1. What “wire numbering” must achieve

Wire numbering is the controlled assignment and physical marking of an identifier to an insulated conductor. In a switchgear LVC, that conductor may belong to a protection CT circuit, a VT circuit, a breaker trip or close circuit, a DC auxiliary supply, interlocking, indication, heater, PLC I/O, communications or a customer interface. The identifier must link the physical wire to the approved schematic, connection list and test record.

A good system answers five questions without guessing:

• Identity: which exact physical conductor is this?
• Connectivity: where does it start and where does it end?
• Function: what circuit or signal does it serve?
• Documentation: where is the authoritative record?
• Change control: will the identity survive an inserted page, a moved symbol or a minor revision?

Wire numbering is not the same as conductor colour, terminal designation, cable number, cable-core number or device tag. These identifiers must cooperate, but none should silently replace another. For example, -X1:17 identifies a terminal; C12 may identify a multicore cable; core 7 identifies one core inside that cable; and TR-0142 identifies the connection made by that core inside the LVC.

2. Standards: what they require—and what they do not

The most important conclusion is often missed: there is no universal IEC wire-number sequence. The standards establish safety colours, unambiguous identification, reference-designation principles, terminal identification and rules for technical documents. The project or manufacturer must turn those principles into a documented numbering convention.

Safety colours are not a substitute for identifiers

Protective conductors and certain grounded or designated conductors have reserved identification rules. Project practice must respect the current edition of IEC 60445 and the applicable national adoption. Colour can help a technician recognise circuit class, but two blue conductors or two red conductors still need individual identification. Likewise, a green-and-yellow sleeve must never be repurposed as a convenient marker background for a control circuit.

3. The seven main numbering philosophies

Method 1 — Simple sequential wire numbers

Every physical conductor receives the next available number: 1001, 1002, 1003. Number blocks may be reserved by panel, voltage class or circuit function—for example 1000–1999 for DC control and 3000–3999 for CT/VT circuits.

Advantages: short markers, quick manual entry, broad compatibility with printers and legacy schematics, and stable numbers if the allocation is controlled. Disadvantages: the number says nothing by itself; duplicate allocation is easy in spreadsheets; reserved ranges eventually become fragmented; and technicians depend heavily on an up-to-date wire list.

Best use: small or medium panels with disciplined database allocation, or as the stable numeric core of a hybrid system.

Method 2 — Potential- or node-based numbering

All conductors on the same electrically continuous potential share one designation. If a +110 VDC trip supply passes through three adjacent terminals without a switching element, the connected segments may carry the same potential number. After a fuse, contact or coil, a new potential number is assigned.

Advantages: closely mirrors circuit logic; easy to follow with a multimeter; common potentials are immediately recognisable; excellent for classical relay logic. Disadvantages: several physical wires can have the same marker, so a wire number is not a unique manufacturing identity; removable links, test switches and isolating terminals complicate the definition of “same potential”; and automatic potential propagation must be modelled correctly.

Best use: conventional hardwired protection and control, provided the production database separately identifies each physical connection.

Method 3 — Page/grid/line-reference numbering

The identifier is derived from the drawing location where a connection originates, such as 27.4.03 for page 27, grid column 4, connection 03. CAE tools can generate this automatically.

Advantages: a technician can navigate rapidly to the relevant drawing; numbers are systematic and easy to generate. Disadvantages: inserting or reorganising pages can renumber many wires; two revisions may show different identities for the same physical wire; redlines become hazardous; and grid-based identity ties the plant asset to document layout.

Best use: controlled machine documentation where page structure is frozen early. It should not be the only identity for long-life switchgear expected to receive protection modifications.

Method 4 — Source-to-destination designation

The marker contains both endpoints, for example -X1:17/-K86:13. A longer fully structured form can also contain location and function aspects.

Advantages: self-documenting; unique when endpoints are unique; excellent for point-to-point manufacture and continuity testing; independent of page position. Disadvantages: labels become long; small ferrules are hard to read; a changed terminal can force a new wire identity; orientation rules must be defined; and device designations must be mature before printing.

Best use: one-off engineered panels, marshalling cabinets and digital production workflows. Prefer storing full from–to data in the CAE database even if the physical sleeve prints a shorter ID.

