Wire harness manufacturing: how a harness is built and checked

A wire harness is usually built in this order: prepare the wires, terminate them, lay them out on a board, bind and protect them, then test the result. Two of those steps matter most for proving the product is sound: the termination, because it decides the mechanical and electrical quality of every joint, and the final electrical test, because it is the last chance to catch a mistake before the customer does.

You will also hear a wire harness called a wiring harness, a cable harness or a wiring loom.¹

What a harness is for

A harness gathers many separate wires into one unit that can be installed in a single operation. Binding the wires together secures them against vibration, abrasion and moisture, and uses less space than loose wiring.¹ It also moves the risk of error away from the installer: the routing and the connections are decided at the bench, not worked out inside the machine.

If your wires sit inside one extruded jacket with a connector on each end, you are probably looking at a cable assembly, which is a different build. The comparison of wire harnesses and cable assemblies explains where the line falls and why it matters less than the specification.

The build, stage by stage

Start from a drawing the floor can use

The drawing and the wire list carry everything the operator needs: the gauge, colour and length of each wire, where each end goes, which terminal and connector it uses, and what protection covers which section. Anything left out gets decided on the bench, and two operators may decide differently. A full-scale drawing can also serve as the layout for the formboard.

Cut and strip

Wires are cut to length and stripped, by machine at volume and with bench tools for small batches. Stripping is where strands get damaged. Molex lists cut or nicked strands, along with incorrect crimp height, tooling and a missing bell mouth or conductor brush, as things that lower pull force.² Check the strip length as well: with the insulation position centred, it sets the conductor brush, the strands that extend past the conductor crimp.²

Terminate

Most harness terminations are crimps. A crimp has to apply enough pressure to break down the oxides on the conductor and the plating inside the terminal, so that metal meets metal. Too little pressure and resistance can rise. Too much reduces the conductor's cross-section, and resistance rises again.² That is why a crimp can look tidy and still be wrong.

Three checks tell you most of what you need to know:

• Crimp height. It is quick to measure, it does not destroy the part, and it is critical to both electrical and mechanical reliability. Molex notes that it is usually control charted, because setup samples are too few to mean much and a chart catches a drifting process before thousands of terminations have to be scrapped.²
• Pull force. This is a quick, destructive test of the mechanical strength of the crimp.² The minimum values come from your acceptance standard.
• Appearance. The bell mouth, the conductor brush, the cut-off tab and the insulation position can be checked with a pocket scale and a magnifier, and they interact: Molex warns that adjusting one, such as the bell mouth, also moves the others.²

How much margin should you expect? In a NASA review of 780 pull test results, most contact and wire pairs beat the minimum by at least 100%, and averages were at least 182% of it.³ That is aerospace data and not a benchmark for your line, but it is a reminder that the minimum is a floor and not a target. Chart your own results so that you notice when they drift towards it.

Pull testing costs parts, and the same NASA paper says it could be possible to test less often if the tool and materials are verified at the start of a run and the settings stay unchanged, provided the project weighs up the added risk.³

Build sub-assemblies

Groups of wires that always travel together, such as those going into one connector, are often made up first. This shortens the lay-up and lets you check a section before it is buried in the bundle.

Lay up on the formboard

The wires are routed on a board that carries the shape of the finished harness, using pegs and fixtures to hold each branch in place. This stage is why so much harness work is still manual. Routing wires through sleeves, taping and fastening strands are hard to automate, so even large suppliers tend to automate only parts of the process, mainly pre-production steps such as cutting and stripping.¹

Bind and protect

Tape, cable ties, sleeving and conduit hold the shape and shield the wires from whatever the harness will rub against. Choose the protection for the place the harness will live in, and write it on the drawing so it is not left to preference. Ties pulled too hard can damage insulation, so treat tension as a controlled step.

Label

Wire and connector markings tie the physical harness back to the drawing, and the label on the finished part ties it back to its production batch. Both matter on the day you have to work out which units share a fault.

Test and inspect

An electrical test confirms that every connection goes where the drawing says and that nothing is shorted. Depending on the application, it may also include insulation resistance or a high-voltage withstand test. The electrical test cannot see mechanical faults: a terminal that is not fully seated in its housing can still pass and then back out in the field. It may look fine too, so add a retention check, such as a push-back test, where the connector design allows it. A visual inspection against written acceptance criteria covers other faults the tester cannot see.

Which standard applies

A widely used reference is IPC/WHMA-A-620, Requirements and Acceptance for Cable and Wire Harness Assemblies, published by IPC (now trading as the Global Electronics Association) and the Wiring Harness Manufacturers Association.⁴ It sets acceptance criteria for three product classes, from general products to high-performance ones where downtime cannot be tolerated.¹ Customer specifications come first, and the standard fills in where they are silent.¹ Agree the revision and the class with your customer before you quote, because they decide how much inspection the job needs.

The standard is sold under copyright, and its publisher restricts the use of its content, so this article does not reproduce its tables. Work from your own licensed copy. Automotive, aerospace and medical customers often add requirements of their own, which the industries section covers.

Questions to ask a harness supplier

If you buy harnesses and do not build them, these questions show quickly how well a supplier controls the process:

1. Which revision and class of IPC/WHMA-A-620 do you build to, and is that written on the quote?
2. How do you control crimp height: do you chart it, or only check setup samples?
3. How often do you pull test, and what happens to the batch when a test fails?
4. What does your electrical test cover, and do all units get it?
5. How do you detect a terminal that is not fully seated?
6. If one harness fails in service, can you trace which others share its materials and setup?

The answers describe what happens on an ordinary shift, which a certificate cannot do.

If your assemblies end up inside a control panel, the requirements change again: see what a panel builder has to verify under BS EN IEC 61439.

Notes

1. Wikipedia, Cable harness. Accessed November 2026. Used for the terminology and the general description of manual production. ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Molex, Quality Crimping Handbook, order no. 63800-0029, revision C, 2009, copy hosted by DigiKey. It is written for Molex open barrel terminals and Molex tooling, so check your own terminal maker's limits and the current revision. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

3. A. Constante, C. Fitzgerald and A. Boutte, Analysis of Pull Force Test Results for Crimped Connections, NASA Goddard Space Flight Center, presented at IPC APEX EXPO 2023. ↩ ↩²

4. IPC, IPC/WHMA-A-620, publisher's sales page. Check it for the current revision. ↩