Three-phase power for aircon: what the premises allows

A supply is described by how many live conductors carry power into the premises. The number is one, or it is three. Single-phase arrives on one live conductor and a neutral. Three-phase arrives on three live conductors, each carrying the same alternating voltage, timed a third of a cycle apart from the other two.

Singapore's low-tension supply comes in exactly those two shapes. SP Group's connection handbook classifies low tension as 230V single-phase and 400V three-phase, running at 50 Hz. The network also distributes at 6.6 kV and 22 kV, but those belong to buildings large enough to take supply through a substation of their own. A shop unit, an office suite and a landed house all sit inside the low-tension pair.

The timing offset is what the whole arrangement exists for. On one phase, delivered power rises and falls with the alternating cycle and drops through zero twice inside it. On three, the cycles overlap, so one is rising as another falls and the total reaching the load never collapses to nothing. Power arrives as a steady stream rather than a series of pushes.

The 400V figure confuses more people than any other part of this. It is not a higher supply sitting on top of the normal one. It is the voltage measured between any two of the three live conductors, while each conductor on its own still sits at 230V against neutral. A three-phase premises runs ordinary 230V lighting and socket circuits throughout, each drawing from one of the three. Only equipment needing all three ever sees 400V.

That last point decides how the question gets asked. A premises is not choosing between running 230V things and running 400V things. It either has the three conductors available or it has one, and everything about the plant selection follows from that single fact.

Three properties of a large compressor push it onto a three-phase supply: the current it draws, what it does at the instant it starts, and how evenly it loads the building. All three get worse as the machine gets bigger. All three improve when the load is shared across three conductors instead of stacked on one.

Current sets the cable, and the cable sets what an existing building can retrofit. Splitting the same power across three conductors means each one carries a fraction of what a single conductor would have to. That buys smaller cable, a smaller protective device, and less copper along the run. In an occupied building it often decides whether the risers already in place can carry a new plant load, or whether the whole route has to come out.

Starting is where the difference is sharpest. A three-phase motor gets a rotating magnetic field free from the supply itself, because its three windings energise in sequence. The shape of the supply is what turns the motor. A single-phase motor has no rotating field to draw on and needs added components to produce one at every start, which is why a single-phase compressor takes a hard bite of current each time it cuts in.

Balance is the third property, and it belongs to the premises rather than to the machine. A three-phase load pulls evenly on all three conductors and leaves the incoming supply even. A large single-phase machine puts its entire draw onto one of them, and every other circuit sharing that conductor sits on top of the imbalance. In a small shop this rarely surfaces. On a floor split between several tenancies it is precisely what the landlord's electrician is watching for.

One number governs all of it, and it is not the number on the front of the quotation. Electrical input is what the supply has to carry. Cooling capacity states an effect on a space and reveals nothing about the draw behind it. Those are two separate quantities printed in the same unit, and cooling kW vs power kW settles which is which. Take the input figure into the electrical conversation and leave the cooling figure in the sizing one.

Household aircon in Singapore is single-phase, and it stays single-phase well past where most people expect the change. A system cooling three or four rooms from one outdoor unit still runs on one phase. Room count does not move it. Neither does the tick rating.

Compressor size is what moves it, and manufacturers handle the transition inside a series rather than between series. The smaller capacities in a range are offered on one phase only. Somewhere in the middle the range publishes both, the same machine in two supply variants, separated by a character in the model number. Above that overlap, only the three-phase variant is built.

The overlap is where specification errors live, because the two variants are close to indistinguishable on a quotation. Cooling capacity is identical. Dimensions are identical. The catalogue photograph is the same photograph. A supplier working from a price list can hand a three-phase variant to a single-phase premises with nothing on the page looking wrong to anybody reading it.

Above the overlap the equipment class has already decided. Variable refrigerant flow systems are three-phase, and Singapore's own grant paperwork treats them that way. The Energy Efficiency Grant sets a separate minimum tick requirement for three-phase variable refrigerant flow models, listed apart from split systems. Chilled water plant sits higher again. A premises evaluating either of those has a supply answer whether or not anyone has looked it up.

