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For the line wires, I just check the maximum load current and select a conductor from the manufacturer datasheet that can support that maximum current.

What about the earth continuity conductor though, how is it sized?

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    What is ECC cable? – Speedy Petey May 24 '16 at 11:17
  • A bit more details on what "line wire" you're referring to would be helpful. – Kris May 24 '16 at 21:36
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In North America equipment grounding conductors (circuit grounds) can be much smaller than circuit conductors. In fact it's only the first few circuit sizes (up to 30A) that require the EGC to be the same size as the circuit conductors.

In the US, grounding conductors are sized according to the NEC Table 250.122. enter image description here

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Generally speaking, the grounding conductor may be required to carry (at least briefly) a fault current that's large enough to trip the breaker for the circuit(s) that it is protecting. This would imply that it needs to be the same size as the line wires.

Nowadays, most jurisdictions require the use of GFCI/RCD or AFCI breakers, which are much more sensitive to faults, so it could be argued that the current-handling requirements of the grounding conductor can be relaxed for circuits that have them.

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    In North America equipment grounding conductors (circuit grounds) can be much smaller than circuit conductors. In fact it's only the first few circuit sizes (up to 30A) that require the EGC to be the same size as the circuit conductors. GFCI and AFCI requirements have nothing to do with this. The sizing rules for EGCs have not changed for many years. – Speedy Petey May 24 '16 at 11:20
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    @SpeedyPetey that's pretty much an answer. – Harper - Reinstate Monica May 24 '16 at 17:21

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