Feeder Calculations

The Feeders topic of the Georgia PSI outline carries feeder load and conductor-sizing calculations on both technical exams. They are always one of five problem types: (1) size the conductor for continuous + noncontinuous load, (2) apply ambient correction and conductor-count adjustment, (3) size a tap, (4) size the neutral, (5) voltage drop with Chapter 9 Table 8. The method below is the 2023 text; the numbers come from Table 310.16, Table 310.15(B)(1)(1), Table 310.15(C)(1) and Chapter 9 Table 8.

1. The two-step feeder conductor rule — 215.2(A)

A feeder conductor must satisfy both of these, and the larger governs:

• 215.2(A)(1) — the termination check: ampacity ≥ noncontinuous load + 125 % of the continuous load, using the termination temperature column of Table 310.16 (110.14(C): 60°C column for circuits ≤ 100 A or conductors 14–1 AWG; 75°C column for over 100 A or 1/0 and larger, unless the equipment is marked for 75°C/90°C). No adjustment or correction factors are applied in this step. Exceptions: 100 %-rated assemblies (OCPD + assembly listed for continuous 100 % operation); separately installed pressure connectors at both ends; grounded conductors not connected to an OCPD are sized at 100 %.
• 215.2(A)(2) — the conditions-of-use check: ampacity ≥ the maximum load (100 % of continuous + noncontinuous) after the 310.15 ambient-temperature correction and the Table 310.15(C)(1) conductor-count adjustment, starting from the conductor's insulation temperature column (90°C for THHN/THWN-2/XHHW-2).
• 215.3 / 240.4: the OCPD ≥ noncontinuous + 125 % continuous; 240.4(B) lets you round up to the next standard size (240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 …) where the conductor ampacity does not correspond to a standard rating, the conductors are not part of a multioutlet branch circuit with cord-and-plug loads, and the next size does not exceed 800 A.