Designing a wire harness is a multidisciplinary engineering task that combines electrical design, mechanical packaging, environmental analysis, and manufacturing considerations into a single integrated assembly. Whether you are designing your first harness or refining an existing product, this comprehensive guide walks you through every phase of the process—from initial requirements gathering to final documentation and testing. At Simkab, we support our clients through each of these stages as part of our engineering and design services.
1. Requirements Gathering
Every successful wire harness project begins with a thorough understanding of the application requirements. Before selecting a single wire or connector, your design team should document the following: the number of circuits and their functions (power, signal, data, ground), the voltage and current requirements for each circuit, the operating environment (temperature range, humidity, chemical exposure, UV exposure, vibration), the physical space available for routing, any regulatory or certification requirements (UL, CE, RoHS, REACH), and the expected product lifetime. Gathering these requirements upfront prevents costly redesigns later. For complex projects, consider using a formal requirements specification document that all stakeholders review and approve before design begins.
2. Wire Selection
Selecting the right wire for each circuit is one of the most critical decisions in harness design. The primary factors are conductor size (gauge), insulation material, and temperature rating. The conductor gauge must provide sufficient ampacity for the circuit’s maximum current draw, with appropriate derating applied for bundled conductors and elevated ambient temperatures. Common insulation materials include PVC (general purpose, low cost), XLPE (high temperature and chemical resistance), PTFE/Teflon (extreme temperatures and chemical environments), and silicone (high flexibility and heat resistance). For applications in the energy sector, XLPE and LSZH (Low Smoke Zero Halogen) insulations are frequently specified. In agricultural equipment, UV-resistant and oil-resistant insulations are preferred. Always consult the wire manufacturer’s datasheet for exact ratings and verify that the selected wire meets any applicable UL or EN standards.
3. Connector Selection
Connectors are the interface between your harness and the rest of the system, and their selection directly impacts reliability, serviceability, and cost. Key selection criteria include: the number of contacts (pins/sockets), current and voltage rating per contact, contact type (crimp, solder, IDC, press-fit), mating and unmating force, number of rated mating cycles, environmental sealing (IP rating), locking mechanism (positive lock, bayonet, threaded), and shielding capability. Major connector families from manufacturers like TE Connectivity, Molex, JST, Amphenol, and Phoenix Contact cover virtually every application—from compact board-to-wire connections in telecommunications equipment to heavy-duty circular connectors in industrial machinery. Select connectors early in the design process, as they often dictate wire gauge, terminal type, and tooling requirements.
4. Routing & Mechanical Layout
Routing defines the physical path the harness takes through the equipment or vehicle. Good routing minimizes wire length (reducing cost, weight, and voltage drop), avoids heat sources and moving parts, provides adequate bend radii (typically 6–10 times the cable outer diameter for static installations), and allows for service access to connectors and breakout points. In 3D-modeled products, harness routing is typically done using CAD tools that allow the engineer to visualize the path within the actual geometry of the product. For simpler designs, a 2D schematic combined with physical measurements from the equipment may suffice. During routing, identify all pass-through points where grommets or bulkhead fittings will be needed, and plan mounting points where the harness will be secured with clips, clamps, or cable ties. Consider the assembly sequence—the harness must be installable in the order that the product is assembled.
5. Shielding & EMC Considerations
In applications where electromagnetic compatibility (EMC) is a concern, shielding must be designed into the harness from the start. Signal wires carrying low-level analog signals or high-speed digital data are particularly susceptible to electromagnetic interference (EMI) from nearby power conductors, motors, or external sources. Common shielding approaches include foil shields (aluminum/polyester), braided shields (tinned copper), and combination foil-braid shields. The shield must be properly terminated—typically at one or both ends of the cable—through the connector backshell or a dedicated ground pigtail. Separation of power and signal circuits, twisted-pair configurations for balanced signals, and proper grounding practices all contribute to a harness that meets EMC requirements. For telecommunications applications, shielding design can be the difference between a product that passes certification and one that fails.
6. Environmental Protection
Wire harnesses often operate in harsh environments, and the design must account for every environmental stress the product will face. Temperature extremes affect insulation flexibility and connector seal performance. Moisture and humidity can cause corrosion at terminals and degrade insulation over time—sealed connectors and heat-shrink boots with adhesive lining provide protection. Chemical exposure from oils, fuels, solvents, or cleaning agents requires chemically resistant insulation and jacket materials. UV exposure in outdoor installations demands UV-stabilized jackets or conduit. Vibration in vehicles, machinery, and agricultural equipment requires strain-relieved connections, abrasion-resistant coverings, and routing that avoids resonance with vibrating structures. Salt spray in marine or coastal environments calls for tinned copper conductors and stainless steel hardware. Dust and particulate exposure in industrial and field environments is managed through appropriate IP-rated connectors and sealed pass-throughs.
7. Prototyping & Validation
Before moving to serial production, the design must be validated through physical prototypes. Prototyping allows you to verify fit within the actual equipment, confirm that connectors mate correctly and that the harness can be installed in the required assembly sequence, check that bend radii are adequate and that no wires are under tension, and perform initial electrical testing (continuity, insulation resistance, hi-pot). Simkab’s prototyping services provide rapid turnaround on sample harnesses so you can iterate quickly. It is far less expensive to discover a routing problem or connector clearance issue on a prototype than during a production run of hundreds or thousands of units.
8. Testing Requirements
A comprehensive test plan should be defined during the design phase, not after production has started. At minimum, every wire harness should undergo electrical continuity testing to verify correct wiring. Depending on the application, additional tests may include insulation resistance measurement, hi-pot (dielectric withstand) testing, pull-force testing on crimped terminals, crimp cross-section analysis (destructive, on sample basis), and functional testing in the end application. For safety-critical harnesses in energy or industrial applications, 100% hi-pot testing is typically required. All test results should be recorded and linked to the harness serial number through the manufacturer’s traceability system. Simkab’s quality lab is equipped to perform all of these tests in-house.
9. Documentation
Complete documentation is essential for manufacturing consistency, quality control, and field service. A wire harness documentation package typically includes: a wiring schematic or circuit diagram showing all connections, a harness drawing showing physical dimensions, routing, and breakout locations, a bill of materials listing every component (wire, connector, terminal, accessory) with part numbers and quantities, a pinout table for every connector, assembly instructions with sequence and workmanship notes, a test specification defining pass/fail criteria, and wire marking and labeling specifications. Good documentation ensures that any qualified manufacturer—including Simkab—can produce your harness identically, batch after batch, year after year.
10. Design for Manufacturability
A great harness design is not just electrically correct—it is also practical to manufacture at scale. Design for manufacturability (DFM) considerations include: minimizing the number of unique wire types and lengths, standardizing connector and terminal selections where possible, designing breakouts and branches that can be built on a harness board efficiently, avoiding unnecessarily tight tolerances on wire lengths, specifying commonly available components to reduce lead times, and considering the order of operations on the assembly board. During Simkab’s engineering support phase, our team reviews every design for manufacturability and suggests improvements that reduce production time and cost without compromising function or quality.
Ready to Start Your Project?
If you are planning a new wire harness design or need to improve an existing one, Simkab is here to help. Our team has decades of experience designing harnesses for the energy sector, telecommunications, industrial machinery, and agriculture. Visit our Industries We Serve page to learn more about our sector-specific experience, or browse our Wire Harness Glossary for definitions of the technical terms used throughout this guide. For answers to common questions, visit our FAQ.
When you are ready to move forward, request a quote and our engineering team will review your requirements and provide a detailed proposal.