[RE-wrenches] MC4 Y Connectors

[Scott McCalmont]

The specs Dave gives in reason #1 include the 240 W-ish Sunpower modules. In their training they used to point out that three strings could be combined without fuses. You could use two Y connectors to parallel three strings, but no more combining after that, fuses or no fuses.

Scott

On Mar 9, 2012, at 11:08 AM, Dave Click wrote:

> 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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