Two families: primary wire and battery cable
Automotive low-voltage copper falls into two SAE families. GPT primary wire to SAE J1128 - general purpose, thermoplastic - runs the vehicle's circuits: lighting, sensors, accessories. It has a PVC wall sized for protection inside harnesses and is stocked in a full colour range, often with striped combinations for circuit identification. SGT battery cable to SAE J1127 connects battery, chassis ground and starter: heavier bunched stranding for flexibility, a thicker PVC wall with resistance to oil, acids and alkalis, and usually supplied in red and black. Both families are rated 60 V DC and 25 V AC, with 80 C or 105 C grades depending on the compound, which reflects the low-voltage DC world vehicles actually run on.
The ratings surprise buyers who arrive from mains cable. A 60 V DC rating is not a defect; it matches the application, and the standards spend the engineering budget on flexibility, abrasion and chemical resistance instead of insulation voltage. Reading the two families side by side prevents the most common sourcing error, which is ordering primary wire where the design names battery cable.
Construction and reference data
GPT is built from soft bare copper to ASTM B3, bunched into a flexible conductor. The reference 10 AWG construction in public datasheets uses 19 strands, a PVC wall of about 0.81 mm, and an overall diameter of about 4.5 mm, at roughly 57 kg/km, with a DC resistance near 3.55 ohm/km at 25 C. SGT scales up: datasheets list 6 AWG with 133 strands at a 7.65 mm overall diameter, about 162 kg/km and 1.43 ohm/km at 25 C, and the family continues through 4, 2 and 1 AWG in partner catalogues. Reference values from public datasheets:
| AWG | Strands | Insulation wall (mil) | OD (mm) | Weight (kg/km) | DC at 25 C (ohm/km) |
|---|---|---|---|---|---|
| 6 | 133 | 60 | 7.65 | 162 | 1.43 |
| 4 | 133 | 73 | 9.04 | 249 | 0.90 |
| 2 | 133 | 65 | 10.67 | per datasheet | 0.56 |
| 1 | 259 | 65 | 11.63 | 375 | 0.45 |
The strands column is a construction detail, not a promise: bunched and rope-lay stranding varies between plants, as explained in our post on stranded vs solid conductors. Judge offers on overall diameter, weight and DC resistance rather than strand counts, and treat any size not shown here as a datasheet item to request rather than a gap to guess at. Where a programme calls for sizes beyond the table, treat the extension as a datasheet exercise, because wall systems and strandings change between the small harness sizes and the battery range.
Voltage, temperature and the 12 V reality
Datasheets declare 60 V DC and 25 V AC ratings, with 80 C or 105 C continuous grades and cold performance down to about -40 C, which matters on winter mornings when the starter circuit does its hardest work. Underhood and near-exhaust routes are where the 105 C grade earns its keep; harness designers assign grades by zone, and the order should carry the grade per position rather than one blanket choice for the whole programme. The practical design constraint at 12 V is not insulation but voltage drop: with so little system voltage to spare, every fraction of a volt lost in a long battery run is lost to the starter or the load, which is why battery runs are kept short and generously sized. The same arithmetic drives fixed installations, and our voltage drop calculator shows how run length and conductor size interact. Sizing for a specific vehicle or machine is the designer's decision; the buyer's job is to order the size the design names, in the correct family, with the diameter and resistance the datasheet supports.
Choosing the family and avoiding substitutions
Three substitutions cause most returns. Using GPT for a battery or starter run fails because the thinner wall and lighter stranding are not built for the current path, vibration and heat of an engine bay - SGT exists for that duty. Using welding cable as battery cable usually works electrically, since it is flexible copper, but it is a rubber-insulated product specified for welder secondary circuits, documented and priced differently; both families sit on our automotive and welding cables page, and our post on welding cable for buyers explains the distinction. And using bare copper in corrosive marine or winter-road environments invites green corrosion at the terminations; tinned copper is the usual answer where datasheets offer it. Fit matters as well: lugs and terminals are chosen from the actual overall diameter, which varies between plants, so check the datasheet before ordering crimps and boots.
Colours, print and packing
GPT is stocked in a full colour range with striped combinations for circuit identification, which is why harness makers order by colour split - so many feet of each colour per size. SGT is usually red and black for positive and negative battery connections. Print carries the type, size and rating along the wall, plus sequential marks in feet, matching the US markets the SAE system serves. Packing is coil-based: primary wire commonly in 100, 500 or 1,000 ft coils, battery cable in shorter coils and reels, with lengths confirmed in writing per order. Private label buyers add branded coils, headers and boxes, as described on the OEM and private label page, and colour splits should be fixed before the quote is finalised because each colour is a separate production run. Feet-based coils are the convention because the SAE markets cut and invoice in feet; a buyer working in metric should state target lengths in both units, so the label does not turn a 300 ft coil into an assumed 100 m one.
Buying notes for importers
Two framework notes prevent rework. First, standards: SAE J1128 and J1127 are the North American system, and some European vehicle programmes specify ISO 6722 instead - confirm which system the programme requires before quoting, because constructions and test regimes differ. Second, documents: ask for the datasheet of the exact construction and DC resistance records with shipment, and let your customs broker confirm the HS classification; insulated conductors of the low-voltage kind are commonly declared under heading 8544, and the broker owns the code. Typical minimums and lead times are on the MOQ and lead time page, and the desk confirms family, sizes, colours and packing in writing before production. Send the programme requirements through the quote form.