Single-Phase Motor Inputs
= 5.000 kW
Full-Load Current (FLA)
—
Amperes at full mechanical load
Starting Current
—
A (DOL)
Overload Relay
—
A (set at 115% FLA)
Recommended MCB
—
A (next std size)
Input Power (kW)—
Apparent Power (kVA)—
MCB / MCCB Size— A
Contactor Rating— A
Recommended Cable Size—
Overload Relay Range—
Note: These values are calculated estimates. Always verify with motor nameplate data and applicable local electrical codes. Starting current multipliers are typical values; actual inrush may vary by motor design and load.
How to Calculate Motor Full-Load Amps
The full-load ampere (FLA) rating of an AC motor depends on its power, supply voltage, power factor, and efficiency. The formulas are:
Single-phase motor:
FLA = (kW × 1000) / (V × PF × η/100)
Three-phase motor:
FLA = (kW × 1000) / (√3 × V × PF × η/100)
Once FLA is known, protection and control equipment is sized as follows:
- Overload relay: Set at 105%–120% of FLA (typically 115%)
- MCB / MCCB: Next standard size above 125% × FLA × service factor
- Contactor: Rated at or above FLA
- Cable: Sized for continuous FLA with appropriate derating for conduit/ambient temperature
Recommended MCB Sizes for Common Motors
| Motor (kW) | ≈HP | 3-Phase FLA (415V) | MCB Size | Contactor |
|---|---|---|---|---|
| 0.37 kW | 0.5 HP | ~0.9 A | 2 A | 9 A |
| 0.75 kW | 1 HP | ~1.7 A | 4 A | 9 A |
| 1.5 kW | 2 HP | ~3.4 A | 6 A | 9 A |
| 2.2 kW | 3 HP | ~5.0 A | 10 A | 12 A |
| 3.7 kW | 5 HP | ~8.4 A | 16 A | 16 A |
| 5.5 kW | 7.5 HP | ~12.5 A | 20 A | 16 A |
| 7.5 kW | 10 HP | ~17 A | 25 A | 25 A |
| 11 kW | 15 HP | ~25 A | 40 A | 32 A |
| 15 kW | 20 HP | ~34 A | 50 A | 40 A |
| 22 kW | 30 HP | ~50 A | 80 A | 65 A |
| 37 kW | 50 HP | ~84 A | 125 A | 95 A |
Values at PF = 0.85, η = 90%, 415V three-phase. Actual nameplate values take precedence.
Motor Starting Methods Compared
| Method | Starting Current | Starting Torque | Cost | Best For |
|---|---|---|---|---|
| Direct On-Line (DOL) | 6–8× FLA | 100% (full) | Lowest | Small motors <5 kW, fans, pumps |
| Star-Delta | ~2× FLA (line) | 33% of DOL | Medium | Medium motors 5–75 kW, lightly loaded starts |
| Soft Starter | 1.5–3× FLA | Adjustable | Medium-High | Conveyors, pumps, compressors |
| VFD (Variable Frequency Drive) | 1.0–1.5× FLA | Full torque at 0 RPM | Highest | Variable-speed loads, energy saving, precise control |
Star-Delta starters reduce voltage to 57.7% during the star phase, reducing starting current to approximately 1/3 of DOL — but also reducing torque to 1/3. They are only suitable when the load can start unloaded or with low inertia.
Frequently Asked Questions
Full-load current (FLA) is the rated current a motor draws when operating at its nameplate horsepower or kilowatt rating under full mechanical load. For a single-phase motor: FLA = (kW × 1000) / (V × PF × η). For a three-phase motor: FLA = (kW × 1000) / (√3 × V × PF × η). FLA is the reference value for sizing overload relays, cables, MCBs, and contactors.
A 5HP (3.73 kW) three-phase motor at 415V with PF 0.85 and 90% efficiency draws approximately 6.8A FLA. The MCB should be sized at 125% of FLA = 8.5A, so the next standard size of 10A MCB is recommended. For DOL starting (7× FLA ≈ 47A inrush), use a Type C or D characteristic MCB to avoid nuisance tripping on the startup surge.
At startup the motor rotor is stationary and back-EMF is zero, so the motor behaves like a short-circuited transformer. Current is limited only by low winding resistance and leakage reactance, causing 6–8× full-load current during DOL starting. As the rotor accelerates, back-EMF builds up and current falls to FLA within 2–5 seconds. High starting current can cause voltage dips that affect other equipment on the same supply.
Service factor (SF) is a multiplier on the nameplate power rating indicating how much overload the motor can sustain continuously without damage. A 1.15 SF motor rated 10 kW can handle 11.5 kW continuously. When sizing protection equipment, multiply FLA by the service factor to avoid nuisance tripping on allowed overloads. Standard motors have SF = 1.0; premium motors often carry SF = 1.15.