PPD Workshop Question - Lighting Calcs (FC) #2


Regarding question (Part 2, Question #3): I’m arriving at two different FC (foot candle) counts utilizing the formulas given on the “Electrical Formulas” slide page [FC = lumens/ area in ftsq] and [Number of Luminaries = (footcandles x floor area / lumens x (CU) x (LLF)].
19,500 lumens / 10,000 sqft = 1.95

Is the number of luminaries the same as name as number of fixtures? If so, can either formula on the page be utilized to arrive at the same answer?

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@sethmandel8 The terms luminare and fixture are used interchangeably in lighting design. The difference between the two formulas is that the first, FC = lumens/ area in ftsq, will tell you how many foot-candles a single light will produce while the second will tell you how many foot-candles are being produced by multiple fixtures.
The second formula also accounts for a few factors that may result in less light making it into the space. First, CU the coefficient of utilization. This factor is adjusting for how much light from a fixture is directed towards the area being lit. An uplight for example would have a lower coefficient of utilization than a downlight. Second, is the LLF, Light loss factor. This factor accounts for lower levels of light due to age or dirt.