Electrical Firewall Penetrations

[rockitdoc]

I am ready to get wires from FWF to cabin. From what I read, the 'high current' wires should be separated from the 'low current'. I gather the 6AWG wire from the master contactor to the main bus is of the 'high current' variety, but most of the rest of the wires are no bigger than 18AWG (field wires from the alternators, and the +12v power to the P Mags, for example).

So, what constitutes 'low current' and should be separated, or is this really the criteria?

Perhaps, a more appropriate question is which wires should not be co-mingled?

There are 16 wires coming through the firewall as far as I can tell at this point:

1. PMag P leads x 2 (20AWG)
2. PMag +12v leads x 2 (18AWG)
3. Starter Engaged light (22AWG)
4. Main Alternator Offline Light (22AWG)
5. Starter Button (20AWG)
6. Alternator F leads x 2 (18AWG)
7. Hall Effect Current sensor leads x 3 (22AWG)
8. Master Contactor to Main Bus lead (6AWG)
9. Master switch lead (22AWG)
10. Stall horn (20AWG)
11. Baggage light (20AWG)

[rocketman1988]

If you need a ss firewall pass through, I have an unused Avery kit I will make a deal on…

[mburch]

A good rule of thumb would be to separate "wires that measure small voltages" from "wires that carry a lot of current and/or noise".

I assume you will have some kind of engine monitoring, which will involve quite a few of the former kind. I'd bundle those all together, along with the wires for your Hall effect current sensors.

Some of the things you have listed (baggage light, stall horn, etc) aren't exactly in either category, and can probably be run however you like.

[rockitdoc]

Originally Posted by mburch

A good rule of thumb would be to separate "wires that measure small voltages" from "wires that carry a lot of current and/or noise".

I assume you will have some kind of engine monitoring, which will involve quite a few of the former kind. I'd bundle those all together, along with the wires for your Hall effect current sensors.

Some of the things you have listed (baggage light, stall horn, etc) aren't exactly in either category, and can probably be run however you like.

Oh, yeah! I forgot about the 26 wires coming from:
Oil Temp x 1
Oil Press x 3
Fuel Press x 3
Manifold Press x 3
EGT x 8
CHT x 8

Thanks for reminding me, Mike.

[lr172]

Originally Posted by rockitdoc

I am ready to get wires from FWF to cabin. From what I read, the 'high current' wires should be separated from the 'low current'.

In the auto world, they do NOT separate high current and low current wires (starter feed is an exception and sometimes that Alt feed, but still plenty of 30 and 40 amp circuit wires bundled into the same harness as the ECU sensors) and I do not understand why this would be necessary. High current is not synonymous with high noise. Someting like the wires from an old school strobe box is WAY noiser than the 6 or 8 ga wire from the alternator, even if it sending 30 amps. My alt feed to the bus is bundled with all the other wires and have no noise issues, excepting the VERY slight strobe noise that I get on the ground.

You are probably thinking about induced magnetic fields in this situation and there is little on our planes that are affected by this. One exception would be to keep analog audio stuff, like headset wires, away from high current lines. analog audio is a whole different animal. Back in the very old telecom days we called this induced magnetic bleed over, cross talk. The answer was twisted pair wire, which breaks it up into something that has no effect.

Need to be carefull about "what I read" on the internet. Just helped a guy address brake issues. He went to an aviation forum where they recommended DOT 4 brake fluid in RV's. Needless to say, it didn't turn out well, as our systems are designed for oil based fluids and not Glycol based fluids.