Reading Primary Wire Print

What it is

Primary wire is the low-voltage, single-conductor wire used for lighting, controls and accessories on vehicles. In North America it is usually made to SAE J1128. The insulation type tells you the material, wall thickness and temperature rating.

Many wires are printed along their length with the standard, type, gauge, temperature and maker, such as "SAE J1128 GXL 16 AWG 125°C."

How to read it

1. Find the print. Turn the wire under good light. Print repeats every few inches.
2. Read the standard. SAE J1128 is the common US standard. ISO 6722 is the common metric standard, sized in mm².
3. Read the type. GPT, HDT, GXL, SXL and TXL are the common types.
4. Read the gauge. SAE gauge is close to, but not always identical to, AWG. See Primary Wire Gauge Reference.
5. Read the temperature rating in °C.
6. If there is no print, identify by feel and wall thickness, or ask for a spec sheet. Unprinted wire is hard to prove.

Common SAE J1128 types

Temperature ratings shown are the common published values for each type. Confirm on the print or spec sheet.

Key points

• Cross-linked types (SXL, GXL, TXL) handle heat and abrasion better than PVC types. They are preferred in engine bays.
• Thinner walls make lighter, smaller harnesses but give less abrasion protection.
• The wire color does not tell the circuit unless the fleet or OEM has a color code.
• See Primary Wire — GPT, GXL, TXL and SXL.

Common mistakes

• Using GPT wire near the exhaust or engine.
• Treating metric mm² sizes as AWG sizes.
• Buying unprinted wire with no spec sheet.
• Splicing TXL into a harness that needs heavier insulation.

Governing standards: SAE J1128 (low voltage primary cable); ISO 6722 (road vehicles, single-core cables); the vehicle maker's wiring spec.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.