The Cable Harness Spec Checklist UK Engineers Wish They’d Had Earlier

Most cable harness problems are not made on the production floor. They are made on the drawing, weeks earlier, in the gaps between what the engineer meant and what the specification said. A missing tolerance, an unstated insulation rating, a connector described rather than part-numbered: each one looks minor in isolation, and each one quietly turns into a requote, a clarification email, or a prototype that must be built twice.

SIC Ltd has been manufacturing wiring harnesses, cable harnesses and cable assemblies in the UK since 1964. During that time, the same handful of gaps have appeared in incoming specifications repeatedly across every sector, from aerospace to vending. This checklist gathers them in one place. It is the document a lot of engineers tell us they wish they had been handed at the start of their first harness project, because getting these details down before you send an enquiry is the single biggest thing within your control that determines how smoothly the build goes.

Why the Specification Stage Decides Everything

A complete specification lets a manufacturer quote accurately, order the right components first time, and build a prototype that matches your intent. An incomplete one forces the manufacturer to do one of two things: ask a round of questions that add days to the quote or make assumptions that may not match what you needed. Neither is helpful when you are working to a deadline.

The checklist below is organised in the way a harness is built up, from the conductors outward. You do not need to answer every line for every project, but you should know which lines apply to yours, and you should be able to say so on the drawing or in the enquiry rather than leaving it to interpretation.

The Cable Harness Specification Checklist

Work through each section before you send anything to a supplier. Where a requirement is genuinely open, say so explicitly rather than leaving it blank, so the manufacturer can advise rather than guess.

1. Conductors and Wire

• Wire size for each conductor, stated as cross-sectional area (mm) or AWG, and the current it can carry.
• Conductor material and stranding, for example, tinned copper, and whether flexibility is a requirement in service.
• Insulation type with its temperature and voltage rating, such as PVC, cross-linked, silicone or PTFE, rather than a colour alone.
• Whether any circuits need to be screened or shielded, and whether the screen is to be terminated or left floating.
• The colour and identification scheme, so the harness can be assembled and later traced unambiguously.

2. Terminations and Connectors

• Connector manufacturer and full part number for every position. A description such as “nine-way connector” is not enough to order against.
• Gender, keying and polarisation, so mating parts cannot be assembled the wrong way round.
• Crimp terminal part numbers and contact plating, for example, tin against gold, since this affects both cost and long-term reliability.
• Backshells, seals, cavity plugs and any unused cavity sealing the connector system requires.
• Who supplies the connectors and terminals? Confirm whether the manufacturer sources them or you free issue them, and whether obsolescence is a risk.

3. Dimensions, Geometry and Tolerances

• Overall length and every branch or breakout length, each with a stated tolerance rather than a single nominal figure.
• Breakout positions and connector orientation relative to the main trunk.
• Minimum bend radius and any fixed routing the harness has to follow in the assembly.
• Service loops or slack required at connection points.

4. Protection and Covering

• The covering system, whether braided sleeve, convoluted tubing, conduit, expandable sleeving or tape, and where each is applied along the harness.
• Heat shrink requirements, including whether plain or adhesive-lined sleeving is needed for sealing or strain relief.
• Environmental sealing and any ingress protection rating the finished assembly must meet.
• Resistance to abrasion, chemicals, temperature extremes or vibration that the operating environment will impose.

5. Identification, Labelling and Traceability

• The wire numbering or circuit identification scheme to be applied.
• Label content, position, and material, with particular attention to labels that must withstand heat, fluids, or abrasion.
• Batch or serial traceability requirements, and whether certificates of conformity are needed at delivery.
• Drawing revision marking, so the build is always tied to a controlled, current version.

6. Testing, Standards and Compliance

• The electrical test required typically includes continuity and correct point-to-point wiring, and whether an automated test is expected.
• Insulation resistance or high-voltage testing where the application demands it.
• Crimp validation such as pull-off testing and crimp-height measurement.
• The workmanship standard to build and inspect against. IPC/WHMA-A-620 is the recognised standard for cable and harness assembly, and its class 1, 2 or 3 acceptance level should be stated.
• Sector-specific standards that apply, for example, aerospace, automotive, medical or rail requirements, and any first-article or approval step before volume production.

7. The Commercial Detail Engineers Forget

• Expected annual volumes and the call-off pattern, since these change the most cost-effective way to build and stock the harness.
• Delivery schedule and whether scheduled deliveries or call-off against a blanket order would suit you.
• Packaging requirements, particularly for delicate connectors or long assemblies.
• Confirmation, again, of which components you supply and which the manufacturer is to source.

The One Thing Most Specs Leave Out

If there is a single line worth adding to every harness enquiry, it is a short description of what the harness connects to and the environment it will live in. A drawing tells a manufacturer what to build. The application tells them whether it is the right thing to build.

With that context, an experienced manufacturer can flag a better type of insulation for a hot engine bay, spot a sealing gap before a prototype is wired, or suggest a connector that is easier to source than the one specified. Without it, they can only build exactly what is on the page, even when their experience says there is a better way. After sixty years and more than 14,000 product lines, that experience is worth giving SIC something to work with.

The Complete Pre-RFQ Checklist

Use this as a final pass before any enquiry leaves your desk. If every line has an answer or a clear note, your harness is ready to quote accurately and build right the first time.

How SIC Helps You Get the Spec Right

SIC has been manufacturing cable harnesses, wiring looms, cable assemblies and control panels in the UK since 1964, across more than twenty sectors and over 14,000 product lines. That breadth means most specifications that arrive are like something the team has built before, and the gaps are easy to spot early.

Manufacturing is carried out in the UK to ISO 9001 and ISO 14001 standards, with a UL-approved facility. Precision termination is supported by equipment such as the Komax Alpha 530, one of the few machines of its kind in the UK, providing consistent one- and two-sided crimping with full crimp-height verification.

Crucially, the process is built to catch specification gaps before they become production problems. The technical team reviews your drawing, addresses any remaining open questions on this checklist, and produces a prototype for validation before any volume run begins. It is a low-risk way to confirm that what you specified and what you needed are the same thing.

Build Your Next Harness From a Complete Spec

If you have a harness coming up, the most useful next step is a conversation about the specification before it is finalised. Send what you have, including the lines you are still unsure about, and the SIC team will help you fill the gaps.

Contact the SIC team to discuss your cable harness requirements. Call 01792 957009, email enquiries@sicltd.com, or use the form at sicltd.com for a prompt, no-obligation response.