Distant array cabling?

[Northern Optimist]

Hi, Bit of a head scratcher if anyone can help. Apolgies if too much info.

North Wales (UK) mountain smallhold(homestead) surrounded by forest. Picked the only spot open from East to West on a southerly aspect with no obstructions, and put my solar array in that field on a trailer sheltered to the North. Only problem is that it is 75m from the house. 25-35m directly toward the house from array is a converted fornerly refridgerated lorry back (no chassis) used as a workshop . In an ideal world the batteries and inverter would sit there as it's already insulated and I'ld like another consumer unit in the workshop. I could put them in the trailer if necessary as I am told it's best to keep dc lengths to a minimum but would prefer not to.

When I've run some online calcs, results are surprising. It seems that a 10mm 2 core swa dc connection to the inverter/batteries at the house gives the best voltage drop percentage at 0.68% based on voltage of 1056v at 20 amps. If I were to put the inverter/batteries in the workshop and split the run 30m DC 1056v 10mm swa & 40m AC 230v 25mm swa I would get 0.64% and 1.54% respectively. AC all the way at 25mm gives 3.09%.

To make this work I would need to put the 22 48v panels in series for the voltage, but the cost of cabling would be massively reduced. The one downside of the direct to house connection would be no AC consumer unit at the workshop.

Have I got any of this right, as it seems contrary to what I had previously thought I understood and I don't want to opt for a saving that ends up biting me in the rear end.

Thanks in advance for constructive comments.

 

[Samsonite801]

What is the max PV input voltage limit on your MPPT controller? A Voc of 1056v sounds pretty high for a consumer unit to handle. Are you sure you can go that high on volts? Many are 250v or 500v max or something like that. You will fry the MPPT controller circuit if you go over max PV volts. You may not be able to string 22 panels in series without frying anything.

 

[Northern Optimist]

OK. Yeah, your right. manual lists under PV.String Input Data: PV I@nput Voltage 370 (100V-500V). MPPT Range 125-425V. 2x MPPT trackers in this unit. (Sunsynk 8k SG01LP1) N

 

[Northern Optimist]

OK. Yeah, your right. Manual lists under PV.String Input Data: PV I@nput Voltage 370 (100V-500V). MPPT Range 125-425V. 2x MPPT trackers in this unit. (Sunsynk 8k SG01LP1) N

Number of strings per MPPT tracker 2+2. PV Input current 22A+22A. Well that all messes things up a little.

 

[Northern Optimist]

OK. Yeah, your right. Manual lists under PV.String Input Data: PV I@nput Voltage 370 (100V-500V). MPPT Range 125-425V. 2x MPPT trackers in this unit. (Sunsynk 8k SG01LP1) N

Number of strings per MPPT tracker 2+2. PV Input current 22A+22A. Well that all messes things up a little.

 

[Samsonite801]

Number of strings per MPPT tracker 2+2. PV Input current 22A+22A. Well that all messes things up a little.

In case to clarify, the 2+2 just means there are 2 physical input connections going to one MPPT controller, and 2 physical input connections going to the other MPPT controller. So the max amps is per MPPT controller as you depicted (22a+22a). Just think of the 2 physical inputs as a DC combiner onto one of the respective MPPT controllers.

Also you should also read this article and use the calculator there to help you factor in for low temperature coefficient compensation, because cold weather temps drive Voc up higher than the amount on the sticker, so you need to factor in some wiggle room so you don't exceed max PV volts at colder temps.

Solar panel maximum voltage calculator - How to work out the max voltage

To get the maximum solar panel voltage you should expect from your solar panel, use our solar panel maximum voltage calculator.

gold-coast-solar-power-solutions.com.au

 

[Northern Optimist]

OK. New calcs based on checked figures using 2 parallel strings at 373V (11x33.96) and 10.75A giving total ampage of 21.5A across the pair. Used max power voltage and max power current from sticker on back of panels.

