Need help choosing a solar controller
I have 6, 53 watt Siemens panels I am going to use to keep 4 100amp batteries charged up at my cabin. I am looking for some help picking a 30 amp controller suitable for the job. I looked on ebay ...
Forum Discussion
smkettner wrote:BFL13 wrote:pianotuna wrote:
53 watt panel
PT, those guys are as bad at math as your are! :) The USA NEC says you need a 20% margin but these guys say you should multiply by 1.25 for controller size in amps. That is a 25% margin.
Hint for the "math disadvantaged"--
- 25 expected amps. 20% is 5 amps so you need a 30 amp controller.
1.25 x 25 = 31.25 (6.25 more not the NEC required 5 more.)
NEC says 20% margin on the wire/equip for continuous duty (3+ hours).
30 amp x 20 % = 6 amp derate or 24 amps. Or as they suggest 1.25% of the 24 amps.... wait for it..... 30 amps.
So the 1.25 is same as NEC 20% because they are going in opposite directions.
I understand that going from 25 to 20 is a 20% reduction.
It seems there is the 1.25 x the PV Isc to get you that amps, called the "PV 1.25 " and then you do 1.25 on that to get the "NEC 1.25" for the fuse and wiring--after doing some de-rating on the wiring for temperature. Not too confusing! :(
Scroll to Appendix E here, where he starts by explaining how to apply this all to various parts of the set-up. Also mentions the actual articles in the 1999 NEC.
Farther down is an example set-up using eight of those 53w panels!
http://www.senecass.com/NEC2000.pdf
