Extending existing circuit

Old 08-12-08, 01:03 PM
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Extending existing circuit

Newbie here - Wiring question

I have a bedroom (turning office) and was curious about how to check the existing circuit to see if I can add lights to the room.

Basically I would like to place 4 small can lights in the ceiling over a desk and tie them into the current circuit. The room also has a fan light. I'm guessing I can add a switch to control the can lights by using the current wiring but was unsure about how many lights the circuit will support.

How do I calculate the load with the existing wiring?

Thanks in advance for any help!
Old 08-12-08, 01:10 PM
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First, know that it is not legal to extend an ungrounded circuit. So do not move forward if you have an older home with ungrounded circuits.

Lighting usually adds only a small load, so it is usually possible to add to the circuit. But you are wise to check.

First, figure out everything on the circuit. The easiest way to do that is to turn off the breaker and see what's dead. While you're at it, why not do this for all the breakers in your panel so that you have a complete list of what is on what circuit.

Once you figure out everything that is on that circuit (and you must be thorough, since the circuit may include things that are not physically near your bedroom), then you can add up the loads. Lighting is easy to compute--just add up the wattage of the bulbs. For appliances, use the wattage information on the appliance. Unused or occasionally used receptacles are a bit trickier, since you cannot reliably predict what might be plugged in. But you can do this prediction better than we can, because you live there and we don't. It is typical to allow about 180 watts per receptacle, but this is a very rough rule of thumb and you can certainly use other figures if you know how these receptacles are used, or are likely to be used in the future.

Then look at the amp rating of the breaker you turned off. A 15-amp circuit can support up to 1800 watts, and a 20-amp circuit can support up to 2400 watts. The typical recommendation (not requirement) is to not plan to use more than 80% of these figures.

If the circuit includes some fixed-in-place appliances, then other rules apply. But that's not typical in a bedroom.

Bottom line is that this is most likely okay.
Old 08-12-08, 01:38 PM
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Thanks for the quick response -

The home was built in 1985. I'm set with grounding issues but I will need to check the outlets on the circuit as suggested. Thanks for the help. Will post back to update on what I find.
Old 08-12-08, 04:06 PM
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Similar to the above question, I want to add a light fixture to an existing circuit. Specifically, I have installed a light in my daughter's closet and I ran a feed from the new install to the existing switch, which works the overhead light in the room.

Used to be that a pencil, notepad, and about thirty minutes, and I'd have it figured out, but not this time.

Existing switch is single pole, with the hot wire at the switch. What I'd like to do is install a double-switch, so each works each light independently. These lights and one other outlet are the only things on this circuit. I am using the new energy efficient bulbs, so I know my wattage is well within the limit.

I have a hot black and a white from the existing lights, and a black and white from the new lights all in the switch box. Every logcical connection combination fails me. I am using a double switch with a common hot (black).


Thanks in advance.
Old 08-13-08, 04:54 PM
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Before you go any further you should know that the are strict Code rules regarding the types of fixtures allowed and clearences from storage shelves in closets. No bare bulb fixtures are allowed. Enclosed fixtures require at least 12" clearances.

If you can provide more details on the type of fixture you might be able to proper install your lighting.

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