AC Induction Motor
Full Load Current Calculator
Instantly calculate FLC for single-phase and three-phase motors — with cable, breaker, and contactor sizing built in.
🔌 Motor Full Load Current Calculator
AC induction motors are the workhorses of modern industry — reliable, robust, and found in everything from HVAC systems to conveyor belts. Understanding the Full Load Current (FLC) is the first step toward correctly sizing the entire electrical system that feeds them: cables, breakers, overload relays, and contactors.
💡 What is Full Load Current (FLC)?
Full Load Current is the current drawn by a motor when it operates at its rated power output — delivering the maximum mechanical load it was designed for, at the specified supply voltage and frequency.
Safety & Protection
Overload relays and breakers are set based on FLC. Wrong values lead to nuisance tripping or undetected overheating.
Cable Sizing
Cables must carry FLC continuously without exceeding their temperature rating. Under-sized cables are a fire hazard.
Component Selection
Contactors, MCBs, and fuses are all selected with reference to FLC — typically at 125% for cables per IEC 60364.
Energy Auditing
Comparing measured current to FLC reveals load utilisation — a motor drawing 60% of FLC may be oversized for the task.
📐 Calculation Formulas
Where:
P = Rated output power (Watts)
VL = Line-to-line voltage (V)
η = Efficiency (decimal, e.g. 0.90)
cos φ = Power factor (decimal)
1 HP = 0.7457 kW = 745.7 W
🧭 How to Use the Calculator
Select Phase Type
Choose single-phase (1φ) for small pumps/fans or three-phase (3φ) for industrial motors. The formula changes accordingly.
Enter Motor Power
Find the rated output power on the motor nameplate — enter in kW or HP. This is the mechanical output, not electrical input.
Set Supply Voltage
Use the actual line voltage at the motor terminals. In Bangladesh: 400V (3φ) or 230V (1φ) are standard.
Enter Efficiency & PF
Find these on the nameplate or motor datasheet. If unknown, use η = 90% and PF = 0.85 as safe estimates.
Set Ambient Temperature
IEC baseline is 40°C. Higher temperatures derate cable capacity — the calculator applies correction automatically.
Read Results & Sizes
The calculator outputs FLC, starting current estimate, and recommends cable size, breaker, overload relay, and contactor.
🔍 Key Parameters Explained
⚡ Power Factor (cos φ) — What does it really mean?
▼Power factor is the ratio of real power (kW) to apparent power (kVA). For induction motors, the PF is typically 0.75–0.90 at full load, but drops sharply at light loads — a 30% loaded motor may have PF as low as 0.55.
A low PF means more current is drawn for the same useful work, heating cables and causing voltage drop. This is why power factor correction capacitors are installed near large motors.
📊 Efficiency (η) — Where does the energy go?
▼No motor converts 100% of electrical energy to mechanical work. Losses include copper losses (I²R in windings), iron losses (eddy currents in core), friction & windage, and stray load losses. Modern IE3/IE4 motors achieve 93–96% efficiency at full load.
🌡️ Ambient Temperature & Derating
▼IEC 60364 defines cable ampacity at 30°C (underground) or 40°C (in air) baseline. Higher ambient temperatures reduce allowable current. For example:
| Ambient Temp | Derating Factor (PVC) | Effect |
|---|---|---|
| 30°C | 1.00 | No derating |
| 35°C | 0.94 | −6% |
| 40°C | 0.87 | −13% |
| 45°C | 0.79 | −21% |
| 50°C | 0.71 | −29% |
| 55°C | 0.61 | −39% |
This is especially important in Bangladesh where ambient can exceed 45°C in summer.
🔄 Starting Current (Locked Rotor Current)
▼When an induction motor starts (DOL — Direct On Line), it draws 5× to 8× FLC for a few seconds. This inrush current must be considered when:
- Selecting MCB type (Type D or motor-rated MCBs)
- Sizing the supply transformer
- Checking voltage dip on the busbar during starting
- Deciding whether to use a soft-starter or star-delta starter
🚀 Motor Starting Methods & Starting Current
| Starting Method | Starting Current | Starting Torque | Typical Use Case |
|---|---|---|---|
| DOL (Direct On Line) | 5–8 × FLC | 100% | < 7.5 kW, stiff supplies |
| Star-Delta Starter | 1.5–2.7 × FLC | 33% | 7.5–75 kW, light starting load |
| Soft Starter | 2–4 × FLC | Adjustable | Pumps, fans, conveyors |
| Variable Speed Drive (VFD) | ≤ 1.5 × FLC | Full at low speed | Best control & energy saving |
| Auto-transformer Starter | 1.3–4 × FLC | 42–64% | Large motors, limited torque need |
⚠️ Common Errors & How to Avoid Them
Confusing kW with kVA: The motor nameplate shows output power in kW or HP — not kVA. The kVA (apparent power) is higher due to PF. Always use the shaft output (kW) in the FLC formula.
Using input power instead of output power: Pinput = Poutput ÷ η. The formula already divides by efficiency — don't divide input power again.
Wrong voltage for 3-phase: Always use line-to-line voltage (e.g. 400V), not phase voltage (231V). The √3 factor in the formula is already accounting for the three-phase geometry.
Ignoring derating for high ambient: Cables sized for 40°C may be under-rated in Bangladesh summers. Always apply temperature correction factors.
📝 Worked Example
A 5.5 kW, 3-phase, 400V pump motor with η = 90%, PF = 0.85. Find FLC, select cable and breaker.
IFL = 5500 ÷ (√3 × 400 × 0.90 × 0.85)
IFL = 5500 ÷ (1.7321 × 400 × 0.765)
IFL = 5500 ÷ 530.0
IFL = 10.38 A
Cable rated ≥ 10.38 × 1.25 = 12.97 A → use 2.5 mm² (rated ~18 A)
Breaker: standard next above FLC → 16 A MCB (Type D)
Overload relay set at: 10.38 A ± 10%
