Industrial wire harness design

Wire harnesses for machinery and panels: standards, cable, connectors and process

What should be settled before the first wire of a machine, panel, robot or energy-equipment harness is cut: which standard governs, which cable goes in each circuit, which connector survives the environment, and how it is documented so production and service read it the same way.

Standards that apply to machine wiring

Two regulatory worlds coexist and many manufacturers export to both. In Europe the backbone is IEC/EN 60204-1 (electrical equipment of machines): minimum cross-sections per circuit, wiring practices (clause 13), identification and marking (clause 16) and documentation. It is the harmonised standard behind CE marking under the Machinery Directive 2006/42/EC (Regulation (EU) 2023/1230 from 2027) and the Low Voltage Directive 2014/35/EU. The switchgear the harness connects to (contactors, disconnectors, terminal blocks) falls under the IEC/EN 60947 series.

In North America the panel is built to UL 508A, and that constrains the harness even if it is made elsewhere: internal wire of a recognised type (MTW, THHN or AWM under UL 758), cross-sections from the UL 508A tables rather than IEC, connectors recognised under UL 1977 and terminal blocks under UL 1059; sheathed flexible cords under UL 62 and its CSA equivalent. For the harness as an assembled product, acceptance is agreed to IPC/WHMA-A-620: class 2 in machinery, class 3 where failure is critical (life support, parts of the energy sector). The class is set by the customer and written on the drawing.

Two more layers. Safety circuits (emergency stop, doors, light curtains) are designed to ISO 13849-1 (PL a-e) or IEC 62061 (SIL 1-3), and that reaches the harness: two channels in separate conductors, OSSD outputs with test pulses that detect cross-faults, and a route where a single mechanical damage cannot affect both; the push-button follows ISO 13850. In materials, RoHS and REACH touch PVC and contact platings; and if the equipment goes into rail, EN 45545-2 demands fire behaviour and low smoke by hazard level (HL1-HL3), which rules out ordinary PVC and leads to EN 50264 or EN 50306 cables.

Cable: families, temperatures and colours

Wire is chosen by voltage, temperature, environment and, increasingly, by movement.

Cross-section is stated in AWG in the UL world and in mm² in IEC; one convention is fixed per project so that 1.5 mm² and 16 AWG never share a drawing. IEC 60204-1 gives minimum cross-sections per function; the permissible current depends on bundling and cabinet temperature, not just on rated current. Servo cable is chosen by the capacitance per metre the drive tolerates: more capacitance, more leakage current and trips. For cable carriers the manufacturer declares a minimum bend radius and a cycle life (millions) for fine-stranded copper with a core filler and PUR sheath; fixed-installation cable is not designed for that.

On colours, IEC 60204-1 §13.2 mandates green-and-yellow for the protective conductor and light blue for the neutral, and recommends a code that almost all of European industry follows: black for AC and DC power, red for AC control, blue for DC control and orange for circuits that stay live when the main disconnect is opened. Another code is admissible if it is stated in the schematic. UL 508A uses a different convention (yellow for circuits fed from outside the panel), so a harness for both markets calls for an explicit decision.

Then there is the choice between single wires and sheathed multicore. In slotted trunking inside the cabinet, single wires win: they can be reassigned, identified and sized circuit by circuit. Outside the cabinet or in moving sections, multicore is better protected and passes cleanly through a cable gland. Almost every machine carries both, joined at terminal blocks or a rectangular connector.

Connectors: from M8 to Han, from DIN rail to PCB

A machine harness touches four connector scales: field, cabinet, power and board.

In M12 the coding keys are mechanically different: A for sensors and IO-Link, D for 100 Mbit/s Ethernet with 4 pins, X for Gigabit with 8 pins and individually shielded pairs, and L, S, T or K for power; IP69K only makes sense with pressure washdown (food, pharma). In Harting Han the insert sets poles and current (Han E is the workhorse: 16 A, up to 48 poles; Han A is compact; Han Q packs power into few poles; Han-Modular takes power, signal, RJ45, coaxial, fibre or pneumatic modules in the same housing), while the housing (3A to 24B) and hood set IP rating, cable exit and locking. Push-in terminal blocks accept ferruled wire without a tool, which makes crimping DIN 46228 ferrules the most repeated operation on a cabinet harness; spade and ring terminals call for a calibrated crimper and a pull test. Cable glands set the real IP of the assembly: an IP67 M12 on a box with the wrong gland leaves the box at IP54. Hybrid connectors (power and data in one cable) shrink the harness but tie the design to one manufacturer.

Protection, routing and EMC

Inside the cabinet the harness runs in lidded slotted trunking, not overfilled. Towards the machine the usual choices are polyamide corrugated conduit (PA6, typically -40 to +105 °C, oil-resistant), PET braided sleeving (abrasion, no IP), spiral wrap for sections that must be opened, heat-shrink 2:1 or 3:1 with adhesive at exposed connector exits, and UV-stabilised cable ties with mounts outdoors.

The cable carrier has rules of its own: respect the minimum radius of the most demanding cable, leave clearance around each cable, distribute weight symmetrically, separate power and signal with dividers and fix the cables only at both ends, never in the flexing section. EMC separation follows the same logic across the machine: power and signal in separate trunking, crossings at 90°, servo, encoder and bus shields terminated over 360° with EMC clamps on an earth bar at the cabinet entry, never as a pigtail, and a functional earth made with flat bar or braid. At cabinet doors the harness crosses the hinge with excess length, protected by spiral or conduit, and with its own green-and-yellow bonding conductor.

Identification and labelling

IEC 60204-1 requires every conductor to be identifiable at both ends in accordance with the technical documentation. In machinery this is solved with wire marking by number or by function, identical at both ends and in the schematic: printed heat-shrink sleeves for wires going to connectors, clip-on markers (Partex type) and self-laminating labels for sheathed cables, printed with Brady, Phoenix Contact (Thermomark) or Weidmüller (PrintJet) systems that read the project's wire list. Terminal, device and connector references follow IEC 81346 (clause 16 of 60204-1). Regulated equipment adds lot traceability: harness serial number, cable and terminal lot, operator and date, kept with the drawing revision.

Process: from the from-to list to production and service

It starts with the connection diagram or the from-to list: every wire with origin, destination, cross-section, colour and function, using device references to IEC 81346 (for example +A1-X1:3: cabinet A1, terminal strip X1, terminal 3). Lengths come from the cabinet and machine layout: the real run through trunking with the excess at doors and carriers, not straight-line distance. From that the 1:1 formboard is built so the shop cuts, marks and tapes without measuring each wire.

Minimum verification is continuity and absence of crosses against the connection table; for mains circuits, dielectric strength and insulation; for safety, separated channels. Revision control is tied to the machine's: every commercial option is a harness variant with its own drawing and every change bumps the revision in the title block. Documentation has two readers: production wants the formboard, cut list and BOM; after-sales wants the connection table readable at the machine, because ten years later someone will replace a sensor without the designer at hand.