The basic formulas
- Real power: P (W) = V × I × PF, where PF is the power factor (between 0 and 1).
- Apparent power: S (VA) = V × I. Stabilisers, inverters and generators are rated in VA or kVA, so use this for them.
- Running current: I (A) = P ÷ (V × PF).
- Energy: kWh = average power (kW) × hours. A compressor that cycles uses running power × fraction of time it runs.
- Three-phase, for larger plant: P = √3 × line voltage × I × PF, with India's line voltage nominally 400 V.
Power factor: why amps are higher than watts suggest
Compressor motors draw some current that does no useful work, so the current is higher than P ÷ V. Small refrigeration compressors commonly have a power factor somewhere around 0.7 to 0.95 depending on design and load (approximate). If the nameplate gives current, use it; if it gives only watts, ask the maker for the current or power factor rather than assuming 1.
Starting current
When a compressor starts, its motor draws a short surge called locked-rotor current (LRA on many nameplates). For small single-phase compressors this is commonly several times the running current, often in the range of four to eight times (approximate). It lasts a fraction of a second to a second, but it is what trips weak supplies, overloads small inverters and causes lights to dip.
Always use the nameplate LRA where given. The multiples in the table below are assumptions for illustration only.
| Unit (assumed rating) | Running W | PF | Running A | Start A (6× assumed) |
|---|---|---|---|---|
| Undercounter refrigerator | 250 | 0.80 | 1.36 | 8.2 |
| Two-door reach-in | 500 | 0.85 | 2.56 | 15.3 |
| Chest deep freezer | 300 | 0.80 | 1.63 | 9.8 |
| Single-door visi cooler | 400 | 0.85 | 2.05 | 12.3 |
| All four starting together | 7.6 running | 45.6 |
The restart problem after a power cut
Running, the four units above draw under 8 A. When power returns after a cut, all four try to start at once and briefly ask for about 45 A. That can trip a breaker, stall an inverter or pull a generator's voltage down so far that none of them start. Many controllers include a restart delay; staggered delays of a minute or more between units spread the surges. Ask how each unit's controller behaves on power return.
Voltage drop in the wiring
Long or thin cables lose voltage, and they lose most during the starting surge. Copper resistance is about 0.0175 Ω·mm²/m. A 2.5 mm² cable is therefore about 0.007 Ω per metre; a 20 m run has 40 m of conductor in the loop, so about 0.28 Ω. At a 15 A starting current, drop = 15 × 0.28 = 4.2 V. In 1.5 mm² the loop is about 0.47 Ω and the drop about 7 V.
That sounds small, but add it to a supply already sagging to 200 V on a summer evening and the compressor may struggle to start. Short, adequately sized runs to a dedicated socket matter.
Good practice for refrigeration circuits
- Give each large unit its own properly earthed socket rather than a multi-plug extension board.
- Have a licensed electrician select the MCB and RCCB so that they protect the circuit without nuisance tripping on compressor starts.
- Check the total running and starting load on each circuit when adding a new unit.
- Record the nameplate current and LRA of every unit; you will need them when sizing a stabiliser, inverter or generator.
Frequently asked questions
- How many amps does a commercial fridge use at 230 V?
- Divide the rated watts by 230 and by the power factor. A 500 W unit at PF 0.85 runs at about 2.6 A. Starting current is several times higher for a moment; check the nameplate.
- Why does my fridge trip the breaker only when power comes back?
- Because every compressor on the circuit tries to start at the same moment, and starting current is far higher than running current. Restart delays, separate circuits or a breaker selected for motor loads usually solve it; ask an electrician to check.
