Why do I measure low, varying phase leg voltage which fluctuates with load?

Question

I have 123 volts on one phase and 89 or less on the other phase depending what I turn on. How do I troubleshoot this?

Answer 1

RED ALERT! Call the power company TONIGHT and report an outage

You have, at the very least, a lost phase. And quite probably a lost neutral.

Now, in most of Europe, this would mean a 100% outage, since their power drops only have 2 wires. America uses 3, which we get a lot of weird effects when 1 wire breaks.

The weirdest effect is a lost neutral, where both hot legs are still hot, but the 120V voltage wanders all over the place high and low - the two legs adding up to ~240V, but one higher and one lower.

The lost neutral is an immediate and imminent threat to your safety. 95% of the time, the problem is on the service drop between your meter and the pole/transformer, since that's the one flapping around in the breeze and accumulating metal fatigue. That's why to call the power company straightaway. The last time this happened to me, the PoCo was out in an hour, on a Sunday, after saying they'd be delayed due to another emergency. Fixed not only our complex, but a weak hot wire on our neighbor, too.

The other possibility is a lost hot, which I discuss at length here. In that case, the other hot will appear to work, but be weird and weak. What's happening is a 240V appliance (like the water heater) has cycled on, and is effectively shorting the two poles through its high resistance heating element. So your 120V appliances are now in series with the water heater, all trying to share 120V. That may work well enough for the water heater to reach its high setpoint and cycle off, and that leg will go dead again Don't do that, though.

Meanwhile, shut off the main breaker!

You may shut off all breakers and turn the main back on, and then do one of the following:

• turn on ONE 120V load only (no breakers with double width or tied handles) e.g. Furnace or fridge. It may not work.
• If you know panel topology reasonably well (the weird way that poles are striped across the breakers), you can turn on 120V loads on one pole OR the other, never both... but all 240s must be off.

Answer 2

There are two most common causes of what you are seeing.

The first is when there is a poor connection on L1 or L2, which are the two hot lines from your transformer supplying power to the house. This will typically manifest with voltage drop on one leg (line) and stable voltage on the other. This situation is not safe for your appliances (refrigerators do not do well with low voltage, as an example). It is also not safe for where the voltage drop is happening, which is usually a poor connection or a failing transformer winding. Fires can happen.

Another less common source of this problem is where aluminum wires feed the panel box in a residence, and the connection has loosened over time. It will get hot, and the expansion and contraction will worsen the condition. Sometimes this can be detected by measuring the voltage at the conductor coming into the panel box and on the bus bar in the panel box to see if there is a differential.

The second situation which could cause what you are seeing is a floating neutral. This doesn't happen as often, but is a very dangerous situation. It can result in electrocutions, with some appliances like older stoves, where the neutral also functions as a ground. In the case of an electric range/stove the problem is compounded by the appliance being in a room which customarily has wet hands, and a sink nearby with an earth ground.

If you are not comfortable pulling panel covers and measuring 240 volts, get someone who is. Within a few minutes they should be able to determine what is happening.

Power companies vary in how much they are willing to debug customer power problems, but you might get faster response calling the power company. They can at least verify that they are feeding you good power.