A wire harness RFQ package is three documents: a dimensioned drawing, a wire list and a connector BOM. Send all three and you get quotes you can put side by side. Send a photo of the loom you currently buy plus a target price, and you get a number that is a sales estimate wearing a quotation's clothes — a guess about copper, a guess about terminals, and a guess about how long each branch runs.
An assembled loom is opaque by construction. The conductor cross-section is buried under insulation. The terminal is invisible once it is latched into the housing. A sealed housing and its unsealed twin look identical in a photograph and carry different terminals, different applicators and different prices. Branch lengths cannot be scaled off an image. Three suppliers looking at the same photo make three different sets of assumptions, and the cheapest quote is usually just the one that assumed the least.
Why a photo and a part number are not a quotable spec
Buyers who have sourced moulded or machined parts arrive with a habit that works badly here. For a plastic housing, geometry is the spec — one file describes the part completely. A harness has almost no geometry of its own: it is a bill of materials plus a topology plus a set of process settings, and the topology is the only part a picture shows.
The picture leaves out exactly what moves the price: conductor size and construction, insulation type, terminal choice and plating, whether cavities are sealed, how many crimps per assembly, how much tape or braid wrap, and how the finished assembly is tested. A quote that does not name those things has not costed them. When your first article arrives 40 mm short on the branch that has to span to a fixed bolt, nobody has broken a contract — there was no dimension to break.
The fix is a package, not a longer email.
| RFQ element | What it fixes | What happens without it |
|---|---|---|
| Dimensioned assembly drawing | Overall length, branch lengths, breakout positions, tolerances, wrap and sleeve extents | The factory assumes lengths from a photo; the first article comes up short |
| Wire list | Conductor size, insulation type and rating, colour and tracer, circuit-to-cavity assignment | The factory quotes the cheapest wire it already stocks |
| Connector BOM | Housing, terminal, seal, cavity plug and retainer part numbers | A look-alike is substituted; your mating half will not latch or will not seal |
| Quantity and release schedule | Per-part-number annual volume and per-release quantity | The quote is priced at a batch size you will never actually order |
| Test requirement | Continuity, withstand-voltage, insulation resistance, pass criteria, records retained | Assemblies are tested to the factory's default, not to yours |
| Packaging and labelling | Coil diameter, tie method, bag and carton counts, label content | Leads arrive loose, tangled and unidentifiable at goods-in |
| Component supply model | Consigned (free-issue) or factory-purchased connectors | You compare a price with connectors against a price without them |
Fill in those seven lines and the quotes differ by price and lead time — the comparison you wanted. Leave three blank and they differ by scope, which is the comparison that wastes a month.
Document one: the dimensioned drawing
The drawing is the geometry contract. It does not need to be beautiful or 3D. A clean 2D layout on one sheet with a title block carrying part number, revision letter and date does the job. Revision control matters far more than draughting polish: every quote, every sample approval and every argument afterwards refers to a revision, and an undated PDF called harness_final_v2_new.pdf is not a revision.
Where you measure from
This is the most common cause of a re-quote, and it stays hidden until the first article lands. A branch quoted "600 mm" can mean 600 mm to the cut end of the wire, to the back of the crimped terminal, or to the mating face of the housing. Between the first and the last sits the terminal length plus the cavity depth — commonly 10 to 25 mm per end depending on the series. On a short jumper that is a scrap part.
Put a general note on the drawing in capitals: all lengths measured to connector mating face unless otherwise noted. Say whether dimensions run cumulatively from a single datum node or incrementally branch to branch, and mark the datum. Say also how the assembly is measured — laid flat and relaxed on the board, not pulled taut. A 1.5 metre trunk measures differently under tension, and a factory that measures one way while you inspect the other will fail an inspection nobody caused.
Tolerances that mean something
A single global tolerance applied to every dimension on the sheet is one nobody will honour. Length tolerance normally scales with length, and the practical band is negotiated rather than derived. The decision you need to make is narrower: which dimensions are critical and which are free.
A critical dimension is one where both ends are anchored — a branch running from a fixed board header to a bolted earth stud has no slack to absorb error. A free dimension ends in a cavity or at a device that can move. Mark the critical ones, give them their own tolerance, and let the rest sit under the general note. Two or three tightly toleranced branches cost very little; twenty of them re-price the whole assembly, because each adds a check to the outgoing routine.