Method 5 — Opposite-end or destination marking

At each end, the sleeve shows where the wire goes. The end at -X1:17 may read -K86:13, while the end at -K86:13 reads -X1:17.

Advantages: exceptionally useful during installation and fault tracing because the far end is visible directly. Disadvantages: the same wire has two different printed texts; a loose wire found midway has no single identity; production staff can swap end markers; and automated end-aware printing is essential at volume.

Best use: field cables, marshalling interfaces and harnesses. Combine it with a unique wire ID if traceability of the physical conductor matters.

Method 6 — Functional or signal-name identification

The marker carries a mnemonic such as CB1_TRIP, CT_L1_S1, PLC_DI017 or BUS_UV.

Advantages: intuitive to commissioning engineers; circuit purpose is visible; ideal for PLC I/O and instrumentation signals. Disadvantages: naming becomes subjective; translations and abbreviations diverge; a changed function can require relabelling; repeated signals need suffix rules; and long names exceed marker capacity.

Best use: as a controlled prefix or secondary printed line, not the sole primary key.

Method 7 — Hybrid stable ID + structured attributes

Each connection receives a stable ID such as TR-0142. The CAE record stores endpoints, potential, function, voltage, colour, cross-section, terminal accessories, routing, cable/core and test status. Depending on marker size, a second line may show the far-end terminal.

Advantages: combines short durable labels with complete digital traceability; survives page moves; supports automated wire preparation; makes revisions auditable; and can handle both potential-based and unique-physical-wire logic. Disadvantages: requires disciplined master data, CAE ownership and controlled printing; the ID alone is not self-explanatory without the drawings or wire list.

Best use: modern project-engineered switchgear LVCs. This is the recommended default.

4. The recommended system for switchgear LVCs

The best general solution is not the longest marker. It is a stable connection identity backed by complete source–target data. The recommended convention has four layers:

1. Structured device and terminal designations: every endpoint follows the project’s IEC 81346-based tagging convention, for example =J05+LVC-X1:17 and =J05+LVC-K86:13.
2. Stable primary wire ID: a concise ID such as TR-0142, unique within the defined project boundary and never reused.
3. CAE attributes: source, destination, circuit function, potential, voltage class, colour, cross-section, material, ferrule, route, cable/core and revision status.
4. Physical marking: the primary ID at both ends, in the same reading orientation; optionally a second line or adjacent marker showing the far-end terminal where space and client rules permit.

Recommended format

A practical pattern is CC-NNNN or PCC-NNNNN:

• CC is a controlled two- or three-character circuit-class prefix.
• NNNN is a zero-padded, non-significant unique counter.
• The hyphen is a fixed separator; avoid spaces, slashes and punctuation that printers or exports may interpret inconsistently.

If the organisation cannot govern prefixes reliably, omit them and use a purely numeric stable ID. A meaningless but unique ID is safer than a meaningful-looking prefix that is often wrong.

5. Worked example: circuit-breaker trip path

Consider panel J05, with a 110 VDC trip circuit. The positive supply passes through fuse -F1, terminal block -X1, the normally open contact of lockout relay -K86, and breaker trip coil -Q0-Y1. The return reaches the negative DC terminal. The example is simplified; a real design may include trip-circuit supervision, test facilities, local/remote selection, anti-pumping, duplicate coils or separate supplies.

* Conductor type is an example only; select voltage rating, temperature class, flame performance, halogen properties, flexibility and approvals from the project specification. ** Confirm the exact colour convention against IEC 60445, national rules and client requirements; do not copy this illustrative table blindly.

How each method would label the same wire

For the connection from -K86:14 to -Q0-Y1:A1:

• Sequential: 1427
• Potential-based: TRIP-23
• Page/grid: 27.4.03
• Source–destination: -K86:14/-Q0-Y1:A1
• Opposite-end: -Q0-Y1:A1 at K86 and -K86:14 at Y1
• Functional: CB1_TRIP
• Recommended hybrid: TR-0143, with all of the above facts stored as attributes.

6. Special circuit rules inside an LVC

CT secondary circuits

CT circuits require exceptional clarity because an accidentally open secondary can create hazardous voltage and protection failure. Identify phase, core, polarity and terminal consistently; distinguish protection and metering cores; show test-switch/shorting-terminal states; and never depend only on generic wire numbers. A useful record might pair stable wire ID TC-0205 with function J05-CT1-L1-S1, source -T1:1S1 and destination -XCT1:01.

