We’ve received many questions related to “split phase” power distribution, which is used in North American homes, and how a single installation can handle such a system. Hopefully I can shed some light on the matter, and explain:
- How North American homes can have both 240V and 120V appliances
- Why it must be installed on a 240V circuit, more commonly used for high-power loads like dryers, ovens, and central air
- How it uses split phase power as an additional clue to help distinguish devices
Split Phase Voltage
The world’s power distribution systems use alternating current, or AC, in which the electrical current alternates direction many times a second. This is because transformers allow AC to be easily raised to a very high voltage for long-distance transmission, and then lowered back down for use. Voltage is like water pressure; it measures “how hard” the electrons are pushing. Like water pressure behind a tap that is turned off, voltage doesn’t go away when the switch is off — it’s still there, behind the switch. Even with voltage, if they have nowhere to go electrons will not flow and no useful work will be done. However, with the “tap” on, electrons that are pushing harder can do more work each.
In an alternating-current system, typical household appliances are connected to a “live” wire, which has an alternating voltage and a “neutral” wire, which neither pushes nor pulls electrons. Neutral is necessary, though, to complete the path that electrons take.
120V/240V Split Phase
120/240 Vac split phase is a type of single-phase three-wire mid-point neutral power distribution system commonly found in America with a standard phase-neutral voltage of 120 Vac for residential and light commercial applications. The phase to phase( Live to Live) voltage is 240Vac for heavy industrial loads such as compressors, fridge, and pumps. Because of the 120-0-120 voltage configuration, it is also sometimes referred to as dual-phase, 2 phase / two-phase or even mistakenly, single-phase 220Vac.
North American homes are not wired up with a single live wire, but two. The difference between them is that when one is pushing, the other is pulling, and vice-versa. The “normal” appliances and outlets in a house are divided amongst the two live wires, and all are connected to neutral. Because the voltage difference between a single live wire and neutral is alternating with a magnitude of 120V, appliances that run on 120V power can be connected to the neutral wire and either of the two live wires.
Very power-hungry appliances can be built more efficiently if they can run on 240V. With twice the voltage, they can receive twice the amount of power with the same amount of current. This allows the use of thinner and less expensive wires. Appliances like this are connected to both live wires. Because one is pushing while the other is pulling, the voltage difference between them is twice the difference between a single live wire and neutral. In the graph below, you can see what this looks like: A positive voltage below indicates the line is “pushing,” and a negative one indicates it’s “pulling.” The difference between two lines is what determines how much electrical “pressure” there is between them.
To measure the power usage and detect the electrical signatures of all appliances in the house, we need to sense both the voltage and the current of both live wires. The natural way to access both wires for voltage measurement is to connect to a 240V breaker — even though its power needs are minimal, a 120V circuit breaker just wouldn’t give us all the information we need to detect every appliance. To measure the current on each line independently, we use two different current sensors, each of which senses the current flowing through its attached wire.
A happy side effect of split phase electrical power is that it gives us an additional clue we can use when we detect appliances, by dividing appliances into three categories: Those on one supply line, those on the other, and those on 240V circuits (and thus present on both live wires.) Of course, its load disaggregation algorithms use many more data points than simply which line a device is on — but it’s nice to have some additional help! For example, if we know your toaster is on one of the “legs” of the system, and we notice a similar electrical signature on the other leg, we won’t mistakenly report that your toaster is on.
What is Single Phase Power?
A single phase three-wire system is an AC power distribution that saves conductor material across a single-phase system. On the distribution transformer, it only needs one phase at the supply side to qualify. A transformer that supplies a distribution system of three wires contains a single phase, primary input winding.
What is Three Phase Power?
Three phase power is a power circuit that consists of an Alternating Circuit (AC) with three wires. The majority of commercial buildings throughout the United States contain a three-phase power circuit. The power setup generally consists of four wires — 208 Y / 120 V — an arrangement seen as the densest and most flexible.
In comparison to a single phase, three-phase power arrangements supply puts greater sums of power — 1.732 times the single phase — on the same current:
- In loads that use low amounts of power — such as the lights, television, radio, computer and scanner — the power can be supplied by any of the three 120 V single-phase power circuits.
- For loads with medium quantities of power — such as in water heaters and air dehumidifiers — the power can be supplied by any of the three 208 V single-phase power circuits.
- Loads that require high volumes of power — including heaters, air conditioners and heavy-duty garage equipment — are powered with a single 208 V three-phase power circuit.
Most industrial plants throughout the United States use three-phase, four-wire power setups, as this arrangement — 480 Y / 277 V — is the densest and most powerful. In comparison to the 208V three phase, the 480 V three phase offers a considerably greater power supply with either the same current or with 43% reduced current. The benefits of this setup are as follows:
- Lower costs for construction, thanks to the smaller electrical devices and circuitry required.
- Lower costs on energy, due to the conservation of electrical currents, which get transformed into heat instead of being lost.
Related Split Phase Inverter
Split Phase FAQ
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What is 240v split phase?Instead of a single 240-volt power supply, we use two 120 volt supplies (in phase with each other!) in series to produce 240 volts, then run a third wire to the connection point between the loads to handle the eventuality of one load opening. This is called a split-phase power system.
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Can you split 240V to 120V?
you can easily convert to a single 120V circuit but you can’t convert to two circuits, you have two wires and you need three or four. Your 240V branch circuit is supplied by two wires originating at a two pole breaker in the panel.
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What is a split phase inverter?
120/240 Vac split phase is a type of single-phase three-wire mid-point neutral power distribution system commonly found in America with a standard phase-neutral voltage of 120 Vac for residential and light commercial applications.
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