How to Specify Transport Equipment Cables

Start with the environment, not the part number

Before choosing a cable or connector, write down what the part will actually experience. Every item below should end up as a drawing note or a referenced specification.

Exposure	Questions to answer	What it drives
Temperature	Lowest and highest ambient? Heat from exhaust, brakes or motors? Flexed when cold?	Insulation and jacket rating, and cold-bend performance
Fluids	Diesel, hydraulic oil, coolant, de-icing fluid, cleaning chemicals, salt spray?	Jacket compound and connector seal material
Water	Rain and spray, submersion, or high-pressure washdown?	Sealing and IP rating
Sunlight and ozone	Permanently outdoors?	UV- and ozone-resistant jacket
Mechanical	Abrasion, crush, impact, being driven over?	Jacket thickness, armor or conduit, routing
Movement	Static, occasional flex, continuous flex or coiled?	Conductor stranding, cable construction, strain relief
Vibration	Road, rail, engine or airframe?	Clamping intervals, connector locking, crimp quality
Mating cycles	Plugged once, or daily?	Connector durability rating and contact plating

Jacket materials at a glance

Material	Strengths	Watch for
PVC	Low cost, widely available	Stiffens and cracks in cold; limited oil resistance unless specified
Polyurethane (PUR)	Very good abrasion and oil resistance, good for continuous flex	Grade matters for hydrolysis and heat
Thermoplastic elastomer (TPE)	Flexible across a wide temperature range, good for overmolding	Fluid resistance varies by grade
Rubber (e.g. CR, EPDM compounds)	Rugged, flexible, good weathering; common for heavy-duty power leads	Heavier; oil resistance depends on compound
Cross-linked polyolefin	Heat and abrasion resistant; common in primary wire	Less flexible as a jacket
Fluoropolymers (e.g. ETFE)	High temperature and chemical resistance, thin walls; aerospace	Cost; follow the governing aerospace specification

A fuller comparison, including primary wire insulation, is in PVC vs PUR vs TPE cable jackets.

Sealing: say which rating, under which standard

"Waterproof" is not a specification. Ingress protection is defined in IEC 60529 (for example IP67, temporary immersion). For road vehicles, ISO 20653 adds ratings such as IP6K9K for high-pressure, high-temperature washdown. State the rating, the standard, and whether it applies mated, unmated or both. An unmated connector is often not sealed at all.

Strain relief and flex life

For any cable that moves, the most important location is where it leaves the connector. Specify the strain relief method (molded boot, gland or backshell clamp) and, where it matters, a pull-force requirement. For continuously moving cables, specify fine-stranded conductors and a cable designed for flexing, and state the bend radius and expected number of cycles.

Connectors: match the standard first

Where the interface is standardized, name the standard and let it drive the connector: SAE J560 for the North American trailer 7-way, ISO 7638 for trailer ABS/EBS, ISO 12098 or ISO 1185 and ISO 3731 for European 24 V lighting, and IEC 60309 for reefer power, including the earth position. The pin assignments differ between standards that look alike, so state the pinout on the drawing as well. See the J560 pinout and reefer plug guides for two common mix-ups.

Where the connector is a design choice, specify:

the sealing rating, mated and unmated;
the mating cycle rating against how often the connection is actually made;
contact plating suited to the current and the environment;
a positive lock or secondary lock where vibration could back a connector off;
terminal and seal part numbers matched to the wire size and insulation diameter.

Electrical tests worth writing down

Test	What it finds	When to require it
Continuity and circuit verification	Opens, miswires and crossed circuits	Every part
Insulation resistance	Damaged insulation, contamination, moisture	Power leads and harnesses where leakage matters; state the test voltage and minimum value
Dielectric withstanding voltage (hipot)	Insulation that fails under stress	Higher voltage parts, and where the customer's specification calls for it
Phase order	Swapped phases on three-phase leads	Every three-phase lead, such as reefer power cables
Crimp pull test	Weak crimps	Sampling per the workmanship standard or your specification

Drawing notes that prevent arguments

Workmanship standard and class, for example IPC/WHMA-A-620 Class 2 or Class 3.
Cable and connector part numbers, with approved alternates or "no substitution".
Sealing rating and standard, and mated or unmated condition.
Electrical tests: continuity on every part, plus insulation resistance and hipot values where needed.
Marking: part number, revision and date or serial, with method and location.
Packaging that protects the molded ends and prevents kinking.

Common questions

What IP rating does a trailer connector need?

It depends on exposure. Connectors hit by road spray and pressure washing are often specified to IP67 under IEC 60529 plus IP6K9K under ISO 20653. State which condition, mated or unmated, the rating applies to.

Is "oil resistant" enough of a jacket specification?

No. Name the fluids, the temperature and the exposure (splash or immersion), and ask for the datasheet test that covers them.

Should the drawing name a specific cable brand?

Name a part number if you have qualified it, with "or approved equal" if you will accept alternates. Otherwise specify the performance and the governing standard, and approve the supplier's proposal.

What workmanship class suits transport equipment?

IPC/WHMA-A-620 defines three classes: Class 1 (general), Class 2 (dedicated service) and Class 3 (high performance), and the customer chooses. Class 2 is often chosen for commercial equipment and Class 3 where continued performance is critical, as in aerospace. State the class on the drawing.

For the full list of what to send a manufacturer, see the cable assembly RFQ checklist. Workmanship classes are explained in IPC/WHMA-A-620 classes.