Powerline Networking - Outlet lost data throughput
Noisy Neighbors

It could be a new appliance or device that you or one of your neighbors has installed, putting more noise on the line than what was their previously. Or it could even be new powerline networking devices that your neighbor has installed, causing interference with yours. It could even be that your neighbor moved an old powerline networking device to a different outlet in their house.

Wait, one of your neighbors?  ;-)

Networking Through The Power Company Transformer

The two 120 Volt hot buses in your service panel are fed from opposite poles of the big power company step-down transformer outside (some people will refer to these poles/buses somewhat but not entirely misleadingly as "phase legs.").

The 120V outlets in your house are all fed off of just one of those buses in the service panel.

All of the branch circuits on each bus are electrically connected to one another inside your service panel, inside your house, but connected to branch circuits on the other bus only through the power company's big noisy transformer outside.

All of the branch circuits in your nearby neighbor's homes are also connected through that very same path through the transformer.

Communication Paths &amp; Interference

So, if the powerline networking devices connected to your computer and your switch/router are on branch circuits on the same bus, then they can "hear" each other through the service panel inside your house. This is a relatively short and probably a fairly "quiet" path.

If the devices are on branch circuits on opposite buses, they can only "hear" each other through the power company transformer, which you also share with all your neighbors and their electronic equipment and electric motors (air conditioners, refrigerators and freezers, dishwashers, washing machines, clothes dryers, fans, power tools), as well as with your neighbor's powerline networking devices.

It's even conceivable that their powerline networking devices are interfering with yours in the same way that two wireless access points on side-by-side channels can interfere with one another (powerline networking is just broadband radio transmission through the wire, like WiFi is broadband radio transmission through the air).

Howdy, Neighbor

And either way, yes this does mean that signals from your neighbor's powerline networking equipment are propagating all over the electrical circuits everywhere in your house, and with the right equipment you could snoop on their traffic, or they could snoop on yours. But those transmissions from your neighbors' equipment might be loaded with enough noise and distortion to cause your equipment to falter, especially if you're both using the same brands and frequencies, and your devices are trying to communicate with theirs. Or maybe worse, if you're using different brands which are competing for the same frequencies and not coordinating their efforts very well.

Somebody once tried to tell me that circuit breakers stop these transmissions. That is absolutely, positively bunk. If that were the case, two devices couldn't talk to each other unless they were literally on the same circuit.

Having said that, you're likely to get the best signal in this order:


Same branch circuit (no circuit breaker in the path)
Same bus in the service panel (two circuit breakers in the path)
Different buses in the service panel (at least two circuit breakers plus the inner coil of the power company transformer in the path)