VT secondary circuits

Make the voltage source, phase, winding and protection state explicit. The identifier system must prevent confusion between fused and unfused measurement circuits and between open-delta/residual-voltage wiring and phase-voltage wiring. Where test blocks are used, model every isolating point rather than assuming continuous potential.

Trip and close circuits

Use a recognisable functional class and preserve coil identity, supply source and polarity. If two independent trip coils exist, the data model and marker convention must distinguish Trip Coil 1 from Trip Coil 2. A duplicated TRIP label is not sufficient.

External and multicore cables

Mark the cable at entry and, where required, along accessible routes. Each core should retain its manufactured core number and also carry the project connection identity at the termination. Do not overwrite cable-core identity with a wire number; store both. For spare cores, label them as spare with cable/core identity, insulate them safely and document their location.

Ethernet, fibre and serial communications

A patch lead is not an anonymous accessory. Identify both ends with network function, endpoints and port designations. Fibre pairs must preserve Tx/Rx and strand identity. The connection database should include media type, connector, port, VLAN or network role where relevant, while avoiding operational secrets on publicly visible labels.

Protective bonding and safety-related wiring

Never let a project prefix, marker colour or heat-shrink sleeve obscure the required identification of protective conductors. Safety functions should be identifiable and traceable, but the wire-number system does not by itself establish functional-safety integrity.

7. Practical implementation workflow

1. Define the governing scope.
   List the applicable product standard, market, client specification and company standard. Decide whether the object is an MV switchgear LVC, a machine panel, a PSC assembly or a mixed project. Resolve IEC versus NFPA conventions before design begins.
2. Freeze the identification policy.
   Issue a one- or two-page project rule covering uniqueness boundary, format, prefixes, starting ranges, same-potential treatment, spare wires, field cables, internal jumpers, marker orientation, duplicate prevention and revision behaviour.
3. Structure devices and terminals.
   Create consistent IEC 81346-style designations, define every device connection point, and number terminal strips before final wire allocation. Bad endpoints create bad wire lists.
4. Model connections in the CAE tool.
   Generate identifiers from the connection database—not from a disconnected spreadsheet. Configure potential propagation, interruption points, PLC I/O, cable cores and terminal accessories deliberately.
5. Run automated validation.
   Check duplicate IDs, missing source/target, unconnected pins, conflicting cross-sections, colour-rule violations, two wires in terminals that accept one, missing ferrules and inconsistent cable cores.
6. Approve before bulk printing.
   Release wire numbering only after schematic design and terminal planning reach the agreed maturity gate. Print a controlled job from an approved revision and identify reprints.
7. Prepare and mark wires.
   Use cut/strip/crimp/mark data exported from CAE. Fit markers before termination where the marker type requires it. Place each marker close enough to the termination to remain associated after duct covers are removed.
8. Inspect and test.
   Perform visual comparison, point-to-point continuity, polarity checks, CT/VT circuit tests and functional tests. Record deviations against the unique ID.
9. Close redlines and regenerate as-built data.
   No handwritten field change is complete until the CAE source, wire list and final drawings are updated and reissued. Retired IDs should remain reserved and never be silently reused.

8. CAE implementation: EPLAN and equivalent tools

In EPLAN Electric P8 or another capable CAE platform, a wire should be treated as a connection object with properties, not merely text placed beside a line. Configure a project scheme for connection numbering and keep that scheme under version control. EPLAN’s numbering formats can combine identifiers, page/grid elements, counters and separators; its PLC model also treats PLC connection points as uniquely identified through device tag, plug designation and connection-point designation.

A robust data model should include at least:

• unique connection/wire ID;
• full and displayed source device tag plus connection point;
• full and displayed target device tag plus connection point;
• potential and function text;
• conductor type, cross-section, colour and prepared length;
• source and target termination treatment/ferrule;
• routing path or wire duct, if automated length calculation is used;
• cable tag and core number for cabled connections;
• manufacturing status, change status and test status.

