Can someone check my lighting calcs?

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  #1  
Old 10-20-07, 11:00 PM
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Can someone check my lighting calcs?

I am going to run a lighting circuit in a garage. The lights are going to be 4ft 2 lamp shop lights on switched receptacles. Correct me if I'm wrong, but if I remeber correctly, these lamps are 32 to 40 watts. We'll say 80 watts per fixture?

A 14 guage, 120v, 15 amp circuit gives me 1800 watts to play with. Derating the lighting load (continuous duty) gives me 1440watts. This gives me a maximum of 18 four foot, 2 lamp fixtures, correct?
 
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Old 10-21-07, 06:20 AM
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I'm not sure about this rule in the US, but in Canada, you can only load the breaker to 80% capacity. This means you cant put 1800 watts on that 15A breaker. You can only load it to 12A or 1440 watts.


So to answer your question, YES, 1440 is the max you could load on the circuit. Be careful when considering this circuit, you may want room for future expansion. If the material is not already purchased and/or installed, do yourself a favor and run 12 guage and use a 20A circuit.
 
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Old 10-21-07, 12:30 PM
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I've got 20 amp circuits for the receptacles. I just wanted to make sure i didn't make a mistake. 18 shop lights seems like alot for a 14 gauge wire, but I think its right. I am a big fan of running 12 gauge too, but there is no way i am going to use even 10 lights on this circuit.

I think around here you're aloud to use 100% of the breaker, but for continuos loads, which lighting is considered, you derate to 80 percent.
 
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Old 10-21-07, 12:37 PM
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You can double the wattage-capacity by simply adding a 3rd conductor to the circuit.

Using cable?--- "14/3" or "12/3"

Tubing/Conduit ? --- Black/Red/ White conductors

Also , you can have 2 switches if you chose.
 
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Old 10-21-07, 01:06 PM
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Yes, strider, your calculations are correct.

Most U.S. inspectors do not consider residential lighting as a continuous load.
 
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Old 10-22-07, 06:27 PM
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Thanx for the clarification on the code issue John. The only things I know about the NEC is what I'm learning on here. The CEC (Canadian Electrical Code) is a different story.
 
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Old 10-22-07, 07:14 PM
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Your calcs work this time, but you should not use lamp wattage. You should use ballast wattage for load calcs.
 
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Old 10-23-07, 12:15 PM
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"Wattage" , or "volt-amperes" for Branch-Circuit calculations with ballasts?
 
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Old 10-24-07, 06:21 AM
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Do switched lighting receptacles in a detached garage have to be gfci protected? I assume they do.
 
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Old 10-24-07, 06:40 AM
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The term receptacles in your post is confusing.

Hardwired lights in a detached garage do not need GFCI protection.

Lights plugged into a receptacle would need GFCI protection if the receptacle were readily accessible. For example, a receptacle in the ceiling for a plug in fluorescent light would not need GFCI protection. A receptacle on the wall at normal height that a floor lamp was plugged into would need GFCI protection.
 
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