Identification Rules for Wire Numbering in Power Distribution Cabinets

The identification of wire numbers in power distribution cabinets is a core link to ensure the installation, commissioning, maintenance and safe operation of electrical systems. It must comply with the four principles of **uniqueness, readability, continuity and correspondence**, as well as national standards and specifications. The following is a detailed explanation from the aspects of core principles, basic marking, system-specific rules, normative basis and examples, to ensure that wire numbers are "easy to understand at a glance and accurate when checked".

I. Core Principles of Wire Number Identification

The fundamental purpose of wire number marking is to **clarify the function, ownership and connection relationship of wires**, avoid confusion or misoperation, and must meet the following 5 principles:

1. **Uniqueness**: All wire numbers in the same power distribution cabinet (or the same electrical system) shall not be repeated (even for different circuits, they must be distinguished by prefixes/partitions).

2. **Readability**: Wire numbers shall use clear fonts (Song typeface/Kai typeface are recommended). The font size shall be adjusted according to the cross-sectional area of the wire and the installation position (e.g., 10-12 point font for main wires in the cabinet, 8-10 point font for secondary control wires). The color shall have a distinct contrast with the wire/cabinet (e.g., black font on white background, black font on yellow background).

3. **Continuity**: For wires in the same functional circuit, the wire numbers shall be consecutive (e.g., the control circuit is numbered incrementally starting from "1" without skipping numbers; the main circuit progresses according to phase sequence + serial number).

4. **Correspondence**: The **same wire number must be marked at both ends of a wire** (e.g., one end connected to the coil of contactor KM1 is marked "102", and the other end connected to terminal block TB1-05 shall also be marked "102"). For wires crossing cabinets/terminal blocks, additional information about the corresponding end shall be marked (e.g., "102-TB2-03").

5. **Standardization**: Strictly comply with national standards (such as GB 7947) and avoid self-defined confusing marks (e.g., do not use colloquial markings like "Live Wire 1" or "Neutral Wire 2").

 II. Basic Marking Specifications (Color + Letter + Number)

Wire number marking usually combines **color** (for intuitive distinction), **letters** (for function ownership) and **numbers** (for specific numbering), and the three are used together:

1. Color Marking (National Standard GB 7947-2010)

The color of the wire insulation layer shall correspond to its function, serving as a "visual aid" for wire numbers and cannot replace text numbering:

2. Letter Marking (Abbreviation for Function)

Uppercase letters are used to mark the system or component that the wire belongs to. Common abbreviations are as follows:

3. Number Marking (Distinction by Serial Number)

Arabic numerals are used to mark the serial number of wires under the same function, usually incrementing according to "circuit partition" or "wiring sequence" without skipping numbers:

- Single circuit: Directly increment with 1, 2, 3... (e.g., the control circuit is numbered starting from 1);

- Multiple circuits: Add "circuit number" before the number (e.g., 1-01, 1-02 represent wires of Circuit 1; 2-01, 2-02 represent wires of Circuit 2);

- Crossing terminals/cabinets: Add "terminal number/cabinet number" after the number (e.g., 103-TB2-04 means wire 103 is connected to terminal 04 of terminal block TB2).

III. System-Specific Wire Numbering Rules (Core Focus)

Wires in the power distribution cabinet are classified into "main circuit (primary circuit)" and "control circuit (secondary circuit)", with different numbering rules that need to be marked separately:

1. Main Circuit (Primary Circuit: Power Circuit, e.g., Power Supply for Motors, Inverters)

The main circuit carries large current, so the wire number must be associated with "component number + phase sequence + serial number" to facilitate tracing the power flow:

 Numbering Format: 「Phase Sequence Letter/Live Wire Mark + Component Number + Wire Serial Number」

- **Single-circuit main circuit (e.g., single-phase motor)**:  

  Example: L1-QF1-1 (Live wire L1 → Circuit breaker QF1 → 1st section of wire), N1-QF1-1 (Neutral wire N1 → Circuit breaker QF1 → 1st section of wire);  

  Subsequent extension: L1-KM1-2 (Live wire → Contactor KM1 → 2nd section of wire), L1-FR1-3 (Live wire → Thermal relay FR1 → 3rd section of wire).

- **Three-phase main circuit (e.g., three-phase motor)**:  

  Example: A-QF1-1 (Phase A → Circuit breaker QF1 → 1st section), B-QF1-1 (Phase B), C-QF1-1 (Phase C);  

  Subsequent extension: A-KM1-2 (Phase A → Contactor KM1 → 2nd section), A-FR1-3 (Phase A → Thermal relay FR1 → 3rd section), A-M1-4 (Phase A → Motor M1 → 4th section).

