The melting point of a copper conductor can be calculated by using these values:

• 1 ampere (RMS current)
• for 5 seconds
• for every 16.19 circular mils of cross-sectional area of the conductor.

NEC 250.122(B) states that if the ungrounded conductors are sized larger for any reason, the equipment grounding conductor shall be increased proportionately, based on the cross-sectional area of the ungrounded conductors for that particular branch circuit or feeder.

The need to increase the size of the ungrounded conductor might be for voltage drop reasons, or because high values of fault current are possible. In NEC 250.2 we find a definition of an effective ground-fault current path:

An intentionally constructed, permanent, lowimpedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault detectors on highimpedance grounded systems.

* Under fault-current conditions, the possible burning off of an equipment grounding conductor, a bonding jumper, or any conductor that is dependent on safely carrying fault current until the over current protective device can clear the fault results in a hazard to life, safety, and equipmen

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