[Farmall] Farmall B wiring

[szabelsk at gdls.com]

Craig,

When rewiring you need to keep in mind the current that the wire will have 
to carry when the tractor is running. This varies by wire size based on 
the temperature rating of the wire itself (actually the rating of the wire 
insulation). There is no need to wire everything to a larger size wire if 
it doesn't carry a lot of current, or carries a current only for a short 
time. When I redid my Cub a few years back, I used 12 AWG for everything 
dealing with the starter /generator circuit, and 16 AWG for everything 
else (lights, etc). This simplified what I had to be worried about. 
Usually length would also factor into the equation, but on tractors, those 
lengths aren't usually long enough to consider. In addition, there are 
factors related to bundling (number of wires grouped together) and whether 
or not you're in free air, conduit, or in a sleeve of some type. It can 
all get really complicated if you let it.

The following table shows the allowable currents for various wire sizes 
and insulation ratings. I also included the approximate wire diameters 
(for the wire without any insulation).

Wire                    Continuous Duty Current (Wires In Groups, Bundles, 
Or Harnesses) 
Gauge                   105oC                   150oC 200oC Wire Diameter 
- Inch
                        Insulation              Insulation Insulation 
(Copper Conductor Only)

22 AWG          3A                      5A                      6A  0.084
20 AWG          4A                      7A                      9A  0.094
16 AWG          7A                      11A                     14A  0.120
12 AWG          13A                     19A                     25A  0.157
10 AWG          17A                     26A                     32A  0.181
8 AWG                   38A                     57A 71A 0.248
6 AWG                   50A                     76A 97A 0.293
4 AWG                   68A                     103A 133A 0.355
2 AWG                   95A                     141A 179A 0.420

Form the above table, you can see that you can use a smaller gauge wire in 
a given instance, if the wire insulation has a higher rating so that it 
can handle the additional heat caused by the current flowing through it (a 
12 AWG with 200C insulation can carry more than a 10 AWG with 105C 
insulation).  Since engines have a tendency to generate heat, I would 
suggest using a high temperature wire.

With respect to bundling the wires, any good shrink sleeve should do, 
provided it also has a temperature rating in line with the environment 
it's going to used in. If you pick a sleeving material that isn't right 
for the temperature of the environment, it will either melt or split. Most 
shrink sleeving has a two to one shrink ratio, so calculate the bundle 
diameter and pick a sleeving that is roughly twice the calculated 
diameter. Overlap the sleeving by roughly an inch and apply a layer of 
silicone at that point before you shrink it down to size. This applies to 
where you need to make branches as well.  That will help keep water out. 
Also use a silicone at the ends of the sleeving for the same purpose. 

A simple way to build your new harness is to remove the old one and lay it 
out flat (mark all the ends with tape as well as the corresponding 
terminals when you disconnect them). Sketch the harness on a piece of 
paper and record the lengths of each branch. Add in whatever you think 
needs to be added, based on what you saw of the original harness 
installation. After you cut your wires to length, loosely assemble them 
into the harness and tie them together with string, roughly every 12 to 18 
inches, and about 2 inches from where a branch needs to break out of the 
main harness (remember that you will want to be able to overlap the branch 
sleeving, so the tie point needs to be done before the branch breaks out). 
This will keep the wires all nicely lined up and will help avoid things 
getting tangled up. Once you have it tied together, you can lay it in 
place on the tractor and make whatever adjustments you like before you 
sleeve it. This is were making the individual wires longer than what is 
necessary a good idea. After you lay the harness into position, you can 
trim the wires to their required lengths. Keep in mind to leave enough 
length for crimping terminals and the position the wire needs to be in to 
make the connection. If you can, sleeve the harness while it's on the 
tractor. That way you won't have to worry about things moving on you and a 
branch winding up too short.

Hope this helps.

Carl Szabelski



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