Why does the motor calculator use table FLC instead of the nameplate amps?

Q: What size breaker and wire do I need for a 10 hp 460V three-phase motor?

Select three-phase AC, 460 V, and 10 hp; the calculator uses the FLC from NEC Table 430.250 rather than the nameplate, per 430.6(A)(1). Conductors are sized at a minimum 125% of FLC (430.22), the short-circuit/ground-fault OCPD is capped by Table 430.52 (for example 250% of FLC for an inverse-time breaker), and the disconnect is at least 115% of FLC (430.110). The tool rounds the OCPD up to the next standard size.

Q: Can I use this calculator for a VFD or HVAC compressor circuit?

No. Selecting the VFD starting method stops the calculation because variable-frequency-drive-fed motors follow NEC 430 Part X, where the drive nameplate input current governs the supply conductors. Hermetic refrigerant motor-compressors are covered by NEC Article 440, not Article 430, so HVAC compressor circuits are out of scope here.

Question

Why does the motor calculator use table FLC instead of the nameplate amps?

Accepted Answer

NEC 430.6(A)(1) requires that branch-circuit conductors, the short-circuit/ground-fault device, and the disconnect be sized from the full-load current values in Tables 430.247, 430.248, and 430.250, not the motor nameplate FLA. The nameplate FLA is used only for overload protection sizing under 430.32. This calculator follows that split, taking the table FLC for everything except the overload.

Why does the motor calculator use table FLC instead of the nameplate amps?

Answers