What is the ampacity of each current-carrying conductor when nine 8 AWG THHN copper conductors are supplied to three 3-phase motors in a rigid metal conduit?

To determine the ampacity of each current-carrying conductor in this scenario, we start by referencing the National Electrical Code (NEC) table for the ampacity of conductors, which provides values for various sizes of copper conductors under different conditions.

For 8 AWG THHN copper conductors, the base ampacity in a conduit at 30 degrees Celsius (86 degrees Fahrenheit) is typically around 40 amps. However, when multiple conductors are bundled together, as is the case here with nine conductors, a derating factor comes into play to account for heat buildup due to the close proximity of the conductors.

For three-phase motors using solid metal conduit, the NEC specifies that when more than three conductors are present in a conduit, the ampacity must be derated. Specifically, for nine conductors, the ampacity is reduced to 70% of the original value due to this derating requirement.

Calculating the derated ampacity for the 8 AWG conductors:

1. Start with the base ampacity: 40 A (for one conductor).

2. Apply the derating factor for nine conductors, which is 70%:

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