Motor Calculations
Worked Examples (430.6, 430.22, 430.24, 430.32, 430.52, 430.62, 430.110, 430.122)
NFPA 70 National Electrical Code, 2023 edition · Reviewed 2026-09-21
Motor calculations sit in the Motors and Generators topic of the Georgia PSI outline on both technical exams. They all run the same six-step chain, and the exam traps are (1) using the nameplate where the Code says table (or vice versa), (2) forgetting that 430.52 rounds up while 430.32 does not, and (3) the 25 % adder applies to the largest motor once. Written from the 2023 text.
The six-step chain
| Step | What | Current used | Factor |
|---|---|---|---|
| 1 | Branch-circuit conductors — 430.22(A) | table FLC (430.6(A)) | 125 % |
| 2 | Overload — 430.32 | nameplate FLA | 125 % (SF ≥ 1.15 or rise ≤ 40°C) / 115 % others; may go to 140 % / 130 % if needed (430.32(C)) |
| 3 | Branch-circuit short-circuit/ground-fault device — 430.52 / Table 430.52(C)(1) | table FLC | nontime-delay fuse 300 %; dual-element fuse 175 %; instantaneous-trip breaker 800 % (1100 % for Design B energy-efficient and premium efficiency); inverse-time breaker 250 %; wound-rotor 150 %; next size up permitted (430.52(C)(1)(a)); if still insufficient — ≤ 400 % / 225 % / 400 % (≤ 100 A FLC) or 300 % (> 100 A) |
| 4 | Feeder conductors — 430.24 | table FLC | 125 % of the largest motor + 100 % of the others + non-motor loads (125 % continuous) |
| 5 | Feeder OCPD — 430.62(A) | — | largest branch-circuit device + the other motors' FLC; round down to a standard size |
| 6 | Disconnect — 430.110 | table FLC / Table 430.251 LRC | ≥ 115 % of FLC; hp-rated by combined FLC and LRC (430.110(C)) |
2023 note: Table 430.52(C)(1) carries a separate row for Design B energy-efficient and Design B premium efficiency motors (instantaneous trip 1100 %); 430.52(C)(3) caps instantaneous-trip settings at 1300 % (1700 % for those Design B motors); 430.22(G) covers 18/16 AWG small-motor conductors by overload-relay Class.
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