Heat pumps up to about 5–7 kW heating output are often single phase (230 V). Larger units are usually three phase (415 V). Check the nameplate for rated input current, maximum running current (at the hottest/most loaded condition), and locked rotor amps (LRA), the current a fixed-speed compressor draws for a moment at start.

Starting current:

• A fixed-speed scroll compressor on direct-on-line (DOL) start can draw 5–7 times running current for a fraction of a second.
• Soft starters reduce this to typical 2–3 times.
• Inverter compressors ramp up gently; inrush stays near running current.

Indian supply issues: voltage can sag to 180–200 V on single phase in rural areas, and phases can be unbalanced. Use a phase-failure and phase-sequence relay, and a voltage monitor or stabiliser sized for the starting kVA where supply is weak.

Protection chain (typical): incoming MCB/MCCB → RCCB/RCBO (earth leakage, 30 mA for personnel protection or 100–300 mA for equipment per local practice) → contactor → overload relay or compressor motor protector. Fit a local isolator next to the outdoor unit. Earth properly — test earth resistance.

Worked example — cable and MCB for a 25 kW (heat) unit

COP at design 3.0 → electrical input 25/3.0 = 8.3 kW. Max running input (hot day, high water) say 10 kW, pf 0.9.

I = 10,000 / (1.732 × 415 × 0.9) = 15.5 A. Choose a 20 A C-curve or D-curve MCB (D-curve if DOL start with high LRA). Cable: 4 mm² copper, 4-core armoured, typical for runs up to about 30 m; check voltage drop (keep under 3–5 %) and derating for grouping and ambient. Starting check (fixed speed, LRA 6 × 15.5 ≈ 93 A): transformer/DG must accept this dip. With a soft starter, about 40 A.

Remember

• Size cable on maximum running current, not nominal.
• Check LRA against DG and weak grid supply.
• Phase monitor + RCCB + local isolator on every installation.
• Voltage drop under 3–5 % at full load.