Single-Phase or Three-Phase?

A single-phase generator is suitable for low-power applications where only single-phase (230 V) equipment is present. A three-phase generator (400/230 V) is the right choice wherever there are three-phase motors, the power requirement is high or the building connection is three-phase. The decision depends on your existing installation, the type of equipment and the total power.

What are single-phase and three-phase?

Two basic connection types are used on the low-voltage grid in Türkiye:

• Single-phase: There is one phase conductor and one neutral conductor. The phase-to-neutral voltage is 230 V. Common in homes and small businesses.
• Three-phase: There are three phase conductors (L1, L2, L3) and usually a neutral. The voltage is 400 V between phases and 230 V between each phase and neutral. Used in industry, commercial buildings, apartment blocks and anywhere with motor loads.

Generators are built as single-phase or three-phase on the same principle. The phase configuration of the generator you choose must match that of the distribution board and equipment it will supply.

When to choose a single-phase generator

• The building’s mains connection and meter are single-phase,
• All the equipment to be supplied is 230 V single-phase (lighting, socket loads, combi boiler, refrigerator, small pumps),
• The power requirement is low.

Single-phase generators are generally found in the smaller power classes. As power increases, the current drawn over a single phase rises quickly; this complicates cable cross-section, fuse sizing and alternator design.

When to choose a three-phase generator

• The site has three-phase motors: lifts, booster pump sets, large compressors, pumps, fans, construction machinery,
• The building’s distribution board is three-phase (apartment blocks, residential complexes, factories, hotels, hospitals),
• The total power requirement exceeds a few kVA and is likely to grow,
• Synchronized operation or automatic changeover from the mains via an ATS is planned.

In a three-phase system the same power is shared across three conductors, so the current in each conductor is lower, cables are thinner and motors run more smoothly. This is why the vast majority of industrial diesel generators are three-phase.

Current calculation: a quick comparison

Formulas:

• Single-phase: I = P ÷ (U × cosφ)
• Three-phase: I = P ÷ (√3 × U × cosφ)

Example: 8 kW active power, cosφ = 0.8.

Connection	Voltage	Approximate current
Single-phase	230 V	8,000 ÷ (230 × 0.8) ≈ 43 A
Three-phase	400 V	8,000 ÷ (1.732 × 400 × 0.8) ≈ 14 A

For the same power, approximately one third of the current flows through each conductor in a three-phase system. Cable cross-section, fuses and contactors are selected accordingly.

Supplying single-phase loads from a three-phase generator

Single-phase equipment can run on a three-phase generator; 230 V is available between any phase and neutral. The key point here is load balance:

• Each phase of a three-phase generator is designed to carry roughly one third of the total power. You cannot draw 30 kVA from a single phase of a 30 kVA generator.
• Single-phase loads should be distributed as evenly as possible across L1, L2 and L3. A heavily unbalanced load can cause alternator heating, voltage imbalance and problems with sensitive equipment.
• Alternator manufacturers state the permissible unbalance limits in their data sheets; these limits should be checked at the design stage.

In practice, if a three-phase generator is chosen for a building made up mainly of single-phase loads, the distribution of loads across the phases on the board side must be reviewed.

Can three-phase equipment run on a single-phase generator?

Not directly. A three-phase motor cannot be supplied from a single-phase source. Intermediate solutions such as phase converters or variable frequency drives can be used for some small motors, but they add cost and reduce efficiency. If the site has three-phase equipment, choosing a three-phase generator is the soundest approach.

Quick selection checklist

1. Are your main distribution board and meter single-phase or three-phase?
2. Are there any three-phase motors or equipment?
3. What are the total running power and the starting power of the largest motor?
4. Can single-phase loads be distributed evenly across the phases?
5. Is a future capacity increase or automatic start-up via an ATS planned?

In most cases the answers to these questions settle the choice. If you are undecided, a three-phase generator is usually the safer option in terms of flexibility; however, the installation should be designed by a qualified electrical engineer or electrician.

Next step

Terex Diesel manufactures generator sets in the 10–4,125 kVA range. You can estimate the power you need with the power calculator, compare single-phase and three-phase currents with the power converter on that page, and list suitable models with the generator finder. For automatic changeover from the mains, see the ATS transfer panel page; for project-specific questions, contact us.