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Question – How to Determine If You Can Add Another Fixture to a Circuit


By Dave Rongey - Summary:

I have four white wires and four black wires in a fixture box. Is my circuit overloaded?

I wanted to tie into an existing ceiling light to add a fluorescent a few feet away and replace the original with a fan. When I opened the fixture there were two wire nuts with four black wires in one and four white in the other. Is it possible to do this, or is it already overloaded?

Because the fixture box has 4 black and 4 white wires does not mean the circuit is overloaded and the circuit should be protected by an appropriate breaker or the fuse at the electrical panel.
In order to determine your circuit capacity, we have to identify the wire gauge of the circuit (not the wires of the fixture). Typically this would be a #12 which is rated for 20 amps, or #14 rated for 15 amps, #14 being smaller that the #12.
Next you need to identify this circuit at the electrical panel that supplies the power, with is either a circuit breaker or a fuse.
Next, find out what devices (lights, plugs etc.) are on this circuit. Make a list of these devices and make note of the power that is being consumed. Simply make a list of the number of light bulbs and the watts of each bulb. If there are receptacles on this circuit, make note of the items that are typically used. Look for name plates that state watts or amps. You need to convert this total number into amps or watts.
To determine watts take 120(volts) times the amps. To determine amps divide watts by 120(volts).
Then we determine 80% load factor so we do not overload the circuit:
So a #12 gauge wire can be safely loaded up to about 17 amps or 1920 watts, and a #14 gauge wire can be safely loaded up to about 12 amps or 1440 watts.
Of course you need to take your proposed light fixture into consideration, and the ceiling fan, and don’t forget to deduct the original light fixture you would be removing. Also, keep in mind that we understand that the circuit will most likely never have all these devices turned on all at the same time, however we do want to identify the circuit potential load and keep the load below the maximum allowed. The circuit wiring is protected by the breaker or the fuse, and this is why we should never replace these protective devices with oversized circuit protection, but stay with the right size breaker for the wire size.
Ok, now that we have that out of the way, I have some things for you to consider. If you determine that your circuit will in fact allow you to add the ceiling fan, you need to make certain that the ceiling support box is in fact rated for the weight of a ceiling fan. These boxes are identified with a stamp or printing stating their compliance for ceiling fans. If you are not sure, I would encourage you to replace the box with an approved box, and install wooden blocking between ceiling supports if needed. (I will be placing info about these methods on the website soon.) If you do not have access to the ceiling you may purchase an approved metal fan box kit which comes with an adjustable spanning bar, and follow the directions provided.
As always, positively identify your circuit and never work on an energized circuit, or perform any work where you do not feel comfortable or lack experience. It’s always best to hire a qualified licensed electrician to insure best results.



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