Option 1 inverter at panels. 25mm swa for full 75m 2.9% drop or 2.1% at 35mm,1.56% at 50mm
Option2 inverter at house 16mm swa DC for full 75m 1.21% drop or 0.76 at 25mm
Option 4 25m 10mm swa to inverter in workshop + 50m 25mm swa to house at 230V 50A 0.66%+1.93% drop
Option5 25m 10mm swa to inverter in workshop + 50m 35mm swa to house at 230V 50A 0.66%+1.43% drop
Option 6 25m 10mm swa to inverter in workshop + 50m 50mm swa to house at 230V 50A 0.66%+1.04% drop

Is this right, or am I missing something. It is still showing lowest overall drops with inverter furthest away from panels?

 

[Northern Optimist]

Will look into cold stuff now thanks

 

[Northern Optimist]

I used vmp not voc for calcs, which is much lower 33.96 vs 41.13. still reading article. thanks

 

[Northern Optimist]

Right using Voc of 41.13 and temp Voc coefficient of 0.272, max power on 11 panel strings is 501.65. Off to redo the cable calcs. Might stop for a glass of wine and a good nights sleep first. Thanks for the steer.

 

[Samsonite801]

Right using Voc of 41.13 and temp Voc coefficient of 0.272, max power on 11 panel strings is 501.65. Off to redo the cable calcs. Might stop for a glass of wine and a good nights sleep first. Thanks for the steer.

So on that calculator you did set the - x° C to the lowest your panels will ever see I assume?

I might feel more comfortable running 10 panels if it were my system (of course with voltage drop it should be under 500v on the long PV run length calculation adjusted for cold temp)...

 

[Northern Optimist]

W

So on that calculator you did set the - x° C to the lowest your panels will ever see I assume?

I might feel more comfortable running 10 panels if it were my system (of course with voltage drop it should be under 500v on the long PV run length calculation adjusted for cold temp)...

Yes .
Worth considering, but I over egged cold temp as below -11C is extremely unlikely and I used -15C. Alsos the coldest part of the day is well before the panels are getting direct sun and operating at their highest output, it's unlikely to come close to the limit. If all else fails, there is a 500 volt fuse on each string, so I think I'm comfortable with the 11 panels per string the supplier spec'd for their pack.

Adjusted calcs:

Option 1 inverter at panels. 25mm swa for full 75m 2.9% drop or 2.1% at 35mm,1.56% at 50mm
Option2 inverter at house 16mm swa DC for full 75m .9% drop or 0.57 at 25mm
Option3 inverter at house 10mm swa DC for full 75m 1.44% drop
Option 4 25m 10mm swa to inverter in workshop + 50m 25mm swa to house at 230V 50A 0.48%+1.93% drop
Option5 25m 10mm swa to inverter in workshop + 50m 35mm swa to house at 230V 50A 0.48%+1.43% drop
Option 6 25m 10mm swa to inverter in workshop + 50m 50mm swa to house at 230V 50A 0.48%+1.04% drop

Haven't opened box yet, but I assume from what's been said that MPPT ports are basically bus bars with 2 possible connections each and so I could connect strings parallel earlier and have a single long connection to MPPT for the two strings with fusing and kill switches just vefore the join.

Still a bit unsettled that maths puts long DC connection as the best option for cable cost and voltage drop. Anyone any thoughts? Firstly if this is right and secondly, how important are the drop savings once I've got below the 3%

 

[Samsonite801]

W

Yes .
Worth considering, but I over egged cold temp as below -11C is extremely unlikely and I used -15C. Alsos the coldest part of the day is well before the panels are getting direct sun and operating at their highest output, it's unlikely to come close to the limit. If all else fails, there is a 500 volt fuse on each string, so I think I'm comfortable with the 11 panels per string the supplier spec'd for their pack.

You should be fine on a panel count of 11 then, since you over-spec'd on the coldest temp number.

Adjusted calcs:

Option 1 inverter at panels. 25mm swa for full 75m 2.9% drop or 2.1% at 35mm,1.56% at 50mm
Option2 inverter at house 16mm swa DC for full 75m .9% drop or 0.57 at 25mm
Option3 inverter at house 10mm swa DC for full 75m 1.44% drop
Option 4 25m 10mm swa to inverter in workshop + 50m 25mm swa to house at 230V 50A 0.48%+1.93% drop
Option5 25m 10mm swa to inverter in workshop + 50m 35mm swa to house at 230V 50A 0.48%+1.43% drop
Option 6 25m 10mm swa to inverter in workshop + 50m 50mm swa to house at 230V 50A 0.48%+1.04% drop

Have you read any other sites on it yet like this one?

