Question

A parallel 3-phase set of 500 kcmil THWN copper feeders protected by an 800-ampere overcurrent device is run in a wire way and is to be tapped. The tap wires will be 20 ft long, terminated in a single circuit breaker, and run in a conduit. What's the smallest size of copper THWN wire allowed by the NEC for this tap?

1. 2/0
2. 3/0
3. 4/0
4. 300 kcmil

Answer 1

The smallest size of copper THWN wire allowed for this tap is 4/0 (400 kcmil).

Step-by-step explanation:

The National Electrical Code (NEC) specifies the minimum size of wires allowed for different applications. In this case, we have a parallel 3-phase set of 500 kcmil THWN copper feeders protected by an 800-ampere overcurrent device. The tap wires will be 20 ft long and terminated in a single circuit breaker.

According to the NEC, the tap conductors must have an ampacity not less than the calculated load. Since the feeder is protected by an 800-ampere overcurrent device, the smallest size of copper THWN wire allowed for this tap would be 4/0 (400 kcmil).