Auto-Size Equipment
Automatically select conductor sizes, overcurrent device ratings, and transformer kVA using selected NEC 2023 rule checks. Covers feeders, branch circuits, motor circuits, and transformer protection as an engineering aid.
Method — NEC 210.20 / 215.3 / 240.4 / 310.15 / 392.80
For continuous loads (≥3 hours): conductor ampacity and OCPD rating must be at least 125% of the load current (NEC 210.20(A), 215.3).
Conductor size is selected from NEC Table 310.16 baseline ampacity values (formerly Table 310.15(B)(16)). Ambient temperature, bundling, and cable tray correction factors per NEC 310.15(B)(1)(a), 310.15(C)(1), and 392.80(A) are applied based on the Installation Conditions you enter below.
NEC 110.14(C) terminal-temperature limits are applied separately from the insulation ampacity table. Leave the terminal rating blank to infer 60 C through 100 A equipment and 75 C above 100 A.
OCPD rating is the next standard rating from NEC 240.6(A) at or above required load ampacity (NEC 240.4(B)). For small conductors, NEC 240.4(D) limits maximum OCPD to #14 Cu = 15A, #12 Cu = 20A, #10 Cu = 30A, #12 Al = 15A, and #10 Al = 25A.
Method — NEC 430 / 310.15 / 392.80 (Motor Branch Circuits)
Full load current (FLC): from NEC Table 430.250 (3-phase) or Table 430.248 (single-phase). Note: always use the NEC table values, not the motor nameplate current, for sizing branch circuit components.
Branch circuit conductor: ≥ 125% of motor FLC (NEC 430.22(A)). Ambient temperature, bundling, and cable tray correction factors per NEC 310.15(B)(1)(a), 310.15(C)(1), and 392.80(A) are applied based on the Installation Conditions you enter below.
NEC 110.14(C) terminal-temperature limits are applied to the selected branch-circuit conductor.
Branch circuit OCPD (inverse time breaker): ≤ 250% of FLC per NEC Table 430.52. Next standard OCPD size is selected.
Overload relay: set at 115% of FLC for motors with service factor ≥ 1.15 or temperature rise ≤ 40°C, otherwise 125% (NEC 430.32(A)(1)).
Method — NEC 450.3(B) (Transformers ≤ 600V)
Transformer kVA: next standard kVA rating at or above the required load.
Primary OCPD: ≤ 125% of primary rated current (or 167% if primary current ≤ 9A) per NEC Table 450.3(B). Next standard OCPD size is selected.
Secondary OCPD: ≤ 125% of secondary rated current per NEC 450.3(B).
Secondary conductors: sized at 125% of secondary rated current (continuous load using the NEC Table 310.16 baseline ampacity check).
NEC 110.14(C) terminal-temperature limits are applied to the selected secondary conductor.