Reading Wire Harness Drawing
Posted by SZFRS Engineering Team
Wire harness drawings are dense documents. A single drawing typically conveys the harness physical layout, every connector specification, every wire’s gauge and termination, branch geometry, bundling and labeling instructions, and testing requirements. Reading them fluently is a learnable skill — once you understand the standard conventions, the drawings become readable like any technical specification. This guide walks through the symbols, callouts, schedule tables, and conventions used in typical wire harness drawings, with practical examples that mirror what we see in incoming customer specifications.
Table of Contents
TL;DR — What’s on a Harness Drawing
A complete wire harness drawing has five main sections: (1) the physical layout showing connector positions, branch points, and cable routing in 1:1 or scaled view; (2) the connector schedule listing each connector with its part number, position designator, and pin assignment; (3) the wire schedule tabulating every wire’s source pin, destination pin, gauge, color, and length; (4) the bill of materials (BOM) listing all components with manufacturer part numbers and quantities; (5) the notes section covering bundling, labeling, testing, and workmanship requirements. The physical drawing is the navigator; the schedules and BOM are the database; the notes are the workmanship instructions. Reading them together produces the complete picture.
Section 1 — The Physical Layout View
The physical layout shows the harness as it would appear when laid flat on a form board during assembly. Standard conventions:
- Scale. Most harness drawings are 1:1 or 1:2 scale, allowing the manufacturer to use the drawing directly as a form-board reference. Some larger automotive harnesses use multi-sheet drawings at 1:1 with sheet boundaries marked.
- Connector representation. Connectors are typically shown as rectangles or specific shape symbols (round for circular connectors, rectangular for IDC, hexagonal for shielded backshells). Each connector has a position designator (P1, P2, J1, etc.) and reference to the connector schedule.
- Trunk and branch lines. The cable runs are shown as bold lines. The main trunk is typically the longest path through the harness. Branches extend from the trunk at specific dimensions called out on the drawing.
- Branch point identification. Y-points or trunk-branch junctions are typically labeled (BP1, BP2, etc.) and dimensioned from a reference end. The reference is usually one specific connector designated as P1 or as the “primary” end.
- Length dimensions. Distances from connector to connector and from connector to branch points are dimensioned on the drawing. Tolerance is either explicitly noted or carried in a general notes block.
- Connector orientation indicators. Pin 1 position, locking lever direction, mating face orientation. Critical for correctly oriented connector mounting during assembly.
The physical layout is what we use most often during the form-board setup phase of harness assembly. Production teams pin the drawing onto the form board (or load it into the digital form-board system) and lay wires according to the geometry shown.
Section 2 — The Connector Schedule
The connector schedule (sometimes called connector list or connector summary) tabulates every connector in the harness:
| Position | Connector Type | Manufacturer P/N | Pin Count | Mating Conn (ref) | Position on drawing |
|---|---|---|---|---|---|
| P1 | Deutsch DT | Deutsch DT06-12S | 12 | Mate to ECU | Sheet 1, top left |
| P2 | JST PHR-3 | JST PHR-3 | 3 | Mate to PCB header | Sheet 1, lower left |
| P3 | M12 5-pin A-coded | Phoenix 1681965 | 5 | Sensor cable | Sheet 1, right side |
| … | … | … | … | … | … |
Reading the connector schedule:
- Position (Px / Jx). Reference to the position on the physical layout. P1 in the schedule is the same connector shown as P1 on the drawing.
- Connector type. Generic family identification — Deutsch DT, JST PH, M12 A-coded, etc. This describes the connector class without committing to specific part number.
- Manufacturer part number. The specific part number to be sourced. This is the source of truth for what to order.
- Pin count. Often redundant with part number but useful for double-checking. A 12-pin DT06 vs an 8-pin DT06 are different parts.
- Mating reference. What the connector mates to. Useful context but not always present.
- Position on drawing. Cross-reference back to the physical layout. Useful for multi-sheet drawings where finding a specific connector takes navigation.
The connector schedule alone is insufficient — it describes which connectors exist but doesn’t say which wires connect where. The wire schedule fills in that information.
Section 3 — The Wire Schedule
The wire schedule (also called wire list, wire table, or net list) is the most detailed section of a harness drawing. Every individual wire in the harness gets a row:
| Wire # | From Conn / Pin | To Conn / Pin | AWG | Color | Length (mm) | Notes |
|---|---|---|---|---|---|---|
| W001 | P1-1 | P3-3 | 22 | RED | 650 ±10 | +12V power |
| W002 | P1-2 | P3-1 | 22 | BLK | 650 ±10 | GND |
| W003 | P1-3 | P2-1 | 22 | WHT | 320 ±5 | Signal A |
| W004 | P1-4 | P2-2 | 22 | BLU | 320 ±5 | Signal B |
| W005 | P1-5 | BP1-out | 22 | YEL | 200 ±5 | Trunk to BP1 |
| … | … | … | … | … | … | … |
Reading the wire schedule:
- Wire identifier (W001, W002). Unique number for each wire in the harness. Used in cross-references throughout the drawing.
