6.05
A
7.56
A
6.96
A
4.097
kW
3.729
kW
6.05
A
7.56
A
6.96
A
4.097
kW
3.729
kW
The motor full load current (FLC) calculator determines the steady-state current drawn by an AC induction motor at its rated load, voltage, and frequency. Full load current is the fundamental parameter for motor circuit design — it governs conductor sizing, overcurrent protection, overload relay settings, disconnect switch sizing, and starter selection.
The full load current formulas are: Single-phase: I_FLC = HP × 746 / (V × η × PF). Three-phase: I_FLC = HP × 746 / (√3 × V × η × PF). Where HP is horsepower, V is line voltage, η is motor efficiency (decimal), and PF is power factor. The factor 746 converts horsepower to watts (1 HP = 745.7 W).
NEC Article 430 uses FLC values from NEC Tables 430.248 (single-phase) and 430.250 (three-phase) as the basis for motor circuit component sizing, rather than actual nameplate FLA values. This is because NEC tables represent conservative values that encompass a broad range of motor designs. However, for setting overload relays, the actual nameplate FLA must be used per NEC 430.6(A)(2).
Motor efficiency directly affects current draw. A premium efficiency motor (96% efficient) draws less current for the same horsepower output than a standard efficiency motor (91% efficient). For a 50 HP motor at 460V with η = 96% and PF = 0.85, FLC = 50 × 746 / (√3 × 460 × 0.96 × 0.85) = 61.3A. The same motor at η = 91%: FLC = 64.7A — a 5.4% difference that affects conductor and protection sizing.
Power factor reflects the phase relationship between voltage and current. Induction motors typically have power factors of 0.80-0.90 at full load, improving toward unity with load and deteriorating to 0.20-0.40 at no-load. Power factor affects reactive current (which contributes to line current but not useful work) and can be corrected with capacitor banks to reduce apparent power (kVA).
The NEC-required 125% factor for motor conductor sizing (430.22) accounts for the continuous nature of motor loads — motors are continuous duty by definition unless specifically marked otherwise. The 115% overload relay setting (per NEC 430.32) protects the motor from moderate overloads while allowing normal motor starting.
Single-phase FLC = P_output / (η × PF × V) = HP × 746 / (η% × PF × V). Three-phase FLC = HP × 746 / (√3 × V × η% × PF). 125% FLC = FLC × 1.25 (NEC 430.22 conductor sizing). Overload relay = FLC × 1.15 (NEC 430.32, standard motors). Input power = output power / efficiency.
Use 125% FLC to size branch circuit conductors and the disconnecting means. Use FLC × 1.15 for overload relay setting (or × 1.25 for motors with Service Factor ≥ 1.15 or temperature rise ≤ 40°C). For short-circuit protection (breakers, fuses), use NEC Table 430.52 — up to 250% for inverse time breakers, 175% for dual-element fuses.
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Results
10 HP at 460V (3-phase): FLC ≈ 15.1A (compare to NEC Table 430.250 = 14A). Conductors rated ≥ 18.9A → AWG 12. Overload relay set at 17.4A.
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Results
3 HP single-phase at 240V: FLC ≈ 12.6A (NEC Table 430.248 = 17A conservative value). Use 17A from NEC table for breaker sizing, nameplate value for overload relay.
FLC (Full Load Current) is the calculated current from the motor formula. FLA (Full Load Amperes) is the actual nameplate rated current. NEC 430.6(A) requires using FLC from NEC tables for circuit sizing but actual FLA for overload relay settings.
NEC tables (430.248, 430.250) provide conservative current values that cover a broad range of motor designs at the rated horsepower. This ensures circuit components are adequately sized for any motor of that HP rating, not just one specific model.
Use the line-to-line voltage. For 480V wye systems, use 480V. For 208V three-phase systems, use 208V. The √3 factor in the three-phase formula already accounts for the conversion between line and phase voltages.
LRC (also called starting current or inrush current) is 5-8 times FLC for standard induction motors during starting. Motor protection and branch circuit breakers must withstand LRC without tripping — this is why NEC allows breaker ratings up to 250% of FLC for motor branch circuits.
Motor current increases roughly proportionally with mechanical load from no-load to full-load. At no-load, current is 25-40% of FLC (mostly reactive magnetizing current). At overload, current can exceed FLC — overload relays are designed to trip when this exceeds 115-125% of FLC for an extended period.
Service Factor (SF) is a multiplier indicating how much above rated load a motor can operate without overheating. SF = 1.15 means the motor can run at 115% of rated HP continuously. Motors with SF ≥ 1.15 may have overload relays set at 125% of nameplate FLA per NEC 430.32(A)(1).
These values are on the motor nameplate or in the manufacturer's data sheet. NEMA MG 1 defines efficiency testing procedures. Premium efficiency motors (NEMA Premium®) have efficiencies of 89.5-96% depending on HP. If unknown, use 90% efficiency and 0.85 PF as typical values.
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