How to Specify Cable Assembly

Posted by SZFRS Engineering Team

Specifying a custom cable assembly is an exercise in precision communication. The supplier needs enough detail to quote accurately and build correctly; not so much detail that the specification overspecifies and increases cost without adding value. Most cable specification problems we see in incoming RFQs trace back to one of two patterns — either too little detail (supplier has to guess at critical dimensions or material choices, leading to inaccurate quotes and rework after sample) or too much detail with internal contradictions (different sections of the same drawing specifying conflicting requirements). This guide walks through the practical specification framework that produces clean RFQs and clean orders, drawn from years of working with procurement and engineering teams across the cable industry.

TL;DR — The Specification Checklist

A complete cable assembly specification covers 10 key elements: (1) physical drawing with dimensions and tolerances, (2) connector specification at each end with manufacturer part numbers, (3) cable construction (conductor count, AWG/area, stranding, insulation material), (4) overall jacket material and color, (5) shielding requirements (none / foil / braid / combination), (6) overall length with tolerance, (7) labeling and marking requirements, (8) testing requirements (continuity, hi-pot, functional), (9) workmanship class (IPC/WHMA-A-620 Class 1, 2, or 3), and (10) packaging and labeling for shipment. Below walks through each element with the practical detail that prevents back-and-forth during quoting.

Element 1 — Physical Drawing

The physical drawing is the foundation of any cable specification. It shows the cable layout, dimensions, connector positions, branch points (if any), and routing geometry. Standard drawing requirements:

• Overall length specification. The cable’s total length from connector body to connector body. Most cable assembly specifications use ±5% tolerance for non-critical applications and ±2% for precision applications. Specify whether the length includes connector boots/strain relief or just measures to the connector face.
• Connector orientation. Pin 1 position, locking tab orientation, key direction. For non-symmetric connectors (USB Type-A, RJ45, push-pull aviation with keying), the orientation matters and should be called out on the drawing.
• Branch points (for harness-style assemblies). If the assembly has a Y-split or trunk-and-branch routing, show the branch points with dimensions from a reference end to each branch point.
• Bend radius requirements. If the cable has to fit through a specific physical opening or routing path, specify minimum bend radius for both static and dynamic conditions.
• Marking locations. Where labels, part numbers, or batch codes appear on the cable. Include the cable face and orientation.

Drawing format matters. PDF or DWG drawings work for our quoting. Hand-drawn sketches with clearly marked dimensions also work for prototyping but slow the quoting process. The cleaner the drawing, the faster the quote.

Element 2 — Connector Specification

Connector specification is where most cable RFQs go wrong. Specifying just “USB-C connector” or “M12 connector” is rarely enough — connector families have many variants and the specific manufacturer part number determines the cable termination process and BOM cost. The minimum specification per connector should include:

• Manufacturer name and part number. Example: “JST PHR-3” or “TE 1-794617-3” or “I-PEX 20528-022T-02.” We can often substitute compatible alternatives if the customer is open to it, but the specified part is the reference point.
• Pin count. 4-pin, 6-pin, etc. Some connector families have multiple pin counts at the same form factor.
• Coding or keying (where applicable). M12 A-coded vs D-coded vs X-coded. FAKRA color code. USB-C orientation. Keying prevents physical mating with incompatible connectors and matters for both safety and assembly.
• Wire termination type. Crimp, IDC (insulation displacement), solder, screw terminal. The termination affects the manufacturing process and unit cost.
• Backshell or boot requirements. Strain relief, sealed connector backshells, overmolded boots. Specify the boot material (TPU, PVC, silicone) and the strain relief style.
• Pin assignment. Which signal goes to which pin. Use a pinout table referenced to the connector specification. Conventions like “pin 1 = +V, pin 2 = signal, pin 3 = GND” should be unambiguous.

For the connector family question — which connector family fits your application — see our connector selection basics guide for the decision framework.

Element 3 — Cable Construction

The cable itself — what’s between the connectors — needs explicit specification. Cable construction includes:

• Conductor count. How many electrical conductors in the cable. A 4-pin connector mating to a 4-pin connector at the other end with a single ground bond means 5 conductors typically (4 signals plus ground). Sometimes the connector counts and the cable conductor counts don’t match because of internal common rails.
• Wire gauge. Specified in AWG (American Wire Gauge) or mm² (metric area). 22 AWG for general signal, 18-16 AWG for moderate current, 14-10 AWG for high current. The gauge selection follows the application’s current requirement with appropriate margin for length-dependent voltage drop and thermal rise.
• Stranding. Solid (single conductor) or stranded (multiple smaller wires). Stranded is the default for any cable that flexes during use. Specify the strand count if the application has specific flex life requirements (typical 7×7×7 stranding for general flexing, finer 19+ strands for drag chain applications).
• Insulation material. PVC for general purpose, TPU for outdoor and rough environments, silicone for medical and high-temperature, FEP/PTFE for extreme chemical exposure or high-frequency applications. Our cable insulation comparison covers the trade-offs.
• Insulation color. If the cable has multiple conductors, specify the color code per conductor. Standard color codes (red/black for power, white/black for differential pairs) are widely understood; custom color schemes need explicit listing.
• Twisting (where applicable). Differential pair signals (USB, Ethernet, RS485) need twisted pair construction. Specify the pair grouping and lay length if specific signal integrity targets matter.

