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First I apologize for my ignorance regarding electrical matters. I am analyzing a dataset from a smart house. Some of the power measurements are divided into phase A and B. Their values are somewhat similar (both magnitude and time of occurrence) but different by a small difference.

Some examples: washer machine power has only one measurement, but dryer has power from phase A and B. Oven also has A and B. Microwave has only one measurement. I tried searching for two-phase power systems and found out that they have not been used since the early 1900s.

So this leads me to a few questions: Why are there two phases? Does this house have a two-phase power system? Why only a few of the equipment have 2 phase measurements while the remaining do not? Is the total power the sum of the two phases or the average?

Your help and insights are much appreciated.

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    \$\begingroup\$ The US uses something called "split-phase." This is basically just a 240 VAC transformer secondary with a center-tap. The center-tap is considered "neutral" (N) and is usually "staked" into the Earth to make a "ground." The other two leads are called L1 and L2. (Or H1 and H2 when people think of them as "hots.") Power is distributed to 120 VAC wall plugs using the center-tap and either L1 or L2 (but obviously not both.) The idea is to balance things around the home so that about half the load is on L1 and about half on L2. Some devices, like a stove, power things from L1 to L2 and skip over N. \$\endgroup\$
    – jonk
    Commented Aug 17, 2020 at 23:15
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    \$\begingroup\$ The authors of whatever you are reading likely called L1 as "A" and L2 as "B". A lot of stuff is only tied to "A". Other stuff is tied only to "B". Some things are tied between "A" and "B". So they probably tried to account for all this leading to what is confusing you but really isn't all that confusing when you understand the "split phase" concept. It is decidedly not a two-phase, which is two phases 90 degrees out of phase (at quadrature.) It really is a "split-phase" and you need to keep that solidly in mind. L1 and L2 are 180 degrees out of phase. \$\endgroup\$
    – jonk
    Commented Aug 17, 2020 at 23:18
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    \$\begingroup\$ On average shared with a dozen homes they balance well enough to utilize core flux but an individual house will likely be unbalanced at any given time except if only 240V heater appliances are on \$\endgroup\$
    – D.A.S.
    Commented Aug 18, 2020 at 0:53
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    \$\begingroup\$ @TonyStewart There are exactly two homes on my transformer, here. Not enough for that kind of balancing. It may be part of the reason our area loses transformers, from time to time, that might have otherwise lasted longer. Home sites are each 4 hectares or larger. \$\endgroup\$
    – jonk
    Commented Aug 18, 2020 at 1:01
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    \$\begingroup\$ We call it single split phase in North America because they distribute high voltage 3 phase and residential transformer uses only 1 of the 3 phases with centre-tapped secondary = Neutral=earth bonded \$\endgroup\$
    – D.A.S.
    Commented Aug 18, 2020 at 2:55

3 Answers 3

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'2-phase' just means a split 240V feed with a neutral. These feeds are normally called out as L1, L2 and of course N for neutral. Neutral is tied to earth ground at the panel, L1 and L2 are 120V with respect to that neutral and 240V to each other.

Why a split feed? Early light bulbs worked on 110V; ones that ran on 220-240V didn't come until slightly later. Meanwhile, larger loads like motors worked better with a higher voltage. To work around the problem, both Edison and Westinghouse offered split-feed systems, using 110V for lighting and light appliances, and 220V for heavier loads.

This split approach stuck in the US, forever saddling Americans with longer boiling times for electric kettles for example. Europe meanwhile, coming on line later than the US, took advantage of the arrival of newer metal filament bulbs that could run on the higher (and more efficient) 220V, and so dispensed with the split feed approach.

More here: https://electronics360.globalspec.com/article/10511/how-the-u-s-came-to-adapt-120v-while-others-are-using-230v

Virtually all North American houses have a split-feed system. The breakers in the panel are divided up between L1 and L2. The breakers alternate legs as they go down the panel.

In a split-feed system, some appliances use both L1 and L2, other use only one. High-power feeds like A/C units, ovens, cooktops, and electric dryers use both legs L1 and L2 as a 240V, and will use a linked set of adjacent breakers that pick up L1 and L2. Wall plugs and lighting on the other hand only get one 120V leg and a neutral. The single-leg 120V loads will be divided up between L1 and L2 to help with load balancing.

A special note about North American dryers. They're weird: they use 120V for the motor and 240V for the heater. Why? This split approach allows easier optioning for gas or electric. Gas dryers only need 120V; if reconfigured for electric they need 240V with a neutral.

