Single-phase vs three-phase: what is the difference?

A single-phase supply has one live conductor (L) and a neutral (N); its voltage waveform is a single sine wave. In Vietnam the nominal low-voltage single-phase value is 220 V at 50 Hz.

A three-phase supply has three live conductors L1, L2 and L3 offset by 120 degrees, usually with a neutral. On Vietnam's low-voltage network the nominal line-to-line voltage is 380 V, while each phase to neutral remains 220 V.

Manufacturing sites almost always use three-phase supply: three-phase motors start smoothly, run more efficiently and need smaller conductors for the same power.

The rule is simple: the inverter must match the supply at the connection point. A single-phase site takes a single-phase inverter; a three-phase site takes a three-phase inverter. Pushing a large single-phase inverter onto a three-phase board creates imbalance, heats the neutral and may breach connection requirements.

• Residential systems below roughly 5–6 kWp: a single-phase inverter is usually the cost-effective choice.
• Systems above roughly 10 kWp, or any site already on three-phase supply: use three-phase inverters for natural balance.
• Commercial rooftops from hundreds of kWp to several MWp: multiple three-phase string inverters grouped by roof area for easier O&M.
• If outage ride-through matters, choose a three-phase hybrid inverter with a backup port and pair it with a BESS.

• Imbalance appears when generation or load is unevenly spread across the phases, raising neutral current and causing local voltage deviation.
• Three-phase inverters export symmetrically by default; with multiple single-phase inverters you must distribute them evenly across phases.
• Export-limitation devices must measure all three phases, otherwise the system can still feed back on a lightly loaded phase.
• Connection point, cable sizing, switchgear and protection must be rated for the actual short-circuit level and system capacity.

Connection documentation and agreement with the utility must follow the regulations in force before energisation.

• When total load exceeds what the existing single-phase service can deliver, or voltage sags whenever large equipment starts.
• When planning a solar system well beyond the sensible limit of single-phase inverters.
• When commissioning motors, compressors, chillers or high-power EV charging.
• When preparing the site for battery storage or future participation in direct power purchase.