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Section 9: Study Guide Review Questions / Question 8 nominal voltage
« Last post by Victor on April 04, 2014, 04:51:18 PM »
In question 8 on page 151, the question is to find max wire run distance for a PV output circuit from combiner box to inverter for a 2% voltage drop.  The given solution uses 240 V for the nominal voltage, however the DC voltage in that circuit appears to be (14 modules) X (29.5 V Vmp) = 413 V.
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Why does the example use THWN for the rooftop source circuit in conduit?  I thought THWN was more appropriate for circuits in conduit after leaving the roof and USE-2 was more appropriate for source circuits on the roof.  Also, due to high temperatures on rooftop, wouldn't USE-2 be more appropriate?
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Section 2: Verify System Design / Re: Calculating Conductor Ampacity
« Last post by Victor on April 02, 2014, 08:34:15 AM »
I think the reason that 10 AWG is needed is because the 14 A fuse could allow a 14A fault current into this circuit, but the 12 AWG wire selected in Step 4 only has a conditions of use adjusted ampacity of 13.92 A.
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General PV Discussion / Re: Example Question Corrections
« Last post by Arvore on March 23, 2012, 07:42:35 PM »
Here's how I did it:

(6900kwh x .90/365) / 4.8 hours (latitude -15 tilt) x 1.25 = 4.43kw

The latitude -15 comes from the fact that the roof is at 26 degrees which is 15 degrees less than the latitude of 41.53 degrees. The 1.25 factor is the reciprocal of .8. Hope this helps.
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Section 9: Study Guide Review Questions / Re: Question #5 Error?
« Last post by Arvore on March 23, 2012, 07:37:23 PM »
Disregard my previous comment. I must have read their answer incorrectly, because they did use the -15 tilt angle to arrive at the answer, oops.
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I have another issue with Figure 91.  Normally, OSHA shows the tie-off point as at the top (or upper) support point of the ladder, not at a column next to the ladder near the top of the 3' extension above the ladder.  It would be clearer to show this the same way that OSHA does, and not to imply that a ladder is normally tied-off a couple of feet above the top support point of the ladder...  Which would not normally be as safe. Maybe this could work for some situations as in the Figure, but it is not the norm. 
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My understanding is that OSHA defines the working length of the ladder as the length of the ladder from the foot of the ladder to the upper (or top) support:

1926.1053(b)(5)(i) Non-self-supporting ladders shall be used at
an angle such that the horizontal distance from the top support
to the foot of the ladder is approximately one-quarter of the
working length of the ladder (the distance along the ladder
between the foot and the top support).

The way it is usually shown is with the distance between the base of the ladder to the wall as "L/4", and the ladder rails from the feet to the upper support point as "L".
 
Your description above figure 91 is correct in the first sentence of the paragraph.  But, the next sentence beginning with "For example,...) is incorrect because it says that "the base of the ladder should be one-quarter the height, or 4 ft away from the building."  This is not correct.  A correct statement would be "the base of the ladder should be one-quarter the working distance "L", or in this case 4 ft away from the building."  If the "L= 16 ft" is correctly labeled as the real working length of the ladder (along the rails of the ladder up to the top support,) and not the height of the building.
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Section 2: Verify System Design / Section 2.7.1 Determine Circuit Currents
« Last post by Arvore on March 22, 2012, 04:10:35 PM »
Page 64, second paragraph, header reads, "PV Power Source Maximum Circuit Current". Should be, "PV Power Output Maximum Circuit Current".
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Section 4: Installing Electrical Components / Include 690.35 Exception
« Last post by Arvore on March 22, 2012, 04:03:24 PM »
On page 92, first paragraph, the requirement that "one conductor of a 2-wire system...must be grounded if the maximum PV system voltage is over 50V (NEC 690.41)." It might be good to include the exception (690.35) here, as well. In all fairness, this exception is mentioned in the following page; however, it might be beneficial to mention the exception in context with 690.41. I think it would make it less confusing.
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Section 8: Case Study Examples / Re: Voltage Drop Calculation
« Last post by Arvore on March 22, 2012, 03:53:23 PM »
Derrick,

You're not mistaken. They based the nominal voltage on the inverter output. It was done correctly on the second case study, though.
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