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High Plains Grifter

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Posts: 3
Reply with quote  #1 
I am building a solar system for my allotment shed. The shed is often in shade, so I am putting the panels about 10m away. I have plugged my numbers into some loss calculators as well as I can, but I'm aware that the rated voltages that I am dealing with in theory are not at all what I am likely to achieve in real life - on a cloudy day, I will be dealing with very low voltages from the panels and I may find the distance has a big impact... or will I? Does the voltage vary much in practice?

I really don't know whether I should be spending lots of money on high quality cabling, or whether I am worrying overly about what is actually a pretty small distance. Here's the details of the setup
  • System Voltage: 12V
  • Panel Peak power: 180W
  • Panel Maximum power voltage: 20.2V
  • Panel Maximum power current: 8.91A
  • Panel Open circuit voltage: 23.9V
  • Panel Short circuit current: 9.47A
  • Cable length 10m
I am in the UK, near Portsmouth and the panel will face directly South (Isolation Calculator)

So... should I be investing my time and money in moving the shed or buying better cables (or fritter it away on the other pleasures of life)?

Rick H Parker

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Reply with quote  #2 
Cannot tell you swat without knowing the specs of the Cabling.
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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
High Plains Grifter

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Posts: 3
Reply with quote  #3 
Thanks for answering, Rick! I guess that’s exactly the problem - according to my understanding I don’t know what voltage will be going through the cables, as it is coming straight from the panel. That means I don’t know what size cable to get and thus I don’t know what my losses are likely to be in the real world.

I’m asking what the specs of the cabling should be.
jjackstone

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Posts: 80
Reply with quote  #4 
You need to spec your cable to the highest voltage and current that will likely be obtained. There are numerous calculators on line.
Here are the results from one of them assuming:
 
20 volts max
9 amps max
3% voltage drop
30 foot run

https://www.southwire.com/support/voltage-drop-calculator.htm

Results

1 conductors per phase utilizing a #10 Copper conductor will limit the voltage drop to 2.84% or less when supplying 9.0 amps for 30 feet on a 20 volt system.
For Engineering Information Only:
30.0 Amps Rated ampacity of selected conductor
1.1417 Ohms Resistance (Ohms per 1000 feet)
0.05 Ohms Reactance (Ohms per 1000 feet)
0.6000000000000001 volts maximum allowable voltage drop at 3%
0.567. Actual voltage drop loss at 2.84% for the circuit
0.9 Power Factor
**Note to User:All ampacity values are taken from the Section of 310-15 of the NEC. The conductor characteristics are taken from Table 9 of the NEC. The calculations used to determine the recommended conductor sizes for branch circuits are based on 60°C ampacity ratings for circuits rated 100 amps or less or marked for use with #14 AWG - #1 AWG. Circuits rated over 100 amps or marked for conductors larger than #1 AWG are determined using 75°C ampacity ratings. Calculations to determine service and feeder conductor sizes are based on overcurrent device ratings rather than actual expected loads which are conservative and may yield oversized conductors. No calculations take into account temperature correction factors or conductor de-rating.
This voltage drop calculator is applicable only to NEC applications. It does not optimize conductor sizes for several different loads at various points in a circuit. The total combined load and length of the circuit must be used. Consult with an engineer if your application requires more complex engineering calculations.
 

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JJ
Rick H Parker

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Reply with quote  #5 
Voltage does not flow, current does.
Voltage is not the issue with loss, current is.
That is why long distance power lines are hundreds of thousands of volts but low current.

You do know the maximum current, you can use the current to calculate loss if you have the cable specs.

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
High Plains Grifter

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Posts: 3
Reply with quote  #6 
All the calculators give much larger losses at lower voltages - is that because the current is assumed to be higher to reflect the power output of the panel? I have mostly been worrying that on a cloudy day the losses will be so high that I won't get any voltage at all at the other end of the cable.

Also, thanks @jjackstone - I did not know to enter the highest values I will achieve; I was sort of entering a range of values and then getting totally lost having such a large range of results that I could no longer choose anything. I think maybe I'm just panicking a bit about the distance.

I'll think maybe the best thing to do would be to bring the solar panel into the slightly shadier spot that is closer and stop worrying about cables - an allotment shed is certainly not worthy of an engineer's visit! If things turn out badly I guess I will have to see about the cabling at that point.
jjackstone

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Posts: 80
Reply with quote  #7 
On cloudy days you are simply going to have less output from the panel because of the clouds, not due to wiring issues. If you place the panel in a more shady spot the same thing will happen. In general if any part of a single panel is shaded it will minimize the output from the entire panel.
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JJ
colinmcc

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Posts: 172
Reply with quote  #8 
Quote:
Originally Posted by colinmcc
At such a low voltage/high current I'd suggest that you bury a length of 2 core 6mil SWA cable, (there is no metric equivalent cable in Britain to the US #10, which is slightly bigger than 4mil and  smaller  than 6mil), so standard 6mil UK cable it is, it will also have lower resistance than #10 and thus less voltage drop too.

A  site that sells SWA in the UK https://swacable.net/swa-cable-2-core-6mm.html

It has been 20 + years since I left the UK for Canada, SWA (Steel Wire Armoured) may no longer be the cable of choice for direct in ground burial of course.

cable.png
myk3y

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Posts: 31
Reply with quote  #9 
Cable is cheap, compared to the cost of the panels.

Use big cable, two of them will halve your losses.

A 100m drum of two-core 6mm SWA cable will cost <£200and in double-run will offer negligible resistance - for 12v/30a over 30m, about 0.2v

On the farm we used polypropylene water piping buried a few inches into the ground to run 240v with negligible losses over 30-50M to outlying sheds. You just stick a spade in and lever it open, push the pipe in and repeat - seemingly endlessly.

https://www.ebay.co.uk/itm/100-Meter-Drums-2-Core-SWA-Cable-6mm-Outdoor-Steel-Wire-Armoured-Cable-6942X/352070581030
colinmcc

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Posts: 172
Reply with quote  #10 
myk3y He only wants to go 10m from one 180w panel. Why would he need 100m? [confused] And have you checked the regs on using water pipe to enclose cable, let alone only a few inches below ground? Also have you buried a yellow streamer reading 'Danger buried cable below" above your cable? 
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