When a light starts dim and then gets brighter in your house, you should address the situation as soon as possible. The chances are good that there's a loose or corroded neutral connection in the panel or in the service cable coming from the line transformer. If the problem is between the transformer and your meter, it's the power company's responsibility, but if the problem is in your panel, you need an electrician.
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If lights are dimming in one room only, that usually means you have a circuit overload, and that's fairly easy to fix. Things get more complicated if the lights are flickering and dimming in the entire house. To get a handle on what may be happening, it helps to understand the basics of residential circuitry.
From the Transformer to Your Lights
High-voltage electricity in the power lines gets stepped down to a usable 240 volts by a line transformer, and two hot wires from the transformer connect to the main panel in your house. One of the hot wires is black and the other is red. A white neutral wire returns from the panel to the transformer and a bare wire connects the panel to ground, usually via a ground rod.
The voltage between the two hot wires (or legs) is 240 volts, but the voltage between each hot leg and neutral is only 120 volts. All the 120-volt circuits in the house are connected to the panel via a circuit breaker that contacts one or the other of the hot legs – but not both. Large appliances that run on 240-volt power connect to both hot legs via a double-pole breaker.
When everything is operating as it should, power runs through each 120-volt circuit independently of all the other circuits because the electricity has a well-defined path back to the transformer through the neutral wire. If the neutral path is broken, strange things can happen.
An Open Neutral Can Be the Cause of Lights Dimming in a House
Electricity is resourceful. If the neutral path between the panel and the transformer is broken, you'd think the current would just stop flowing, but that isn't what happens. It tries to complete the circuit through ground, but it also uses the other hot leg, which effectively turns all the 120-volt circuits in the house into 240-volt ones.
The lights in the house get brighter or dimmer according to the balance of loads between the red and black legs. When the load on one leg, say the black one, is greater than that on the red leg, the voltage drop causes lights on the black leg to dim. At the same time, the lights on the red leg get brighter than they should because the effective voltage is greater than 120 volts.
The load balance changes when you turn lights or appliances on or off. That's why you may see that a light starts dim and then gets brighter. An open neutral is an unsafe situation that you need to correct as soon as possible.
Who Should You Call to Repair an Open Neutral?
The service cable extends from the panel to the meter and on to the transformer, and any repairs needed between the meter and the transformer – that is to say, outside your house – are the responsibility of the power company. Problems also arise in the transformer itself, particularly during or after a large storm, and those are also up to the power company.
Most power companies respond quickly, but keep in mind that, unless you just had a storm, disconnections outside the house are rare. It's more likely that the problem is inside your panel, and the power company won't touch that. You need an electrician to work inside your house.
Before you make any calls, check with one or two neighbors. If they're experiencing the same symptoms as you are, the problem is almost certainly in the power lines, and you should call the utility company. If their electricity is normal, it's more likely that the disconnection is inside your house, and you need an electrician.
Lights Dimming in One Room
When lights on a single circuit are dimming but everything else in the house is working normally, that's usually a sign of an overload in that circuit. The most common scenario is that a large appliance, such as a refrigerator, shares the circuit with the lights. Every time the appliance cycles on, it draws more power than it does when it's running, and the resultant drop in power dims the lights.
An overloaded circuit isn't dangerous, but it becomes troublesome when the current draw exceeds the rating of the circuit breaker and the breaker trips. If this happens repeatedly, you need to do something.
The most effective fix is to add a new circuit for the appliance, which, if it's a refrigerator, should be on a dedicated circuit anyway. If you live in an older house, the panel may be too small for a new circuit. Upgrading the panel is an expensive proposition, so the alternative is to move the appliance so you can plug it into a less-used circuit.
Lights Flickering and Dimming in a House
Sometimes lights flicker and dim because of a loose bulb or a loose connection in the fixture. If the flickering is isolated to a single fixture, it's usually a straightforward repair. Tighten the bulb and/or turn off the breaker, check the wire connections to the fixture and tighten those connections.
If all the lights controlled by a switch are flickering, the problem is in the switch, and that's usually also an easy repair. Tighten the connections or replace the switch.
Lights in an entire room can flicker for the same reason that they go dim. They're on the same circuit as a large appliance, and the extra power drawn by the appliance when it cycles on causes voltage fluctuations. The remedy is the same as it is for dimming lights: Move the appliance to a different circuit.
For Your Own Safety
Electricity is dangerous, which is why it's important to call a licensed electrician to troubleshoot problems with the service panel. It's never safe for inexperienced people to work in the panel because the hot bus bars are always energized, even when the main breaker is off. It's doubly hazardous to attempt to diagnose and repair the service lugs themselves.
If you have a reasonable understanding of electrical repair techniques, and you remember to turn off the breaker before starting any work, you can repair individual fixtures and switches yourself. Always check each wire and each connection terminal with a voltmeter to make sure it's dead before you touch it.
It's important that you repair flickering lights as soon as possible. The flickering is a sign of a bad connection that may begin arcing at any time and could start a fire. If you can't correct the problem by tightening the bulb and you're not confident working with electricity, don't hesitate to call an electrician for help.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.