How many square millimeters should the grounding wire for the distribution box be
For most residential distribution boxes (100–200 A), a copper grounding wire of 4 AWG (≈21 mm²) or aluminum 2 AWG (≈33 mm²) is typically used.
Equipment Grounding Conductor (EGC)
The EGC connects non-current-carrying metal parts of the distribution box to the system ground, providing a low-impedance path for fault currents. According to NEC Table 250.122, the minimum size depends on the rating of the overcurrent protection device (OCPD) protecting the circuit:
- 15–20 A: 14 AWG copper (≈2.08 mm²)
- 30 A: 10 AWG copper (≈5.26 mm²)
- 100–200 A: 4 AWG copper (≈21 mm²) or 2 AWG aluminum (≈33 mm²)
- 400–800 A: 2/0 to 3/0 AWG copper (≈107–152 mm²) Copper is preferred due to higher conductivity and corrosion resistance, allowing smaller sizes compared to aluminum for the same ampacity .
Grounding Electrode Conductor (GEC)
The GEC connects the distribution box to the grounding electrode (ground rod, Ufer ground, or metal water pipe). NEC Table 250.66 specifies sizing based on the largest ungrounded service-entrance conductor:
- For up to 250 kcmil copper service conductors, a 2 AWG copper GEC (≈33 mm²) is sufficient.
- If connecting only to a ground rod or pipe, the GEC need not exceed 6 AWG copper (≈13 mm²).
- For a concrete-encased electrode (Ufer ground), the GEC need not exceed 4 AWG copper (≈21 mm²), .
Conversion to mm²
AWG sizes can be converted approximately to mm²:
| AWG | Copper mm² | Aluminum mm² |
|---|---|---|
| 4 | 21 | 33 |
| 2 | 33 | 53 |
| 1/0 | 53 | 85 |
| 3/0 | 152 | 243 |
Practical Recommendation
- Residential 100–200 A service: Use 4 AWG copper (≈21 mm²) or 2 AWG aluminum (≈33 mm²) for the EGC.
- GEC to ground rod: 6 AWG copper (≈13 mm²) is sufficient.
- GEC to Ufer ground: 4 AWG copper (≈21 mm²) is recommended. Always ensure the grounding conductor is installed according to NEC requirements and local codes, and consider professional installation for safety and compliance .
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