Motor Circuits

Motors and Generators is a named topic in the Wiring Methods and Materials part of the Georgia PSI outline, and motor calculations sit on both technical exams. Every motor calculation follows the same five-step chain, and every step has its own percentage. 2023 edition notes: Table 430.52(C)(1) has a separate row for Design B energy-efficient and Design B premium efficiency motors (1100 % instantaneous-trip), 430.52(C)(3) caps instantaneous-trip settings at 1300 % / 1700 %, and the small-motor 18/16 AWG rule (430.22(G)) is written around the overload relay Class. Written from the 2023 text.

1. Which current do you use? — 430.6

• Conductors, disconnect ratings and branch-circuit short-circuit/ground-fault protection: the full-load current from Tables 430.247 (dc), 430.248 (single-phase), 430.249 (two-phase) and 430.250 (three-phase) — NOT the nameplate amperes (430.6(A)(1)). If only amperes are marked, the hp is taken from the table.
• Separate overload protection: the motor NAMEPLATE current (430.6(A)(2)).
• Exceptions: multispeed motors (per winding), listed motor-operated appliances with a marked FLC (use the nameplate), low-speed/high-torque and canned-pump motors (nameplate), torque motors (nameplate), and equipment marked with MCA/MOCP (430.7(D) / 430.25).
• 430.17: the "highest rated" or "smallest rated" motor in a group is the one with the highest (or lowest) table FLC — not horsepower.

Table 430.250 (three-phase, induction) — the values that get tested:

Table 430.248 (single-phase) — 115 V / 230 V: ½ hp 9.8 / 4.9; ¾ 13.8 / 6.9; 1 16 / 8.0; 1½ 20 / 10; 2 24 / 12; 3 34 / 17; 5 56 / 28; 7½ 80 / 40; 10 100 / 50.

Table 430.247 (dc, 240 V): 1 hp 4.7; 5 hp 20; 10 hp 38; 20 hp 72; 50 hp 173.