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  #1  
Old 05-16-2018, 06:28 PM
sjhurlbut sjhurlbut is offline
 
Join Date: Jan 2005
Posts: 590
Default Alt wire - 4 or 6 awg?

In recent post about running battery cables through gear tower on rv8 it was commented that 4 awg is overkill for alternator wire. Alt is B&C standard 60A alt.

In reviewing aeroelectric connection most drawings show 4 awg but there are a couple drawings show 6 awg wire from 60A current limiter to alt.

My personal drawing that I've used in several aircraft is 4 awg.

So for electrical experts can I use 6 awg?
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  #2  
Old 05-16-2018, 07:25 PM
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Walt Walt is offline
 
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Location: Dallas/Ft Worth, TX
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Quote:
Originally Posted by sjhurlbut View Post
In recent post about running battery cables through gear tower on rv8 it was commented that 4 awg is overkill for alternator wire. Alt is B&C standard 60A alt.

In reviewing aeroelectric connection most drawings show 4 awg but there are a couple drawings show 6 awg wire from 60A current limiter to alt.

My personal drawing that I've used in several aircraft is 4 awg.

So for electrical experts can I use 6 awg?
Not sure where you saw 4, but 6 or 8 is generally recommended, 6 for a 60 amp unit and 8 for a 40 amp unit. The alt wire is usually quite short and rarely does an alternator output full design current unless the battery is low while it briefly charges it. A 60 amp airplane is really quite rare these days with glass panels and LED's. Average use is probably less than 20. Pitot heat and seat heaters are about the only thing I can imagine that would increase demand that much. So if you're running in the dark with all the lights, seat heaters and pitot heat on you should probably be looking for somewhere to land really soon.
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  #3  
Old 05-16-2018, 08:42 PM
rv7charlie rv7charlie is offline
 
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Location: Pocahontas MS
Posts: 3,067
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Steve,

Most of the AEC drawings tie the alt B lead to supply side of the starter contactor. That terminal is then fed by the load side of the master contactor. So the alternator and the starter share the same wire, meaning one less wire in the plane. However, that it must be large enough to carry starter current. Is this what you're seeing?
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  #4  
Old 05-18-2018, 08:04 AM
Robert Anglin Robert Anglin is online now
 
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Location: houston, texas
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Default Walt has it.

We did not know for sure if we would end up with a 40,60,or 70 Amp. Alt at the time. We were lucky to have some NAS 5 AWG lift over from an older job. So we used a little of it to do this run. It is up to you, but it never hurts to go one size up on a short run like this that you really need to trust. Kind of like the battery wire. I see a lot of aircraft with 4-O in them and that's working for most, but there are a few that have a lot of drop and heat build-up from an old used starter. I still to this day would rather use 2-O and just eat that little extra weight, just for peace of mind, to know that starter is getting all that the battery can send it even if it is dragging a little. Just what we have found that works for us. Yours, R.E.A. III # 80888
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  #5  
Old 05-18-2018, 09:54 AM
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Walt Walt is offline
 
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Part of the problem with larger gauges on the alternator is the undue stress on the alt. terminal end from the very stiff wire, combined with the engine vibration it will increase the possibility of the rather small unsupported terminal failing.
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EXP Aircraft Services LLC
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Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
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  #6  
Old 05-18-2018, 10:57 AM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by Walt View Post
Part of the problem with larger gauges on the alternator is the undue stress on the alt. terminal end from the very stiff wire, combined with the engine vibration it will increase the possibility of the rather small unsupported terminal failing.
#6 welding cable for the alternator and #2 welding cable for the starter greatly mitigate fatigue failure associated with the stiffer traditional cables.

Carl
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  #7  
Old 05-18-2018, 02:08 PM
Turbo69bird Turbo69bird is offline
 
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Location: CT
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The a&p at our field recently had a EI gauge to install in. A certified.it called for number 4 wire, he ended up going number 6 as far as I know. Doesnt that seem like over kill for a number 4 to an ammeter.
Like the welding cable idea. When I move m battery I think Ill use that.

Id like to move it to improve CG.
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  #8  
Old 05-18-2018, 05:58 PM
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Walt Walt is offline
 
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Quote:
Originally Posted by Turbo69bird View Post
The a&p at our field recently had a EI gauge to install in. A certified.it called for number 4 wire, he ended up going number 6 as far as I know. Doesn’t that seem like over kill for a number 4 to an ammeter.
Like the welding cable idea. When I move m battery I think I’ll use that.

I’d like to move it to improve CG.
The EI manuals I have (availalbe on their website) for their Volt/Amp instruments all say "use the same size wire as the original".
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EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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  #9  
Old 05-21-2018, 06:41 AM
DTARM1 DTARM1 is offline
 
Join Date: Feb 2016
Location: VA
Posts: 7
Default Wire size calculation

Use figure 9-116 on page 9-72, Ch 9 of FAA-H-8083-31 Vol 2. It has all you need to know for the wire sizes that work best for your system.
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