Wiring from PV Panels?

[Kerry]

I have:

(4) 250w PV panels with MC4 male & female connectors on each 3' 12 awg wire. (each panel is rated 30.1v, 8.3 amp).
(4) 6v, 232 amp Deep Cycle batts configured for 12v using 2/0 cables.
(1) Flexmax 80
(1) ProWatt SW 1000 Inverter
(1) Midnite Solar e-panel part MNE125ALT
(1) Midnite Solar-MNPV6 Combiner Box with up to (6) 15 amp fuses.
SnapNRack fastening system.

My question:

Can I connect all four neg wires from the panels on the roof, using a (4-1 MC4 splitter), and then use say an 8 awg multi-strand wire from the splitter down to the Combiner Box. And do the same with the four positive wires from the panels. If yes, I wouldn't have to run 8 wires down to the Combiner box (a distance of about 10'). Also I would only have to have (2) 14 amp fuses in the Combiner Box. I'm wanting to keep everything 12v.

The distance between the batts and the Inverter and Charge Controller is about 5'.

The system is for emergency use only: e-bike batt, power tool and cell phone batts, rechargeable AA/AAA batts and, a small laptop for our many power outages. I have no plans to tie into the grid.

Recommendations please.

Thanks,

Kerry

[vtmaps]

Since you already have the combiner, use it.  Mount it near the panels (it's weatherproof) and run the panel cables directly to the combiner.  Do not cut the MC4 connectors off the panels.  Buy some MC4 extension cords and cut them in half.  Connect the panel to the connector on the extension cord, and the cut end of the extension cord goes into the combiner.

You should have 4 circuit breakers (avoid fuses) , one for each panel.  Then run a single cable from the combiner to your charge controller.

Your combiner can hold either four 600 volt fuses, or six 150 volt circuit breakers.  You do not need the high voltage fuses... the circuit breakers are more useful because they can function as switches.  This is useful for diagnosis and servicing of problems in your array. 

Also, an outdoor combiner is a great place to install a lightning arrester.  It's better to do this outdoors near the panels, and run the ground wire (outdoors) to a ground rod.  Keep the lightning outdoors, that's better than letting the lightning into you house.

--vtMaps

[Kerry]

Hi vtmaps,

Thanks so much for the advice and instructions.

I'm 76 and don't envision being agile/confident enough to go up on the roof too many more timesâ€"so having the Combiner Box on the roof is not my ideal. Simply installing the rack and panels, although coming along nicely, is killing my back.

I'm not sure if your suggestion means that my suggestion would not work. Could I connect all four 3' long positive panel cables, with their existing MC4 connectors, to a MC4 4-1 "Y" connector and then run a single 10' long 8 awg cable down to the positive at the Combiner Box to be located in the Control Room? And then, do the same for the four negative panel cables (using another "Y" 4-1 MC4 connector)? I already have MC4 male and female connectors to fasten to the each of proposed 10' long 8 awg cables going from the two "Y"s down to the Control Room just below the panels.

If yes, what size should the two circuit breakers in the Combiner Box be?

The Midnite Solar e-panel MNE125ALT Lite (bare bones) is also en route.

I haven't rec'd the Combiner Box yet so I could return it if there is a mo betta solution.

With aloha,

Kerry

[vtmaps]

I'm not sure if your suggestion means that my suggestion would not work. Could I connect all four 3' long positive panel cables, with their existing MC4 connectors, to a MC4 4-1 "Y" connector and then run a single 10' long 8 awg cable down to the positive at the Combiner Box to be located in the Control Room? And then, do the same for the four negative panel cables (using another "Y" 4-1 MC4 connector)? I already have MC4 male and female connectors to fasten to the each of proposed 10' long 8 awg cables going from the two "Y"s down to the Control Room just below the panels.

What you are suggesting is unsafe.  When you have more than two strings of panels you must (with rare exception) have overcurrent protection (fuse or circuit breaker) on EACH string before the strings are combined.  In your scheme, you have done the combining before the combiner box... the combiner has no function.

You need overcurrent protection on each string because if one string shorts out, the other strings (in parallel) will feed their output into the shorted string.  The shorted string could then cause a roof fire. 

If you are unwilling to put the combiner on the roof, mount it on the side of your house and run cables from each string down to the combiner. 

--vtMaps

[Kerry]

Ok. I understand now.

All four positive and all four negative 8 awg wires (each 10' long) down to the Combiner Box in the Control Room. These 8 wires would be connected to the panel wires via MC4 connectors. Yes?

In the Combiner Box I would fasten the four 8 awg negative wires to the negative bus in the Combiner Box and fasten each of the four positive wires to their own circuit breaker also located in the Combiner Box, yes?

And, the four circuit breakers should be 15 amps each, Yes?

Thanks muchly.

Kerry

[vtmaps]

Ok. I understand now.

I think you have it correct.  Take a look at Midnite's wiring diagrams for their combiner boxes to see a good example of how to do it.

--vtMaps

[Kerry]

Thanks.

Will 8 awg from panels to Combiner Box be overkill?

Kerry 

[vtmaps]

Will 8 awg from panels to Combiner Box be overkill? 

Depends on the distance.  Your panels have Vmp=30 and each cable will be carrying up to 8.3 amps.  You can run 60 ft of #8 cable with an acceptable 2% voltage drop. 

If the distance is less than 60 ft, then yes, #8 cable might be overkill.

--vtMaps

[Kerry]

Thanks vtmaps,

That's good to know; 80' of 8 awg is pricey.

The distance from the panels to the Combiner Box is about 10'.  Perhaps 10 awg?

Am I going to be OK with the four 15 amp circuit breakers in the Combiner Box?

I sure appreciate your support. I see now that this project is a bit over my head, but it sure will be worth the security of being able to charge my e-bike batt for when we have our monthly grid outages, or worse. 

Kerry

[vtmaps]

The distance from the panels to the Combiner Box is about 10'.  Perhaps 10 awg?

Am I going to be OK with the four 15 amp circuit breakers in the Combiner Box?

If the distance is really only 10 ft, you can get by with #14 awg.  You need to use PV wire.  PV wire is rated for outdoor use without conduit.   If you use the MC4 extension cords that I mentioned previously you will be OK.

With an Isc of 8.3, I would use 15 amp breakers.....  unless the panels have a series fuse rating that is different from that.

I sure appreciate your support. I see now that this project is a bit over my head, but it sure will be worth the security of being able to charge my e-bike batt for when we have our monthly grid outages, or worse. 

Unless your grid outages are very frequent or prolonged, I usually recommend a small generator as the most cost effective way to deal with occasional outages.  Have you looked at the Honda or Yamaha inverter generators?

--vtMaps

[Kerry]

Thanks for the tips.

I do plan on getting a small generator for cloudy weeks and for the occasional equalizing.

Again, much appreciation.

Kerry