I think we all agree there are no codes.
But standards are another thing - best practices. He just wants to do good work. If a standard says that for headlights it's this size conductor stranded n strands y insulation and slow blow fuse or (what do you call those ceramic things in a car?)... then I think that's what he's looking for.
Wiring a tractor wrong can not only cause downtime and make him look incompetent, it can cause fire and collateral damage. He has a fiduciary duty to do good work.
I think he's being very diligent in looking for standards/ best practices, even though the FBI is not going to come inspect his work.
There are no standards as far as I know other than that set forth by the manufacturer, and they vary from manufacturer to manufacturer. Using headlights for example, the conductor size for the exact same loads will vary not only from maker to maker, but from one type of equipment to the other. It's whatever the engineer decided would work for the least amount of money.
All one needs to do is look at a book with automotive wiring diagrams. The wire sizes are pretty much haphazard and are very small. I repaired many vehicles that burned do to factory wiring being overloaded and catching fire.
The most important thing is to use wiring designed for automotive use. It's not marked. You just have to get it from a decent auto parts store. That wiring is designed with flexibility and vibration resistance in mind. Using any kind of premises wiring in an automotive environment, especially on agricultural equipment, will have not have a long life. Do not use THHN or anything like that.
I can also tell you from experience that crimped connectons don't last long unless they are metered and provide a cold weld type connection. Otherwise soldering is required. I made lots of money years ago fixing failed crimped connectors.
I also hand built many wiring harnesses. If I was a bit unsure of size, I just used the same 310 basics one would used in a factory. That usually provided for a conductor at least a size or two over.
As far as best practices go, that will vary from person to person. I had much higher standards than any other automotive repair facility in the area. I was also the only non-dealership facility that was allowed to do electrical work on dealer owned or owned by the dealer's customers. But all I did was electrical.
Very important is to get all the wiring diagrams you can. I can't stress that enough. If I was working on a something I didn't have a diagram for, I reverse engineered it and drew up my own from what was there. I kept all of them for future reference. I even have a diagram for a 1959 Lincoln with a Skyliner type drop top on microfilm.
In 1990 I started working at a Chrysler dealership where I specialized in electrical, but did whatever it took to get a paycheck. I can tell you from first hand experience that you can throw any idea of standard wiring right out the door. Let's use doors for instance. Between the door and the body of a car is a bunch of wires for the window, mirrors, etc. GM started out using large diameter wires thinking bigger was better. MoPar never did. They always used as small a conductor as possible. I fixed hundreds of the large GM conductors and maybe a handful of the smaller MoPar ones. GM finally realized their error and started using smaller wire.
I have seen and repaired the results of engineers using undersized connectors. Like 1/4 inch spades for headlight control boards and blower motor modules and resistors. Many failures resulting in many design changes. The parameters are always changing, so it's almost impossible to have 'standards.'
