parallel panels via their junction boxes

73 Posts
Aug 21, 2009 07:51 am
parallel panels via their junction boxes

I currently (3) solar panels and have each individual Kyocera KC-40T panel on it's own wire and it's own breaker hooked into a 3 space Midnight Solar combiner box.

I want to increase my array and was wondering if I can parallel 2 panels by running wire between their junction boxes and then send the wire from this string to 1 slot of the combiner box?

I would need to increase the breaker by a factor of 2x? and make sure that the wire going from the parallel string to the combiner box can carry the amperage?

this would mean I could double my 40watt panel array size to (6) panels?

James
 
Aug 23, 2009 07:43 am
Re: parallel panels via their junction boxes

Its an NEC issue.
That is whats known as "double lugging."
A lot depends on the UL rating of the terminals in the PV modules J-box. Even if each screw/clamp terminal is UL listed as being able to have double wires and its UL listed amperage ratings are sufficient, a lot of inspectors still don't like to see it. What does the manufactures spec. sheet or warranty have to say about it? Anything? Diodes?
Think about how, not just the amperage from the one PV module is passing through the screw/clamp terminal but the amperage of two PV modules. Or more if several are "daisy chained" in parallel. There is also the issue of the UL listed amperage rating of the combiner box, the total amperage, not just the individual PV module circuits. 6 - KC40T's in parallel after the 156% would be close to 25 amps.
(On a side note, something that has always irked me about residential, 15 amp, 120 vac duplex wall receptacles, on a 20 amp circuit, is that little flimsy piece of metal jumper between the screw terminals. In Commercial use, the current carrying conductors must be bonded together with a single "pig tail" that hooks under the screw.)
Of course its not just at the PV modules in your case put at the combiner box which most inspectors will treat just like a power distribution or "breaker" panel. There again some, like the Square-D, QO breakers are made with the screw/clamp terminal that can hold two wires. But a lot of inspectors still don't like to see it and they don't like to see power distribution panels used as j-boxes so a "pig tail" may have to be done in a separate j-box first.
http://photovoltaics.sandia.gov/docs/John_Wiles_Code_Corner.htm
This one in particular.
http://photovoltaics.sandia.gov/docs/PDF/Code_Corner_96.pdf
I hope something here helps you.
 
578 Posts
Aug 23, 2009 10:28 am
Re: parallel panels via their junction boxes

in addition, to what was mentioned above about double lugging, here is the math behind why its now allowed.

take the isc of one module.  multiply it by 1.56 then by the number of strings you want in parllel.  if this number is over the series fuse rating, you are not allowed to wire them in parallel in a battery based system without series ocpd on each string. 

in your case, i believe their series fuse size is 6a, so you cant put more than one module in parallel without using an ocpd rated at 6a for each, (hence the combiner box located at the site of the array).

james
altE staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
 
73 Posts
Aug 23, 2009 12:23 pm
Re: parallel panels via their junction boxes

O.K. I got it....need to upgrade my combiner box to add in more panels...

Thanks Thomas & James for the info!







« Last Edit: Aug 23, 2009 12:25 pm by James Johnson »
 
Aug 23, 2009 01:23 pm
Re: parallel panels via their junction boxes

Thank you James. I missed that very important point.
http://www.nmsu.edu/~tdi/pdf-resources/IAEI-5to6-08.pdf
Something a spec. sheet doesn't tell.
http://www.kyocerasolar.com/pdf/specsheets/KC40T.pdf
Indeed, 2 - KC40T's in parallel with the 156% added to their Isc of 2.56 at STC, would add up to 8.268 amps.
Where as I fully understand and respect that these "laws" are in place to keep the likelihood of an electrical fire from erupting to a minimum, I ponder.
If that series fuse rating of 6 amps for the KC40T, is already 1.866 amps higher than the addition of 156% to the Isc of 2.65 amps, which is 4.134 amps, then would it not still be sufficiently protected with only one other KC40T in parallel on a single over current protection device rated at no more than 6 amps? Knowing the extreme set of circumstance involved that would have to happen simultaneously. Alas, probabilities and possibilities.
http://www.judicialview.com/Law-Review/Expert-Witness/Daubert-Probabilities-and-Possibilities-and-the-Ohio-Solution/A-Sensible-Approach-to-Relevance-Under-Rule-702-in-Civil-and-Criminal-Applications/25/3890-3
Probably best that you stick with the NEC Mr. Johnson, what with liability and all being the way it is.
Better safe than sorry, right?
 
