[RE-wrenches] MC4 Y Connectors

Dave Click daveclick at fsec.ucf.edu
Fri Mar 9 11:08:09 PST 2012


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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