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, the NEC sizes from the overcurrent device.
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. The two main standards size them differently:
- BS 7671 / AS/NZS size the circuit protective conductor (CPC) from the line conductor cross-sectional area via Table 54.7, or by the adiabatic equation S = √(I²t) ÷ k.
- NEC sizes the equipment grounding conductor (EGC) from the overcurrent device rating via Table 250.122.
Worked examples
BS 7671:a 25 mm² line conductor falls in the 16–35 mm² band, so the CPC is 16 mm².
NEC:a circuit protected by a 100 A breaker requires an 8 AWG copper equipment grounding conductor.
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 between 16 and 35 mm², and half the line size above 35 mm². The adiabatic equation may give a smaller result where calculated.
- How is the equipment grounding conductor sized under the NEC?
- NEC Table 250.122 sizes the EGC from the rating of the overcurrent device protecting the circuit - for example a 100 A breaker requires an 8 AWG copper EGC.
- What's the difference between earthing and grounding?
- They describe the same concept in different standards. 'Earthing' is the term used in BS 7671 and AS/NZS; 'grounding' is the NEC term. Both connect exposed metalwork to earth for safety.
- Does the grounding conductor increase if I upsize the phase conductors?
- Yes. NEC 250.122(B) requires the EGC to be increased proportionally when ungrounded conductors are upsized (for example for voltage drop). The Voltix app handles this adjustment automatically.
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