A well-designed wire harness doesn't just connect points A to B—it survives the environment, fits the available space, can be manufactured efficiently, and can be serviced in the field. Poor harness design creates assembly nightmares, field failures, and expensive redesigns.
This guide walks through the complete harness design process, from initial requirements through production release. Whether you're designing your first harness or refining your methodology, these twelve steps provide a proven framework for success.
Gather Requirements
Before opening CAD software, understand exactly what the harness must do. Incomplete requirements are the #1 cause of harness redesigns—and the cheapest problems to fix are the ones you prevent.
Requirements Checklist
Electrical
- • Voltage levels (all circuits)
- • Current requirements (continuous & peak)
- • Signal types (power, analog, digital, data)
- • Shielding/EMC requirements
- • Grounding strategy
Environmental
- • Temperature range (ambient & operating)
- • Moisture/IP rating requirements
- • Chemical exposure (fuels, oils, coolants)
- • Vibration levels
- • UV/ozone exposure
Physical
- • Available routing paths
- • Connector locations (fixed vs. flexible)
- • Weight limits
- • Flex/articulation requirements
- • Service access needs
Compliance
- • Industry standards (automotive, aerospace, medical)
- • Safety certifications (UL, CSA, CE)
- • Customer specifications
- • Quality class (IPC-620 Class 1/2/3)
- • RoHS/REACH compliance
Create System Schematic
The schematic defines what connects to what—it's the logical foundation before physical design begins. A clear schematic prevents crossed wires, missed connections, and debugging headaches.
Schematic Best Practices
Organize by system
Group related circuits (power, signals, ground)
Label everything
Wire IDs, pin numbers, connector refs
Show all connections
Including grounds and shields
Include ratings
Fuse values, current limits, wire gauges
Select Connectors
Connector selection drives many downstream decisions—wire termination method, housing style, and often the harness routing itself. Select connectors early and verify availability.
| Selection Criteria | Key Questions |
|---|---|
| Current Rating | Does it handle peak current with margin? Consider derating for temperature. |
| Pin Count | Enough for current needs + 10-20% for future expansion? |
| Environmental | IP rating, temperature range, vibration resistance match application? |
| Mating Cycles | Service connector vs. permanent? (50 vs. 500+ cycles) |
| Keying/Polarization | Can it be mated incorrectly? Need unique keying? |
See our connector types guide for detailed information on common connector families.
Size Wires & Cables
Wire sizing is a balance between electrical requirements (current capacity, voltage drop), physical constraints (flexibility, weight), and cost. Size for the application, not just the minimum.
"I always recommend sizing wires one gauge larger than calculations require. It costs a few cents more but gives you margin for manufacturing tolerance, elevated temperatures, and the inevitable 'can we add one more device to this circuit?' that always comes later."
Hommer Zhao
Harness Design Engineer
Use our wire selection guide for AWG tables and voltage drop calculations.
Define Wire Routing
Routing transforms your schematic into physical reality. Good routing considers manufacturing, installation, service, and lifetime reliability—not just the shortest path.
Routing Do's
- • Follow existing cable trays and channels
- • Maintain minimum bend radius (6× OD typical)
- • Route away from heat sources
- • Allow for thermal expansion
- • Plan for harness installation sequence
Routing Don'ts
- • Don't route over sharp edges
- • Don't cross moving parts without protection
- • Don't bundle high-power and sensitive signals
- • Don't create service access obstacles
- • Don't rely on friction alone for support
Select Protection
Protection keeps your harness safe from the environment and the environment safe from your harness. Match protection level to actual exposure conditions.
| Protection Type | Use Case | Pros/Cons |
|---|---|---|
| Braided Sleeving | Abrasion, expansion zones | Flexible, easy install / Not waterproof |
| Convoluted Tubing | General protection, routing | Good flexibility / Traps debris |
| Heat Shrink | Breakouts, splices, strain relief | Tight fit, sealed / Permanent |
| Tape Wrap | Bundle containment, noise reduction | Low cost / Labor intensive |
| Conduit | Harsh environments, high abrasion | Maximum protection / Rigid, heavy |
Design Breakouts
Breakouts—where individual wires or sub-bundles separate from the main trunk—are critical stress points. Poor breakout design leads to wire fatigue and insulation damage.
