Data Source and Methodology

Authoritative source: National Electrical Code (NFPA 70), 2023 Edition, Table 430.250 (Three-Phase AC Motors, 60 Hz). Access the official text via the NFPA online portal: NFPA 70 (NEC) 2023.

Where an engineering estimate of nameplate FLA is presented, the tool uses standard power equations based on widely accepted motor fundamentals (see NEMA MG 1). The NEC table FLC is used for sizing per Code; the nameplate estimate is provided for context only.

All calculations are strictly based on the formulas and data provided by this source.

The Formula Explained

For an estimated nameplate FLA at full load:
Three-phase: $I_{FLA} = \dfrac{P_{out}}{\sqrt{3}\, V \, \eta \, PF}$, where $P_{out} = \text{HP} \times 746\,\text{W}$
Single-phase: $I_{FLA} = \dfrac{P_{out}}{V \, \eta \, PF}$

NEC Table 430.250 values (FLC) are standardized and do not depend on efficiency or power factor. Use FLC for conductor and protection sizing unless your Code article states otherwise.

Glossary of Variables

HP — Motor horsepower (output power at the shaft).
V — Line-to-line voltage for three-phase, line voltage for single-phase (volts).
PF — Power factor (0–1) of the motor at full load (nameplate or typical).
η (eta) — Efficiency (0–1) at full load (nameplate or typical).
NEC FLC — Standardized full-load current from NEC Table 430.250 (three-phase).
Minimum Conductor Ampacity — 125% of NEC FLC per NEC 430.22 for continuous duty motors.

How It Works: A Step-by-Step Example

Scenario: Three-phase, 460 V, 10 HP, 60 Hz motor.

  1. Select Three‑Phase and 460 V.
  2. Enter 10 HP. The tool uses the 10 HP row from NEC Table 430.250.
  3. NEC FLC is 14.0 A at 460 V for 10 HP.
  4. Minimum conductor ampacity = 1.25 × 14.0 A = 17.5 A (NEC 430.22).
  5. (Optional) Estimate nameplate FLA with η = 92% and PF = 0.86: $I_{FLA}=\dfrac{10\times 746}{\sqrt{3}\times 460\times 0.92\times 0.86}\approx 11.4\text{ A}$ which is lower than the NEC FLC, as expected.

Frequently Asked Questions (FAQ)

What is the difference between FLA and FLC?

FLA is the current on a motor’s nameplate at rated load. FLC is the standardized current from NEC tables used for Code sizing. Use FLC for conductor sizing unless otherwise permitted.

Which NEC table does this tool implement?

NEC Table 430.250 (2023 Edition) for three‑phase AC motors at 60 Hz. Values for some voltages are proportionally scaled from the 230 V or 460 V baselines to mirror Code tables closely.

Can I use this for 50 Hz systems?

The NEC tables are for 60 Hz. The tool can estimate nameplate FLA at 50 Hz using the formula, but you should consult the appropriate standards for final sizing.

What if my HP doesn’t exactly match a table value?

The tool rounds up to the next higher standard HP rating listed in the NEC table, consistent with conservative Code practice.

Why is the estimated FLA lower than the NEC FLC?

Because the NEC table is standardized and intentionally conservative for sizing, while the estimate uses physics-based equations with typical η and PF, often yielding a lower value.

Does this tool size breakers or overloads?

It provides the NEC FLC and the 125% minimum conductor ampacity. Overcurrent protection sizing (NEC 430.52, etc.) depends on motor type and protection device and is outside this tool’s scope.

Tool developed by Ugo Candido.
Content verified by CalcDomain Electrical Engineering Review Board.
Last reviewed for accuracy on: .