• Yueqing Naza Electric Power Technology Co., Ltd

Low-Voltage Circuit Breaker Wiring Code: Live, Neutral and Earth Conductor Connection Requirements

1. Preface
   This paper translates the most common national codes (GB 50303-2023, GB/T 36291, NB/T 42086, IEC 60364-5-52, NEC Art. 240 & 250) into step-by-step craft rules for electricians who actually tighten the screws.  It concentrates on the external cabling side—how the three system conductors (L, N, PE) must be landed on breaker, bus-bar and terminal strip inside residential or light-commercial switchboards ≤ 400 V.  Factory internal wiring is mentioned only where it affects field work.

1. Core definitions and colour code (2025)

• L (Live, Phase, Line) – red, brown, black or any colour except blue, green, yellow or green/yellow.
• N (Neutral) – light blue (mandatory).
• PE (Protective Earth) – green-yellow bi-colour, 100 % coverage, no other function permitted.
  Note: PEN conductor (combined) must be green-yellow with light-blue marker at termination only, but PEN is forbidden inside final distribution boards built after 2023; TN-C-S must separate into L+N+PE before the main breaker.

1. Safety first – lock-out, test, verify

1. De-energise: open upstream breaker, withdraw fuse, lock & tag.
2. Prove dead: two-pole voltage tester on L-N, L-PE, N-PE.
3. Earth and short: apply temporary earth clamp if working on overhead incomer.
4. Re-check colour: even new builds may have custom colours in flexible conduit—always test, never assume.

1. Terminal identification and mechanical rules

• Strip 10 mm ± 0.5 mm insulation; do not expose copper beyond terminal collar.
• Wire-brush aluminium conductors, apply anti-oxidant paste, torque + 10 %, re-torque after 30 min.
• Do not solder wire ends—solder cold-flows and loosens.
• Fork ferrules for stranded wire ≥ 4 mm²; crimp with hex die, six-point indent.

1. Single-pole (1P) breaker – L only

• Incomer L → upper L terminal.
• Outgoing L → lower L terminal to load.
• Neutral conductor bypasses breaker, runs directly to insulated neutral bar.
• Earth conductor bypasses breaker, lands on earth bar bonded to enclosure.
  Key point: the neutral bar must be insulated from the metallic enclosure; only the earth bar is bonded.  Using the same bar for N and PE is a code violation (except inside a TT earthing system where the consumer makes the earth electrode—still, the bar is split into two insulated sections).

1. Single-pole + neutral (1P+N) breaker – L switched, N solid

• L same as 1P.
• N conductor enters upper N, exits lower N.  Polarity must be respected—reverse connection (N upstream on lower terminal) still works but defeats the “trip-free” mechanism and violates clause 5.3.2 of GB/T 10963.
• Earth still bypasses.
  Colour check: blue wire must land on N terminal, not on spare L terminal painted white by installer—code does not care about paint, only about embossed marking.

1. Two-pole (2P) breaker – L & N both switched ------------------------------------------------* Used when full isolation is required (main incomer, sub-main to garage, inverter output).
   Wiring:

• Left pole usually marked L, right pole N.  Some brands are reversible; follow + sign near terminal.
• Bridge bar inside breaker ensures both poles open together—do not remove.
• Incoming feed from utility meter: L → left upper, N → right upper.
• Outgoing: L → left lower, N → right lower.
• Earth still bypasses.
  Note: if supply is two-phase (180° split-phase as in North America), the two hots land on L1 and L2, neutral not present; breaker is then called 2P but functionally double-line.

1. Three-phase & four-pole breakers ------------------------------------------------* 7.1 3P breaker
   L1, L2, L3 switched; neutral bypasses.  Common for three-phase motors, welders.
   7.2 3P+N breaker
   Adds solid neutral link; neutral not protected.  Used where neutral is expected to carry imbalance but not to be opened (IT systems).
   7.3 4P breaker
   All four conductors switched simultaneously.  Mandatory when neutral protection is required (generator change-over, inverter output, TN-S isolation).
   Wiring sequence: face the breaker; terminals are numbered 1-2-3-4 from left to right.  Standard is 1=L1, 2=L2, 3=L3, 4=N.  Verify embossing—some Asian brands reverse 3 and 4.
   Earth conductor lands on earth bar; never on the fourth pole unless the device is marked PEN and certified for TN-C.

1. Residual-current devices (RCCB/RCBO) – earth conductor critical ------------------------------------------------* 8.1 Toroid wiring
   The internal toroidal transformer must enclose all live conductors (L+N or L1+L2+L3+N) but never the earth conductor.  If PE is accidentally passed through the toroid, the device sees a 30 mA “fault” and trips instantly.
   Field tip: wrap cables neatly through centre window; use cable tie to keep PE outside.

1. Earth-bar installation details ------------------------------------------------*

• Material: copper or tinned copper, minimum 4 mm thick.
• Cross-section: when PE is ≥ 16 mm² Cu, the bar shall have equal or greater ampacity (calculate I = kS, k=115 for Cu).
• Bonding: one conductor (usually 16 mm² green-yellow) from bar to enclosure grounding point, then to main earth terminal (MET) and on to earth electrode or utility PEN.
• Stainless-steel M6 screws with captive washers; torque 12 N·m for 16 mm² lug.
• Label: green-yellow “PE” symbol per ISO 7010; indelible, not Dymo tape.
• Multiple bars: if auxiliary earth bar is added (e.g., for data equipment), bridge with 16 mm² Cu; do not rely on panel steel for continuity—impedance too high at fault frequency.

1. Neutral-bar insulation and isolation ------------------------------------------------*

• Insulated stand-off pillars ≥ 6 kV impulse rating.
• Isolate from earth bar by at least 8 mm air gap or 4 mm solid insulation.
• In TT systems, neutral bar may carry 50 V under fault; cover with snap-on shroud to IP20.
• No more than two conductors per screw unless terminal is marked for two; otherwise use ferrule + single screw.

1. Cross-sectional relationships (simplified) ------------------------------------------------* Table 1 – Minimum copper cross-sections (IEC 60364-5-52)