The Zen of Heat or, What, me worry?.

General Information:

What does SAE mean? It's the abbreviation for the Society of Automotive Engineers which is the American group that establishes and maintains the standards for American automotive components and manufacturing. Corresponding groups are the ISO and DIN (German), for Europe, and JIS, for Japan.

The reason it is important to us is that they have tested the materials and processes used to manufacture wire, and determined which wire materials will work and which will fail in automotive applications.

Our job is to use the information these groups have provided to select the best wire and other components for our specific application. It's not particularly difficult, we simply have to be aware that there are varying levels of temperature and mechanical toughness, and balance cost vs. performance.
The American specs covering automotive wire are SAE J 1127 and J-1128, and specify material and finished temperature and mechanical behaviors of the wire. The European spec is ISO 6722, the Japanese is JASO D608-92. Generally, if a wire is listed by one group for an application, it is acceptable for that application by the other two.

Whether you're preparing a race car, or wiring a street machine,
your main wiring concerns are:

safety, reliability, & serviceability,
in that order.

The most important factors affecting safety, reliability, and serviceability are:
1. heat damage,
2. mechanical damage, and
3. chemical damage.

Of these factors, heat damage is far and away your biggest worry.

1: Engine Compartments....Can the wire take the Heat?

High temperature beyond the wire's rating can directly damage any insulation as well as make it much more susceptible to the other kinds of damage mentioned above, and so the temperature rating of the insulation is the first thing for you to look at.

Under hood temperatures in a car can reach well in excess of 220° F, or 105°C, even though you are keeping a respectful distance from the exhaust manifolds. Not too surprising since the recommended 50/50 antifreeze mixture raises the boiling point of the coolant to 265°F/130°C, and the system is normally pressurized, which raises the temperature even more. (Check the bottle label the next time you're adding antifreeze.)

So there are two different insulations specifically designed for
automotive use. hese insulations as well as other factors are
specified in the aforementioned automotive standards.

Now this business of technical specs may be a big yawn-provoker for you, but keep in mind that if you are buying wire that is not labeled as conforming to these specs, which is usually the case in your local automotive parts store, you are spending good money and your valuable time taking an unnecessary risk with a demonstrably inferior product.

Using wire conforming to these specifications means that you benefit from the experience and testing done by both the automotive manufacturers and the standards organizations. And the cost is typically not that different.
In fact, most often the price of a low-temperature, flimsy wire wire at your local hardware or auto parts store is more than you would pay for the better wire here at kayjayco. (We did go out and do some market research, and this was one of the factors we considered in setting our prices.)

Sometimes the suppliers to the auto parts retailers will print
"meets SAE J-1128" on their wire, in combination with AWG, the
abbreviation for American Wire Gauge.
And SAE and AW Gauges are different.
Makes you wonder if they really know what they're talking about.
And of course unless their label explicitly states which SAE J-1128 specification they are meeting, ( TXL,GXL, etc. ) you may be sure that it is the cheaper low-temperature stuff , ( GPT ), or most likely, worse.

2: Flying Rocks....Can the wire take the Beat(ing)?

The next thing is mechanical damage, which is where you should consider
the thickness of the insulation.

There are thin, medium, and thick insulations available in both of the
standard 185°F/85°C and 257°F/125°C temperature ratings. The thicker
insulations are simply more rugged.
Any wiring in an off-road vehicle that has any risk of being
hit by flying rock or other debris should use both the heaviest
insulation available and a protective covering, whereas a race car
would go with the thin insulation to saveweight.

3: Last and least ....Can the wire take the DEETing?

Chemical damage is less of an issue in most circumstances.
Even brake fluid won't normally bother modern wire insulations,
but some chlorinated solvents will aggressively attack PVC insulation.
( DEET (Chemical name N,N-diethyl-3-methylbenzamide) is listed by a JAMA study as "the most effecting insect repellent", 301 minutes vs 96 for the nearest competitor. I don't know whether the wire will resist DEET or not, just thought the info might be useful. ( And, it's all I could think of that rhymed.)

OK, So.......? 

One approach you can take is to get the wire with the toughest,
highest temperature insulation available and use it, for example a Mil spec 22759, Kapton insulated, tin-plated fine conductor wire. It's a tough, lightweight, readily available aviation wire, although available in white only. This will work just fine, but there are downsides: safety in an accident, cost, difficulty installation, and reduced serviceability. The safety issue relates to using Kapton insulated wire, which was the insulation of choice for commercial and military aircraft back in the eightys. An Aussie study found that under "hard" short circuit conditions, such as might be experienced in a crash, the Kapton "explodes in a ball of flame". There's a lot of the stuff available, but the Navy for example has replaced Kapton wire in most aviation applications for this reason. So one must choose carefully even using this approach; there are wire insulations available that resolve this problem, but they're hard to obtain; if you really want to pursue this call or email me, (see the company info pages). I'll touch on the other issues again later.

There are two basic types of insulation covered in these specs:
Polyvinyl Chloride, (PVC ), and Cross-linked Polyethylene, (XPE).

PVC: PVC compounds are the most common and least expensive insulations in general use. Standard PVC is the normal plastic for automotive wires in the cooler parts of the car, and the J-1128 standard is for a 185°F/85°C temperature rated wire. The SAE specification designations are TWP, GPT, and HDT for thin, medium, and thick insulation, respectively.

XPE Cross-Linked Polyethylene, sometimes called XLPE or XLP, is made from Polyethylene. Cross-linking changes thermoplastic polyethylene to a thermosetting material with a greater resistance to environmental stress cracking, cut-through, ozone, and solvents such as motor oil, coolant, or fuels than PVC. The cross-linking can be done either thermally/chemically, which is normally rated up to 150°C, or with irradiation. The irradiated version is available up to 200°C. The J-1128 standard for under hood wiring is for a 257°F/125°C temperature rated wire, with designations of SXL, TXL, and GXL for thin, medium, and thick insulation. The XPE insulations are much tougher mechanically and chemically that the PVC insulations at any temperature, in addition to the wider temperature range.

The SAE wire tables (here) give the information discussed above in detail.
In particular, note the differences in insulation thickness and in weight per foot of wire. Also, see Amperes vs SAE & Metric Wire