A well-designed wire harness is easier to manufacture, cheaper to produce, more reliable in the field, and simpler to service. A poorly designed one causes production delays, drives up costs, and creates field failures that damage your product's reputation. The difference often comes down to a handful of design decisions made early in the process.
This guide covers the best practices that experienced harness engineers follow — and the common mistakes that cost OEMs time and money.
1. Design for Manufacturability from Day One
The single most impactful thing you can do is design your harness with manufacturing in mind — not as an afterthought. This means:
- Involve your manufacturer early. Share your concept or preliminary drawings and ask for DFM feedback before the design is finalized. Changes are cheap on paper and expensive in production.
- Design for harness board assembly. Most harnesses are built on flat assembly boards with jigs and fixtures. Design your routing so the harness can be laid out, assembled, and removed from a board efficiently.
- Minimize unique components. Every unique wire length, connector, or terminal adds complexity. Standardize where possible — use common wire gauges, common connector families, and common terminal types across your harness.
2. Get the Wiring Schematic Right
The wiring schematic is the foundation of your harness design. Every wire, connector, pin assignment, and splice should be clearly defined. Common schematic issues that cause problems downstream:
- Missing pin assignments — which wire goes to which cavity?
- Ambiguous wire callouts — gauge, color, and insulation type should all be specified.
- Undefined splice locations — where do wires join and what method is used?
- Missing fuse and relay specifications — if they're part of the harness, call them out.
A clean, complete schematic saves time in quoting, reduces first-article issues, and prevents costly errors in production.
3. Choose Connectors Carefully
Connector selection has an outsized impact on cost, reliability, and serviceability. Key principles:
- Match the connector to the environment. Use sealed connectors (Deutsch DT, Molex MX150, TE Superseal) where moisture, dust, or chemicals are present. Use unsealed connectors (Molex Mini-Fit, JST XH) for protected internal connections.
- Don't over-specify. Sealed connectors cost 3–10x more than unsealed equivalents. If the connection is inside a dry enclosure, an unsealed connector is the right choice.
- Consider serviceability. Will the harness ever need to be disconnected in the field? Choose connectors with adequate mating cycles and intuitive keying so field techs can't mis-mate them.
- Use keying and color coding. Prevent incorrect mating by using different connector sizes, key positions, or housing colors for connectors that could be confused.
4. Size Wire Correctly
Wire gauge selection affects safety, performance, and cost. Follow these principles:
- Size for peak current, not average. Account for motor inrush, transient loads, and worst-case operating conditions.
- Calculate voltage drop for long runs, especially in 12V and 24V DC systems where even small drops affect performance.
- Apply derating factors for bundled conductors and elevated ambient temperatures.
- Standardize gauges where possible. If one circuit needs 18 AWG and another needs 20 AWG, consider using 18 AWG for both to reduce inventory and simplify production.
5. Plan Your Routing
How the harness is routed through the product directly affects reliability and service life:
- Respect minimum bend radii. Use 10x the bundle diameter for dynamic applications, 6x for static. Tight bends fatigue conductors and crack insulation.
- Avoid heat sources. Route away from exhaust, engines, and high-temperature components. If proximity is unavoidable, use high-temperature insulation (XLPE, PTFE, silicone) and heat shielding.
- Prevent abrasion. Use grommets where the harness passes through sheet metal. Add protective sleeving in areas where the harness contacts sharp edges or moving parts.
- Support the harness. Use P-clamps, cable ties, and adhesive mounts at regular intervals to prevent the harness from hanging unsupported. Unsupported weight causes strain at connectors and breakout points.
- Plan for service access. Don't bury connectors behind components that have to be removed to access them. Field techs will thank you.
6. Specify Proper Strain Relief
The cable-to-connector junction is the most vulnerable point in any wire harness. Without proper strain relief, vibration and mechanical stress will fatigue the wire at the termination point, causing intermittent or permanent failures. Options include:
- Overmolding — the gold standard. Creates a sealed, reinforced transition between cable and connector.
- Backshells and strain relief boots — attach to the connector and support the cable at the exit point.
- Heat shrink — provides moderate strain relief and environmental protection.
- Cable ties and clamps — secure the cable near the connector to prevent pulling force from reaching the termination.
7. Label Everything
Proper labeling saves enormous time during installation, testing, and field service. Best practices:
- Label every connector with a unique identifier that matches your schematic.