The Basics of Calculating Voltage Drop

How do you know if your wiring will provide a reasonable efficiency of operation?

www.ecmweb.com

Like in my case, I always take what I think I need and go a gauge larger just to make sure, but that's just me.

As far as placement, I'd probably just put the inverter in the workshop, and run the AC line to the house. Then neither length is excessively long.

Haven't opened box yet, but I assume from what's been said that MPPT ports are basically bus bars with 2 possible connections each and so I could connect strings parallel earlier and have a single long connection to MPPT for the two strings with fusing and kill switches just vefore the join.

Yeah, the pairs are bridged, so you could run single wire as long as the connector on the MPPT input can easily handle the 22a max (more specifically your Isc max)

Still a bit unsettled that maths puts long DC connection as the best option for cable cost and voltage drop. Anyone any thoughts? Firstly if this is right and secondly, how important are the drop savings once I've got below the 3%

DC isn't always the best option. Our grids always use AC for long transmissions, because with AC they can use transformers to go up/down voltage easily. But in your case, the PV volts can be maybe double than the AC volts a lot of the time, so it gets some advantage here for longer run (higher volts means lower amps). And also to note that PV volts can fluctuate based on how the MPPT controller is loading the circuit (could run at half of Voc or lower, depending on where maximum power point watts is found).

If you wanted to get really geeky, you could buy a set of used transformers (in the US we can typically find these on eBay for good price) and then can up the AC volts to 460v at inverter, make the long run to house using a smaller cable, and go back down to 230v using the other transformer at the house...

You probably don't need that here, it seems like 1.05% voltage drop isn't an alarming number... I'm not an expert journeyman electrician or anything, but it seems like in the industry they shoot for something like 3% but not totally sure, they have numbers for branch circuit, and including feeder connectors, etc...

Voltage Drop: Picking the Right Cable for the Long Run | IEWC.com

Reliability may not be a tangible item that’s installed alongside a new furnace or wired into a dock-side crane, but it is nonetheless an essential “accessory” that can mean the difference between overtime and lost time; in-stock and out-of-stock; perfect fits and refits.

www.iewc.com

The argument for what is acceptable loss, depends on what you're willing to tolerate, how much energy you harvest, store, and consume, if you're more overpaneled and have lots of storage, then a bit of loss may not be noticed. But on systems without any reserve margin, maybe every watts counts.

 

[Northern Optimist]

OK. Thanks, helped by all your input and a butt load of reserch,I've come up with the attached solution. The current requirements on the mppt port connectors was a little low at 18A so had to combine and resplit the cable to keep amps down to 10.75 per connector while using one thicker wire to cover the distance. The drawing shows a modification of the existing set up where there is a switch to an outside generator socket which disconnects the mains. The 25mm cable replaces gen connection which now goes into inverter. Before you get too hard on what's planned, please bare in mind the success of an excersize is measured against it's objectives 1) Want to be disconnected completely from the grid. 2) Would like the safety net of switching back on in the first year if everything goes belly up. I would really welcome anything constructive you'ld like to add. Many thanks

 

[Quattrohead]

Looks good but you might be light on the battery, easy enough to add more later though.

 

[Northern Optimist]

Thanks. Yes we had suspected that might be an issue, but as there is a second MPPT port on the inverter (2+2) and the site lends itself to pico hydro power which will run 24/7 when its wet and gloomy, we thought that between 300w and 1KW that way would be enough to keep us going. The interesting thing is going to be when we have caravan guests and I'm using power tools, but it's a place to start. We hope to be power rich mid summer from the solar and dump it to heat water which would be taken over by the wood burner when it get colder and darker and we have less power. We also intend to adjust the times when we use greedy electrics to middle of the day where possible so we are drawing lesss from the batteries.