What minimum size ungrounded THHN conductors are required for a 120/208-volt, 3-phase feeder supplying continuous load of 37.5 kVA?

To determine the minimum size ungrounded THHN conductors required for a 120/208-volt, 3-phase feeder supplying a continuous load of 37.5 kVA, it’s important to first calculate the current that the feeder will carry under these conditions.

The formula for calculating current in a three-phase system is:

[

I = \frac{P}{\sqrt{3} \times V}

]

where:

• (I) is the current in amperes (A),

• (P) is the power in watts (W),

• (V) is the line voltage (in this case, 208 volts).

Given that the load is 37.5 kVA, we convert this to watts for use in our formula:

[

P = 37.5 , \text{kVA} \times 1000 = 37500 , \text{W}

]

Now, inserting the values into the formula:

[

I = \frac{37500 , W}{\sqrt{3} \times 208 , V}

]

Calculating this step-by-step:

1. Calculate (\sqrt{3}): Approximately 1