What is the minimum size of the equipment bonding jumper required for bonding a metal CT enclosure for a service lateral of 5 parallel runs of 350 kcmil THWN copper?

To determine the minimum size of the equipment bonding jumper required for bonding a metal current transformer (CT) enclosure in this case, we look to the National Electrical Code (NEC) which provides guidelines based on the size of the service conductors.

In this scenario, you have a service lateral with five parallel runs of 350 kcmil THWN copper conductors. The NEC stipulates that the bonding jumper must be sized according to the largest ungrounded conductor in the service. For conductors made of copper, the appropriate size of the bonding jumper is derived from the guidelines set forth in NEC Table 250.66.

According to this table, for 350 kcmil copper conductors, the minimum size of the bonding jumper required would be 250 kcmil copper. This guideline ensures that the jumper is capable of carrying sufficient fault current to ensure the effective functioning of protective devices and provide a good ground path in the event of a fault condition.

Thus, the selection of 250 kcmil copper as the minimum size for the bonding jumper is based on adherence to these code requirements. It ensures that adequate safety and reliability are maintained in the electrical installation, specifically for the grounding and bonding requirements of the CT enclosure.