73 Posts
Aug 23, 2009 01:54 pm
Re: parallel panels via their junction boxes

I have wondered about the 6amp fuse rating as well...seems to be a bit high given that 2.65 x 1.56 = 4.134

perhaps it's because 5amp DC breakers are hard to come by and 6amps are not?..just thinking out loud

thanks for all the info in this thread guys

 
578 Posts
Aug 23, 2009 02:04 pm
Re: parallel panels via their junction boxes

so to james,

yes, that is one choice.  if you plan on doubling your array, there is another choice.  switch to an mppt controller that can down convert voltages, (flexmax 60, others ...) and then you can leave each series string of up to 4 (maybe 5) modules on one overcurrent protection breaker.  if you took your current array of 3 kyo 40, and wired them in series instead, you would have that one string feed one 6a breaker in your combiner.  your pv disconnect would be 6a, however, the charge controller disconnect would be at minimum 20a, because by down converting voltage 3 to 1 ratio, your current would go up by that same ratio.

if you went this way, you would leave the combiner box, but have the ability to have your current 3 modules, or 4 modules (1 string of 4 in series), or you could double your array as proposed with 2 strings (in parallel each with ocpd in combiner) of three in series.

--

for mr. schmidt, it is important to remember that in a battery based system, it is not only other strings of modules that can be problematic in fault or component failure situations.  the battery bank has the capability of backfeeding, and the ocpd based on max series fuse size, protects the modules, as well as the system and potentially building from all the unpleasant (though less than likely) happenings.  whenever i reply to posts i do my best to highlight the "should" possibility rather than the "could" possibility.  thanks for your posts on this one



james
altE staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
 
578 Posts
Aug 23, 2009 02:12 pm
Re: parallel panels via their junction boxes

regarding the choice of max series fuse size, it has to do with what the internal wiring of the module can handle.  there are numerous examples of modules from back in the day that had 15 or 20a series fuse sizes even though they were 50w. 

off the top of my head, sanyo ba3 and ba19 modules have 15a series fuse sizes even though the isc is below 4a.

so i looked at some old bp 40w modules we had, and sure enough 20a series fuse size. 

these modules could handle higher current so they math sometimes worked that you could cram two strings on one ocpd.  this is very rarely the case anymore, with the sanyo being the only one i can think of; and even the sanyo's dont quite make it for the math with two strings in parallel on one ocpd.

kaneka! they can do it too 7a series fuse, about 1a isc

i need to walk away from the computer . . .; back to class prep for next week.

james
altE staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
« Last Edit: Aug 23, 2009 02:16 pm by James Cormican »
 
Aug 23, 2009 02:53 pm
Re: parallel panels via their junction boxes

http://www.nvenergy.com/renewablesenvironment/renewablegenerations/documents/Tab-2NABCEPStudyGuide.pdf
Page 20 paragraph 3.5.1
But this does not take into consideration the manufactures assessment of its own PV modules to withstand more than the series fuse rating printed on the module label. Engineers are always a bit conservative, at least on paper. If I were to try 2 - KC40T's in parallel on 1 - 6 amp fuse, I would insure that both the pos. and neg. leads from each to a j-box where they are spliced into one set of pos. and neg. were exactly the same length. Of course there is a good possibility that I might be voiding any expressed warranties offered by the manufacture.

More fun with Possibilities and Probabilities.
http://www.markvernon.com/friendshiponline/dotclear/index.php?post/2009/07/31/The-improbable-possibility-of-probabilities
http://hubpages.com/hub/The-Mental-Warrior
http://www.nonesoblind.org/blog/?p=3689
 

Disclaimer and Disclosure

The Alternative Energy Store, Inc reserves the right, within its sole discretion, to refuse or delete any posting or portion thereof, or terminate or block the access to this forum.

The opinions and statements posted on this forum are the opinions and statements of the person posting same, and do not constitute the opinion or act of the Alternative Energy Store, Inc (AltE). The Alternative Energy Store, Inc does not endorse or subscribe to any particular posting. No posting shall be construed as the act or opinion of the Alternative Energy Store, Inc.

Click here for BBB Business Review

McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams
Desktop Website | Mobile Website

Share

Click on an icon to share! If you don't see the method you want, hover over the orange "+".

Feedback

What can we do to help you?

Please enter a summary
Sorry, the copyright must be in the template.
Please notify this forum's administrator that this site is using an ILLEGAL copy of SMF!
Copyright removed!!