Solar Panel connections and breaker

[rmaddy]

I'd like to confirm a few things about connecting the solar panels together and using a breaker.

Let's say I plan to use four 300W panels wired in 2S2P. Am I correct that I simply wire two in series, the other two in series, and then use a pair of MC4 Y connectors to make the parallel connection, all on the roof? And then I can run a single pair of wire inside to the charge controller? No combiner box is needed, correct?

Between the panels and the charge controller I need a breaker. If these panels have an Isc of 9.78, that means I need 2 x 9.78 x 1.56 = 30.5. I found a 32A DC breaker for PV so that should work.

Can this breaker be used as the isolator/disconnect if I have a need to isolate the panels from the charge controller?

 

[Supervstech]

It depends... what voltage are the panels? Most 300watt panels are high voltage, so it should be fine, but mc4 limits you to #10 wire. BE SURE YOU DONT EXCEED THE AMP RATING FOR TWICE THE LENGTH...

At 60v, you would be pushing 6ish amps... two sets would be only 12ish... so fine on moderate length run #10 conductors.. but mc4 combiners can and do fail spectacularly... an enclosed combiner would be better and safer.

 

[rmaddy]

They are 24V panels with (about 40 Voc). The spec sheet states the MC4 connectors are good for 30A. Two panels in parallel will be a hair under 20A.

 

[Supervstech]

They are 24V panels with (about 40 Voc). The spec sheet states the MC4 connectors are good for 30A. Two panels in parallel will be a hair under 20A.

Oof... that limits the margin a bit.

How long a run? 1200 watts at 48v would be 25amps... I think that would generate a bit too much loss unless the runs were under 10feet...
I’d strongly recommend a combiner box, and up to #8... heck, you might upgrade panels in the future, feed #6 instead. Much safer all around, and give you headroom for expansion without restringing the feed.

 

[rmaddy]

Vmp with the panels in 2S2P would be 65V so the current would only be about 19A. I've been basing this on the Renogy panels. Their own calculator indicates that 12AWG wire is enough though I was planning on 10AWG. The cables only need to run about 7-8 feet from the connectors on the roof to the charge controller.

 

[Supervstech]

Vmp with the panels in 2S2P would be 65V so the current would only be about 19A. I've been basing this on the Renogy panels. Their own calculator indicates that 12AWG wire is enough though I was planning on 10AWG. The cables only need to run about 7-8 feet from the connectors on the roof to the charge controller.

Yup, that short a run should be fine on 10, but I still wouldn’t do it.
I have seen the mp4 combiners melt down with normal loads on them.., not worth it to me, where a cheap outdoor box with some buss bars installed in it work better, and #8 or 6 wire is so cheap...

 

[rmaddy]

I'm not sure what you mean by "cheap outdoor box". Do you have a link to an example for this setup? Thanks.

 

[Supervstech]

 

[rmaddy]

Thanks for the video link. So a combiner box is nothing special. Just a pair of busbars to join all of the wires. I thought there was more to it than that. Simple.

I may make this even simpler and setup my panels as 4S instead of 2S2P. I've been debating the pros and cons of that. Ignoring other factors of that choice, 4S keeps the amps low so a combiner won't be needed but the higher voltage means a much more expensive charge controller. Decisions, decisions...

Any thoughts on the breaker/isolator from my original post?

 

[Supervstech]

Yes, if you went 4s, there would just be the #10 wire. And, at that voltage, if you added 4 more panels, you would still be well below the threshold for #10.

The breaker size shouldn’t be bigger than 1.25 higher than your amp load. If the array puts out 7 amps, an 8 amp fuse will work. If it’s 20amps, then a 25 amp fuse...

What is the 1.56 multiplier in your formula?

Yes, a DC breaker before the controller works fine for a PV disconnect.

 

[rmaddy]

The 1.56 came from http://www.diysolarforum.com/threads/solar-panel-circuit-breakers.2048/post-18114 which states the NEC requirement for PV is to use 1.56 instead of the typical 1.25.

Thanks

 

[Mac6792]

The 1.56 came from http://www.diysolarforum.com/threads/solar-panel-circuit-breakers.2048/post-18114 which states the NEC requirement for PV is to use 1.56 instead of the typical 1.25.

Thanks

The 1.56 accounts for both over irradiance and 3 hrs continuous use over normal expected use. I'd suggest everyone watch Amy's video on wiring your systems to ensure proper sizing of wiring fuses and breakers. The explanation for the 1.56 starts at about 9 minutes in.