Twisted Pairs and Wire Consumption

Is there any ballpark rule of thumb estimate for how much extra wire a twisted pair consumes as opposed to a straight run? For example, if I have a 100mm run from the main trunk to an injector, will I need to use 20% more wire (120mm)? Or is it just kind of eyeballed?

I used to add 3-6" to my measurements, and sometimes this was a little close. I've switched to using the 20% suggested in the professional motorsports wiring (that is for a concentric twist), and it's a lot less stressful to cut a little extra wire, than to worry you're going to have to add a splice near the end or worse yet, not have a wire in a bundle come out the end as you put sheathing over it (and you later say "where did that xxx wire go?"). I would say if I was just twisting a pair or wires, I would use 10% or 6-10" whichever is larger.

100mm is about 4", so I would probably make that wire at least 200mm (8").

All good points. I would think 20% for an additional fudge factor + 20% for the twist, 140mm in this case?

It depends on the absolute length as well.

For a concentric twist loom I usually add 25% and while that works nicely for lenghts of 1000mm and above I find that with shorter lengths I prefer using 40%. In your case where you have 100mm target length, I would cut 140mm ish. Always better to cut extra wire than to worry about being too short and having to splice on extra wire in the end.

1m of 22 AWG Tefzel wire is about 0.4 €, so cutting too much in some cases does not hurt that much. And you can always use what you cut in other places where you only need short lenghts, like for splicing of filler wires. I always keep what I cut off and re-use where applicable.

Coming back to your question: 140mm sounds about right, if you add service loops, add another 10-15mm.

Thanks to both of you.

FWIW, after a little experimentation I found it was about 7% for a pair and 10% for a triple, both at a lay length of ~12 with 20AWG TXL. Cutting a few wires I needed to do for my injector and coil sublooms, I found that cutting 130% left me plenty on the ends for a fudge factor.

I wasn't planning on twisting more than a few ends and subharnesses (no digital comms in this loom), but after experimenting I think I prefer the straight lay anyway.