Motor Full Load Amps (FLA) Calculator
Estimate motor full-load amperage from horsepower, voltage, phase, efficiency, and power factor — use the result to size conductors, breakers, and motor starters.
Motor Details
HP
Live Results
Full Load Amps (FLA)
21.51A
Horsepower
5HP
Voltage
240V
Phase
Single-Phase
Efficiency
85%
Power Factor
0.85
Motor FLA calculations are based on standard electrical formulas. Always verify final sizing with motor nameplate data and local electrical code.
How to Use This Motor FLA Calculator
- Enter motor horsepower. Input the rated horsepower (HP) from the motor nameplate or equipment specification. Common residential and light commercial motors range from 1/2 HP to 10 HP — larger industrial motors may exceed 50 HP and warrant additional engineering review.
- Select supply voltage. Choose the operating voltage: 120 V for small single-phase equipment, 208 V and 240 V for residential and commercial branch circuits, or 480 V for industrial three-phase systems. Voltage directly affects current draw — lower voltage means higher amperage for the same horsepower.
- Choose single-phase or three-phase. Select the motor phase configuration. Single-phase motors use one hot leg and a neutral or second hot; three-phase motors distribute power across three legs and typically draw less current per leg for the same horsepower output.
- Set efficiency and power factor. Enter motor efficiency and power factor as decimals between 0.50 and 1.00. Defaults of 0.85 are reasonable estimates when nameplate data is unavailable. Nameplate FLA values already account for these factors — use them directly when available instead of calculating.
- Review FLA and verify sizing. The results panel shows calculated full-load amperage rounded to two decimal places. Use this value with the Breaker Size, Wire Gauge, and Voltage Drop calculators to confirm conductor ampacity, breaker rating, and starter requirements before installation.
Formulas & Example
Motor FLA converts mechanical horsepower to electrical current using standard power formulas. Three-phase calculations include the √3 factor because line voltage applies across phase pairs in a balanced system.
1 HP = 746 watts
Single-phase:
FLA = (HP × 746) / (V × efficiency × power factor)
Three-phase:
FLA = (HP × 746) / (V × efficiency × power factor × √3)
Round FLA to 2 decimal places.Worked Example
A 5 HP single-phase motor at 240 V with 0.85 efficiency and 0.85 power factor:
HP = 5
Voltage = 240 V
Phase = single
Efficiency = 0.85
Power Factor = 0.85
FLA = (5 × 746) / (240 × 0.85 × 0.85)
FLA = 3,730 / 173.40
FLA = 21.51 AUse this FLA with the Breaker Size Calculator (NEC Table 430.52 motor rules), Wire Gauge Calculator, and Voltage Drop Calculator to complete the branch-circuit design. When the motor nameplate lists FLA directly, prefer that value over calculated estimates.
Frequently Asked Questions
What is motor FLA?â–¾
Motor FLA (Full Load Amps) is the current a motor draws when operating at its rated horsepower under normal load conditions. It appears on the motor nameplate and is used to size conductors, circuit breakers, overload heaters, and motor starters. Calculated FLA from horsepower, voltage, and efficiency provides a planning estimate when nameplate data is unavailable — always verify against the actual nameplate before final installation.
What is the difference between single-phase and three-phase FLA?â–¾
Single-phase FLA uses the formula FLA = (HP × 746) / (V × efficiency × power factor). Three-phase FLA divides the denominator by √3 (approximately 1.732) because power is distributed across three legs: FLA = (HP × 746) / (V × efficiency × power factor × √3). For the same horsepower and voltage, a three-phase motor draws roughly 58% of the current per leg compared to a single-phase motor, which is why three-phase is preferred for larger loads.
What affects motor amperage?â–¾
Motor amperage is primarily determined by horsepower (mechanical load), supply voltage, efficiency, and power factor. Higher horsepower increases current; higher voltage decreases current. Lower efficiency or power factor increases current because more electrical power is required to deliver the same mechanical output. Service factor, altitude, ambient temperature, and voltage imbalance can also affect actual running current beyond the calculated FLA.
What is power factor?â–¾
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amperes) in an AC circuit, expressed as a decimal from 0 to 1. Inductive loads like motors draw reactive current that does not perform work but still loads the conductors. A PF of 0.85 means 85% of the current delivers useful power. Motors typically run between 0.75 and 0.90 PF depending on load and design. Lower power factor increases FLA for the same horsepower output.
Should I oversize breakers for motors?â–¾
Motor branch-circuit breakers are sized differently from general loads. The NEC allows inverse-time breakers to be sized up to 250% of motor FLA for standard motors (NEC Table 430.52) to accommodate starting inrush without nuisance tripping — this is not the same as oversizing for continuous loads. Conductors are typically sized at 125% of FLA. Never install a breaker larger than permitted by NEC 430.52 without proper engineering, and always verify wire ampacity matches or exceeds the conductor sizing rules in NEC 430.22.
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