You can accurately measure the 240V current by taking the sum of the currents on L1 and L2. Below is an example:

enter image description here

From here: https://help.ekmmetering.com/support/solutions/articles/6000081677-single-phase-3-wire-120-240v-or-120-208v-metering

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  • \$\begingroup\$ your reference indeed open my EE eyes. So there are indeed Two Phase or Edison power systems. I am glad to learn that it is Telsa who invented Three systems, and 110V 60Hz. I always want to know why everybody uses 220V, 50Hz, but the low tech US guys use 110, 60Hz. I once wrongly thought that 110V is lower voltage, therefore not that easy to get electrically executed. Not too many monthly I watch the film The Current War, and was glad to see how those guys Edison, Westinghouse, Telsa look like. / to continue, ... \$\endgroup\$
    – tlfong01
    Commented Aug 18, 2020 at 7:04
  • \$\begingroup\$ My references: (1) Single-Phase 3-Wire 120/240V or 120/208V - ekmetering - 2017aug17 help.ekmmetering.com/support/solutions/articles/… (2) Single phase 3-wire systems (some call them two phase or Edison) have 2 hot wires and a neutral wire. This is the most common residential system in the USA. If you measure from hot wire to hot wire you will get 240 volts and if you measure from either hot wire to neutral you will get 120 volts. / to continue, ... \$\endgroup\$
    – tlfong01
    Commented Aug 18, 2020 at 7:05
  • \$\begingroup\$ My references: (1) Single-Phase 3-Wire 120/240V or 120/208V - ekmetering - 2017aug17 help.ekmmetering.com/support/solutions/articles/… (2) Single phase 3-wire systems (some call them two phase or Edison) have 2 hot wires and a neutral wire. This is the most common residential system in the USA. If you measure from hot wire to hot wire you will get 240 volts and if you measure from either hot wire to neutral you will get 120 volts. / to continue, ... \$\endgroup\$
    – tlfong01
    Commented Aug 18, 2020 at 7:06
  • \$\begingroup\$ (3) How the US Came to Adapt 120V While Others are Using 230V - Ken Thayer 2017Dec01 electronics360.globalspec.com/article/10511/… (4) The Current War - Wikipedia en.wikipedia.org/wiki/Main_Page. End of my comments. \$\endgroup\$
    – tlfong01
    Commented Aug 18, 2020 at 7:07
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    \$\begingroup\$ Thank you, this answered all my questions. \$\endgroup\$
    – gvkcps
    Commented Aug 19, 2020 at 17:29
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Something to note: a dryer has two separate loads. The motor normally runs from one phase only. The heating element (s) run from both phases.

You will see this while you are observing the dryer current. During the "cool-down" period, the heating elements are turned OFF while the motor continues to run.

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  • \$\begingroup\$ Ah, this is indeed low tech: Motor uses one phase, Heating uses two phases. :) \$\endgroup\$
    – tlfong01
    Commented Aug 18, 2020 at 7:08
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    \$\begingroup\$ Dryers are made that way to minimize the differences between a gas dryer and and electric one. They both use the same 120V to run the motor and timer, but only the electric one needs the high-power 240V feed. \$\endgroup\$
    – hacktastical
    Commented Aug 19, 2020 at 18:47
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It's not two-phase

A two-phase power system has two phases 90 degrees out of phase. This can't be done with less than four wires. That's a lot of wires for not a lot of power transmission. Needless to say, this system hasn't gone very far.

Three-phase carries 1.732 x as much power on 1.5x as many wires as single-phase, so that is the system that is widely used for high-power applications.

What you're dealing with is actually single-phase, with a center-tap for neutral. This is called split-phase. Calling it 2-phase is incorrect.

Understanding split-phase

The easiest way for a DC-electronics person to think of split-phase as neutral being 0V, pole L1 being +120V, and pole L2 being -120V. This won't lead you too terribly wrong as long as you remember it is AC, and so it's only true half the time :)

Neutral is what electronics people call GND or Vss. AC mains also has a "safety shield" which in that world is called ground, even though it never carries current except during fault conditions.

Small loads in the <=1500 watt territory attach to a 120V leg -- they connect to one phase or the other, and also neutral. A toaster might be hooked up between L1 and Neutral. A refrigerator might attach between L2 and neutral.

Large loads (>2000W) generally attach to both L1 and L2 legs - and enjoy 240V between them.

Neutral only handles "differential current".

Monitoring power

It's common to put current monitors on L1 and L2, and not put any on neutral. It's easy to see what is moving on neutral; it is the difference in current between L1 and L2.

When the power monitor sees a 120V load, it appears as activity on only one pole - L1 or L2.

When the power monitor sees a 240V load, it appears as exactly equal activity on both poles.

Some loads (dryer, range) have 240V components and 120V components. The 240V parts of the load present as equal load on both poles. The 120V parts present as load on only one pole. So what you see in net is two draws that are large and slightly different. Examples are:

  • Dryers, with about 20A of 240V dryer heating coil load and about 3A of tumbler motor load.
  • Ranges, with 5-38A of various 240V range and oven heating coils and about 0.3A of controls, clock and oven light.
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