Spacing of lighting fixtures - practice exam form 1

I am trying to understand how to solve for the spacing required of the lights. There is a cut sheet that is provided but I do not understand why is it that we should use the CBCP factor and how is it that we get to the spacing of it being 3.35ft apart. can anyone help me understand this? Thank you!

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Hi @luis.torres & welcome to the Community! Let’s me ping one of our expert architects to help!

@coachlizschneider do you mind jumping in?

@luis.torres Typically when spacing lights you are looking for a desired footcandle level. This is true in this case as well. However, when looking at the cutsheet provided for the fixtures footcandles are give as a percentage of the CBCP or Center Beam Candle Power. You’ll notice on page 11 of the reference provided there is a note that states “Footcandles may be calculated by multiplying the CBCP of the desired model number by the percentage in the tables above”.

So first you must find the CBCP for you fixture. In this case your CBCP is 7510 according to the chart on page 12. Your mounting height is 15’ and the chart on page 11 tells you that your beam diameter is 6.7’ at 50% CBCP. So you can divide 7510 by 2 to get 3755 CBCP. The same chart tells you that your footcandles are .4% of your CBCP or 15fc.

Because you are trying to achieve 30fc you will need to space these fixture so that their beam diameters overlap. Placing the fixtures half a diameter apart insures that the beams will overlap, so your separation should be 6.7’/2 = 3.35’