Motors, Transformers, and Special Equipment
Motors, Transformers, and Special Equipment draws on Articles 430, 450, 680, 690, and 625. The exam separates motor overload protection from branch-circuit short-circuit and ground-fault protection, tests transformer overcurrent rules, and asks about pool bonding and EV charging equipment.
Motors (Article 430)
- Use the FLC tables in 430.248–430.250, not the nameplate current, for conductor and branch-circuit protection sizing.
- Branch-circuit conductors: 125% of FLC (430.22).
- Short-circuit and ground-fault protection: 430.52 percentages (often 250% for inverse-time breakers) then the next standard rating.
- Overload protection: 430.32, generally 115–125% of nameplate FLA.
- Several motors on one feeder: sum the FLCs plus 25% of the largest (430.24).
Transformers (Article 450)
- 450.3 sets primary and secondary overcurrent protection based on transformer rating and voltage.
- 240.21(C) governs secondary conductor taps; length and ampacity limits are strict.
- Separately derived systems require 250.30 grounding and bonding.
Special equipment
- Pools (680): equipotential bonding grid, GFCI protection, and restricted use of underground wiring.
- EV charging (625): listed equipment, conductor sizing at 125% of continuous load, and disconnecting means.
- PV (690): rapid shutdown, marking, and conductor and OCPD sizing at 125%.
Worked example
A 10 hp, 480 V, three-phase motor has an FLC of 14 A from Table 430.250. Branch conductors: 14 × 1.25 = 17.5 A, so 12 AWG copper at 75°C (20 A) is acceptable. Inverse-time breaker: 14 × 2.50 = 35 A, a standard rating. Overloads are set from the nameplate FLA, not the table FLC.
Traps
- Sizing motor conductors from nameplate FLA instead of the FLC table.
- Confusing overload protection with branch-circuit short-circuit protection.
- Applying 250% to every motor or every breaker type.
- Forgetting the 25% largest-motor adder on a feeder.
- Treating a transformer secondary as unlimited length without checking 240.21(C).