# Coefficient of Utilization vs Finish Floor Height

Does the coefficient of utilization matter based on the mounting height of the fixture?

I received a practice question, where it asked about the # of fixtures needed, and they gave me all the values, except the surface I was illuminating was 3 feet AFF. And the fixtures were mounted at 10 AFF and had a CU of 60% at that height. The question explicitly mentioned that the CU was at that height. My guess is that since it has to do with the output of how much light is coming out of the fixture, the height won’t make the CU vary. Is this assumption correct?

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Hi @Shikha ,

Was this a Black Spectacles question? If so, do you know what form? A little more context might help me give you a more accurate answer!

The coefficient of utilization is a measure of the efficiency of a luminaria to light a portion of the work plane, so mounting height of the fixture would definitely factor into the question.

I just visited wikipedia to give myself a quick refresher on coefficients of utlization.

Thanks,
Darion

It was actually in a third party material (Gang Chen’s PPD Mock Exam) question 56. Without writing the full question the key parts that mention CU are “The tenant wants 50 fc at desk level 3’-0” AFF…The CU is 60% when fixtures are installed at 10’-0" AFF…how many fixtures does the tenant need?"

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Hi @Shikha ,

Ah ok… This is a bit outside my area of expertise. Maybe some of our PPD or PDD coaches can help? @coachadamcoers @coachmarkbailey @coachbryanhoward @coachrebeccasmith any assistance would be appreciated!

Thanks,
Darion

Hi @Shikha !

This is a tricky one. There is a lot of information that goes into calculating the CU so it is difficult to believe that the question would be asking you to adjust the value based on heights. The CU is usually determined based on computer simulations and is variable based on surface area, distance from fixture to surface, and other inputs.

Was there a formula provided to you as part of the question? Often times questions like these are really looking for you to find the correct formula that you need to find the answer, it could be possible that the CU information was put in there to throw you off.

I suggest looking at the units you need for the answer, in this case # of fixtures, find the formula(s) that will provide that unit as an answer and determine if you have the inputs you need to run that formula.

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Thanks @coachbryanhoward ! Great explanation.