Use boot molding for sealed applications
Provides strain relief and environmental protection at connector interface
Maintain angles ≤ 90°
Sharp angles create stress concentrations; gradual curves distribute load
Support breakout points
Tape, clamps, or sleeving to prevent wires from moving independently
Allow service length
Extra wire length at breakouts enables future rework or connector replacement
Create Harness Drawing
The harness drawing is your manufacturing instruction—it must contain everything needed to build the harness without further clarification. Incomplete drawings create production delays and quality issues.
Drawing Content Checklist
- Full-scale harness layout
- Wire table (ID, gauge, color, length)
- Connector details (P/N, cavity assignments)
- Splice and terminal callouts
- Critical dimensions and tolerances
- Protection and covering specs
- Label/marking requirements
- Test requirements
- Bill of materials
- Revision history
Build Prototype
A physical prototype reveals problems that CAD can't predict—wire stiffness, connector clearance, installation fit. Never skip prototyping, even for "simple" harnesses.
Prototype Objectives
- • Verify fit in actual installation
- • Check wire lengths and routing
- • Confirm connector accessibility
- • Identify installation sequence issues
- • Test service access
Prototype Documentation
- • Photograph installation process
- • Mark any forced fits or interferences
- • Record actual vs. designed dimensions
- • Document installer feedback
- • Track all change requests
Perform DFM Review
Design for Manufacturing (DFM) review ensures your harness can be built efficiently at production volumes. Involve manufacturing engineers early—fixing issues on paper costs a fraction of fixing them on the production floor.
DFM Questions to Ask
Validate & Test
Testing proves your design works—not just electrically, but in the real-world conditions it will face. Define pass/fail criteria before testing begins.
Continuity
Verify all connections are made correctly
Insulation Resistance
Confirm isolation between circuits
Hi-Pot
Verify insulation withstands voltage stress
Pull Force
Validate crimp and termination quality
Functional
Confirm harness works in the application
Environmental
Verify performance under temp, humidity, vibration
For detailed testing procedures, see our testing methods guide.
Release to Production
Production release isn't just hitting "approve"—it's ensuring everyone has what they need to build consistently. A proper release package prevents first-article issues and ramp-up delays.
Production Release Package
- Approved harness drawing (signed-off)
- Bill of materials with approved vendors
- Work instructions / assembly sequence
- Test specification and limits
- Inspection criteria (visual standards)
- First article inspection requirements
- Quality control plan
- Tooling and fixture specifications
Frequently Asked Questions
How long does the harness design process take?
Simple harnesses can be designed in 1-2 weeks. Complex automotive or aerospace harnesses may take 3-6 months from requirements to production release. The biggest variable is usually iteration cycles with the customer.
What software is used for harness design?
Common tools include Capital Harness XC (Mentor), CATIA Electrical, E3.series, Zuken Harness Builder, and SolidWorks Electrical. Many smaller projects still use AutoCAD with manual wire lists.
When should I involve the manufacturer?
As early as possible—ideally during schematic development. Manufacturers can advise on component availability, suggest cost-effective alternatives, and identify DFM issues before they're locked into the design.
How do I reduce harness cost?
Focus on component standardization (fewer unique parts), efficient routing (shorter wire lengths), appropriate protection (don't over-specify), and DFM optimization. Our cost reduction strategies guide covers this in detail.
Related Resources
About the Author
Hommer Zhao is a Harness Design Engineer with over 15 years of experience across automotive, industrial, and medical device applications. He has designed harnesses ranging from simple sensor cables to complex vehicle wiring systems with 500+ circuits.
Connect with Hommer