Can I connect two strings with 3 wires

[davidwillis]

I have 3 4/0 wires running underground 1000 feet, and I want to setup two strings, one facing west, and one facing east, and will be using two charge controllers (one for each string).  Can I connect all negatives together and run them through one of the wires, then run the positive from one string through the second and the other positive through the third?

[RossW]

I'm sure there's some regulation out there that says "no", and I'm sure there will be pureists will say you mustn't, but one of my early arrays is run exactly that way. It's only a modest array of two x 600W at a nominal 100V, but it's been working perfectly well since I installed it in July 2009, and I've had no problems with any excessive voltage drop, or that drop causing issues with the charge controller.

[TomW]

Ross;

Totally agree with that.

I also  think "two charge controllers" might be the fly in the buttermilk in this situation?

Seems I recall they don't like sharing a negative lead?

Maybe just feeble memory at work but I would check on that with Ryan.

Just 2 circuits sharing a negative is certainly not a problem, with a few provisions like cable capacity on the negative.

I will poke Ryan to see what he thinks on this.

Tom

[Halfcrazy]

Classics all have common negatives so from that side you would be fine with 3 conductors, keep in mind the negative will be carrying 2x the current so as long as the size is adequate you should be good. Honestly I can not think of an NEC reason not to do it either.

Ryan

[RossW]

I can not think of an NEC reason not to do it either.

When did they start needing "reasons" for daft regulations??

[davidwillis]

Thanks all for the replies.

I just found out doing this with the flexmax will cause problems with the mppt (I have no idea why).  So it sounds like the midnite controller is the way to go.

[CDN-VT]

I have 3 4/0 wires

4 gauge  or 4 Ought as in  thumb size wire for a 1000 feet ?
The rest will come

Any IDEA of any voltage ??
or what the 1000 feet is moving ?
PV high or Battery voltage ?






VT

[davidwillis]

This is actually for a friend of mine, and he said it was 4/0, but I still need to verify.

Voltage will be about 90V off the panels, The charge controller will be next to the battery bank after the 1000' run, and is 24v.

[CDN-VT]

Looking at this 4 gauge  or 4 ought  size & seeing a 3 strand run , The brain clicked & thought of Primary house feed :
Here: http://www.homedepot.com/p/Southwire-500-ft-4-0-4-0-2-0-Gray-Stranded-AL-SEU-13097101/202316325

That is two runs of 4/ought Aluminum and the third is 2/ought Al also .
I priced out 4G copper @ 3X 1000' = 3000 feet @ 1.49/Ft = just under 4500.00$  so no need to price out the 4/ought  @ 3K feet.

Here is my 4/0 cable 4 lengths @20' from Bank to DC breaker boards

VT

[bee88man]

Hi, new to forum, solar owner for a year and been doing electrics for quite some time now.
With one cc for east facing, and another cc for west facing...would not even only just a set of 2 wires be able to work?

Ture east / west are never powered at the same time.

[Westbranch]

True east / west are never powered at the same time

Until you get a foggy/cloudy day when they all will be making power.... but luckily the production should remain at less than 50%...

[tecnodave]

bee88man,

If you have two MPPT controllers wired with only set of wires to two array's it will not work!  No way!
When one controller does an MPPT sweep it will cause a fault in the other controller....Nn way around that...

Now then with the Classic and any other controller that has the negative PV input connected directly to the battery negative bus, you can use a 3 wire feed from two arrays connecting the negatives togather. In one on my setups I did just that, 3 conductors ,250kcmil about 375 feet from 2 Classic 150's with 2 arrays at about 2kw each, using a common negative.

Please note that the Midnite Kid does measure current in the negative lead but does it's MPPT switching on the positive lead then in that case 4 wires are needed.

I have very little experience with the flexmax controllers so I don't know where that they measure current so you would have to determine which wire that they measure current on. I'm assuming that they measure on the positive lead as the Classic was designed by the same crew that the flexmax was, but that is an assumption!

David

[bee88man]

Beg to differ based on unique facts of the matter...
My statement above earlier was/is contingent on having 2 seprate arrays...one east facing and another west facing as is exactly the OP's case here.

If there are conflicting issues with mppt sweeps with/between these at this 180 degree back to back arrangment then a time clock or photo switch can relay a heavy contactor to swap the unwanted cc out of circuit...just thinking outside the box here.

[tecnodave]

Bee88man,

You are totally wrong in your assumption that you can run two Classic controllers with only one set of wires from the 1 or 2 or however many array's you have installed.......true only one array will produce significant power when placed back to back as you propose......but what you fail to realize that unless the controllers are synchronized to sweep at exactly the same time then it will not work.....or having only controller do the sweep then down load the parameters to the other controller......Very early versions of the Kid controllers had such a syncing mode called Bully mode but that has been discontinued In later versions of the firmware, I could not get them to sycronize and stopped trying to get my 2 Kid controllers to do that. I have never tried twin mode.

I have tried several times to do controller sycronization and have not been able to do that with any type of controller PWM or MPPT.

I have had several conversations with boB about this and I am convinced that two controllers working on a single pair of wires from the array or array's will not work unless specifically designed to do so. There is not any mechanism that can sycronize the controllers to share sweep data and load sharing.

12 solar controllers total 2 Classis 150's 2 Kids 8 Trace C-40's ........And 50 years in electrical engineering/contracting.....just saying....experience!

[bee88man]

No arguement here about any syncing.

Okay, but if you can get off the "syncing" issues long enough you may just see that what the OP wants and what I describe is so simple and easy.

Heck, can even do both a east and west arrays w/ just the 3 wire and only one cc.if cc only see 1 array at a time. By this i mean relay off/out of circuit/disconnect whichever array is out of direct sun while other array is in direct sun. No second cc is required ever for this arrangement.

51 years doing same as you.