This Is How a Wiring Loom Goes from Customer Brief to Finished Batch

You send a drawing. Some weeks later, a box arrives with your looms in it, built to spec and ready for the line. What happens in between is a black box for most buyers, and it stays that way because nobody tends to ask.

It is worth asking. The decisions taken in the first few days of a project, long before anyone picks up a crimp tool, are the ones that set your unit cost, your lead time and how much rework you live with for the next five years. By the time a batch is running, most of that is already fixed.

So here is the whole thing, start to finish: the route a wiring loom takes at SIC Ltd, from the first email to a packed batch leaving Swansea. The same path applies whether you need a six-wire cable harness or a multi-branch wiring harness running the length of a vehicle chassis.

SIC has been building looms, harnesses and cable assemblies in the UK since 1964 and now produces over 14,000 product lines across more than 20 sectors. What follows is the process behind all of them.

A brief arrives in one of three forms. Some customers send a full engineering pack: a wire schedule, a 2D loom drawing, connector part numbers and a test specification. Some send a physical sample of a loom already in production, often one they want moved from a UK or overseas supplier. Others send a description of the application and a rough sketch and ask for help turning it into something manufacturable.

All three are workable. The information that needs to be established at this point is consistent:

• Wire specification: gauge, insulation type, temperature and voltage rating, colour coding
• Terminations: connector housings, terminals, ring crimps, splices, ferrules or tinned ends
• Branch lengths and tolerances, including any fixed points the loom must reach
• Environment: temperature range, vibration, chemical exposure, EMC and screening requirements
• Protection and finish: braided sleeving, convoluted conduit, spiral wrap, heat shrink, overmoulding
• Volumes, batch sizes and how often the customer expects to call off stock
• Compliance requirements such as IPC/WHMA-A-620 class, UL, RoHS or REACH

Where a customer cannot answer everything, the gaps are listed and resolved before quotation rather than discovered in production. Each account is assigned a dedicated account manager and an engineering contact, so these questions are answered by the same people throughout the life of the project.

The engineering team then reviews the design against how it will be built. This is where most of the avoidable cost in a loom is either removed or locked in.

Typical findings at this stage include:

• A connector specified for the original design that is now on long lead time, where a form, fit and function equivalent is available in weeks rather than months
• Branch lengths with tolerances tighter than the application requires, adding assembly time for no functional benefit
• Two similar looms in a range that could share a common sub-assembly, reducing part count across both
• A termination that needs a dedicated applicator, where a standard crimp would serve the same purpose
• Sleeving specified along the full length where protection is only needed across a chafe point

None of this changes the design without agreement. The output is a set of options with the cost and lead time implications of each, and the customer decides. The same review applies to panel wiring, control panels and box build assemblies where a loom forms part of a larger unit.

Once the specification is settled, a bill of materials is built and priced. Component cost typically dominates a loom quotation, so the sourcing position matters more than the labour rate. Long-standing supplier relationships, stock held on common items and volume across 14,000 product lines all feed into the price a single customer sees.

The quotation covers the unit price at the volumes discussed, tooling or applicator costs where a specific termination requires them, the prototype cost, and the lead time for both first samples and repeat batches. Where a customer is unsure of volumes, pricing at more than one batch quantity shows where the breakpoints fall before any commitment is made.

A prototype is built to the agreed specification and sent to the customer. This is deliberately a physical check rather than a paper one. Drawings can be correct, and the loom still be wrong for the application: a branch that will not route around an obstruction, a connector orientation that fouls a panel, a length that is fine on the bench and short once the unit is assembled.

Prototypes are produced with minimum delay and with a guarantee of manufacturability, so the sample the customer holds can be reproduced at volume rather than being a one-off hand build. Any changes coming back from the customer are recorded against a revision, and the revised sample is issued before production tooling is committed.

Before a batch runs, the process itself is set up and proven. This covers the build board or jig layout, the crimp tooling and its settings, the wire cut lengths and marking, the test method and the packing format.

First-off inspection is carried out on the first unit from the line. Crimp height and pull-off force are checked against the terminal manufacturer’s data, terminations are inspected against IPC criteria, and continuity is verified against the wire schedule. Only once the first article is signed off does the batch continue. Written work instructions are then produced so that the same loom is built the same way by any operator on any shift.

