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. So, higher-voltage systems can get more work done with less electrical current.

split phase voltage

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. I’m tempted to refer to this as something like a sink drain, but I may have pushed (or pulled) the metaphor too far!

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.

split phase wave form

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.


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