Hi... I have replaced ceiling light fixtures before, and always observed which wires were connected to which. My problem is, my son moved out and took his bedroom ceiling fixture with him, so now I don't know how the wires should connect to the new fixture. My house was built around 1950-1955. The wires in the ceiling box are : red-all connected together; white- all connected together except one; and one yellow wire.
I'm not sure which is the hot wire, and which is neutral. What will happen if I connect the wires wrong?

A yellow wire, are you in US?

Hello handimom

In the US the black is a constant hot and a red is either switched or another constant hot.
White or gray is the neutral.
Green or bare is the ground (if available)

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So the connected reds are a hot wire, used for interconnection only. One of these goes to the light switch, others to another connection point or a power point.
The single red wire is a switched hot wire (via the on/off switch), and goes to the lamp.
The single white wire is the neutral and the other wire that goes to the lamp.
Just leave the yellow wire free. The average ceiling light does not need an earth connection, and unless it is a green/yellow wire, it may not be an earth wire at all.

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General wiring information : USA : Design and installation conventions

For residential wiring, some basic rules given in the NEC are:

* phase wire in a circuit may be black, red, orange (high leg delta) insulated wire, sometimes other colors, but never green, gray, or white (whether these are solid colors or stripes).
* neutral wire is connected to the center tap of the final step-down transformer and is identified by gray or white insulated wire, perhaps with stripes; most commonly bonded to earth for a fixed known path to stabilize the voltages only at the main service panel; many times called the grounded wire. Note that all metallic systems in a building are to be bonded to the panel; e.g. water, natural gas, HVAC piping, etc.
* Grounding wire of circuit may be bare or identified insulated wire of green or having green stripes.
* Larger wires are furnished only in black; these may be properly identified with suitable paint or tape. The phase wire for a switch "leg" is the white wire of a two insulated wire cable; the black wire is connected to the light.
* All wiring in a circuit except for the leads that are part of a device or fixture must be the same gauge. Note that different size wires may be used in the same raceway so long as they are all insulated for the maximum voltage of any of these circuits.
* There is no maximum number of receptacles on a circuit, but the Code gives rules for calculating circuit loading.
* Ground-fault circuit interrupter (GFCI) protection is required on receptacles in wet locations. This includes all small appliance circuits in a kitchen, receptacles in a crawl space, basements, bathrooms and a receptacle for the laundry room, as well as outdoor circuits within easy reach of the ground. However, they are not required for refrigerators because unattended disconnection could cause spoilage of food, nor for garbage disposals. Instead, for refrigerators and other semi-permanent appliances in basements and wet areas, use a one-outlet non-GFCI dedicated receptacle. Two-wire outlets having no grounding conductor may be protected by an upstream gfci and must be labelled "no grounding". Most GFCI receptacles allow the connection and have GFCI protection for down-stream connected receptacles. Receptacles protected in this manner should be labeled "GFCI protected".
* All branch circuits with receptacles must have arc-fault circuit interrupter protection, unless wired with specific types of armored cable.[4]
* Most circuits have the metallic components interconnected with a grounding wire connected to the third, round prong of a plug, and to metal boxes and appliance chassis.
* furnaces, water heaters, heat pumps, central air conditioning units, stoves on dedicated circuits
* Use exterior components for exterior lighting and outlets
* Electrical boxes must be properly sized to prevent heat build-up, especially from joints, and just having so many items in too small a space.

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Hope this helps !

:)

Quote:

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Originally Posted by handimom

Hi...I have replaced ceiling light fixtures before, and always observed which wires were connected to which. My problem is, my son moved out and took his bedroom ceiling fixture with him, so now I don't know how the wires should connect to the new fixture. My house was built around 1950-1955. The wires in the ceiling box are : red-all connected together; white- all connected together except one; and one yellow wire.
I'm not sure which is the hot wire, and which is neutral. What will happen if I connect the wires wrong?

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Thanks to those who replied to my question. Yes, I am in the US. Unfortunately, in older homes I have found that whoever did the original, or long ago, wiring seem to have used whatever color wire they had lying around. I have found purple, orange, etc.

I do have one error in my question to correct. I previously stated that all the white wires except one were connected. ALL the white wires are connected. All the red wires are connected. There is one Yellow wire by itself. Go figure...

Ok, remove the switch and tell us what's there. You will probably find a red wire and a yellow wire but lets be sure.

The switch has one red, and one yellow wire connected to it. There are white wires also in the switch box, but not connected to the switch. Let me add that when my son removed the former fixture, a fan / light combo, he left the red wires tucked into the ceiling box, with the white, and one yellow, wires hanging down. Why is a red wire connected to the switch, but apparently wasn't connected to the fixture?

OK, connect white of fan/light to the white wires all ready connected together. Connect the fan hot wire, which is probably black, to the red wires all ready connected together. Connect the light hot wire, which is probably blue, to the yellow wire. The switch will control the light. The fan will be controlled by the pull chain.

I am NOT installing a fan / light. I am installing a light fixture only, in place of a pre-existing fan / light. Do I just leave the red wires alone? I'm sorry if I sound like an idiot, I'm really not. I just have a terrlble fear of electricity. I had some serious electrical problems here before I had new service installed last year.

Sorry, my error. Just attach white of light fixture to white wires in ceiling box and black of light fixture to yellow in the ceiling box.

Thanks for all your help... and your patience. I just didn't want to burn down my house.