Production runs in sequence, with the mix of automated and manual operations depending on the loom.

Cutting, Stripping and Marking
Automatic cutting and stripping machines process wire to length, with strip lengths set for the terminal in use. Cable marking equipment prints circuit identification directly onto the insulation where the specification calls for it, removing a common source of error at assembly and making the loom easier to fault-find in service.

Crimping and Termination
Terminals are applied on presses with crimp force monitoring, which flags an out-of-tolerance crimp at the point it is made rather than at final test. Crimp quality is the single largest contributor to field failures in looms, so it is monitored throughout the run rather than sampled at the end. Where a design calls for it, ultrasonic splicing, soldered joints or overmoulded terminations are used instead.

Assembly
The loom is laid up on a build board that maps the branch geometry at full scale. Wires are dressed into their branches, connectors are populated to the schedule, and the assembly is taped, sleeved, braided or wrapped to the specified finish. For low-volume, high-complexity work, this stage is largely manual and depends on operator skill. For high volume runs, dedicated manufacturing cells are built around the product.

Finishing
Heat shrink is applied and recovered, cable ties and P-clips are fitted at the specified positions, overmoulding is carried out where sealing or strain relief is required, and labels are applied. Overmoulding processes and specialist machinery allow complex loom configurations to be produced to tolerances that would otherwise be unattainable.

Every loom is tested before it leaves the building. The standard sequence covers:

• Continuity and correct point-to-point connection against the wire schedule
• Short circuit and miswire detection across all cavities
• Insulation resistance and, where specified, high voltage flash testing
• Connector retention checks, confirming terminals are fully seated and locked
• Visual inspection against IPC/WHMA-A-620 acceptability criteria for the agreed class

Bespoke test rigs are built where products need them, particularly for looms with high cavity counts or mixed-signal and power circuits. Test records are held against the batch, so a query months later can be traced back to the specific units and the components used in them.

Packing is agreed with the customer rather than assumed. A loom that arrives correctly built and then needs to be untangled, or that is damaged because a connector was left unprotected in transit, has failed just as surely as one that fails continuity. Coiling method, separation, labelling and carton quantities are all set during pre-production and repeated on every batch.

Batch and component traceability is maintained throughout. From an 188,000-square-foot manufacturing and warehouse facility, SIC can hold finished stock and call it off against a customer schedule, allowing a customer to buy at an economic batch size while receiving deliveries matched to their build rate.

Once a loom is in steady production, the process stabilises, but it does not stand still. Component availability changes, volumes move, and designs are revised. Repeat batches are built to the approved work instructions and revision level, and any change to either is controlled and agreed rather than applied quietly.

Where volumes grow, work that started as a manual build can move to a dedicated cell with tooling that lowers the unit cost. Where a customer adds variants, common sub-assemblies across the range keep the part count down. This is the stage where the groundwork laid during design review pays off.

Two suppliers can quote the same loom at similar unit prices and deliver very different results. The difference sits either side of the assembly bench: whether the design was reviewed before it was quoted, whether the first article was properly signed off, whether crimps are monitored during the run or checked at the end, and whether test records exist if a question arises later. A robust ISO 9001 quality system ensures consistent quality throughout the entire manufacturing process.

A loom that fails in service is rarely expensive because of the part. It is expensive because of the unit it sits in, the engineer sent to the site, and the customer relationship that bears the cost. The process described here exists to keep that outcome rare, and it is applied the same way to a batch of 50 cable looms as to a high-volume automotive programme.

SIC manufactures looms, harnesses and assemblies for over 100 customers across more than 20 sectors, including:

• Aerospace and defence
• Automotive and transport
• Medical and healthcare
• HVAC and building services
• Switchgear and electrical infrastructure
• Catering, vending and refrigeration
• Gaming and leisure
• Lighting and audiovisual
• Semiconductor and industrial equipment

Product sizes range from 25mm interface connections to 25m trailer looms, in low-, medium-, and high-volume batches.

The most useful starting point is a drawing, a sample or a description of the application, along with an indication of volumes. From there, the engineering team can confirm what is required, identify anything that would benefit from review, and provide quotes based on realistic batch quantities.

Contact the SIC team to discuss your wiring loom, wiring harness or cable assembly requirement. Call 01792 957009 or email enquiries@sicltd.com.