Phase Coupling

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What is Phase Coupling?

The vast majority of homes in North America have the electrical power divided into two phases (commercial buildings often have three phases but we will ignore that since I am intending this tutorial for home users). About half of the 110V lights, appliances and outlets use one phase and the rest use the other phase (220 V appliances use both phases and kitchen duplexes typically have one plug on each phase). You don't need to know why your house is set up this way, just trust me that it is.

Phase Coupling is allowing the X10 powerline signals to travel from one phase to another. This is needed so that if the transmitter (transceiver, plug-in controller or 2-way module) is on one phase and the receiver is on the other phase the receiver will hear the X10 signal.

Every home has a small level of coupling between the two phases at the transformer that supplies power to the house. This coupling is inductive which means that the higher the frequency, the more it attenuates (a fancy word for weakens) the signal. When combined with the signal reduction caused by traveling to the transformer and back, it is typically assumed that the signal will be 1/10 as strong on the opposite phase. This would be fine if there wasn’t anything else to weaken the signal or generate noise on the line, but this usually isn’t the case.

Some people will say to test if a problem is due to poor phase coupling by turning on an electric stove or dryer and see if the module starts working. This will sometimes work because a dryer (or stove) provides basically a resistive (yes the motor is inductive, but lets ignore it) load between the two phases. This means that it will attenuate all frequencies by the same amount. The resistance in the dryer is reasonably low, so it will improve the coupling, but it won’t be great. That is why even if the problem doesn't go away when performing this test, a phase coupler may still solve your problem. Every house will benefit from a phase coupler, so I highly recommend installing some type of phase coupler if you want to have more than a couple X10 modules in your house.

Passive Couplers

A Passive Coupler is a device which has been designed and manufactured for the purpose of transferring X10 signals from one phase to another. These are typically CSA and UL approved so that they allowed in the electrical code. They are also typically tuned to pass only 121kHz signals so they will reduce (but not eliminate) the noise that is coupled. There are three types of Passive Couplers (4-wire, 2-wire and plug- in). Each has its own pros and cons.

  • A 4-wire coupler, if designed properly, should allow 220V modules to work, but it requires that you (or an electrician) open up your breaker panel and connect it to two breakers on opposite phases and to the neutral wire.
  • A 2-wire coupler might provide slightly better coupling than a 4-wire coupler, but 220V modules won’t work. It also requires you open up your breaker panel and connect it to two breakers on opposite phases (no neutral wire connection is needed).
  • A plug-in coupler saves you from having to open your breaker panel by letting you plug it into a dryer outlet (you can then plug your dryer into the coupler). The biggest problem with this type is the signals have to travel to your dryer and back. This will increase the signal attenuation depending on how far your dryer is from your breaker panel. Depending on its design it also may be similar to a 2-wire coupler in that 220V modules won't work.

Which one should you get? Well that depends on you. If you feel comfortable opening up your electrical box, you will get better performance from either a 2-wire or 4-wire coupler. If you are like most people and don't use any 220V modules (and don't think you ever will) then a 2-wire Passive Coupler is probably your best bet, otherwise, go with a 4-wire Passive Coupler.

If you don't feel comfortable opening up your electrical box and don't want to hire an electrician, then get a plug-in coupler. Just check if you need a 3-pin or 4-pin plug.


For most people, a Passive Coupler is good enough but if you have a large house (the rule of thumb is over 3000 sq. ft.) and you have a lot of electronic equipment in your house (stereos, TVs, computers, etc) then you might be better off with a Coupler-Repeater. These have their own set of pros and cons.

A Coupler-Repeater uses the fact that most X10 commands are repeated. The Coupler-Repeater will listen for X10 commands on the powerline and then repeat them on both phases at the same time as the transmitter is repeating the command. This is helpful if you have other significant signal loss issues and is typically only needed in larger houses. Since only X10 commands are repeated, noise will not be coupled from one phase to the other. If you are not sure whether you have a phase or a line noise problem, a regular Passive Coupler might make things worse, but a Coupler-Repeater will not. 220V modules will work with a Coupler-Repeater.