Dimension the things that are not wire, too: where tape wrap starts and stops, where a split conduit or braided sleeve begins, how far a heat-shrink boot overlaps the backshell, and where a cable tie sits on the trunk. Those are labour minutes, and a factory guessing at them either guesses cheap and disappoints you or guesses expensive and loses the quote.
What the drawing does not have to be
It does not have to be a native CAD file. A dimensioned PDF from any source is quotable; a marked-up photograph carrying real measurements works for a simple two-ended lead. What it must have is a revision identity and a materials block tying each labelled branch to a row in the wire list. Whether a supplier will draft from a physical sample, and which file formats it prefers, is not published on most factory profiles; ask when you send the enquiry.
Document two: the wire list
The wire list is one row per conductor — the least glamorous document in the package and the one that most changes the price, because it is where copper volume, insulation type and crimp count become countable.
| Column | What goes in it | Why the factory needs it |
|---|---|---|
| Circuit ID | Your own circuit number or name | Ties the wire to the drawing and to the test program |
| From / To | Connector reference plus cavity number at each end | Makes the continuity test programmable and the build repeatable |
| Conductor size | Cross-sectional area in mm² and the equivalent gauge | Sets copper cost and terminal wire-range selection |
| Insulation type and rating | Compound, temperature rating, voltage rating | Determines stock, strip settings and price band |
| Outside diameter | OD over the insulation | Decides whether the wire seals in the cavity and fits the grommet |
| Colour / tracer | Base colour plus stripe, in a stated convention | Build repeatability and field fault-finding |
| Strip length | Per end, or "per terminal spec" | Applicator setup; a wrong strip is a failed pull test |
| Terminal part number | Per end, with plating | Selects the applicator and the crimp height |
| Seal part number | Where the cavity is sealed | Sealed and unsealed builds are different assemblies |
| Marking | Printed circuit number, sleeve or label text | Ferrule and label work is costed separately |
Gauge is not a single number
Metric area and gauge are different systems and do not map cleanly. Twenty gauge sits near 0.5 mm², but a 0.5 mm² automotive-style wire and a 20-gauge appliance wire can carry different strand counts, different wall thickness and a meaningfully different outside diameter over the insulation. The conductor decides whether the terminal crimps correctly. The outside diameter decides whether the wire seals.
That second point catches people. A cavity seal is specified for a range of wire outside diameters, not a conductor area. Order the right copper on the wrong insulation wall and the crimp passes every pull test while the sealed cavity quietly leaks. If your build has sealed cavities, put the OD column in the wire list and treat it as controlled. State strand construction too, if the branch flexes in service, and whether the conductor is tinned or bare.
Colour, tracer and circuit identity
Write colours in a stated convention — base and tracer, for example WH/BU for white with a blue stripe — and define the abbreviations on the drawing. Then decide how each circuit is identified once it is inside the loom.
A twelve-circuit control-cabinet loom built entirely in black wire is buildable, but the assembler is relying on routing memory rather than anything visible, and the electrician tracing a fault two years later has nothing to read. Colour coding, printed circuit numbers along the insulation, or numbered ferrules all solve it at different cost. Pick one deliberately and put it in the wire list.
Document three: the connector BOM
This is the gap in most RFQ packages a harness shop receives. Buyers send a housing part number and consider the connector specified. A connector is rarely one part number; it is usually three to six.
- Housing, with the correct circuit count, keying or polarisation code, and colour code — series that share a body offer several keying variants that will not mate with one another
- Terminal, chosen for the conductor size range and the plating, specified as reel-fed or loose-piece
- Wire seal, matched to the wire outside diameter, on sealed builds
- Cavity plug, for every unused cavity in a sealed housing
- Secondary lock or terminal position assurance, where the series uses one
- Backshell, boot or strain relief, where the build calls for it
Three details are worth knowing before you write this list.
First, a housing family very often has sealed and unsealed variants that look nearly identical and carry near-identical part numbers. The terminals differ, the applicator on the crimp press differs, and so does the piece price. Naming the housing alone hands that choice to whoever is quoting.