Element 4 — Outer Jacket

The cable’s outer jacket determines environmental and mechanical resistance. Jacket specification includes:

• Material. PVC for indoor general purpose; TPU for outdoor, rough handling, oil exposure; silicone for medical and high-temperature; PUR for drag chain industrial applications. The jacket material drives both cost and field reliability.
• Color. Black is standard. Industry-specific conventions exist (orange jacket for high-voltage automotive, yellow for gas, blue for water). Specify if the application has color requirements.
• UL flammability rating. VW-1 for general indoor; FT1 for residential; FT4 for risers; CMR for indoor commercial; CMP for plenum. The UL rating affects allowable installation locations.
• Outdoor / UV requirements. Standard PVC degrades 18-36 months outdoors. UV-stabilized PVC, TPU, or specialty outdoor jackets extend life to 10+ years. UL 583 outdoor sunlight resistance is the relevant standard.
• Operating temperature range. Standard PVC is -20 to +80 °C. Industrial cable rates -40 to +85 °C. High-temperature applications use silicone, FEP, or PTFE for ratings to +200 °C and above.
• Chemical resistance. Oil, fuel, solvent, cleaning agent exposure. PUR handles oil and fuel; silicone handles many chemicals; PTFE handles aggressive chemicals. Specify the chemical class if exposure is part of the application.

Element 5 — Shielding

Shielding protects against electromagnetic interference (EMI) — both incoming (the cable’s signals being corrupted by external EMI) and outgoing (the cable’s signals causing EMI in adjacent equipment). Shielding options:

• Unshielded. No EMI protection. Cheapest and lightest. Acceptable for low-frequency, low-current applications without sensitive analog signaling.
• Foil shield. Aluminum-mylar foil wrapped around the conductor bundle. Provides EMI shielding at moderate cost. Common in industrial control cable, USB internal cable, and general-purpose multiconductor cable.
• Braid shield. Tinned copper braid woven around the bundle. Better EMI shielding than foil; more flexible than foil-only construction. Common in audio cable, video cable, and industrial flex applications.
• Foil + braid (combination). Best EMI shielding. Foil provides 100% coverage; braid adds mechanical durability and improves shield drainage. Common in high-speed data cable, audio/video applications, and EMC-sensitive industrial cable.
• Individual pair shielding. For multi-pair cable carrying multiple signal channels, individually shielded pairs prevent crosstalk between channels. Specify if individual pair shielding is needed beyond overall shield.
• Drain wire. An uninsulated wire running alongside the foil shield, providing an electrical termination point for the shield at the connector. Standard practice for foil-shielded cable.
• Shield termination. Specify how the shield connects at each end — bonded to the connector shell (most common), terminated to a specific pin (some applications), or floating (rare). Shield termination affects EMI performance significantly.

Element 6 — Length and Tolerance

Cable length seems straightforward but specification details matter:

• Reference points. “Length 1.5 m” — measured from where to where? Connector face to connector face? Cable end to cable end (excluding boot)? Inside the boot to inside the boot? The reference matters and should be explicit.
• Tolerance band. ±5% is typical for general cable. ±2% for precision applications. Some applications need absolute tolerance (e.g., +0/-2mm for board-mount applications).
• Cable straight length vs flex length. If the cable will be installed with significant flex, specify the straight (relaxed) length rather than flex-installed length.
• Branch dimensions. For harness-style cable assemblies, each branch needs its own length specification with tolerance.

Element 7 — Labeling and Marking

• Cable face printing. Manufacturer name, cable type, length markings every meter. Standard for many cable types; specify if additional or modified printing is required.
• Label/sticker placement. Cable identification labels with part number, batch code, date code. Specify location (typically near each connector) and format.
• Wrap labels vs heat shrink. Wrap labels are quick-apply but less durable in field service; heat shrink labels are more durable but slower to apply.
• Color coding for connector identification. If multiple cables in the same product need distinguishing (e.g., 5 connectors that might be confused), color-coded boots, labels, or jacket colors prevent misconnection.
• Custom branding. Customer logo or product name printed on the cable. Adds cost; specify if required.

Element 8 — Testing Requirements

End-of-line testing verifies the cable assembly meets specification. Standard testing options:

• Continuity testing. Verifies each conductor connects from end to end without breaks or shorts to other conductors. Standard for all cable assemblies.
• Hi-pot (high voltage) testing. Verifies insulation integrity by applying a voltage above rated voltage between conductors. Common for industrial and medical cable; specifications vary by application (typical 1500 VAC for 1 second, but application-specific values).
• Insulation resistance testing. Measures resistance through the insulation under DC voltage. Catches insulation defects that hi-pot might pass.
• Functional testing. Application-specific testing — running the cable through actual signals or loads. Common for high-speed cable (signal integrity tests) or for medical and aerospace applications with regulatory requirements.
• Pull testing on connector terminations. Verifies crimp pull-out force meets specification. Required for IPC/WHMA-A-620 Class 3 work; recommended for Class 2.
• Visual inspection. Workmanship inspection per IPC/WHMA-A-620 Class 1, 2, or 3 criteria.

Specify which testing the supplier should perform per assembly and which testing should be sample-based vs 100%. Most commercial work uses 100% continuity plus 100% visual inspection plus sample-based hi-pot. Critical applications (medical, aerospace) push to 100% on more tests.

Element 9 — Workmanship Class

IPC/WHMA-A-620 defines three workmanship classes for cable and wire harness assembly:

• Class 1 — General Electronic Products. Consumer electronics where cosmetic imperfections are acceptable. Lowest cost.
• Class 2 — Dedicated Service Electronic Products. Commercial and industrial applications. The default for most cable manufacturing. Tighter workmanship requirements than Class 1; reasonable cost.
• Class 3 — High-Reliability Electronic Products. Medical, aerospace, military. Highest workmanship standards with strict acceptance criteria. Higher cost.