Single-phase vs three-phase heat pumps: choosing the electrical supply

Short answer: Do not choose single-phase or three-phase supply based on the heat output printed on the heat pump. Check maximum electrical input, maximum current, auxiliary electric heater, the manufacturer’s wiring diagram and the power available from the building’s electrical installation.

Thermal output is not electrical input

A heat pump can deliver more thermal kilowatts than it consumes electrically because it transfers heat from the outdoor environment into the system.

Therefore, a heat pump rated at 12 kW heating output does not necessarily require 12 kW of electrical power, and electrical protection cannot be calculated from nominal heating output alone.

What is single-phase supply?

With single-phase supply, the main load is connected to one phase. In Romania, the usual nominal voltage is approximately 230 V.

For the same electrical power, the current on one phase can be substantial. The approved connection capacity and the circuit design must therefore be checked.

What is three-phase supply?

In a three-phase installation, the load can be distributed across three phases. Equipment designed for this supply generally uses a network with approximately 400 V between phases.

The practical benefit is a more balanced distribution of available electrical power, especially for larger equipment and electrical loads.

Why not choose by heating kilowatts?

Heating output, compressor consumption and the maximum electrical input of the complete unit are different figures.

The electrical supply assessment should cover at least:

• maximum electrical power input;
• maximum operating current;
• manufacturer-specified characteristics;
• any auxiliary electric heater;
• pumps and accessories powered from the same distribution board;
• total power available in the building.

The auxiliary electric heater can make a big difference

Many systems have an integrated or external auxiliary electric heater. It may represent a significant electrical load and must be included in the installation calculation.

Even if the compressor’s normal electrical consumption is relatively low, the supply must not be sized solely for average consumption.

What other loads are there in a home?

Proper assessment considers more than the heat pump. An electric hob, oven, water heater, electric-car charger, air conditioning and other appliances may operate simultaneously.

The available connection capacity and load management may be as important as the heat pump’s own specifications.

Is three-phase always better?

Not necessarily. If the selected heat pump is designed for single-phase supply and the electrical installation has sufficient capacity, that can be an entirely suitable solution.

Three-phase supply is especially useful for higher electrical loads or when the building’s loads need to be shared more evenly.

Can a single-phase heat pump simply be converted to three-phase?

No. Supply type is a characteristic of the model and the manufacturer’s wiring diagram must be followed. Do not arbitrarily modify the unit’s internal electrical supply.

Can cable and circuit breaker size be chosen by a universal rule?

No. Cable cross-section and protective devices depend on current, length, installation method, temperature, voltage drop, equipment characteristics and applicable regulations.

Final values must be determined by the designer or licensed electrician using the unit’s documentation.

What does this mean for an existing house?

When renovating, check the existing electrical connection before ordering equipment. Sometimes the heating system is suitable for a heat pump, but the available electrical power or existing distribution board needs upgrading.

Checking early avoids delivery of a unit before the required electrical supply is ready.

Should compressor starting currents be considered?

Use the electrical ratings and requirements stated by the manufacturer for the specific model. Modern inverter equipment should not be assessed using simplified rules borrowed from other types of motors.

Size the circuit and protective devices according to the technical documentation and installation conditions.

What should I check before ordering?

• whether the house has single-phase or three-phase supply;
• the approved available connection capacity;
• the existing main protective device;
• the manufacturer’s maximum specified power and current;
• the capacity of the auxiliary heater;
• other significant electrical consumers;
• whether a connection-capacity upgrade is required.

Frequently asked questions

Does a 12 kW heat pump consume 12 kW of electricity?

No. The 12 kW may refer to delivered heating capacity. Electrical input must be read separately from the specific model’s technical data.

Does a three-phase heat pump consume less electricity?

The number of phases alone does not determine efficiency. Efficiency depends on the compressor, operating conditions, system temperatures and model performance.

Can I install a large heat pump on single-phase supply?

Only if the model is built for single-phase supply and the electrical installation can safely provide the required power and current.

Continue with: the complete ECO-TEC guide and the available heat pumps.