Safety Warning: Testing wires to see if they’re live can be dangerous if you're not an expert. Always turn off your breaker at the main panel before opening any electrical box in your home—never work on a live wire in any circumstance.
Before you take the more advanced stop of opening an electrical box, try checking it with a receptacle tester. If it’s a plug outlet, the easiest and safest way to check for voltage (and a few more things) is by plugging in directly. Every DIYer should have one of these devices—Home Depot has them for under $20 as of this writing (Fig. 1).
The Receptacle Tester
This little tool, once plugged into a receptacle, will tell you if the plug is by turning on its indicator lights. The sequence of those lights will indicate whether the ground or the neutral is open, and whether the Hot, Neutral, and the Ground (Gnd) are wired wrong. Most importantly in this case, if the circuit doesn’t have power (or if the Hot is open), this handy device will tell you. Some receptacle testers are also equipped with the extra option to test GFCI receptacles, such as the one shown in figure 1.
With the tester’s lights (if any are on) showing the situation about the plug’s wiring, make note of any wiring anomaly indicated. Remember—before removing the decorative plate, the BREAKER MUST ALWAYS BE TURNED OFF at the main panel (there’s never an exception to this rule). With this done, the repair or modifications can be done on the wiring of the receptacle. The faulty wires can be removed and reconnected to their correct locations, then re-tested once the box is closed up and the power flipped back on.
Can a Miswired Light Socket Cause a Hazard?
Light sockets only have two wires (besides the Gnd). So can they be miswired? If some previous handyman wired it up with the neutral wire to the switch instead of the Hot wire, this would mean that even when the switch is off, the light socket is very much LIVE at all times. This can be quickly verified at the light fixture.
A stable step ladder or another sturdy stand will be required under the light fixture.
The power will need to be temporarily turned back on, then the light has to be turned off at the switch, and the light bulb removed.
Using the step ladder to get closer to the light fixture box, set the multimeter on AC Volts, place one lead on the grounded screw on the socket and the other on the contact at the bottom of the threaded socket (Fig. 2). If there’s a reading on the meter the Hot is miswired, otherwise, proceed to the next step.
With the first lead still on the grounded screw, the other lead is now transferred to the metal threaded side of the socket. If there’s a reading on the meter the Hot is miswired, otherwise, the wiring is all good. Job done.
What if the Cable coming in the Electrical Box was Miswired?
If there’s any voltage at the light socket, the switch box will need re-wiring—start by turning the breaker back off.
Once you've done that, you can open the switch’s electrical box and pull out the switch to expose the wiring (Fig 3).
In this particular box, wired up for two-way switches, any of the black wires and the red wire are likely to be hot. Making sure first that the breaker is turned off, open it up and pull out the switch to confirm that the white (neutral) wire (and NOT the black or red) is wrongly hooked up to the switch. This can be corrected by removing the white piece of wire connecting the switch to the cluster of white wires and replacing it with a black wire between the same spot on the switch and the “cluster” of black wires.