Most manufacturers start by buying components. A wiring loom here, a control panel there, a cable assembly from a third supplier. The parts arrive, and someone in-house assembles them.
That model works until it does not. The question worth asking is when the point arrives that handing the whole assembly to a single supplier costs less than managing the parts yourself.
The hidden cost of multiple suppliers
A bill of materials with four suppliers on it carries costs that never appear on any of the four quotes.
Each supplier needs to be sourced, approved, and audited. A business buying looms from one source, cable assembly manufacturing from another, and enclosures from a third carries that overhead three times over. Each has its own lead time, its own minimum order quantity, and its own delivery schedule, and your production plan has to accommodate all of them. Each raises its own invoice, and each requires goods-in inspection when the parts arrive.
When something does not fit, the cost rises sharply. A loom that does not seat correctly in an enclosure is a conversation between two suppliers who did not design their parts together, mediated by you. Neither is contractually wrong. The problem is the interface between them, and the interface belongs to nobody.
Where responsibility sits
This is the part that shapes the decision more than unit price does.
When you buy components and assemble in-house, the finished assembly is yours. If it fails testing, the diagnostic work is yours, and so is the rework. If a customer reports a fault in the field, tracing it back through three suppliers is your task.
When one supplier builds the complete assembly, they own the finished product. They are responsible for the fit between the loom and the enclosure, for the terminations into the panel, and for the testing of the assembly as a working unit rather than as a set of parts. Where the enclosure houses switchgear or PLC equipment, that supplier needs to be among the control panel manufacturers capable of building the panel itself, not just wiring into someone else’s. For many manufacturers, that transfer of responsibility is worth more than any saving on the components themselves.
Where floor space and labour go
Assembly work consumes space and skilled hands, and both are constrained for most UK manufacturers.
Bench space given over to putting together bought-in components is bench space not available for the work that differentiates your product. Skilled assembly staff spent on wiring and mounting are staff not spent on the processes your customers are paying for.
The same pressure applies upstream. Wiring harness manufacturers have absorbed much of this work already, which is why buying the harness in rather than building it is now standard practice across most of the sector. Extending that logic to the finished assembly is a difference of degree rather than kind.
The availability of skilled assembly labour has tightened across UK manufacturing. Where recruiting and training a wiring technician is difficult, moving that work to a supplier who already has the people and the equipment addresses a capacity problem rather than simply moving a cost.
Signals that the threshold has been reached
The decision is easier to make against specific conditions than in the abstract. It is usually worth reviewing when:
- Three or more suppliers contribute parts to one finished assembly
- Assembly staff are working on bought-in components rather than your own processes
- Fit and interface problems between components recur
- Lead times are dictated by whichever supplier is slowest
- Production volumes have grown to the point that in-house assembly constrains output
- Testing and quality assurance on the assembled unit sit with you, and add time
Two or three of these together generally indicate that the component-buying model has been outgrown.
When it does not pay
Handing over the whole assembly is not right in every case.
Where the assembly work is itself a source of competitive advantage, or where the design changes constantly and iteration speed matters more than production efficiency, keeping it in-house makes sense. Very low volumes with no growth expected rarely justify the setup work. And where in-house assembly capacity is already paid for and underused, the calculation changes.
The question is not whether outsourcing is better in principle. It is whether, for a particular assembly at a particular volume, one supplier taking responsibility for the finished unit costs less in total than four suppliers and your own bench time.
What a single-supplier build involves
SIC manufactures wiring looms, cable assemblies and control panels, and carries out cable harness assembly through to complete units built as box build assembly.
SIC manufactures wiring looms, cable harnesses, cable assemblies and control panels, and builds complete units through box build assembly. The same organisation that makes the loom mounts it into the enclosure, terminates it into the panel, and tests the assembly as a working unit.
That removes the interface problem, because the interfaces sit inside one business rather than between several. It reduces goods-in handling to one delivery. And it means that when a question arises about the finished assembly, there is one place to ask.
If you are reviewing an assembly currently built from multiple sources, the useful starting point is a full cost comparison covering supplier management, goods-in, bench time, testing and rework, set against a single quoted price from a supplier offering box build assembly services