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  #1  
Old 04-06-2012, 11:39 AM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
Join Date: Jan 2005
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Default Avionics bus wire size question

Just been going through things one final time and uncovered a question related to how I wired my avionics bus. Particularly, whether my wire size was selected properly.

My Avionics Bus consists of:
Intercom (1A fuse)
Dynon 180 (3A)
Icom A210 (10A)
Power Port-cigarette lighter (5A)
GPS (2A)
Transponder (3A)
Total Avionics Bus = (24A)

For this total I put a 25A fuse on my main bus. At present, I have 8 feet (at most) of 14AWG wire from that 25A fuse to my avionics bus via an avionics master switch. According to the AC43.13-1B continuous flow conductor chart (pg. 11-30), 8 feet of 14AWG will only carry 20A. However, the intermittent flow conductor chart (pg. 11-31) shows that 8 ft of 14AWG will adequately carry about 40A.

In the case of my avionics bus, will my power usage be considered constant or intermittent? Obviously, the big power user, the comm will only be drawing anywhere near its rated power when transmitting, but I'm not sure about any of the others. And, it's been so long since I first wired that I'm not sure what I was thinking at the time. My best guess is that the distance between the main bus and av bus is only 6-7 feet, in which case 14AWG is technically sufficient. However, I like to build a little more conservatively than the bare minimum, so I'm not sure why I didn't at the time.

In short, am I OK as presently wired or do I need to rework something? I can make any necessary changes, either by going up to 12AWG or by removing something from the avionics bus (like the power port), but don't really want to start cutting into my system if it's unwarranted. Just need someone who knows more about electricity than I do.

Thanks.
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Last edited by alpinelakespilot2000 : 04-06-2012 at 11:59 AM.
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  #2  
Old 04-06-2012, 02:28 PM
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shuttle shuttle is offline
 
Join Date: Dec 2006
Location: UK
Posts: 342
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Hi Steve,

Have a look at the Aeroelectric wire size reference http://www.aeroelectric.com/articles/wiresize.pdf

According to this, just 15 amps will induce a 35 deg C tempurature rise in a 14AWG wire.

By my sums, at 24 amps you'll get 0.48V drop over your 8ft of 14AWG cable, plus you'll need to add the drop caused by the remainder of to wire to each device.

I'm using a 10AWG for a similar requirement.

Steve
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RV-7 G-HUTY (not flying yet)
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  #3  
Old 04-06-2012, 04:24 PM
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Walt Walt is online now
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 3,175
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Quote:
Originally Posted by alpinelakespilot2000 View Post
Just been going through things one final time and uncovered a question related to how I wired my avionics bus. Particularly, whether my wire size was selected properly.

My Avionics Bus consists of:
Intercom (1A fuse)
Dynon 180 (3A)
Icom A210 (10A)
Power Port-cigarette lighter (5A)
GPS (2A)
Transponder (3A)
Total Avionics Bus = (24A)

For this total I put a 25A fuse on my main bus. At present, I have 8 feet (at most) of 14AWG wire from that 25A fuse to my avionics bus via an avionics master switch. According to the AC43.13-1B continuous flow conductor chart (pg. 11-30), 8 feet of 14AWG will only carry 20A. However, the intermittent flow conductor chart (pg. 11-31) shows that 8 ft of 14AWG will adequately carry about 40A.

In the case of my avionics bus, will my power usage be considered constant or intermittent? Obviously, the big power user, the comm will only be drawing anywhere near its rated power when transmitting, but I'm not sure about any of the others. And, it's been so long since I first wired that I'm not sure what I was thinking at the time. My best guess is that the distance between the main bus and av bus is only 6-7 feet, in which case 14AWG is technically sufficient. However, I like to build a little more conservatively than the bare minimum, so I'm not sure why I didn't at the time.

In short, am I OK as presently wired or do I need to rework something? I can make any necessary changes, either by going up to 12AWG or by removing something from the avionics bus (like the power port), but don't really want to start cutting into my system if it's unwarranted. Just need someone who knows more about electricity than I do.

Thanks.
In reality here are some numbers (not fuse sizes):
Intercomm 1 amp
Dynon 1 amp
Icom 1 amp (xmit 5-6 amps)
PP - 1-2 amps maybe (if being used)
GPS 2 amp (not sure what you have here)
Xpdr 1 amp (old ones maybe 2 amps)

Total: 8 amps continuous, maybe 14 intermittent.

If you agree with these numbers you should be fine.
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