- Label individual wires at each end for circuits that will be field-terminated.
- Use durable label materials — self-laminating labels, heat-shrink labels, or laser-marked sleeves for harsh environments.
- Include the harness part number and revision on the main harness label.
8. Define Your Testing Requirements
Every wire harness should be electrically tested before it ships. At minimum, specify:
- Continuity testing — verify every circuit is complete from end to end.
- Short-circuit testing — verify no unintended connections exist between circuits.
- Hi-pot / dielectric withstand — for applications requiring insulation integrity verification (common in industrial and medical applications).
- Pull-force testing — verify crimp terminations meet minimum retention force requirements.
H&L performs 100% end-of-line continuity and short-circuit testing using automated test fixtures with integrated vision systems, plus crimp force monitoring and cross-section analysis.
9. Prototype Before Production
Never go straight from drawing to production. Build prototypes, install them in your product, test them, and iterate. Prototyping catches:
- Routing conflicts you didn't see on the drawing
- Wire lengths that are too short or too long
- Connector access issues that make installation difficult
- Interference with other components
- Opportunities to simplify the design
H&L supports rapid prototype builds with engineering feedback — so you can validate and improve your design before committing to production tooling and volumes.
10. Document Everything
A complete documentation package ensures your harness can be built consistently, inspected accurately, and maintained in the field. Your package should include:
- Wiring schematic with pin assignments
- Harness assembly drawing with dimensions and routing
- Bill of materials (BOM) with manufacturer part numbers
- Special instructions (torque specs, testing requirements, packaging)
- Revision history
Design Support from H&L
H&L Manufacturing offers SolidWorks design assistance, DFM review, and APQP processes to help engineers optimize their wire harness designs for manufacturability and reliability. From concept through prototype to production, our team is an extension of yours. Send us your drawings and let's build something together.
Harness Design FAQ
Common questions about wire harness design and working with a manufacturer.
What is Design for Manufacturability (DFM)?
DFM is the practice of designing a product so it can be manufactured efficiently, consistently, and at the lowest practical cost. For wire harnesses, this means choosing standard components, simplifying routing, minimizing unique wire lengths, and designing for assembly on a harness board. A good manufacturer will provide DFM feedback during design review.
What should my wire harness drawing include?
A complete drawing should include a wiring schematic, wire callouts (gauge, color, length, part number), connector and terminal part numbers, routing paths, breakout dimensions, labeling requirements, and any special instructions (torque specs, testing requirements, packaging). The more detail you provide, the faster and more accurate the quoting and first-article process.
How early should I involve my manufacturer in the design process?
As early as possible. Engaging your manufacturer during the design phase — not after the drawing is finalized — gives you access to DFM feedback, component recommendations, and cost-saving suggestions that become much harder to implement later. H&L offers SolidWorks design assistance and APQP processes to support early-stage collaboration.
What is the minimum bend radius for wire harness routing?
The general rule is a minimum bend radius of 10x the cable or bundle diameter for dynamic (moving) applications and 6x for static (fixed) installations. Tighter bends stress the conductors and insulation, leading to premature failure. Always check the wire and connector manufacturer's specifications for exact minimums.
How do I reduce wire harness cost without sacrificing quality?
Standardize wire gauges and colors across circuits where possible, use common connector families, minimize the number of unique wire lengths, design for efficient board assembly, and avoid over-specifying (e.g., sealed connectors where unsealed will do). Your manufacturer can identify the biggest cost drivers during design review.
Should I use a BOM or a drawing to specify my harness?
Both. The drawing defines the physical layout, routing, and dimensions. The BOM lists every component with part numbers, quantities, and specifications. Together they give the manufacturer everything needed to quote and build accurately. If you only have one, start there — your manufacturer can help develop the other.
What wire colors should I use?
Follow your industry's color-coding standards if applicable (SAE J1128 for automotive, NEC for building wire). If no standard applies, establish a consistent color code across your product line and document it. Use unique colors or striped wire for circuits that could be confused during installation or service.
Does H&L offer engineering support for harness design?
Yes. H&L provides SolidWorks design assistance, DFM review, APQP processes, and prototype builds with engineering feedback. Our team reviews every design for manufacturability and can recommend improvements to reduce cost, improve reliability, and streamline production.