Three-Phase Power for Aircon: What the Premises Allows

A supply is described by how many live conductors carry power in: one, or 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.

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 at 50 Hz. The network also distributes at 6.6 kV and 22 kV, but those belong to buildings with their own substation. A shop unit, an office suite and a landed house all sit inside the low-tension pair.

The timing offset is what the arrangement exists for. On one phase, delivered power rises and falls with the cycle, dropping through zero twice. On three, the cycles overlap, so one is rising as another falls and the total reaching the load never collapses. 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 alone still sits at 230V against neutral. A three-phase premises runs ordinary 230V lighting and socket circuits, each drawing from one conductor. Only equipment needing all three sees 400V.

That fact decides how the question gets asked. The premises either has three conductors available or one, and everything about plant selection follows.

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 with size, and all three improve when the load is spread across three conductors instead of stacked on one.

Current sets the cable, and the cable sets what a building can retrofit. Splitting the same power across three conductors means each carries a fraction of what one would: smaller cable, a smaller protective device, less copper along the run. In an occupied building it often decides whether the existing risers can carry a new plant load.

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

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

One number governs all of it, and it is not the quotation's headline figure. Electrical input is what the supply carries; cooling capacity states an effect on a space and reveals nothing about the draw behind it. Those are separate quantities printed in the same unit, and cooling kW vs power kW settles which is which.

Household aircon in Singapore is single-phase, and it stays single-phase 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, and neither does the tick rating.

Compressor size is what moves it, and manufacturers handle the transition inside a series. The smaller capacities are offered on one phase only. The middle of the range publishes both, the same machine in two 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 nearly indistinguishable on a quotation. Cooling capacity is identical. Dimensions and the catalogue photograph are the same. A supplier working from a price list can hand a three-phase variant to a single-phase premises unnoticed.

Above the overlap the equipment class has already decided. Variable refrigerant flow systems are three-phase, and Singapore's grant paperwork lists a separate minimum tick requirement for them, apart from split systems. Chilled water plant sits higher again. A premises evaluating either has a supply answer whether or not anyone checked.

Stating the crossover as a wattage would be false precision. The band shifts by manufacturer, by series, and by the refrigerant it 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 which side of that line a specific quoted model sits on.

The supply is a fixed fact about the building, 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.

In a tenancy the question has two halves, and answering only the first is the usual failure. The first 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 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 planned and never built.

None of this calls for opening a board. The answer lives in the paperwork from the last certified work on the installation, and whoever holds it 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 once equipment is on site become a variation order and an argument about who absorbs it.

• •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: 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: independent systems fail independently, so one compressor failure takes out a zone rather than a floor. What it costs is plant space, more outdoor units to site, and more machines to service.

Changing the supply is an application rather than a site decision. SP Group's connection handbook states that where extension or rewiring 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, outside an aircon contractor's scope. Start it the moment the mismatch is known.

The third route exists only where three phases are already present. Single-phase equipment can be distributed across the three conductors so each carries a share of the plant, keeping the incoming supply balanced. That gets decided when 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, while a supply capable of carrying one properly sized system sits half used. Nothing fails, and the building 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. 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 when the three conductors do not arrive in the expected order. A unit refusing to start on one of those codes is describing the supply, not itself.