How to Specify a Custom Wiring Harness: A Drawing Checklist That Gets You an Accurate Quote

Every week, harness enquiries stall for the same reason: the drawing or description is missing two or three details, so the quote comes back with questions instead of a price. If you buy or resell wiring harnesses for vehicles, a complete specification is the fastest way to cut days off sampling and avoid the most common production mistakes.

Here is the checklist our buyers use before they send an enquiry.

1. Connectors: type, pins and orientation

Name the connector family or send a clear photo of both mating halves. State the pin count, whether each end is male or female, and any locking style (latch, screw, bayonet). "Same as the sample" works only if a sample actually travels with the enquiry — say so explicitly if you are sending one.

2. Wire: gauge, colour and length per branch

List every branch separately: wire gauge (mm² or AWG), insulation colour, and the length of that branch measured end to end. A single overall length is the number one cause of harnesses that arrive too short on one leg. If any branch carries high current — power feeds, heaters, work lamps — mark it, because it drives wire gauge and terminal choice.

3. Terminals and contacts

State the terminal type (spade, ring, bullet, pin/socket) and size for each wire end, and whether terminals should be crimped only, or crimped and soldered. If the harness plugs into an existing junction box or fuse block, give the terminal size that box accepts — for example the 6.3 mm spade terminals used in most trailer junction boxes.

4. Protection: sleeving, conduit and environment

Tell the supplier where the harness will live. Engine-bay and chassis runs usually need split corrugated conduit or braided sleeving; exposed trailer wiring needs UV and abrasion resistance; anything near the road spray zone benefits from sealed connectors. Note temperature range and water exposure honestly — over-specifying protection costs money, under-specifying it costs returns.

5. Labels, markings and extras

Branch labels, a printed part number, or coloured tape markers cost almost nothing at production time and save your installers real time. List them in the specification rather than asking later. The same goes for included extras: cable glands where the harness passes through a panel, grommets, mounting clips, or a mating socket for the other end.

6. Quantities and approval flow

Give the sample quantity and the expected bulk quantity separately, and say whether the sample must be approved before production (it should be). If the harness replaces an existing part, the vehicle model and year help the supplier cross-check connector choices.

A one-page template you can copy

Application / vehicle:

Connector A (type, pins, gender):

Connector B (type, pins, gender):

Branches: (gauge / colour / length for each)

Terminals: (type and size per end)

Protection: (sleeving or conduit type, environment)

Labels / extras:

Sample qty / bulk qty:

Send that with a photo or sample and most suppliers can quote without a single follow-up question. You can see how we handle drawing-based sourcing on our Custom Wiring Harnesses page, browse the Cables & Wiring Harnesses category, or send your specification straight to us through the contact page — we reply with a quotation.