Quick Answer / Summary: Specify custom wire assemblies by conductor and insulation, terminal and housing parts, crimp specification, branch lengths, routing and strain relief, plus the electrical and pull tests. Require a first article built on the production jig, not a hand-made sample.
A wire harness is a routing problem with electrical requirements attached. Most field failures come from a wire that was bent too tightly, a terminal crimped with the wrong tool or a branch that pulled on a connector. The specification has to address all three, not only the conductor count.
A harness is a three-dimensional part, not a list of wires.Conductor, Insulation and Environment
The electrical requirement is simple; the environment is what makes a harness difficult. Temperature, movement, chemicals and abrasion decide the materials.
Wire selection
Stranded wire for anything that moves, solid for fixed routing. Insulation material follows the temperature and chemical exposure, and the colour code should be documented because it becomes the maintenance manual.
Shielding and grounding
Specify where the shield terminates and how. A shield grounded at both ends can create a ground loop; grounded at one end it may not protect as expected. Decide deliberately and record the decision.
Terminals and Crimp Quality
Crimp quality is the single largest determinant of harness reliability, and it is invisible in a finished assembly.
Specifying the crimp
Name the terminal part, the tool and the crimp height or pull-test value. A crimp specified only by part number leaves the compression to the operator.
Verification
Ask for a crimp pull test on a sample from each production run and a cross-section on the first article. Both are quick, and both expose worn tooling before it produces a batch.
Routing, Branches and Strain Relief
The harness is a three-dimensional part. Its shape has to be specified with the same care as its electrical properties.
Branch lengths and jig
Give branch lengths from a defined datum and require the harness to be built on a production jig. A hand-laid sample can be perfect and a jig-built batch can still be wrong if the jig is not maintained.
Strain relief and abrasion
Specify where the harness is supported, where it bends and whether a sleeve or a spiral wrap is needed. Abrasion at a panel entry is a common failure that a short piece of sleeving prevents.
Testing and Documentation
Acceptance testing for a harness is mostly continuity and correctness. The value is in the completeness of the test, not the sophistication of the instrument.
Continuity and hipot
Test every circuit for continuity and for isolation between circuits where required. For mains-carrying harnesses, add a hipot test at the specified voltage.
Documents to require
A wiring schedule, a bill of materials with terminal and housing parts, the crimp specification and the test record per harness serial. Without the schedule, repairs become guesswork.
Wire Harness Specification Sheet
| Item | What to state | Acceptance |
|---|
| Wire | Gauge, stranding, insulation | Caliper and material certificate |
| Terminal | Part number and housing | Visual and dimensional check |
| Crimp | Tool and pull-test value | Pull test per run |
| Branches | Lengths from datum, jig built | First article measurement |
| Protection | Sleeve, wrap, tie spacing | Visual inspection |
| Test | Continuity, isolation, hipot | Record per serial |
Practical Applications
Custom wire assemblies are used in industrial control panels, machine tools, medical equipment, automotive test rigs and building services. In each case the harness is a maintenance part as much as a production part, so documentation matters beyond the first installation. Ask for the harness to be labeled at both ends with the circuit identifier, not the wire colour. Colour codes fade in memory and change with staff, while a printed identifier survives. The cost is a few cents per harness and it turns a repair from a tracing exercise into a replacement. Ask for the harness to be photographed at the jig before packing. The photograph is a quick reference for the installer and a record for the supplier, and it settles a surprising number of questions about branch orientation without a site visit. It costs a few seconds per unit and prevents a class of installation errors that are otherwise hard to diagnose. Ask for the harness weight and minimum bend radius, because both affect how it is supported in the cabinet. A heavy harness that is tie-wrapped at wide intervals sags onto terminals, and sag is a slow failure that looks like a component problem. Ask for the harness drawing to show the tie positions and the service loops, not only the branch lengths. Ties placed without thought compress the bundle and stress the terminals, and the resulting failures appear as intermittent circuits rather than as a broken wire.
Reviewed by ZONG Cable Editorial | Last updated: 3 November 2026 | Reviewed against TIA-568 structured cabling practice and ISO/IEC 11801 fiber channel requirements.
How this guide was built: Harness specification follows build-to-print practice: wire and insulation selection for the environment, terminal and crimp specification with pull testing, jig-based branch construction, and continuity plus hipot acceptance.
Disclosure: We do not manufacture harnesses. Confirm crimp specification and first-article approval in the order.
Frequently Asked Questions
What is the difference between a wire harness and a cable assembly?
A harness usually has multiple branches and terminations built on a jig; a cable assembly is typically a single run with connectors at each end.
Why is crimp height specified?
Because it controls the compression that decides pull strength and resistance. Without it, quality depends on the operator.
Do I need a first article?
Yes, and it should be built on the production jig with the production tooling.
What tests should a harness have?
Continuity on every circuit, isolation where required, and hipot for mains-carrying assemblies, plus a crimp pull test.
What documents should ship with the harness?
A wiring schedule, BOM with terminal parts, crimp specification and a test record per serial.
