1) Do not ever use THHN/THWN wire. This wire (insulation) classification is not rated for automotive use. Cars move and vibrate which chafes the insulating material.

2) A wire that is rated for A/C voltage may have a different rating when used in low voltage D/C applications. Your house uses A/C, your car uses D/C.

3) The best primary power cable is a type TEW or MTW (Machine Tool Wire) as its rated for continuous use with higher amperage loads.

4) OFC (oxygen free copper) is by far the best cable for automotive use. However, it is expensive and requires a quality crimper and should be soldered for security. The most common application for this wire is high-end car stereo applications.

5) If you are like me, you may choose to skip OFC and go with TXL wire. TXL (thermal cross-linked) wire is next best. TXL wire has twice the voltage rating of standard GPT (general purpose) wire. Most high-end harnesses are made with this wire. If you want to keep your ride for the long haul, you may want to pony up for a harness made with this wire.

6) The best connection is a properly done solder joint. If you aren't sure how to solder, crimp the connection then solder it. Cold solder joints not only introduce resistance, but they can come apart.

7) Keep your grounds as short as possible. It is far better to have a few more connections than a long ground that can build heat and introduce ground loops. It is common practice to run a ground wire from a high demand devise such as a fuel pump back to the battery. This not only adds additional loads to the pump or devise but also can lead to device failure. Have you ever seen a production auto with a ground cable longer than the positive cable?

8) If at all possible, use stainless hardware to fasten all primary cables. Some galvanic or plated hardware can cause dissimilar corrosion and can sometimes carry more impedance that its stainless counter parts. On smaller secondary circuits, this is not as important.

9) Battery storage. Keep batteries away from humidity. Keeping it charged will prolong its life. If a battery is dead, cool it before you charge it. Put in the refrigerator or outside if its cold. Cool batteries charge better.

10) Batteries specifications and charge rates are for 77 degrees Fahrenheit. Temperature variations will change charge rates.

11) Use relays. Anytime you want to switch a device that draws more current than is provided by an output of a switch or component you'll need to use a relay. The coil of an SPDT relay that we most commonly use draws very little current (less than 200 milliamps) and the amount of current that you can pass through a relay's common, normally closed, and normally open contacts will handle up to 30 or 40 amps. This allows you to switch devices such as headlights, parking lights, horns, etc., with low amperage outputs such as those found on keyless entry and alarm systems, and other components. In some cases you may need to switch multiple things at the same time using one output. A single output connected to multiple relays will allow you to open continuity and/or close continuity simultaneously on multiple wires.

12) Most electromechanical devises are rated according to the MTBF (Mean Time Between Failures). You improve your odds of a long MTBF by keeping your electrical devices in a low humidity, low heat environment.