Second, terminal plating is a price line and a reliability decision at once. Mating a tin-plated terminal into a gold-plated header creates a dissimilar-metal contact interface — a service-life question nobody will raise on your behalf. State the plating.
Third, give the mating half. If your board header or device connector is already fixed, put its part number in the BOM as a reference item — a supplier who can see both halves catches a keying or gender mismatch during quoting, the cheapest possible moment. The same discipline underlies a good product spec sheet for a Chinese factory: give the counterpart, not only the part.
Consigned or factory-purchased
Connectors and terminals are frequently the single biggest line in a harness quote, and the one with the longest lead time. There are two ways to supply them, and a quote built on one is not comparable with a quote built on the other.
Under a consigned or free-issue model, you buy the connectors, ship them in, and the factory assembles. You keep the pricing and the allocation position and carry the freight and customs entry. Under a factory-purchased model, the supplier buys and the connector cost sits inside the unit price — simpler, harder to audit.
If you consign, agree an attrition allowance before the first order. Terminals are consumed setting up an applicator and proving the crimp; a press does not produce a good crimp on its first strike after a die change. A percentage agreed in the quotation is far easier than an argument about a shortfall mid-build. The general trade-offs are covered in consignment versus turnkey component supply; the harness-specific twist is that applicator setup scrap is real and needs a number.
State on the RFQ which model you want quoted. If you do not know, ask for both — a supplier that can quote both tells you something useful about whether it holds connector stock or buys per order.
Common questions
I only have a physical sample of the loom I currently buy. What do I send?
Send the sample plus a photograph marked up with the dimensions you actually control and the connectors your equipment already carries, then ask the supplier to draw the assembly from the sample and return the drawing for approval before quoting. Approve the drawing, not the sample — a sample can be measured three ways, a dimensioned drawing cannot.
Can I send only the housing part numbers and let the factory choose terminals?
You can, and you will get a valid quote — but you have handed over a decision affecting crimp reliability, plating compatibility and price. If you delegate it, require the supplier to write back the exact terminal and seal part numbers it intends to use, so the choice becomes a document you approved rather than a discovery you make during a cross-section review.
How do I make three harness quotes genuinely comparable?
Fix five things across all three: drawing revision, quantity break, component supply model, test requirement, and whether setup is quoted separately or amortised into the unit price. The reasons quotes for the same specification diverge are usually one of those five rather than a difference in margin.
What to ask the supplier next
- Confirm the drawing revision and date being quoted against; reject any quote that does not name one.
- Ask for the wire list back with the supplier's own stock equivalents filled in — you learn at once what it holds and what it must buy.
- Ask which connector and terminal part numbers it will use, in writing, including seals and cavity plugs.
- Ask which crimp applicators it already owns for your terminals, which would have to be bought, and who owns a purchased applicator afterwards.
- Ask for setup or NRE per part number as a separate line, not folded into the piece price.
- Ask for both a consigned and a factory-purchased price, plus the attrition allowance on consigned terminals.
- Ask what outgoing electrical test is standard, what the pass criteria are, and whether records are retained per lot.
- Ask which document accompanies the first article, and whether it carries measured dimensions rather than a pass stamp.
- On any certificate, quality-system document or test report you are shown, check three things on the document itself: the issuing body, the holder's exact company name, and the validity date.
Huisheng Electronics is a fair place to test the package, being the size of shop where an RFQ either lands correctly or does not — a Shenzhen harness maker incorporated in 2016 in Longhua District, working across six lines: electronic wires and charging cables, industrial and automotive looms, servo and control cables, two-wheeler EV connector leads and custom harnesses laid out to the buyer's drawing. Its published equipment roster lists automatic cut-strip-crimp machines, servo crimping presses and benchtop crimpers alongside a test bench covering terminal cross-section analysis, tensile and insertion-force testing, withstand-voltage and insulation checks, salt-spray and cable continuity. Stated trade terms are a 1,000-piece minimum per item, samples in 7–10 days and volume in 15–25 days to warehouse. Send the three documents together, ask for the crimp evidence described in how harness crimps are actually judged, and specify the electrical test you want using the continuity, withstand-voltage and insulation-resistance report as your template.