Earthing & Grounding Conductor Calculator
Size the protective earthing conductor (CPC) under BS 7671 / AS/NZS, or the equipment grounding conductor (EGC) under the NEC. Switch standards with one tap — BS 7671 sizes from the line conductor cross-section, the NEC sizes from the overcurrent device rating.
Protective conductor (CPC)
16mm²
Minimum required
16mm²
16 mm² protective conductor
BS 7671 Table 54.7 - 16 < S ≤ 35 mm²: protective conductor = 16 mm².
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Two standards, two methods
Earthing and grounding conductors carry fault current safely back to the source so the protective device can disconnect before metalwork remains live long enough to cause electrocution. The two main standards size them from different inputs:
- BS 7671 / AS/NZS size the circuit protective conductor (CPC) from the line conductorcross-sectional area via Table 54.7, or by the adiabatic equation S = √(I²t) ÷ k where fault current and disconnection time are known.
- NEC sizes the equipment grounding conductor (EGC) from the overcurrent device rating via Table 250.122.
BS 7671 Table 54.7 — simplified sizing
The most common sizing method for domestic and light commercial circuits:
- Line conductor ≤ 16 mm²: CPC = same as line conductor
- Line conductor 16–35 mm²: CPC = 16 mm²
- Line conductor > 35 mm²: CPC = ½ × line conductor (minimum 16 mm²)
For twin-and-earth cables, the earth conductor is included in the cable and is typically 1.0 mm² in 1.5 mm² cable and 1.5 mm² in 2.5 mm² cable — both below the Table 54.7 minimum. This is acceptable because the adiabatic equation for the typical fault levels in domestic circuits gives a smaller result.
NEC Table 250.122 — EGC sizing by OCPD rating
- 15 A: 14 AWG copper (2.08 mm²)
- 20 A: 12 AWG copper (3.31 mm²)
- 30–60 A: 10 AWG copper (5.26 mm²)
- 100 A: 8 AWG copper (8.37 mm²)
- 200 A: 6 AWG copper (13.3 mm²)
- 300 A: 4 AWG copper (21.1 mm²)
- 400 A: 3 AWG copper (26.7 mm²)
Worked examples
BS 7671 example:A 10 mm² copper line conductor (falls in the ≤ 16 mm² band):
- CPC = 10 mm² (same as line conductor)
- Twin-and-earth 10 mm² cable typically includes a 4 mm² earth — verify via adiabatic equation
NEC example:A circuit protected by a 100 A inverse-time breaker:
- EGC = 8 AWG copper minimum (Table 250.122)
- If phase conductors were upsized from 3 AWG to 1 AWG for voltage drop, the EGC must also be upsized proportionally
Who is this calculator for?
For electricians and design engineers sizing protective conductors under BS 7671 (UK and Ireland) or equipment grounding conductors under the NEC (United States). Critical for new installations, rewires, and verification work — particularly important where high fault current levels are expected (close to the transformer or distribution board) and where the standard twin-and-earth earth size may need to be verified against the adiabatic equation.
Frequently Asked Questions
- How is the CPC sized under BS 7671?
- BS 7671 Table 54.7 relates the protective conductor to the line conductor: equal size up to 16 mm², 16 mm² for line sizes 16–35 mm², and half the line conductor size for anything above 35 mm² (minimum 16 mm²). The adiabatic equation (Section 543.1.3) may give a smaller result where calculated, but the table result is always safe.
- How is the equipment grounding conductor sized under the NEC?
- NEC Table 250.122 sizes the EGC based on the rating of the overcurrent protective device: 15 A breaker → 14 AWG; 20 A → 12 AWG; 30–60 A → 10 AWG; 100 A → 8 AWG; 200 A → 6 AWG; 300 A → 4 AWG; 400 A → 3 AWG.
- What's the difference between earthing and grounding?
- They describe the same safety concept in different standards. 'Earthing' and 'circuit protective conductor (CPC)' are the terms used in BS 7671 and AS/NZS. 'Grounding' and 'equipment grounding conductor (EGC)' are the NEC terms. Both connect exposed metalwork to the earth reference so faults cause sufficient current to trip the protective device.
- Does the grounding conductor need to be increased if I upsize the phase conductors?
- Yes. NEC 250.122(B) requires the EGC to be increased proportionally when ungrounded conductors are upsized for reasons such as voltage drop. Similarly, BS 7671 requires the CPC to remain proportional to the line conductor size. Always recheck the earth size when you upsize the lives.
- What is the adiabatic equation and when do I need it?
- The adiabatic equation (S = √(I²t) ÷ k) calculates the minimum protective conductor area needed to survive the fault current for the disconnection time. You need it when the simplified Table 54.7 gives an impractically large result, or when you are verifying compliance with a calculated fault level. For most domestic circuits, Table 54.7 is used directly.
- Can the earthing conductor be smaller than the main bonding conductor?
- They serve different purposes. The circuit protective conductor (CPC) runs with each circuit. The main protective bonding conductor connects metallic services (gas, water, oil pipes) to the main earth terminal and is sized from Table 54.8 — a minimum of 10 mm² copper for most domestic installations, regardless of the circuit CPCs.
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