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Question on Installing mechanical components answer key

posted Mar 18, 2012 21:03:58 by 21stCES
On question #5 the answer key says 194 degrees is the correct answer. I thought the answer should be 180-magnetic declination= true south ie. 180-14=176. This example is even used in the james dunlop book page 69 with san fran as the example. Am I reading something wrong or is the answer key wrong?? Confused
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4 replies
Jen said Mar 19, 2012 01:03:41
This is confusing... because east (+) declination is on the west coast and west declination (-) is on the east cost. I think that both texts are right but it is the point of reference that is unclear. The Dunlap text is using north as a reference point and the sample question is using south. I think of this - with the sun on your back add east and subtract west. If you have a compass that has a declination dial on it it's much easier to understand after a few minutes of staring.
[Last edited Mar 19, 2012 01:07:43]
Jen said Mar 19, 2012 01:31:56
I have a question about # 1. The answer key said you could fit 10 in there but, It seems to me that you would only be able to fit 9 modules on this roof because; you have a 24x16' roof that you are making into a 20x10' roof for edge clearances. If you take the module width and divide it into the roof width you can fit no more than three. Same with the length, three is the maximum. three long and three high makes 9 modules. You can fit 10 by just going over the 2' top and bottom clearances, but then why have the clearances?.... Is there something I'm missing?

SolPowerPeople said Mar 21, 2012 16:39:51
Hi Jen,

Regarding question #1:

1. What is the Maximum number of 225W PV modules (65” x 37.5”) can you fit on a roof that is 24’ wide by 16’ from lower edge to ridge at top? Assume 2-foot clearance at top and bottom and 3-feet on left and right sides. (be sure to figure in landscape and portrait to see which is more!)

1st step is to determine the available area for the modules by subtracting 3 feet from each side of the 24' width to get 18', and then subtracting 2' from the top and bottom of the 16' dimension to get 12'.

Then determine how many modules you can get in both orientations of at the 18' dimension:

12 x 18 = 216 / 65 = 3.32 portrait
12 x 18 = 216 / 37.5 5.76 landscape
12 x 12 = 144 / 65 = 2.21 landscape
12 x 12 = 144 / 37.5 = 3.84 portrait

So, the answer of 10 modules in landscape mode is correct.

(one potential "gotcha" on the test may have you factoring in the 1" space between modules for the module clamps, although that would not affect this problem as it is written).
CathyRedson said Mar 21, 2012 17:01:14
Regarding Question #5: The calculation for Magnetic Declination is always:

Compass Bearing - (magnetic declination) = True Azimuth. It is a basic A - B = C equation.

When you are dealing with an existing structure, such as in Question #4, you have A (210) and you have B (14) so,

210 - 14 = 196, which is closer to 180 than Roof A

In Question #5, you have different variables. If True South is 180, and you know your magnetic declination or B, (14) now you are solving for C, a True Azimuth of 180. Now, solve for A:

So, to get the reading I want to see on the compass, it plugs in the other way:

Compass Bearing - 14 = 180
194 - 14 = 180

If I was designing a ground mount system in, say Tampa, with a magenetic declination of (-5) it is still the same formula. Always put the magnetic declination in parenthesis so you don't get confused with the subtract sign and the negative number symbol.

Compass Bearing - (-5) = 180
175 - (-5) = 180.

If you get this sort of question, just look at what you have, and plug it in as either A, B, or C and solve for what is missing. And don't worry about memorizing anything, because the word "decline" is in the name! It is always "Compass Bearing - (magnetic DECLINation) = True Azimuth, it is right there in the word!!!
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