What is FLA on a motor nameplate?

FLA stands for Full Load Amperes — the current the motor draws when running at its rated horsepower output. It is the reference value used by the NEC and IEC to size conductors, breakers, fuses and overload relays for the motor branch circuit.

What formula does this calculator use?

For three-phase motors: FLA = (hp × 746) / (1.7321 × V × η × pf). For single-phase motors: FLA = (hp × 746) / (V × η × pf). Here 746 W/hp is the exact SI conversion, 1.7321 is the square root of 3 for balanced three-phase systems, η is efficiency expressed as a decimal, and pf is power factor expressed as a decimal.

Why does the NEC require branch conductors rated at 125% of FLA?

NEC 430.22 requires motor branch-circuit conductors to be rated at no less than 125% of the motor FLA. This provides a safety margin for the continuous load duty of motors (which run for more than 3 hours) and accounts for the fact that motors can draw slightly above nameplate FLA at full load without tripping an overload relay.

How do I choose the right circuit breaker size for a motor?

NEC Table 430.52 sets the maximum overcurrent device size as a multiple of FLA. For an inverse-time circuit breaker the maximum is 250% of FLA. Because the result rarely lands on a standard breaker rating, the NEC allows rounding up to the next standard size. This calculator does exactly that — it multiplies FLA by 2.5, then picks the next size from the standard breaker schedule.

What is a motor overload relay and how is its setpoint determined?

An overload relay (or thermal overload) protects the motor windings from sustained overcurrent. NEC 430.32 sets the trip setpoint at 115% of nameplate FLA when the motor has a service factor of 1.15 or greater, or 125% when the service factor is below 1.15 or not marked. The setpoint is not the same as the branch-circuit breaker — the breaker handles short circuits and ground faults; the overload relay handles sustained motor overload.

Why are nameplate efficiency and power factor important in this calculation?

Both values directly affect how much current the motor draws. A motor with 85% efficiency draws more current than a 95%-efficient motor producing the same shaft output. A low power factor (say 0.75) means more reactive current and therefore more total line current for the same real power. Using the wrong values — or the common shortcut of ignoring them — can result in undersized conductors or nuisance tripping.

Motor FLA Calculator

A motor FLA calculator that computes the full load amperes for any AC induction motor from its rated horsepower, supply voltage, efficiency and power factor — then derives the branch-circuit conductor ampacity, maximum breaker size and overload relay setpoint straight from the NEC formulas.

How it works

An AC induction motor converts electrical power to mechanical shaft power. The relationship between current, voltage and power follows a well-established formula that accounts for two losses on the way from the supply to the shaft: efficiency (heat, friction and core losses inside the motor) and power factor (the reactive component of the current that does no useful work but still flows through the conductors).

Three-phase formula:

FLA = (hp × 746) / (√3 × V × η × pf)

Single-phase formula:

FLA = (hp × 746) / (V × η × pf)

Where hp is rated output horsepower, V is the line-to-line voltage in volts, η is motor efficiency as a decimal (e.g. 0.92), pf is power factor as a decimal (e.g. 0.85), and √3 ≈ 1.7321 is the three-phase constant for balanced line currents.

The constant 746 is the exact SI value for one mechanical horsepower (1 hp = 746 W).

NEC branch-circuit sizing rules

Once FLA is known, three more values come directly from the National Electrical Code (NEC):

Branch-circuit conductor ampacity — NEC 430.22 requires at least 125% of FLA for single motor branch circuits because motors are treated as continuous loads.
Maximum overcurrent device — NEC Table 430.52 allows an inverse-time circuit breaker up to 250% of FLA (2.5×). The calculator rounds up to the next standard breaker rating from the NEC 240.6 schedule.
Overload relay setpoint — NEC 430.32 sets trip current at 115% of FLA for motors with service factor SF ≥ 1.15, or 125% of FLA for motors with SF below 1.15 or not marked on the nameplate.

Worked example

A 10 hp, 480 V three-phase induction motor with 93% efficiency and 88% power factor (typical for a NEMA Premium motor in this size range):

FLA = (10 × 746) / (1.7321 × 480 × 0.93 × 0.88)
FLA = 7,460 / (1.7321 × 480 × 0.8184)
FLA = 7,460 / 680.0 ≈ 10.97 A

From that single number:

Derived value	Calculation	Result
Branch conductor ampacity	10.97 × 1.25	13.71 A → use 14 AWG minimum
Maximum CB (raw)	10.97 × 2.5	27.4 A
Recommended CB (standard)	Next size ≥ 27.4 A	30 A
Overload setpoint (SF 1.15)	10.97 × 1.15	12.6 A

Formula note

The 746 W/hp conversion is exact — it comes from the original definition of one horsepower as 550 ft·lbf/s, which converts to exactly 745.69987… W, rounded to 746 in practice. Some older references use 750 W/hp (a European approximation used with metric hp) — this calculator uses the IEEE/NEC standard value of 746 W/hp.

For three-phase systems the √3 factor arises because the three line currents in a balanced wye or delta circuit are 120° apart in phase, so total power is √3 × V_line × I_line × pf rather than 3 × V_phase × I_phase × pf (the two expressions are equivalent; the line-voltage form is more convenient for panel sizing).