Untangling Automotive Wiring

Most techs have repaired a section of damaged primary wiring by replacing the burned or damaged section. Good solder joints combined with heat shrink tubing make a permanent repair, but have you ever considered that there are different types of primary wire, not just gauges? You can’t see the difference, but in today’s densely-packed high heat harnesses, choosing the right grade can buy insurance against future failures. Like most vehicle systems, there’s an SAE (Society of Automotive Engineers) specification, commonly called J1128. Here are the cable choices for the standard:

GPT: rated for 85 C, this type is used for chassis wiring in low-heat non-underhood locations. GPT is widely available in retail stores. DIY’er s often mistakenly use this wire in the engine compartment.

GPTM: rated at 105 C, GPTM is a marine grade specified for general wiring and underhood use. It’s a good choice for boat trailers or wet RV applications.

TXL: rated at 125 C, this is the grade used for most underhood applications. Usually 18-12 gauge.

GXL: also rated at 125 C, GXL has thicker insulation with better resistance to abrasion and chemicals than TXL. 18-12 gauge.

SXL/SGX: 125 C-rated, this is the grade with the heaviest high-temperature insulation, usually reserved for heavy-gauge, extreme service applications. SXL, 8-10 gauge, SGX, 6-1/0 gauge.

Heat from the engine isn’t the only consideration. Every length of wire has some resistance, and losses from 3-10 percent are common in a medium to high current automotive circuit. Bundle the wires together and the heat builds, making it a good practice to upsize a gauge if the repair is wrapped into a tight harness. The following chart shows how much current can be carried (at 50 C) for a standard gauge wire.

Suggested Maximum Current Capacity by Gauge (50 C)