Powerline and house wiring and sub panels

Recently I solved a problem with using power line network to an outbuilding and I thought I would share what I learned. 

Powerline technology works between two more more devices, a transmitter and one or more receivers, that are plugged into 120 V outlets. 

In North America, each house is fed from the utility with three lines. Two of the lines are 120 V to ground but are out of phase with each other, thus between these two phases the voltage is 240. These are commonly called L1 and L2. The third line is the neutral which is grounded and thus zero volts. 

Within each house's power panel, there are various 120 volt circuits that each are connected to either L1 and the neutral or L2 and the neutral. With respect to electricity supply, it does not matter whether it is connected via L1 or L2. The voltage is the same. 

But, critically, for Powerline networking, it does matter that the circuit into which the transmitting device is plugged (the device that is also connected to the modem), is connected via a circuit breaker to either L1 or L2, is the same into which the receiving device is connected.  

If you just randomly plug in the transmitting device and the receiving device into any circuit, you have a 50/50 chance of having that right. You also have a 50/50 chance of getting it wrong. Within any house, even if you plug the two devices into circuits fed by different power supply legs (that is, one into L1 and the other into L2), it is likely to work somewhat because the connection can be made via the neutral, which is common to all 120 V circuits. But it won't work well. And, it the transmitter and receiver are on different electrical panels, for example one is in a detached garage feed by a sub panel off the main panel, it is unlikely to work at all unless both outlets are wired to L1 or both are wired to L2.

How does one know whether a particular circuit is feed from L1 or L2? One must know which circuit breakers in a panel feeds the outlets that are used. Usually, these will be separate, if the plugs are in separate rooms. If they are both fed by the same breaker, you are all set. If fed by different breakers in the same panel, it goes like this: On the left row, breakers 1,5,9,13, etc are on the same leg (L1 or L2). On the right row, breakers 2,6,10,14, etc are on the same leg (L1 or L2). On my panels, breakers 1 and 2, are on the same leg, 3 and 4 are on the same other leg, breakers 5 and 6 are on the same leg as 1 and 2. That is horizontally, left and right adjacent breakers are on the same leg as each other, but vertically, the breakers alternate between L1 and L2. Not every panel is the same horizontally, it depends on the manufacturer and you can check the wiring diagram for the panel. 

Its a bit more difficult when one device is on a breaker in the main panel and one device is on a breaker in a sub panel. That is because L1 and L2 can be crossed between the two panels, and that makes no difference for electrical supply. But it does for the Powerline devices. There may be some trial and error involved.

In my case the transmitter device was on a breaker from the main panel. The outlets for the receiving device, in a detached structure,  was on a sub panel from another sub panel off the main panel. Powerline would not work. I moved the breaker within the sub panel (don't do this unless you know a lot about power, or get an electrician) to three different places but it still would not work. After studying the wiring panel for the sub panel, I finally realized that although I moved the breaker for the outlets it to three different locations, each of those locations was on the same leg (L1 or L2) that was not the same as the leg for the transmitter on the main panel. Thus, I moved the breaker to a fourth location, that I was certain was on a different leg than the other three locations. And Voila, it worked perfectly. 

Moving breakers around within electrical panels is not for the average home owner, it needs an electrician. I'm not an electrician, but I have experience with wiring panels. The bottom line is Powerline technology is great for houses and detached structures, but you just have to be sure that the devices are in outlets ultimately feed from the same leg, L1 or L2. If it works poorly or not at all, that is a likely issue if everything else is right.