Motor Current Calculator

Full Load Current (FLC) is the steady-state current a motor draws from the supply when running at its rated output power. It is the single most important number for selecting the cable, contactor, overload relay, and circuit breaker feeding a motor — undersize any of these against the real FLC and the motor either nuisance-trips or, worse, overheats the wiring. Motor nameplates usually list a rated FLC, but this calculator lets you verify or estimate it directly from the motor's rated power, supply voltage, power factor, and efficiency, following the same relationships used throughout standard IEC/NEMA motor sizing references.

For a single-phase motor, the relationship between electrical input power and current is I = P ÷ (V × PF × η), where P is the rated shaft power converted to watts, V is the supply voltage, PF is the power factor, and η is the motor's efficiency as a decimal. Efficiency is included because the motor's electrical input power is always higher than its rated mechanical output power — a 10 kW motor at 90% efficiency actually draws about 11.1 kW of electrical power to deliver that 10 kW at the shaft.

For a three-phase motor, the same relationship gains a √3 (≈1.732) term to account for the phase geometry of a balanced three-phase system: I = P ÷ (√3 × V × PF × η). If the motor's power is rated in horsepower rather than kilowatts, the calculator first converts it using 1 HP = 746 W before applying either formula.

Once FLC is known, this calculator also estimates the apparent power (kVA) the supply must deliver, which is simply the product of voltage, current, and the appropriate phase factor divided by power factor, and gives a sizing current with a design margin (typically 20–25%) added on top of FLC — a standard allowance used when selecting cable ampacity, contactor rating, and breaker trip setting, so the protective devices don't nuisance-trip on normal starting inrush and small load variations. The suggested breaker rating rounds this sizing current up to a common commercially available breaker size.

As always, treat this as an engineering estimate rather than a substitute for the motor's actual nameplate FLC, which accounts for the specific winding design and test data of that motor. Always cross-check the final cable, breaker, and overload settings against the manufacturer's nameplate and applicable local electrical code before commissioning.

Why FLC Is the Starting Point for Every Downstream Component

Full Load Current is the anchor figure for an entire chain of downstream decisions on a motor circuit. The cable feeding the motor must be sized so its current-carrying capacity comfortably exceeds FLC (see the Cable Size Calculator for that step). The contactor must be rated for FLC under its AC-3 duty classification, which accounts for the make/break duty of switching an inductive motor load rather than a simple resistive one. The thermal overload relay is set at or just above FLC so it protects the motor winding from sustained overload without nuisance-tripping on normal load variation. And the upstream breaker or fuse must be sized to protect the cable while tolerating the motor's brief but large starting inrush current — typically 5 to 7 times FLC for a direct-on-line start. Getting FLC right, whether from the nameplate or estimated here, is therefore the first and most consequential step in the whole selection chain.

Starting Current vs Full Load Current

It's easy to confuse FLC with starting (inrush) current, but they describe two very different moments in a motor's operation. FLC is the steady-state current drawn once the motor has reached full rated speed under full rated load — this is the number used for cable, contactor, and overload sizing. Starting current, by contrast, is the much larger current an induction motor draws for a fraction of a second (or a few seconds for larger motors) while it accelerates from standstill, because the rotor initially looks almost like a short-circuited transformer secondary to the supply. For a motor started direct-on-line (DOL), this inrush is typically 5–7× FLC; for a star-delta starter it drops to roughly 2–2.5× FLC; and for a soft starter or VFD it can be limited to as little as 1.5–2× FLC or less. This is why breakers and fuses protecting motor circuits use a "motor rated" or time-delay characteristic rather than an instantaneous trip at FLC — they need to ride through the brief starting surge without tripping, while still protecting against a genuine sustained overload.

How to Use This Motor Current Calculator

1. Select the phase type — single phase for smaller domestic/workshop motors, three phase for most industrial and commercial motors above roughly 1–2 HP.
2. Enter the motor's rated power in kW or HP, whichever matches the nameplate, along with the supply voltage.
3. Enter the power factor and efficiency from the nameplate if available; if not, 0.85 PF and 88–92% efficiency are reasonable estimates for a typical general-purpose induction motor in this size range.
4. Click Calculate to see the estimated FLC, apparent power (kVA), a sizing current with margin applied, and a suggested breaker rating.

Common Mistakes in Motor Current Estimation

• Forgetting to divide by efficiency. Using rated shaft power directly as electrical input power understates FLC, sometimes by 10% or more.
• Assuming a flat power factor across all motor sizes. Power factor varies with motor loading and size — small motors and lightly loaded motors often run at a lower PF than their nameplate full-load figure.
• Sizing the breaker directly at FLC with no margin. This leads to nuisance tripping during normal starting and load fluctuation; the 20–25% sizing margin exists specifically to avoid this.
• Confusing single-phase and three-phase formulas. Missing the √3 term on a three-phase calculation, or wrongly applying it to a single-phase motor, produces an FLC that's off by a factor of roughly 1.73.

Typical Full Load Current Reference (415V, 3-Phase, 0.85 PF)

The table below gives approximate FLC values for common motor sizes as a quick sanity check — always confirm against the actual motor nameplate for final design.

Figures are indicative only, based on typical 88–92% efficiency induction motors. Actual FLC varies by manufacturer, motor design, and duty class — always confirm against the nameplate.