to combo box or not?

Getting into the nuts and bolts of wind and solar power components, installation and performance issues, ask technical questions, answers and tips

to combo box or not?

Postby Thewomb » Sun Feb 19, 2017 3:04 pm

Hey there, I'm in the late stages of designing a 4.5KW (18x250) solar array and am unclear on the best way to combine these pre charge controller.

I'll be running a Victron MPPT250/100 with 6 panels per string (223.2VOC) (6.7a) and i need to know the best way to combine these into one feed.

Should I:

1. Have a DC isolator on the end of each string and parallel each of these isolators together on the roof?
2. Bring the three strings down form the roof into the solar room and combine them on a bus bar in a combiner box?
3. something else?

thank you very much in adcance for your help.

;-)

James
Last edited by Thewomb on Sun Feb 19, 2017 3:49 pm, edited 1 time in total.
Thewomb
Solar Supporter
Solar Supporter
 
Posts: 8
Joined: Thu Mar 31, 2016 9:06 pm

Re: to combo box or not?

Postby Gordon-Loomberah » Sun Feb 19, 2017 3:27 pm

>>(16x250) solar array
+
>>with 6 panels per string

If you have 16 panels, it wont work very well if you have 2 strings of 6, and one of 4 panels, they all need to be of the same length, so perhaps you meant 4 strings of 4? In that case, you would need 4 fuses/DCCBs, one for each string, before you combine them, somewhere easily accessible.

What is the max Voc for that charge controller, and what battery voltage?
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5372
Joined: Thu Sep 24, 2009 10:41 pm
Location: Loomberah NSW Australia

Re: to combo box or not?

Postby Thewomb » Sun Feb 19, 2017 3:48 pm

My apologies I meant 18 panels 3X6 strings.

The VOC for the MPPT Charger Controller is 250V

Battery voltage is 24V

RE the DCCB's what ampage would they be? as these would be rated to protect the panels and not the cable correct? the cable is rated at 30a
Thewomb
Solar Supporter
Solar Supporter
 
Posts: 8
Joined: Thu Mar 31, 2016 9:06 pm

Re: to combo box or not?

Postby Gordon-Loomberah » Sun Feb 19, 2017 4:32 pm

What are the panel specs?
Going so high above battery voltage with the panel strings will be less efficient through the MPPT, but will allow smaller PV to CC cables.

Does the 100 bit of the Victron model name refer to 100A? 4500W of PV @ 24V = 187A, far too much for that CC, unless you point the panels in different directions, such as a Virtual Tracker arrangement.
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5372
Joined: Thu Sep 24, 2009 10:41 pm
Location: Loomberah NSW Australia

Re: to combo box or not?

Postby Thewomb » Sun Feb 19, 2017 4:45 pm

Hey Grodon thanks for your help.

Yes thats correct, the 250/100 MPPT reffers to 250V / 100 Amps max output (not input) the output is capped at 100Amps

and yes the 4500W arrays will produce 120.96a as the panels are rated at 37.2 PMAX but I was under the impression that you design they system to so that you get enough power from the panels in the winter months obviously during the summer months it will increase dramatically but the MPPT will dissipate the extra amperage via it's heatsync?

back to the original question. what's the best practice to combine the 3 strings post DCCB's

James
Thewomb
Solar Supporter
Solar Supporter
 
Posts: 8
Joined: Thu Mar 31, 2016 9:06 pm

Re: to combo box or not?

Postby Gordon-Loomberah » Sun Feb 19, 2017 7:58 pm

I'd combine them somewhere easy to get to near the panels in the waterproof box with the CBs, and run a pair of larger cables to the Victron, size depending on distance. Use an isolator before the Victron, and another one after it, before the battery.
How far between the panels and Victron?
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5372
Joined: Thu Sep 24, 2009 10:41 pm
Location: Loomberah NSW Australia

Re: to combo box or not?

Postby Thewomb » Sun Feb 19, 2017 9:02 pm

But what's the best practice for string combination? Bus bars? Y cables? Parallel the output poles from the dccb's

Also what's the logic behind isolating pre and post charge controller? I already have a 125a breaker between the victron and the battery

What I had in my head was to bring each sting down from the roof into the shed, about 15-15m, into the dc mcb's (10amps I'm guessing) as each string will be sending 6 Amps. Then post mcb combine them on a bus bar then send that to the charge controller.

is that a efficient / elegant solution?
Thewomb
Solar Supporter
Solar Supporter
 
Posts: 8
Joined: Thu Mar 31, 2016 9:06 pm

Re: to combo box or not?

Postby Gordon-Loomberah » Mon Feb 20, 2017 9:33 am

You are only going to have 7A max per string, so bus bars are unnecessary between panls and the Victron. Just take the cables from the panels through the string fuses or MCBs and combine to 25mm^2 cables to the isolator before the Victron

Thewomb wrote:Also what's the logic behind isolating pre and post charge controller? I already have a 125a breaker between the victron and the battery


Much quicker to disconnect if you are in a hurry, and I believe it's in the rules.
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5372
Joined: Thu Sep 24, 2009 10:41 pm
Location: Loomberah NSW Australia

Re: to combo box or not?

Postby Thewomb » Mon Feb 20, 2017 9:53 am

Combine in the output of fuses? I was lead to believe this was bad practice?
Thewomb
Solar Supporter
Solar Supporter
 
Posts: 8
Joined: Thu Mar 31, 2016 9:06 pm

Re: to combo box or not?

Postby Gordon-Loomberah » Mon Feb 20, 2017 10:29 am

You did suggest it yourself:
>> Parallel the output poles from the dccb's

What were you told was the problem with combining the strings after the fuses/CBs? It's easier to pul one pair of suitably sized cables, rather than 3, through 15m of conduit.
You could probably use smaller than 25mm^2, say 16mm^2, but 25mm^2 allows for adding more panels later on, and minimises voltage drop.

However, you should leave it the licenced electrician, who will know how to hook it wll up, as it is illegal for you to be working with such a high DC voltage.
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5372
Joined: Thu Sep 24, 2009 10:41 pm
Location: Loomberah NSW Australia


Return to Components, Installation And Performance

Who is online

Users browsing this forum: Google [Bot] and 3 guests

new solar power specials