9. Marker technologies and how to choose

Manufacturer ecosystems can automate printing and application. Phoenix Contact describes its THERMOMARK E.WIRE system as printing and applying movable conductor markers in one process, with automatic diameter measurement; Weidmüller offers CAE-linked marking software, printers and wire/cable markers; WAGO’s Smart Printer ecosystem covers terminal, conductor, cable and device marking. These are implementation options, not numbering standards. Select media by conductor diameter, temperature, chemical exposure, flame requirements, legibility, adhesion, UV exposure, expected service life and the ability to rework.

Physical placement rules

• Mark both ends of every separately identifiable conductor unless a justified project rule explicitly permits otherwise.
• Place the marker close to the termination, but not where it interferes with ferrule insertion, terminal clamping or bending radius.
• Use a consistent reading direction—commonly readable from the panel front and from terminal toward wire; document the rule.
• Ensure the marker cannot migrate far enough to become associated with an adjacent conductor.
• Choose font height and contrast for the real viewing distance and lighting, not only for a printer preview.
• Do not cover insulation damage with a sleeve or label.
• Do not print a marker twice merely to make it “more visible” if duplicated text can be mistaken for two IDs.

10. Quality control and acceptance

Design-stage checks

• No duplicate primary wire IDs inside the declared uniqueness boundary.
• Every ID has exactly one controlled connection record—or a deliberately modelled multi-connection node.
• Every source and destination terminal exists and accepts the conductor/ferrule arrangement.
• Safety colours and project voltage-class colours do not conflict.
• CT, VT, trip, close and external-interface circuits have the required functional attributes.
• Spare cable cores, internal spare wires and unused terminals are distinguished.

Production inspection

• Correct marker at both ends, legible and securely retained.
• Correct wire colour, cross-section, ferrule and terminal.
• No marker trapped inside a terminal or hidden deep inside ducting.
• Two-ended marker text follows the approved orientation rule.
• Any reprint is controlled against the same approved revision.

Electrical and functional verification

Visual inspection does not prove connectivity. Perform point-to-point continuity and, where relevant, insulation resistance, polarity, CT secondary continuity/ratio/polarity checks, VT circuit checks, trip/close functional tests, alarm/indication tests and PLC I/O tests. The test sheet should reference the same connection IDs so failures map directly to the CAE data.

11. Common failures and why they happen

12. Example company specification

13. Decision guide

• Very small one-off panel, fewer than roughly 100 wires: sequential numbering is acceptable if generated and checked from the schematic.
• Classical relay/control LVC: potential-based display is useful, but maintain unique physical connection records.
• Long-life switchgear with future retrofit: avoid page-based identity; use stable IDs and structured endpoints.
• Marshalling or extensive field cabling: consider stable ID plus opposite-end information.
• High-volume automated production: use CAE-to-machine data, unique stable IDs and qualified printer/media systems.
• North-American project: create a dedicated NFPA/UL identification and colour specification; do not reuse an IEC template unchanged.

14. References and further reading

1. IEC, IEC 60445:2021+AMD1:2026 CSV — Identification of equipment terminals, conductor terminations and conductors.
2. IEC, IEC 61666:2010+AMD1:2021 CSV — Identification of terminals within a system.
3. IEC, IEC 81346-1:2022 — Structuring principles and reference designations.
4. IEC, IEC 81346-2:2019 — Classification of objects and codes for classes.
5. ISO/IEC, ISO 81346-10:2022 — Power-supply systems and ISO/TS 81346-101:2025 — Application guidance for power-supply systems.
6. IEC, IEC 61082-1:2014 — Preparation of documents used in electrotechnology.
7. IEC, IEC 61439-1:2020 and IEC 61439-2:2020 — Low-voltage switchgear and controlgear assemblies.
8. IEC, IEC 60204-1:2016+AMD1:2021 — Electrical equipment of machines.
9. EPLAN Help, Online numbering format and PLC connection points.
10. Phoenix Contact, THERMOMARK E SERIES and conductor and cable marking.
11. Weidmüller, wire and cable markers, M-Print PRO and industrial printers.
12. WAGO, marking media and Smart Printer.
13. NFPA, Overview of NFPA 79; UL Solutions, Industrial Control Panels and Panel Shop Program.

Engineering note: Standards and client requirements change. This article explains design practice and does not replace the purchased standards, the assembly manufacturer’s verified design, the project specification or approval by the responsible engineer. All diagrams are original explanatory figures prepared for this article.