- From / To. Source and destination pins. P1-1 means “Connector P1, Pin 1.” This is the routing instruction during assembly.
- AWG (or mm²). Wire gauge. Drives the wire’s current capacity and physical size.
- Color. Insulation color. Standard color codes (RED for +V, BLK for GND, WHT for signal) speed up assembly and field troubleshooting. Some applications use color codes prescribed by industry standards (UL/CSA color codes for power applications).
- Length. Physical wire length with tolerance. The length is from termination to termination, typically excluding the depth inside connector contacts.
- Notes. Additional information — signal name, twisting requirements, shielding, special notes. The notes column is where program-specific requirements appear.
The wire schedule is what the manufacturer’s continuity tester uses for end-of-line verification. Every wire in the schedule has to be present, terminated correctly, and connect from the specified source pin to the specified destination pin. Continuity test failures map directly to wire schedule rows.
Section 4 — The Bill of Materials (BOM)
The BOM lists all components needed to build the harness:
- Connectors. Each connector position with manufacturer part number and quantity.
- Contacts (terminals). The individual contact pins or sockets that go into the connector housings. Many connector families ship contacts separately from housings.
- Wire. By gauge, color, and meters needed.
- Sleeves, boots, and overmolding. Strain relief boots, sealing sleeves, overmolding materials.
- Cable ties, tape, and bundling. Cable ties (tie-wraps), spiral wrap, harness tape, fabric tape, conduit. The specific material affects field service appearance and the assembly process.
- Labels. Cable labels, connector labels, branch identifiers.
- Heat shrink. Heat shrink tubing where used (over splice points, over cable transition zones).
The BOM line items have manufacturer part numbers, quantities per assembly, and ideally a unit-cost-tracking column. We use the BOM to source components and to compute material costs during quoting.
Section 5 — Notes and Workmanship Instructions
The notes section conveys the workmanship and process requirements:
- Workmanship class. IPC/WHMA-A-620 Class 1, 2, or 3. The most important workmanship note. Drives how the harness is inspected and what defects are acceptable.
- Wire stripping length. How much insulation to strip back at each termination. Specific to the connector and crimping process.
- Crimping requirements. Pull force tests, crimp height verification, micrograph inspection requirements (typically Class 3 only).
- Bundling requirements. How tightly to bundle wires, where to place cable ties, what tape to use, conduit specifications.
- Labeling requirements. Where labels go, what they say, how they’re attached.
- Testing requirements. Continuity (always), hi-pot, insulation resistance, functional testing, sample frequency.
- General tolerance block. Default tolerances for any dimension not explicitly tolerated on the drawing.
- Reference standards. IPC/WHMA-A-620, customer-specific specifications, regulatory requirements.
- Compliance markings. RoHS, REACH, conflict minerals, FAA/EASA aerospace requirements.
The notes section is where program-specific quality requirements live. Reading the notes alongside the schedules and physical layout produces the complete picture of what the harness should be.
Standard Symbols and Conventions
While there isn’t a single industry-wide standard for harness drawing symbols, common conventions appear across the industry:
| Symbol | Meaning |
|---|---|
| P1, P2, P3… | Plug connectors (cable side, mate to a fixed receptacle) |
| J1, J2, J3… | Jack connectors (chassis-mounted receptacle, sometimes used interchangeably with P) |
| BP1, BP2… | Branch points (where wires split off the trunk) |
| SP1, SP2… | Splice points (where two wires are spliced together, typically with crimped splice or solder) |
| TP1, TP2… | Termination points (where a wire ends without a connector, e.g., ring lug or fork terminal) |
| L (with subscript) | Length dimension |
| Triangular flag | Reference annotation flag — typically points to a specific note number |
| Dashed lines | Hidden / not visible from this view (multi-sheet harnesses) |
| Cross-hatch pattern | Bundling tape or harness wrap area |
| Ring/circle on wire | Cable tie position |
Specific industries (automotive, aerospace, medical) layer additional conventions on top of these. Automotive harness drawings use SAE-defined symbols; aerospace uses MIL-STD references; medical uses application-specific conventions consistent with ISO 13485 documentation practices.
Cable Assembly Drawings — A Simpler Subset
Cable assembly drawings (point-to-point cable with connectors at each end, no branching) follow the same conventions but are much simpler. Typical cable assembly drawing components:
- Physical layout. Two connector positions and a single cable run between them. May include any specific routing requirement (90° bend, S-curve, etc.) but no branching.
- Connector callout. Both connectors with manufacturer part numbers and pin assignment.
- Cable specification. Cable type, gauge, color, jacket material, shield (if any).
- Length specification. Total length with tolerance.
- Pinout. Often shown as a small table or as wire-by-wire connections drawn between the two connectors.
- Notes block. Workmanship class, testing, labeling, packaging.
