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    Lighting Loads Continuous?

    I can't find anywhere in the NEC that says a lighting load, even a commercial lighting load, shall be considered continuous. However, I've gotten practice test questions for the Journeyman exam wrong because I didn't apply the additional 25% to my calcs. I've searched online for answers to this but people say it's "usually considered" etc etc, but nothing siting the CODE. So if I'm asked on the real exam to calculate the ampacity of a a circuit supplying a commercial lighting load (or dwelling for that matter) how should I calculate this?

    #2
    For a commercial application I would add the additional 25% for continuous loads.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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      #3
      Article 210 talks about branch circuits. It shows us where the outlets are required, what maximum loads should be etc. The way to calculate the load is to use article 220.

      Take a 40 circuit 200 amp panel filled with 20 amp breakers. You don't add up the breakers (800 amps) but rather you calculate the load based on article 220 to determine the service size. It's the same idea...
      They say I shot a man named Gray and took his wife to Italy
      She inherited a million bucks and when she died it came to me
      I can't help it if I'm lucky

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        #4
        How many commercial or industrial buildings have you been in that didn't have the lights on for 3 hours or more. If the lights are on for 3 hours or more than they are continuous--- It's a well-known assumption and all test will be that way
        They say I shot a man named Gray and took his wife to Italy
        She inherited a million bucks and when she died it came to me
        I can't help it if I'm lucky

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          #5
          Originally posted by Dennis Alwon View Post
          How many commercial or industrial buildings have you been in that didn't have the lights on for 3 hours or more. If the lights are on for 3 hours or more than they are continuous--- It's a well-known assumption and all test will be that way
          I know in the practical sense it's a continuous load, it's just that the code is very specific about so many other loads being "continuous" you would think they would officially documented that a commercial load shall be considered continuous. Just wanted to know how to answer on the test. Tests can be VERY specific. Thanks.

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            #6
            Originally posted by hitehm View Post
            I know in the practical sense it's a continuous load, it's just that the code is very specific about so many other loads being "continuous" you would think they would officially documented that a commercial load shall be considered continuous. Just wanted to know how to answer on the test. Tests can be VERY specific. Thanks.
            When the code says "shall be treated like a continuous load" it's talking about things like things like electric water heaters and space heaters which, interestingly enough, don't meet the definition criteria.
            If Billy Idol is on your playlist go reevaluate your life.

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              #7
              Let's take this discussion a step further. I agree with adding 25% to the lighting load, if you calculated that load by adding up the wattages of the light fixtures that are included in the design. But what if you simply assign a number of watts per square foot for "general lighting loads" in accordance with Table 220.12? I have seen some calculations that add the 25% to that figure. I think it is not needed. Any comments?
              Charles E. Beck, P.E., Seattle
              Comments based on 2017 NEC unless otherwise noted.

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                #8
                Charlie, why do you think it is not needed? If they are on for 3 hours or more IMO you need the 125% in the calculation
                They say I shot a man named Gray and took his wife to Italy
                She inherited a million bucks and when she died it came to me
                I can't help it if I'm lucky

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                  #9
                  Originally posted by Dennis Alwon View Post
                  Charlie, why do you think it is not needed?
                  Because I believe the influence of the continuous loading was included in the tabulated values. For example, I believe that a dwelling unit's 3 w/sf was really based on a load of 2.4 w/sf, and became a total of 3 w/sf when they added 25% to the 2.4 value. Another fact that supports my viewpoint is that, for dwelling units, we get to include certain receptacle loading within the 3 w/sf calculation. The lights could be on for 3 hours, but the vacuum cleaner that is plugged into the receptacle outlet is not. Finally, the notion of "continuous loads" includes the requirement that the load be running at its peak value for 3 or more hours. Lighting loads calculated on a w/sf basis will include the entire building, and I doubt that all the lights will be on in all the rooms all day long.

                  Charles E. Beck, P.E., Seattle
                  Comments based on 2017 NEC unless otherwise noted.

                  Comment


                    #10
                    Originally posted by Dennis Alwon View Post
                    Charlie, why do you think it is not needed? If they are on for 3 hours or more IMO you need the 125% in the calculation
                    I'm with Charlie on this one. How could lights that aren't actually installed be on for 3 hours or more?

                    For instance, if I had a 10,000sf office building with an actual connected lighting load of 8500 watts and a minimum lighting load of 35,000 watts per 210.12, I would include the 35,000 watts in my service conductor sizing,
                    not 43,750. The service conductors will only see a lighting load of 8500 watts, and factoring those at 125% for continuous loading (10,625 watts) would be more than adequately covered by the 35,000 watts factored into the service conductors.

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