My take: if you have no more than two strings feeding your inverter,
this is NEC compliant. If you have a ten-string array and you're
planning to use a 5-fuse combiner, you're probably out of luck.
Reason 1:
For a typical 240W-ish module your Isc is around 8.5A. 8.5 x 1.56 =
13.3A. If you have two strings in parallel you would need to put a 30A
fuse on the paralleled output-- according to the spreadsheet in SolarPro
4.6, the biggest fuse allowed for any cSi module is 20A, so this would
only work with modules that have an Isc < 6.4A.
Reason 2:
If you have a fault in a string, you could have (Isc x 1.25) current
from each parallel string sharing the fuse + (fuse value) 20A from the
fuse flowing into that fault. With just one string per fuse, your fuse
protects your module. With more than one string per fuse, the extra
current contribution from those strings bypasses the fuse protection.
I'm not seeing how this product is useful for cSi systems that have more
than two strings. Maybe for some thin-films with low currents, a small
fuse on their output, and a big max series fuse rating. If a module had
a 30A fuse allowed and an Isc of 3.2A, you could do a 15A fuse on up to
3 strings.
More info is in John Wiles's "PV and the NEC" guide, Appendix J.
On 2012/3/9 10:33, Richard L Ratico wrote:
Can someone please explain why this is a bad practice? My understanding is
the fuse will still prevent backfeeding a fault by more than one additional
string,
the one paired by the Y connector. All the other strings are downstream of
the fuse.
Dick Ratico
--- You wrote:
Wrenchers,
Has anyone used MC4 Y Connectors to combine strings? This website claims
that they are TUV Certified but does not list the manufacturer. Seems
like best practice to fuse each string but wanted to see what others think
about this product and the idea of fusing every two strings instead of
each individual string.
http://www.solarpenny.com/MC4-Y-Connectors-609-4431.htm
Al Frishman
AeonSolar
--- end of quote ---
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