Cable assembly drawings often fit on a single sheet and don’t always need separate connector schedule, wire schedule, and BOM tables — the information is compact enough to live in callouts on the layout itself. For our incoming RFQs, the smaller cable assemblies typically come as single-sheet drawings; multi-connector wire harnesses come as multi-sheet packages with separate schedules.
Real-World Case Study — Reading a Specific Drawing
An incoming RFQ for an industrial sensor harness recently included a 3-sheet drawing package. Walking through how we read it:
- Sheet 1 — Physical layout. Showed 5 M12 connectors (P1 through P5), one main trunk, two branch points. Trunk length 800mm; branches 200mm and 350mm. Connector positions clearly designated, branch point dimensions called out.
- Sheet 2 — Connector schedule and wire schedule. All 5 M12 connectors specified as Phoenix Contact 1681965 (5-pin A-coded). 22 wires routed across the harness, each wire row in the schedule with from/to pin specification.
- Sheet 3 — BOM and notes. Component list with all 5 connectors, contacts, 22 wires (4 different colors), bundling tape, cable ties, labels. Notes specified IPC/WHMA-A-620 Class 2 workmanship, 100% continuity testing, 1500 VAC hi-pot at sample frequency, tinned copper for outdoor use, FT1 UL flammability.
Reading this drawing took about 15 minutes from receipt to having a complete BOM and quote-ready understanding. The clarity of the drawing — well-defined connector schedule, complete wire schedule with from/to specification, comprehensive notes — made the quote process straightforward. We’ve seen drawings where any of these elements is missing or ambiguous, and those drawings extend the quote process by days as we iterate with the customer to fill in gaps.
Common Drawing Quality Issues
Patterns we see in incoming drawings that delay quoting:
Connectors specified by family without manufacturer part number. “M12 5-pin A-coded” doesn’t tell us which manufacturer or specific part. Many M12 manufacturers exist (Phoenix Contact, Lumberg, Murr Elektronik, Binder, HTP, Turck) and parts are not perfectly interchangeable.
Wire schedule missing pin assignment. The wire schedule shows wire color and length but not which pin connects where. Without pinout, we can’t verify continuity test routing or spot mismatches between drawing intent and actual routing.
Length tolerances inconsistent across sheets. Sheet 1 says ±5mm, Sheet 2 says ±2%. For a 500mm length, ±5mm is ±1% while ±2% is ±10mm — nearly an order of magnitude different. We need explicit reconciliation.
Reference standards listed without specific revision. “Per IPC/WHMA-A-620” without revision. The standard has gone through revisions (D, E, current); the differences matter for some workmanship details. Specifying revision avoids ambiguity.
BOM and wire schedule disagreement. The BOM lists 22 wire entries; the wire schedule shows 24 wires. The discrepancy needs reconciliation before we can quote. Usually one is right and one needs updating; sometimes both have errors that the customer didn’t catch.
Drawing Format Trends
- PDF drawings remain dominant. Multi-page PDF with physical layout, schedules, BOM, and notes is the standard format for most cable suppliers. Easy to share, easy to print, universally readable.
- 3D CAD models becoming more common. STEP, IGES, or proprietary 3D model formats from Catia, NX, SolidWorks, or specialty cable design tools. The 3D model captures spatial routing (especially for harness installations in complex 3D spaces) better than 2D drawings. Most cable suppliers accept 3D models alongside 2D drawings.
- Spreadsheet wire schedules. The wire schedule sometimes ships as a separate Excel or CSV file rather than embedded in the drawing. This makes the data easier to import into manufacturing systems and continuity test programmers.
- Specialty cable design tools. Catia E3D, Capital Wiring, Mentor Graphics Capital, Zuken E3 — major automotive and aerospace OEMs use specialty harness design tools that generate drawings, schedules, and BOMs from a single design file.
For our work, we accept any drawing format the customer uses. PDF is most efficient because it’s universally readable; CAD models help when the spatial geometry matters; spreadsheet wire schedules speed up our continuity test setup. We adapt to the customer’s documentation toolchain.
Bottom Line
Wire harness drawings have five main sections — physical layout, connector schedule, wire schedule, BOM, and notes — that together describe a complete harness. Reading them takes practice but follows learnable conventions. Standard symbols (P/J for connectors, BP for branch points, SP for splice points), schedule table formats, and notes blocks are widely used across the industry. For procurement and engineering teams sourcing harness or cable assembly products, the drawing is the primary communication tool with the supplier; understanding what’s on it speeds up quoting, reduces RFQ iterations, and produces accurate quotes the first time.
Related Reading
- Cable vs Harness vs Wire Terminology — foundational terminology guide.
- How to Specify a Custom Cable Assembly — specification framework.
- Connector Selection Basics — connector decision framework.
- Wire Harness — wire harness manufacturing scope.
- IPC/WHMA-A-620 Workmanship — the workmanship standard referenced on most harness drawings.
Cable Assembly or Harness Quote?
Send us your drawing — we’ll read it carefully and quote within 48 hours for cable assemblies and 5-10 days for comprehensive wire harness packages. We catch and flag specification gaps during the quoting process so you don’t get surprises after the order.