- **Multi-circuit main circuit (e.g., power supply for multiple motors)**:  

  A "circuit number" must be added for distinction. Example: 1-A-QF1-1 (Phase A of Circuit 1), 2-A-QF2-1 (Phase A of Circuit 2).

2. Control Circuit (Secondary Circuit: e.g., Control Wires for Buttons, Relays, Meters)

The control circuit is a small-current signal circuit, and wire numbers are usually numbered according to "functional partitions" to facilitate quick fault location:

 Numbering Format: 「Partition Prefix (Optional) + Numerical Serial Number」

Common partition rules (industry-generalized, adjustable as needed but must be unified):

Example: Start-Stop Control Circuit of Three-Phase Motor

- Power supply end: Power taken from L1 of the main circuit, wire number "1";  

- Stop button (SB1, normally closed): Incoming wire "1", outgoing wire "2";  

- Start button (SB2, normally open) + auxiliary normally open contact of Contactor KM1 (self-locking): Incoming wire "2", outgoing wire "3";  

- Coil of Contactor KM1: Incoming wire "3", outgoing wire "4";  

- Thermal relay FR1 (normally closed): Incoming wire "4", outgoing wire "5";  

- Power return: "5" connected to L2 of the main circuit to form a loop.  

→ Complete wire numbers: 1 (Power taken from L1) → 2 (Outgoing wire of SB1) → 3 (Outgoing wire of SB2/KM1 auxiliary contact) → 4 (Incoming wire of KM1 coil) → 5 (Outgoing wire of FR1 → L2).

3. Numbering of Special Wires (Grounding Wire, Neutral Wire, Spare Wire)

- **Protective Grounding Wire (PE)**:  

  Uniformly marked with "PE". For multiple grounding circuits, serial numbers are added for distinction. Examples: PE1 (main grounding trunk line), PE2 (grounding wire of Motor M1), PE-TB1 (grounding wire of Terminal Block TB1);  

  Note: PE wires must be yellow-green dual color, and "PE" in the wire number cannot be omitted.

- **Neutral Wire (N)**:  

  Marked with "N" + serial number. Examples: N1 (neutral wire of main circuit), N2 (neutral wire of control circuit), N-TB2 (neutral wire of Terminal Block TB2);  

  Note: N wires must be light blue and cannot be confused with PE wires.

- **Spare Wires**:  

  Marked with "Spare" or "BY" (Pinyin abbreviation) + serial number. Examples: Spare 1, BY2;  

  If a spare wire corresponds to a certain circuit, relevant associated information must be marked. Example: Spare-KM2 (spare control wire of Contactor KM2).

4. Wire Numbering of Terminal Blocks (Key Connection Points)

Terminal blocks are the "transfer hubs" of wires. The wire number must mark both the "wire number" and "terminal number" to facilitate cross-circuit tracing:

Numbering Format: 「Wire Number + - + Terminal Block Number」

- Self-numbering of terminal blocks: According to "terminal block code + serial number". Examples: TB1-01 (Terminal 01 of Terminal Block TB1), TB2-05 (Terminal 05 of Terminal Block TB2);  

- Association between wires and terminals: Examples: 101-TB1-03 (Wire 101 connected to Terminal 03 of TB1), PE-TB2-01 (Protective grounding wire PE connected to Terminal 01 of TB2).

 IV. Normative Basis and Avoidance of Common Errors

1. Core National Standards

- **GB 7947-2010**: *Basic and Safety Rules for Marking of Human-Machine Interface - Conductor Color or Alphanumeric Marking*: Specifies color and letter marking;  

- **GB 50171-2012**: *Code for Construction and Acceptance of Wiring Works for Panels, Cabinets and Secondary Circuits of Electrical Installations*: Specifies requirements for the location and clarity of wire number marking;  

- **GB/T 4728**: *Graphical Symbols for Electrical Diagrams*: Specifies component symbol standards matching wire numbers (e.g., KM for contactor, QF for circuit breaker).

2. Common Errors and Avoidance Methods

V. Example: Comprehensive Wire Number Marking of a Small-Scale Power Distribution Cabinet

Taking a power distribution cabinet with "1 three-phase motor + 1 set of start-stop control" as an example, the wire number marking is as follows:

Through the above rules, the wire numbers of the power distribution cabinet can achieve "clear function, convenient tracing and safe standardization", which can greatly reduce the operation difficulty and safety risks in both early installation and later maintenance. In practical applications, the numbering partitions need to be adjusted according to the complexity of the power distribution cabinet (e.g., whether it contains PLC, inverter), but the core principles (uniqueness, correspondence) must always be followed.