Stating the crossover as a wattage would be false precision. The band shifts by manufacturer, by series, and by the refrigerant a range was designed around. What holds is the shape: one phase at the bottom, both variants through the middle, three phases only at the top. The useful question is never where the line falls, but which side of it a specific quoted model sits on.

The supply is a fixed fact about the building and it is knowable before a single model is shortlisted. Most projects find out in the opposite order. Selection starts from the cooling load, the electrical question surfaces when somebody needs to connect, and a machine has already been bought by then.

In a tenancy the question has two halves, and answering only the first is the standard failure. The first half is which phase arrangement reaches the unit. The second is how much capacity that unit has been allocated out of the landlord's distribution. Three phases can be present in the riser and still be unavailable to a tenancy in the quantity a new plant load wants.

In a landed house the halves are different. The connection belongs to the owner, and the record of what was applied for dates from when the house was built, extended or rewired. Landed premises are also where three-phase turns up already installed and unused, taken originally for a lift, a pool pump, or a kitchen that was planned and never built.

None of this calls for opening a board, and nobody should. The answer lives in the paperwork from the last certified work on the installation, and whoever holds that paperwork can state it in a sentence. In a tenancy that is usually building management. In a house it is whoever last had the installation inspected.

Ask before the shortlist, not after the order. Every question below is a short email at the specification stage. The same questions asked once equipment is on site turn into a variation order and a conversation about who absorbs the difference.

• •What supply is the premises connected at, single-phase or three-phase, and where is that recorded?
• •For a tenancy, what capacity has this unit been allocated, and who set that figure?
• •How much of the allocation is already committed to lighting, kitchen equipment and existing plant?
• •Does the quoted model number correspond to the single-phase or the three-phase variant?
• •What electrical input does the proposed system draw, stated separately from its cooling capacity?
• •If three phases are available, how will the new load be spread across the three conductors?
• •Who holds the last certified record of the installation, and can a copy be produced?

Three ways out of a mismatch exist, and they differ mainly in who has to get involved. Change the equipment to suit the supply. Change the supply to suit the equipment. Or rearrange the load so the supply already there is enough.

Re-selecting the equipment is the route most projects take, because it needs nobody outside the project. One three-phase machine becomes two or three smaller single-phase ones. The trade is not purely a loss, either. Independent systems fail independently, so one compressor failure takes out a zone rather than a floor, which on trading premises separates a bad afternoon from a closed door. What it costs is plant space, more outdoor units to site, and more machines to keep serviced.

Changing the supply is an application rather than a site decision. SP Group's connection handbook states that where extension or rewiring work is required, the customer engages a Licensed Electrical Worker and submits an application for load connection through them. That is licensed work on the fixed installation, and it sits outside an aircon contractor's scope. Start it the moment a mismatch is known, not when the equipment is sitting at the loading bay.

The third route exists only where three phases are already present. Single-phase equipment can be distributed across the three conductors so each one carries a share of the plant, which keeps the incoming supply balanced and stops one conductor holding everything. That gets decided when the circuits are laid out, and it becomes invisible once the ceiling is closed.

The reverse mismatch is quieter and costs more over a building's life. A premises holds three phases, nobody checks, and equipment gets specified as though it did not. The floor ends up cooled by more machines than it needed, on smaller compressors working nearer their limit, while a supply capable of carrying one properly sized system sits half used. Nothing fails and nothing gets flagged. The building simply bought a compromise it had no reason to accept.

One thing changes for good once a system is three-phase, and it is worth telling whoever maintains the plant afterwards. The machine can now report a supply condition as a fault of its own. Several manufacturers publish an error code for a reversed or missing phase, raised by the outdoor unit when the three conductors do not arrive in the order or the number it expects. A unit refusing to start on one of those codes is describing the supply, not itself, and treating it as a compressor problem chases the wrong end of the circuit.