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Wiring Harness Length - Rules of Thumb

cjf154

Member
I'm working on my avionics interconnect diagram and electrical system design. The shear number of wires that will need to be cut and run is pretty daunting. My plan is to create a spreadsheet then measure and record point to point each of my runs in the airplane. With this info in hand I will then be able to cut and label each wire that is needed for all the wiring harnesses.

For example
Point A Point B Length
Wing tip Switch Panel XX Ft YY In
GEA 24 PFD1 15 in
etc

3 Questions:
Do you have a rule of thumb for additional length you add to your runs for service loops, wiring striping, and other slop?

Is this a good approach? I'm trying to be principled in my work versus just diving in and hacking it together.

Is there a good how to guide on building wiring harnesses?
 
Wiring

I spent a career in Telecom. Service loops were a priority. We didn't worry much about length estimates because we weren't paying the bill. Funny now how much it matters. Tefzel is pricey stuff. I use a sample wire. It's a piece of Cat-5 snaked where the majority of the wires will go. Funny how all those little turns add up. I know it's long enough plus extra for a service loop. I tape the end to the roll and match the length. Cut. Label.
 
Thumbs up for service loops. Have worked on enough panels where components like EFIS screens didn't allow enough movement to get a hand in behind to undo the Dsub connectors. (one just yesterday! grumble grumble)
 
Thank you all. Based on what you're saying this is probably pretty straight forward and I'm making it more difficult than it needs to be. I'll overestimate the length of the wire needed, label the wire and then cut it to fit once I've run it.
 
If I only had the skill...

I will not disagree, assuming I possessed the skills to pull it off, that it would be great to have a list of all wires required and their length. That would be sweet.

In reality, every wire run will end up being a compromise based on available holes, conflict with control linkages etc. And since you're only building one (assuming), the effort to creat a list might be more work than the wiring.

Offering a little advice:

First get all your components mounted in their final locations.
Then choose your favored route for all the bundles based on available holes / length etc. Route may change as you add wires and fill up holes, so be ready to flex.
Identify the size wire required for each component. Get a close estimate of the total lengths required then order about 25% more.
Lastly start running. Like everyone has recommended, leave plenty at the terminus for service loops and access while adding your terminals and plugs etc.

I made good use of those metal spring paper clips to hold wires together on runs. This will help you keep them together and not round corners (making the wire shorter than you need).

When everything is hooked up and powered, test ALL your components. Now is the time to find an issue before the wire tying is done.

Organize your wiring tools on a cart. Mount wires on spools for easy pulling. Have all your supplies and component schematics on hand and ready to go.

I actually found the wiring to be pretty fun. Challenging but fun.

Good luck! :cool:
 

Attachments

  • locations.pdf
    7.2 MB · Views: 311
  • wiretie.pdf
    7.6 MB · Views: 346
  • fuses.pdf
    6.9 MB · Views: 311
I will not disagree, assuming I possessed the skills to pull it off, that it would be great to have a list of all wires required and their length. That would be sweet.

In reality, every wire run will end up being a compromise based on available holes, conflict with control linkages etc. And since you're only building one (assuming), the effort to creat a list might be more work than the wiring.

Offering a little advice:

First get all your components mounted in their final locations.
Then choose your favored route for all the bundles based on available holes / length etc. Route may change as you add wires and fill up holes, so be ready to flex.
Identify the size wire required for each component. Get a close estimate of the total lengths required then order about 25% more.
Lastly start running. Like everyone has recommended, leave plenty at the terminus for service loops and access while adding your terminals and plugs etc.

I made good use of those metal spring paper clips to hold wires together on runs. This will help you keep them together and not round corners (making the wire shorter than you need).

When everything is hooked up and powered, test ALL your components. Now is the time to find an issue before the wire tying is done.

Organize your wiring tools on a cart. Mount wires on spools for easy pulling. Have all your supplies and component schematics on hand and ready to go.

I actually found the wiring to be pretty fun. Challenging but fun.

Good luck! :cool:

Not sure what you did when uploading the photos but on my computer they try to download when clicking on the thumbnails instead of displaying in a new tab...
 
Hmmm...

My browser offers options to open in tab or save ...

Think this is the way I've done it recently
 
Not sure what you did when uploading the photos but on my computer they try to download when clicking on the thumbnails instead of displaying in a new tab...

I have the same experience. I think it may be because the attachments are PDFs rather than JPEGs.
 
paper clips

... I made good use of those metal spring paper clips to hold wires together on runs. This will help you keep them together and not round corners (making the wire shorter than you need). ...
Good idea - I used "temp" zip ties but this looks much easier and it holds the wires in the right place.
 
But, the fuselage and Wing aren't connected yet!

Very beginning of wiring the wing. The fuselage is down the road in storage. I can add a length of wire at the tip to try and take care of an installation loop out there, but how much should I leave at the root to reach to the power? Assume everything has to go via the approved routing to a place behind the instrument panel, plus of course some fudge. Are we talking 6ft? 10ft? Specifically asking about wire length beyond the wing root's last inner rib where the conduit ends.
 
I used velcro straps to hold the harness together. Get short straps if you're going to use them - mine are 8" and the extra velcro touches other straps and is a pain.

Also, I tried the spreadsheet route and it was way too much of a pain. I did like Wirejock did and that worked a lot better. Plus I bought extra wire. Wire's cheap, considering.

Dave
 
When everything is hooked up and powered, test ALL your components. Now is the time to find an issue before the wire tying is done.

I agree, but would also add: test each subsystem as you go, preferably as you add each component to it, building up your tested system from smaller verified components.

I've seen a few guys who wired everything up and then did one big "here she goes!" moment, followed very quickly by disappointment as several (or more) things failed to work properly, communicate with each other, etc.
 
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