The biggest problem with Coupler-Repeaters is they have issues with dimming commands. The number of on (or off) commands is not important, since it will turn on (or off) whether it receives the command 1, 2 or 100 times. However, the number of dim (or bright) commands that the module receives is important as it will affect how bright the light will be. There are some subtle complexities with this that I won't get into (see Phil Kingery’s series of articles for more information), but I will say that different manufacturers of Coupler-Repeaters deal with this problem in different ways. All of them will improve the situation, but none of them are perfect.

Some people like to think that if one is good, two is better. This is not the case with phase couplers. This is especially true with Coupler-Repeaters since they can start to ping-pong commands off of each other. There are situations in large buildings where more than one Coupler-Repeater is needed, and in those cases you must use commercial grade units which can be programmed to handle this. This is well beyond the scope of this tutorial and so I will say you should only install one phase coupler, be it a Passive Coupler or a Coupler-Repeater. If you upgrade to a Coupler-Repeater, be sure and remove the old Passive Coupler.

Problems With Extended Commands

As Phil Kingery explains in his article Which One Should I Use, Part XIII "Digital X-10" the data for standard X10 commands are transmitted over 11 powerline cycles (including 2 cycles for the StartCode). The data (including the StartCode) is then immediately repeated. As explained above, Coupler-Repeaters repeat commands on both phases at the same time as the transmitter is sending this repeated command.

All was good until X10 decided in 1993 to add Extended code commands to the protocol. These commands have an extra 20 powerline cycles of data (for a total of 31). The problem with this is if the Coupler-Repeater doesn't support extended commands it will start to "repeat" the data in the middle of the extended command. Most modules that generate extended commands will detect this and stop transmitting and wait for the Coupler-Repeater to finish. Once it is finished the module will try again, starting the whole cycle all over again. Thus the powerline will be filled with a flood of powerline commands that won't stop until either the offending module or the Coupler-Repeater is unplugged.

This is obviously not good, so I highly recommend that only Coupler-Repeaters that support Extended Commands be used. To the best of my knowledge, only the Coupler-Repeaters manufactured by ACT support Extended Commands.

If you have (or decide to get) a Coupler-Repeater that doesn't support Extended Commands (such as those made by SmartHome and Leviton), the other option is to not use modules that use extended commands. These include (but are not limited to) X10 and Leviton (but not SmartHome) 2-way modules and ACT A10 modules. Some of these modules may not use Extended Commands by default, but may do so when used with certain intelligent controllers (such as the CM15A).


If most people can get away with a Passive Coupler and only some people need a Coupler-Repeater, an X10 Firewall is overkill for the vast majority of users. If money is no object and you have lots of really noisy equipment or you want a rock solid system then a firewall might be for you.

An X10 Firewall will isolate each circuit that you use for PowerLine Control (PLC). It will monitor each of these circuits for X10 commands and will then transmit this command on all of the other circuits. The Firewall has collision protection which will wait for the line on each circuit to be clear before transmitting a signal on that circuit to insure that no commands are lost. Since each circuit is isolated from all of the other circuits, you get the following benefits:

  • Noise (or signal absorption) on a non-PLC circuit will not be seen on any of the PLC circuits,
  • Noise (or signal absorption) on a PLC circuit will be isolated to that circuit, and
  • Noise (or X10 signals) from outside the house will not be seen on any of the PLC circuits.

The system is expandable in that you can connect multiple firewalls together should you need more than one (or have multiple breaker panels). There are, however some disadvantages to the Firewall:

  • It is very expensive (about $400 for 4 circuits or $700 for 8 circuits),
  • It is difficult to install (you have to route the wires for all PLC circuits through the firewall between the breaker panel and the loads), and
  • The current models (as far as I know, there is only one manufacturer) don't support extended commands.

