stuart(at)stuarthutchison
Guest

Posted: Tue Jun 14, 2011 5:30 am Post subject: Inflight load return wire size

OK, thanks Bob.



I seem to recall reading somewhere ... over the past 3 years ... that

grounding the avionics at the firewall in a metal airframe can lead to

multiple grounds with different potentials.  Perhaps I confused this with

grounding the various avionics boxes to the airframe whereever it happened

to be convenient, rather than at a common point.



Kind regards, Stu



--

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jun 14, 2011 6:37 am Post subject: Inflight load return wire size

At 09:26 AM 6/14/2011, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Stuart Hutchison" <stuart(at)stuarthutchison.com.au>



  

 OK, thanks Bob.



 I seem to recall reading somewhere ... over the past 3 years ... that

 grounding the avionics at the firewall in a metal airframe can lead to

 multiple grounds with different potentials.  Perhaps I confused this with

 grounding the various avionics boxes to the airframe whereever it happened

 to be convenient, rather than at a common point. 	


    Yeah. Lots if not most avionics get grounded to

    the airframe locally because their power and signal

    grounds are common to chassis. This is what prompted

    the idea behind the avionics panel ground bus added

    to the Z-figures along with this product to the catalog.



 [img]cid:7.1.0.9.0.20110614102312.01db80f0(at)aeroelectric.com.0[/img] 



    The idea is to take all the potential victims to

    ground in a location as local to the avionics

    package as practical. Ground-loops on the panel

    are short, and VERY weakly excited. Once the

    potential victims are fire-walled, you can

    take the power grounds off the panel to about

    anywhere free of concerns for installation induced

    noise.



    This is not so much about holding big

    risks for noise at bay as it is doing-the-

    best-we-know-how-to-do without jacking up

    cost of ownership. The local ground bus

    on the panel is a great convenience when

    assembling the panel that also goes to

    risk of noise reduction.



    When we put the insulating washers under

    headset and mic jacks, THAT activity beat

    down most of the risk for noises in the

    radios right there. There is still the

    occasional strobe noise issue but those

    are pretty rare now too.



    The point is that after 100 years of

    bolting engines to wings and climbing

    aboard, our recipe for success is pretty

    well refined.



 

   Bob . . .

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