Numbers on the wiring terminals of the distribution box

Distribution box terminals are numbered and labeled according to standard electrical identification systems to ensure correct wiring of phase, neutral, and earth connections.

Terminal Numbering in Distribution Boxes

In a typical single-phase or three-phase distribution box, terminals are assigned numbers to identify incoming and outgoing connections:

• Main incoming terminals: The phase (live) wire is connected to the main MCB or main switch input terminal, often labeled as L1, L2, L3 for three-phase or simply L for single-phase, while the neutral wire connects to the N terminal or neutral link .
• Outgoing circuit terminals: Each circuit breaker has numbered terminals for phase and neutral. For single-pole MCBs, the phase terminal is connected to the breaker output, and the neutral is connected to the neutral bus bar. For double-pole MCBs, both phase and neutral terminals are numbered and connected to the load accordingly .
• Earth terminals: All protective earth wires are connected to the earth bus bar, which may be numbered or marked as E or PE for protective earth .

Standardized Numbering System

Electrical standards recommend a sequential numbering system for clarity and documentation:

• Switchboards and MCCs are assigned an Electrical Identification Number (EIN), which forms the prefix for all terminal and circuit references .
• Power circuit reference numbers (PC numbers) are assigned sequentially from the incomer to outgoing circuits. For example, a main breaker might be labeled M101, and the connected outgoing cable terminals could be numbered 1201, 1202, etc., derived from the PC reference .
• Schematic diagrams show these numbers in boxes next to each terminal, and the same numbers are used on the physical distribution box for labeling .

Wiring Sequence

1. Connect the incoming phase wire to the main MCB input terminal.
2. Connect the main MCB output to the input of the RCCB (if used).
3. Connect the RCCB output to the input terminals of the outgoing MCBs.
4. Connect neutral wires from the RCCB output to the neutral bus bar.
5. Connect earth wires from the supply and loads to the earth bus bar.
6. Number each terminal according to the PC reference system for easy identification and maintenance .

Practical Tips

• Always verify the input and output sides of MCBs and RCCBs before wiring.
• Use a multimeter to check continuity and ensure no short circuits.
• Clearly label each terminal with its number and function to comply with safety standards and simplify troubleshooting . By following these numbering conventions and wiring sequences, distribution boxes can be safely and efficiently installed, ensuring proper identification of all terminals for both residential and industrial applications.