Home Grown Solutions

One technique we have heard about but do NOT recommend is to put a 0.1uF, 600VDC capacitor across the phases. A capacitive load is exactly the opposite of an inductive load (the lower the frequency the more it attenuates the signal). With the right capacitor it won’t let the 60Hz power through but the 121kHz X10 signal will be coupled to the other phase with very little attenuation. X10 does not recommend coupling phases with just a capacitor. There are many disadvantages to this 'solution':

  • It is not approved in the electrical code. X10 expressly disclaims all liability for any electrical damage, house fires, or any other mishaps resulting from this DANGEROUS procedure that we do NOT recommend.
  • Electrical noise will be coupled between phases along with the X10 signal
  • 220V modules won’t work (Space does not permit an explanation of why this will not work)
  • Note the "right" capacitor is hard to find because it depends on the resistance of the copper lines carrying the phases (thus forming a high pass RC filter) - and this can vary quite a bit from house to house. In the worst case a "wrong" capacitor can effectively short the two phases together.

Some people make a better phase coupler by putting an 18uH inductor (and a 0.25A fuse) in series with the capacitor (as Ido Bartana describes on his site) to filter out the frequencies above 121kHz. This has two advantages:

  • It will reduce the amount of noise above 121kHz that is coupled
  • It will actually improve the coupling of the 121kHz signal slightly

This is essentially the same as a 2-wire Passive Coupler but in my opinion, if you are going to this much trouble, you might as well buy a commercial Passive Coupler which is UL and CSA approved. X10 does not recommend coupling phases with just a capacitor.

Installing a Wired-In Coupler

Separate Box

When installing a wired-in phase coupler (a passive coupler, coupler-repeater or a firewall), it is important to remember that the electrical code in most jurisdictions doesn't permit devices inside the electrical panel. The proper way to install a wired-in phase coupler is to put an electrical box near the breaker panel and run a short piece of 3-wire (plus ground) Lumex (or Romex for those of you who are American) from the panel to this box. If you are installing a 2-wire passive coupler you might be able to get away with using a piece of 2-wire Lumex, but I would suggest using 3-wire instead for several reasons:

  1. An electrical inspector may not like you having a white wire connected to a breaker.
  2. If you ever upgrade to a Coupler-Repeater (or a 4-wire Passive Coupler) your will be prepared.

Dedicated Breakers

Although you can have the phase coupler share existing breakers, I recommend that you install two new 15A breakers (or better yet one 220V 15A breaker) that are dedicated to your phase coupler. This will minimize the number of connections between the phase coupler and the bus bar in the breaker panel, maximizing the signal strength. Also if there is a problem with the signal on one of the circuits that is being shared, sharing a breaker will affect the performance of the phase coupler.

Opposite Phases

For a phase coupler to work, it must be installed across the two phases. A common mistake is to accidentally connect both sides of the phase coupler to the same phase. The instructions for many phase couplers say to connect it to two breakers that are beside each other. This will sometimes work but when double breakers (two breakers in one slot) are used, both of those breakers are on the same phase. If you are at all unsure, you can connect an AC Voltmeter across the two breakers to ensure there is about 220V. Just be careful; if 110V is dangerous, 220V is even more dangerous.

Why Did Modules Stop Working After I Installed a Passive Coupler?

This is especially common when using a capacitor as a phase coupler but it can also happen with a proper Passive Coupler. What happens is that the capacitor will couple both the X10 signal and powerline noise, so if you have something on one phase which is generating a lot of noise, the capacitor can actually create problems by allowing that noise to be distributed throughout the house.

Proper Passive Couplers will try to filter out the noise and prevent it from being coupled. However, if the noise it too close in frequency to the X10 signal it won't be filtered and will still be coupled.

Since Coupler-Repeaters only repeat valid X10 commands on the other phase, it won't couple any of the noise to the other phase.

Optimally you need to find the source of this noise and install a noise filter to block it.

Related Articles

Phil Kingery’s series of articles
Noise Filters and Automatic Gain Control