Can you convert a lighting fixture to a wall outlet?

for USA, Canada and countries running a 60 Hz supply service.

This can be a complicated project. First and foremost turn the breaker off that supplies the voltage to that circuit. A light fixture outlet is usually switched and the majority of times it is on the ceiling. To bring this circuit down to accommodate a wall receptacle will mean fishing a new wire from the ceiling junction box down to the new wall receptacle outlet. One the rough in is completed the switch will be removed and the two black wires will be spliced together. This will make the old light fixture junction box connections "hot". Install a blanking plate on the old switch junction box. This will keep small fingers out of the electrical splice. Now splice the black to black and white to white in the old ceiling junction box. Install a blank cover on this box. Now the voltage supply will be at your new wall receptacle. Connect the new receptacle to the circuit and install a cover plate. Turn on the circuits breaker and the new receptacle should be in operation.

Note: Light fixtures are typically on circuits with other lights and sometimes outlets for electronics, both of which are adversely affected by sharing a circuit that powers motors and compressors. So, if you are planning on plugging a freezer, washer, dishwasher, or the vacuum cleaner into this outlet, check to be sure your home theater isn't on the same circuit breaker.

As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

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I'm pleased you prefixed the answer with a statement of country. In the UK the power and lighting circuits are separated immediately after the supply company's master switch and fuse, and fed through separate fuses or (nowadays) circuit-breakers. You NEVER mix the two: you can run a table-lamp with a standard 3-pin plug from a 13A outlet socket, but you cannot run an appliance off the light fittings, and definitely never connect the lighting-ring wires to a wall socket as described above. That would be not only dangerous due to the likely higher current hence overheating & fire risk, but probably illegal as a result. The wires are never "spliced" either, if that means twisted together and taped or sleeved, but are connected in proper junction-boxes with screwed terminals.

In times past it was possible in Britain to buy 2-pin bayonet-connector leads enabling small appliances to be plugged into the light fittings in place of the bulb - without an earth pin! They were outlawed decades ago. Incidentally, the supply frequency (50Hz in UK & Europe) is not relevant to the point here.