The voltage can be thought of as the amount of potential required to “push” current (amps) through the wires, plus whatever “load” the wires feed, like a light for example. There will be some voltage difference between one end of a wire and the other due to wire and connections resistances. This voltage drop is proportional to the amps drawn by the load. If everything is ok, it should be within a couple of percent and generally steady, definitely not flickering.

Some older homes will have say, a refrigerator, on the same circuit as some of the lighting, and, when that frig turns on, its current draw will cause an extra voltage drop across the wires. The lights then dim because the voltage to them becomes lower. Frigs and other appliances with motors have an initial large startup current, so the lights on that same circuit dim a lot at first and then very quickly get almost as bright as they were before. If that’s the manner in which lights are dimming, it may not a problem, more of an inconvenience. You wouldn’t want your computer plugged into that circuit, though, and it would be prudent to verify how much of a voltage drop is happening to rule out one of the problems described below.

Large appliances, like air conditioners or washing machines, should each be on their own circuit so the wiring isn’t overloaded. If they’re not already, then the solution is to install a dedicated circuit.

If a connection is loose, the minimal contact of the wires can create a point of high resistance, as if the wire got real thin at that point. Depending on the current draw, that loose connection can spark and/or heat up and can cause the wires to move, which can further change the connection resistance. That can cause the voltage to fluctuate, and thus the lights to flicker. Whether the voltage fluctuates or not, loose connections can cause the wires to heat up enough to melt insulation and start a fire. Dimming or flickering lights can be an indicator of this.

The normal voltage to homes in the San Francisco area is about 115V to 124V, depending on how far your home is to the nearest transformer. If there is dimming happening at a light or receptacle when something else is turned on, the voltage difference between dimmed and not dimmed shouldn’t be more than a couple of volts. If it is, it should be checked.

If lights or appliances dim periodically, and it doesn’t seem related to an appliance turning on or off, this may be a hazardous condition that should be fixed. It could be a wire-to-wire connection somewhere, a breaker connection, the connection at switch or receptacle terminals, etc., and it could be the hot wire or the neutral. It’s often tricky to find and takes some good logic and diagnostic skills.

Most homes have two legs (or phases) of about 120 AC volts each that are 180 degrees out of phase, which creates 240 V potential between them. (If you recall any trigonometry, think of the phase difference between sine and cosine waves.) Even though there are two phases, this is referred to as a “single phase” system.

Each leg is 120 V with respect to the neutral, which is effectively at 0 V. Most of everything in the house will operate from either one of the 120 V legs. Ideally, the overall house load is split pretty equally between the two phases. Then, things like dryers, stoves, and AC units will operate from both legs at 240 V.

The neutral is created at the utility transformer and is held to ground potential (zero V) by the grounding system, which, when done right, usually consists of a combination of a ground rod(s), the plumbing, the building’s foundation, and then the neighbors’ grounding systems as well. Generally, the neutral is bonded to the grounding system at the main panel of the building.

There are also often individual pairs of circuits within a house that have 240 V between them and share a neutral wire. This is fine as long as the neutral connections are good.

If a neutral has a loose connection or gets disconnected altogether, depending on where this happens, there can be 240 V applied between one circuit and another. The voltage on one circuit can then increase towards 240 V while the voltage on the other circuit of the pair correspondingly decreases. This can easily fry electronics and let all the smoke out. Not a good thing.

If lights ever burn brighter than normal, this is probably due to bad neutral connection and should be fixed immediately. A bad neutral connection can also cause the lights to dim.

Identifying the location of this problem requires the use of at least a good voltage meter and some good diagnostic skills.