Wiring from Scratch

[SDA]

I’m wiring my T3 project bike from scratch and my wire size calculations are wildly off of what I see posted here. Hoping someone can steer me in the right direction as I fear I’m missing something.

My maximum allowable voltage drops for lighting circuits of 0.2v and starter circuit of 0.8v come from Tracy Martin’s book “Motorcycle Electrical Systems”

For instance:
4’ long low beam circuit-55w/3.9A@13.8v=20gauge for a 0.17v/1.2% voltage drop
2’ long ignition circuit-95w/7.2A@13.8v=20gauge for a0.15v/1.1% voltage drop
But the lulu is the starter circuit-remember the 4x allowable voltage drop of 0.8v
2’ long starter circuit-1200W/87A@13.8V=16gauge for a 0.71v/5.2% voltage drop!
Even if I want to keep the voltage drop below 0.2v like the lighting circuits it still only requires #10 wire!

I checked the above ignition circuit by wiring up a couple of ignition coils and the voltage drop was spot-on per the calculations.

I know I’m geeking out here but if I’m going to do this I might as well make it as light and small as possible.

I’ve attached my ohm calculator spreadsheet hoping that someone can either validate this or show me where I’m fixin’ to screw up.

 

[jebdog]

Very interesting. I'd have to dig out my old text books from airframe and powerplant school to check your numbers. I haven't had to calculate wire sizes in years. Off the top, it looks OK. Are you going at absolute minimum requirements or have you built in a safety factor like 20%, for example? You'd be better off carrying a little extra weight than too few watts! Or overheating a circuit.

Just asking

What's with all the blank worksheets in the Excel file?

Jim

 

[OldJock]

Hi SDA

You're way over thinking it IMHO. The more detail you go into the worse its going to get and you'll be chasing a moving target. If you must do that then just conduct some ad Hoc experiments and base your ratings on them. Volts drop will vary with heat, conductor length, CSA etc so the more amps you carry the higher the resistivity is going to get and that's before you start on factoring in switches, connectors etc: it will become an exercise in futility.

I redid mine with Thinwall cable and made up a custom loom, no smoke so far

All I did was look at load ratings for the cables and generally used 1/2 of the max cable rating for the current it would be required to carry. Most is 0.5mm^2 for the small stuff and 2.0mm^2 for the heavier loads and 3.0mm^2 for some of the charging circuit and main feeds. Thinwall has a far higher current carrying capacity than normal PVC cable and is what's used nowadays in vehicles.

For the starter I used 16mm^2 extra flexible and got the cables made up for me.

I agree with Jim build in a good factor of safety and if in doubt go up a size.

Just what I did

John

 

[SDA]

Thanks Jim & John for your responses,

Adding a 20% wattage hedge factor keeps me at the same wire size on ½ of the circuits and requires an up-sizing in the other half.

I think the starter circuit is an exception as it is of value to spin the engine as fast as possible so certainly #10 there to reduce voltage drop from 0.71v down to 0.18v. Upsizing from #10 to #4 to reduce drop down to 0.04v is something I’ll try but can’t imagine 14 hundredths of a volt making any difference. But I’ve also gotta try the #16—after using big, thick cables for 40 years and just “knowing” that that was required, reading an acceptable voltage drop on a #16 while turning over an 850 V-twin would just be weird!

Jim, I don’t know what’s with excel opening all those sheets. I’m just using the 1st one.

John I do like your thinking but I’m not trying to get it exactly right for the sake of it being right, I’m just interested in how this all works in actual practice. Like I said, I’m just geeking out. Thanks for reminding me about heat. I’ll heat up an experimental circuit and compare resistivity and voltage drop and add that to my hedge factor—or are there published resistance figures at varying temperatures?

I’m could go on and on but am going to stop before you all write me off as a complete loss!

Stephan

 

[jebdog]

Hi Stephan,

It's fun to geek-out sometimes just for the heck of it. I like the spread sheet. I pulled down a copy and saved it for future use. John has a good point about heat. That will add resistance as the temperature rises. My background is aircraft. Weight is an important consideration on them too. The common practice there is to have open wiring runs. Are you familiar with that? The bundle is not in a conduit or sleeve or jacket. It is tied together every few inches. This is to reduce weight and to keep the wiring cooler, allowing use of a smaller gauge wire.

You may well choose to protect your bundle with a jacket or flex conduit. If so you may want to favor a thicker wire, particularly on circuits that will remain on, like lights. They will get warmer over time.

Now I am geeking - out too!

 

[jebdog]

This is probably over board but here is some guidance on building up aircraft wiring. Starting about page 23 are the issures we're discussing. Followed by formulas that you don't need, there are some charts that show wire wire size, current carrying capacity and length.


http://rgl.faa.gov/Regulatory_and_G...lar.nsf/list/AC 43.13-1B/$FILE/Chapter 11.pdf

Cheers,

Jim