Author Topic: Can I connect two strings with 3 wires  (Read 108 times)

davidwillis

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Can I connect two strings with 3 wires
« on: March 17, 2017, 02:52:23 PM »
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

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Re: Can I connect two strings with 3 wires
« Reply #1 on: March 17, 2017, 03:20:35 PM »
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.
6 tracking arrays, each 6 x 100W modules. Nominal 100V/6A each.
Midnite Classic 150
Outback Flexmax FM80
16 x LiFePO4 300AH cells (primary bank)
24 x AGM 500AH cells (secondary bank)
Selectronics SP-PRO 481 5kW inverter
Home-brew 4-cyl propane powered 14kVa genset
2kW wind turbine

TomW

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Re: Can I connect two strings with 3 wires
« Reply #2 on: March 17, 2017, 04:03:41 PM »
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
Do NOT mistake me for any kind of "expert".

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24 Trina 310 watt modules, SMA SunnyBoy 7.7 KW Grid Tie inverter.

I thought that they were angels, but much to my surprise, We climbed aboard their starship and headed for the skies

Halfcrazy

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Re: Can I connect two strings with 3 wires
« Reply #3 on: March 17, 2017, 04:09:53 PM »
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
Changing the way wind turbines operate one smoke filled box at a time

RossW

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Re: Can I connect two strings with 3 wires
« Reply #4 on: March 17, 2017, 04:46:25 PM »
I can not think of an NEC reason not to do it either.

When did they start needing "reasons" for daft regulations??
6 tracking arrays, each 6 x 100W modules. Nominal 100V/6A each.
Midnite Classic 150
Outback Flexmax FM80
16 x LiFePO4 300AH cells (primary bank)
24 x AGM 500AH cells (secondary bank)
Selectronics SP-PRO 481 5kW inverter
Home-brew 4-cyl propane powered 14kVa genset
2kW wind turbine

davidwillis

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Re: Can I connect two strings with 3 wires
« Reply #5 on: March 18, 2017, 01:00:11 AM »
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

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Re: Can I connect two strings with 3 wires
« Reply #6 on: March 19, 2017, 12:53:20 AM »
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
« Last Edit: March 26, 2017, 06:58:19 PM by CDN-VT »
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c -30c max  4 strings in 2 in Series for 24v Classic 150 -660 Ah  Freezers & fridges ~~~ Second Array same panels of 2sx3 parallel for 24 V Classic 150 -440 Ah  SS300 for lights & overhead door. Building the 36 panel 48 volt system now.

davidwillis

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Re: Can I connect two strings with 3 wires
« Reply #7 on: March 22, 2017, 10:37:04 AM »
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

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Re: Can I connect two strings with 3 wires
« Reply #8 on: March 26, 2017, 06:57:52 PM »
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
« Last Edit: March 26, 2017, 06:59:35 PM by CDN-VT »
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c -30c max  4 strings in 2 in Series for 24v Classic 150 -660 Ah  Freezers & fridges ~~~ Second Array same panels of 2sx3 parallel for 24 V Classic 150 -440 Ah  SS300 for lights & overhead door. Building the 36 panel 48 volt system now.