Single-Phase or Three-Phase: How Many kW and Amps You Really Need

Up to about 2.2 kW of motor power, single-phase 230 V is enough. Above that you need three-phase 400 V or a VFD.A 3 kW domestic meter trips above 3.3 kW drawn, and on start-up a motor pulls 5 to 8 times its rated current. That is why a 2.2 kW machine can trip an installation that on paper would carry it. An official distributor since 2007, we sell machines from 0.15 to 11 kW: this is the guide we use to answer the question we get asked most often.

The question almost always arrives in the same form. I have 220 in the garage, can I run this machine?

The rule of thumb we use when quoting is this. Up to 2.2 kW of motor power, single-phase 230 V works. Between 2.2 and 4 kW you are in a borderline zone, where the type of work matters more than the number on the plate. Above 4 kW you need three-phase 400 V, with no practical alternative.

The limit is not arbitrary. A 6 kW load on single-phase 230 V draws around 26 A: no domestic socket and no household meter will take it. The same load on three-phase 400 V spreads across three phases and drops to about 8.7 A per phase.

There is also a constraint from the network operator. Above 6 kW of contracted power a single-phase supply is no longer granted: you move to three-phase for technical grid reasons.

There are two formulas, and they change according to the supply.

Single-phase: amps = watts divided by (volts × cos φ). At 230 V with cos φ 0.8, a 1.5 kW motor draws about 8.2 A.

Three-phase: amps = watts divided by (1.732 × volts × cos φ). The same 1.5 kW motor at 400 V draws about 2.7 A per phase.

The factor 1.732 is the square root of 3 and comes from the geometry of the three phases. It is the reason three-phase asks for far less current per conductor at the same power.

For a quick mental sum: on single-phase 230 V the kW are roughly the amps divided by 5; on three-phase 400 V roughly the amps divided by 2. These are workshop approximations, not design values.

The Italian regulator ARERA states it explicitly: with a contracted power of 3 kW, which is what most Italian homes have, the meter trips when more than 3.3 kW is drawn instantaneously. The allowance is 10 per cent of the contracted power.

Above that threshold electronic meters tolerate a time-limited overload before tripping, but that tolerance is designed for transients, not for a machine running continuously.

In practice: with a 3 kW contract a 2.2 kW planer-thicknesser starts and works. A 3 kW one does not, because between running draw and start-up current it steadily exceeds the allowance.

This is the point where most calculations done on paper come unstuck.

On start-up an induction motor draws 5 to 8 times its rated current, with initial peaks that can reach 10 or 12 times. On single-phase motors the ratio is higher still, because starting torque is low and the motor stays longer in the critical phase.

A 2.2 kW motor that draws about 12 A when running on single-phase asks for between 60 and 96 on start-up, for a few seconds. The meter sees that peak.

The most common case is the compressor. Starting it with the tank under pressure means starting the motor under load, and that is where inrush is at its highest. It is why a 2 HP compressor trips an installation that carries a planer of the same power without trouble.

Three systems reduce inrush. Star-delta starting brings current and torque down to about a third, but it needs a three-phase motor and makes sense on medium to large machines. A soft starter accompanies the ramp-up. The VFD is the most effective solution on a workshop machine, because it combines soft start with speed control.

It is the most-discussed solution in Italian workshops, and it works. A single-phase to three-phase VFD rectifies the 230 V mains and rebuilds a three-phase supply at 230 V line-to-line, controlling its frequency.

The first check is on the motor plate. It must read 230/400 V with the delta and star symbols: that means 230 V in delta, and that is the configuration you need. If the plate says 400/690 V the motor cannot be run on 230 V and a single-phase VFD is not usable.

The second rule is sizing. The VFD is chosen larger than the motor, because with a single-phase input the current on the internal bus is higher. For a 2.2 kW motor you choose a 3 kW VFD.

Third point, often overlooked. Self-ventilated motors cool less well at low speed, because the fan is keyed to the shaft. If the machine works for long periods at reduced speed under load, an independent forced-cooling fan is needed.

On safety: downstream of a VFD a type AC RCD may fail to detect a fault with a DC component. On a single-phase installation you need a type F RCD, on a three-phase one a type B. The work must be carried out or signed off by a qualified electrician, and in Italy the dedicated circuit requires the declaration of conformity set out in the Italian DM 37/2008.

The rotary phase converter generates the third phase with an idler motor and a few capacitors. It is robust, lasts decades and can feed several machines if sized with margin. It takes up space and it is noisy.

A run capacitor will turn a three-phase motor on single-phase, but it cuts available torque and increases current draw. On machine tools, where torque is needed precisely at the moment of cut, electrical repair shops advise against it.

Replacing the motor with a single-phase one remains the most straightforward choice when speed control is not needed and power stays below 2.2 kW. It costs less than a VFD, but a single-phase motor delivers less than a three-phase of the same rating and runs hotter in continuous duty.

Manufacturers state the kW, almost never the amps. We calculated the current drawn from the plate ratings of the models we carry, with cos φ 0.8. These are reference estimates, not plate data: the real current must be read on the machine's rating plate.

The useful reading is the threshold. Below 1.8 kW the whole catalogue is single-phase; above 2.1 kW it is almost all three-phase. In between lies the band where the choice is real.

On the same machine the difference is measurable. The Bernardo PT 260 planer-thicknesser costs €1,122.40 in the 230 V version and €1,415.20 in the 400 V version: €292.80 of difference for the same working width.

On the Bernardo HBS 400 N bandsaw the gap between the two versions falls to €30.40, while on the base HBS 400 the price is identical. It depends on the model; there is no rule.

Then there is the cost of the connection. For a workshop with a VAT number, which counts as non-domestic use, the contribution set by the Italian regulator ARERA for a power increase is €77.49 per additional kW plus €23 of fixed contribution, excluding VAT. On the bill, every kW of contracted power weighs about €26 a year.

Taking a workshop from 6 to 15 kW therefore costs a few hundred euros of activation and around €230 a year in capacity charge. At equal consumption the price per kWh does not change between single-phase and three-phase.

In our experience three pieces of information avoid ninety per cent of the problems.

The real contracted power, not the assumed one: it is read on the meter or on the bill. If the machine is three-phase, the actual availability of the three phases in the room where it will be installed, which does not always match the one at the meter. And the type of work, because a machine that starts and stops constantly stresses the installation far more than one that runs for eight hours straight.

The advice I give most often: if you have three-phase in the workshop, use it even for machines that exist in single-phase. A three-phase motor of the same rating delivers more, runs cooler and lasts longer. And the day you sell the machine on, the professional used market is looking for three-phase.

Sandra Gaspar, founder of Krollit

To choose the machine with the right supply you can start from the metal lathes, the planer-thicknessers or the dust extraction systems. For compressors, where inrush is the critical factor, the category is compressed air.