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| Another Code Question http://solar-guppy.com/forum/viewtopic.php?f=6&t=723 |
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| Author: | sub3marathonman [ Tue Dec 09, 2008 4:33 pm GMT EthGMT ] |
| Post subject: | Another Code Question |
This is about calculating the maximum current for NEC 690.8 I relied on http://solar.wiseowlnetworks.us/Solar-Math.html for doing the calculations of the maximum "PV Source Current Circuit Current." Now I'm wondering if this is in fact correct. So, for calculating the maximum "PV Source Current Circuit Current" for NEC 690.8 (A) (1) is it strictly Isc *1.25 (125%)? And then the second 1.25 factor (690.8 (B)(1)) is your standard safety factor when sizing conductors. Or is it as stated on that solar math website, that you use NEC Table 690.7 to calculate the maximum "PV Source Current Circuit Current" for the low temperature (up to 1.25 depending on the lowest temperature) and then multiply that by 1.25 (125%) because of "its solar" as stated on that website? But Table 690.7 states it is the "Voltage Correction Factor" not the Current Correction Factor. I'm really asking because if it that website is wrong then my PV circuit breakers may be calculated wrong too. The Isc of the Sharp 224U1F is 8.33A. I then multiplied that by 1.14 (because the record low for Bartow was 18°F and in NEC Table 690.7 that equates to 1.14) to get 9.4962 and then multiplied that by 1.25 to get 11.87025. So I purchased a 12A breaker for each PV circuit. But if it is 8.33 * 1.25 * 1.25 that equals 13.0156, which means you need a 15A breaker for each PV circuit. I don't know how critical this is, or if it could be allowed to "slip through" by the local inspectors. I know they never said anything when they reviewed my detailed plan I submitted for getting the permit. By calculating with the "Maximum Power Current" you get 11.96875, which is still less than the 12A breaker for the PV circuit. Wouldn't you want the breaker to trip any time the current is greater than the "Maximum Power Current" not the "Short Circuit Current" anyway? |
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| Author: | solartek [ Thu Dec 11, 2008 2:00 pm GMT EthGMT ] |
| Post subject: | Re: Another Code Question |
You need the 15A (continuous duty rated) fuse or breaker on each source circuit. The module short circuit current (Isc) is 8.33 A. Don't apply the temperature correction table in 690.7. That is for the open circuit voltage. You need to multiply the Isc by 1.25 to add a safety factor for solar insolation greater than the 1000W/m2 that is used to spec the Isc value. The result is 1.25 x 8.33A = 10.4125 A. This is the short circuit current value to use in ampacity and overcurrent protection calculations. For overcurrent protection as well as ampacity calculations you need to apply the standard 1.25 safety factor. So for overcurrent protection you need 1.25 x 10.4125A = 13.015625 A. Round up to the next typical size which is 15A. This is how I understand the calculation needs to be done to be NEC compliant. Scott. |
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| Author: | sub3marathonman [ Fri Dec 12, 2008 10:00 am GMT EthGMT ] |
| Post subject: | Re: Another Code Question |
I have since done more research on this. And yes, your math is correct, and I would need the 15A breaker, IF the breaker was not rated for continuous duty. However, and just by good luck, not intelligence, I purchased the continuous duty rated 12A breaker from MidNite Solar for their combiner box. When the breaker is rated for continuous duty you don't have to apply the second 1.25 multiplier as stated in 690.8 (B) (1). |
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