In the TN-C-S grounding system used in electrical installations in the US, one of the main functions of the an EGC (equipment grounding conductor) is to serve as a back-up neutral wire, so that in the event of a line-to-case fault, a complete circuit is formed with low impedance to trip the breaker and clear the fault, thanks to the neutral-to-case bond at the main disconnect. Obviously this wire serves other purposes.
But in the previous purpose, a very high short-circuit fault current flows through the EGC. It's value depends on the impedance of the whole path and the voltage of the source. So we should make sure that the EGC doesn't melt or suffers insulation damage due to the high current during the very short duration of the fault. Hence intuitively, we should sice the EGC based on the available/prospective short-circuit current.
My question is why is the minimum EGC size required by the NEC to be based on the breaker size, and not on the short-circuit current?
Is it to simplify the process instead of performing a whole short-circuit analysis? But I don't think so, since the NEC already requires to consider the short-circuit current when selecting the interrupting rating of a breaker.
Is it for aesthetics, since big phase and neutral wires with a thin ground wire would look incorrect? I don't think so, since as we know the NEC is published by the NFPA, which is basically concerned with preventing fires. For that matter, the NEC doesn't even require voltage drop calculations, which are important but would not cause a fire if conductors and OCPD are properly sized.
So what's the reason?
But in the previous purpose, a very high short-circuit fault current flows through the EGC. It's value depends on the impedance of the whole path and the voltage of the source. So we should make sure that the EGC doesn't melt or suffers insulation damage due to the high current during the very short duration of the fault. Hence intuitively, we should sice the EGC based on the available/prospective short-circuit current.
My question is why is the minimum EGC size required by the NEC to be based on the breaker size, and not on the short-circuit current?
Is it to simplify the process instead of performing a whole short-circuit analysis? But I don't think so, since the NEC already requires to consider the short-circuit current when selecting the interrupting rating of a breaker.
Is it for aesthetics, since big phase and neutral wires with a thin ground wire would look incorrect? I don't think so, since as we know the NEC is published by the NFPA, which is basically concerned with preventing fires. For that matter, the NEC doesn't even require voltage drop calculations, which are important but would not cause a fire if conductors and OCPD are properly sized.
So what's the reason?