Equipment Grounding Conductor (EGC) & CPC Sizer
Determine minimum safety ground conductor sizes for AC and DC circuits per NEC Table 250.122 and BS 7671 Table 54.7 / Adiabatic Equation. Calculates proportional ground upsizing when phase conductors are enlarged for voltage drop.
⚡ Quick Engineering Answer: How Is Ground Wire Sized?
NEC 250.122 • BS 7671An Equipment Grounding Conductor (EGC) is sized based on the rating of the upstream circuit breaker or fuse protecting the circuit, NOT the operating load current. Under NEC Table 250.122, a 20A breaker requires a 12 AWG copper (10 AWG aluminum) ground; a 60A breaker requires 10 AWG copper; and a 200A breaker requires 6 AWG copper (4 AWG aluminum). Crucially, under NEC 250.122(B), if phase conductors are upsized to reduce voltage drop on long runs, the ground wire must be proportionally enlarged in circular mils by the same ratio so fault current impedance does not rise. Under BS 7671, CPC size is verified using the adiabatic equation ($S = \sqrt{I^2 t} / k$).
Complete your electrical grounding and circuit protection design:
NEC Table 250.122: Minimum Size Equipment Grounding Conductors
The table below lists the minimum size Equipment Grounding Conductors for grounding raceways and equipment based on the rating or setting of the upstream automatic overcurrent device in the circuit ahead of equipment.
| Rating of Overcurrent Device (Amperes) | Copper Conductor (AWG or kcmil) | Aluminum / Copper-Clad Aluminum | Metric Cross-Section (mm²) |
|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 2.08 mm² |
| 20 A | 12 AWG | 10 AWG | 3.31 mm² |
| 30 A | 10 AWG | 8 AWG | 5.26 mm² |
| 40 A • 60 A | 10 AWG | 8 AWG | 5.26 mm² |
| 100 A | 8 AWG | 6 AWG | 8.37 mm² |
| 200 A | 6 AWG | 4 AWG | 13.3 mm² |
| 300 A | 4 AWG | 2 AWG | 21.2 mm² |
| 400 A | 3 AWG | 1 AWG | 26.7 mm² |
| 500 A | 2 AWG | 1/0 AWG | 33.6 mm² |
| 600 A | 1 AWG | 2/0 AWG | 42.4 mm² |
| 800 A | 1/0 AWG | 3/0 AWG | 53.5 mm² |
| 1000 A | 2/0 AWG | 4/0 AWG | 67.4 mm² |
Frequently Asked Questions: Ground Wire & CPC Sizing
How does NEC Table 250.122 determine minimum equipment grounding conductor size?
NEC Table 250.122 establishes minimum Equipment Grounding Conductor (EGC) copper and aluminum wire sizes based on the ampere rating of the upstream circuit breaker or fuse. For example, a 15A or 20A breaker specifies 14 AWG or 12 AWG copper, a 60A breaker requires 10 AWG copper, and a 100A breaker requires 8 AWG copper (or 6 AWG aluminum). If phase conductors are upsized for voltage drop per NEC 250.122(B), the ground conductor must be upsized proportionally in circular mils.
Why must ground conductors be upsized when phase wires are enlarged?
When phase conductors are upsized to counteract voltage drop over long distances, the impedance of the supply conductor decreases. If the equipment grounding conductor is left at the standard code minimum size, the return path retains high impedance. During a line-to-ground fault, the elevated ground path resistance can reduce fault current below the instantaneous trip threshold of the circuit breaker, delaying clearing time and creating hazardous touch voltages on metal equipment frames (NEC 250.122(B)).
How does the BS 7671 adiabatic equation size a CPC?
In British Standard BS 7671, Regulation 543.1.3 allows Circuit Protective Conductors to be sized using the adiabatic equation: S = sqrt(I^2 * t) / k, where S is the minimum conductor cross-section in mm², I is the prospective fault current in Amperes, t is the operating disconnection time of the protective device in seconds (typically 0.1 to 0.4s), and k is a material constant (115 for 70°C PVC copper, 143 for 90°C XLPE copper).
Can bare copper wire be used as an Equipment Grounding Conductor?
Yes. NEC allows Equipment Grounding Conductors to be bare, covered, or insulated. When insulated, the conductor must have a continuous green outer finish or green with one or more yellow stripes per NEC 250.119.