Combine cables by panels or pull separate cables ?

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donnib

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Hi, I need some help with cable loss calculation and what the best option would be in my case.

Little background :

  • 21 panels x 310W, Voc 40V, Isc 9,88A (datasheet attached)
  • I have 3x4 array on one side of roof and 3x3 array on the other side.
  • Two MPPT chargers (Victron SmartSolar)
  • I have approximately 15m length from panels (roof) to charger.
Options i have to run cables :

  1. Combine all strings close to the panels then run one set of 10mm2 x 15m cable for the 120V 29,64A array and one set for the 120V 39,52A array.
  2. Run all the cables 7 sets which is 14 cables using 6mm2 x 15m as close as i can get to the MPPT charger (i can't get all the way but few meters away) then combine and run either 6mm2 or 10mm2 from there to the MPPT charger.
Calculating cable loss :​

For the 3x4 array in option 1 using 6mm2 cable i get a loss of 2,91% ~ 138W and for the 3x3 i get 2,18% ~ 72W so in total i loose 216W, does that sound right ? <- using 15m length​


Now few question :

  1. I used above calculator and i am in doubt whether i need to type in 15m or 30m (round trip). What's correct ?
  2. In the calculator, for the 3x4 array i have 12 panels which is 310W x 12 = 3720W but when put the panels in series i get a voltage of 120V and then in parallel i get 39,52A so 120V * 39,52A = 4725W which is not 3720W, why ?
  3. Option 2 has less cable loss per cable because i have lower amperage but is it worth running all the cables like that, i mean if you run all cables and have a little loss per cable when you count that up compared running just one set cable isn't it going to be the same? What would you do in this case ? Go for option 1 or 2 ?
 

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>I used above calculator and i am in doubt whether i need to type in 15m or 30m (round trip). What's correct ?
Thisisn't that significant if you use proper size wire for the amps. You can go with 30m - never hurts to have thicker wire if you can afford it.


>In the calculator, for the 3x4 array i have 12 panels which is 310W x 12 = 3720W but when put the panels in series i get a voltage of 120V and then in parallel i get 39,52A so 120V * 39,52A = 4725W which is not 3720W, why ?
Assuming you mean 39.52a / 4 (in parallel) then that's 9.88a per panel. 310w/9.88a = 31.4v. 31.4v * 3 in series = 94.2v (not 120v). 94.2v * 39.5a = 3720w. I'm guessing you did 4 * 31.4v instead of 3 * 31.4v - and that's why your off.


[size=small][size=small]>[/size][/size][size=small][size=small]Option 2 has less cable loss per cable because i have lower amperage but is it worth running all the cables like that, i mean if you run all cables and have a little loss per cable whenyou count that up compared running just one set cable isn't it going to be the same? What would you do in this case ? Go for option 1 or 2 ?[/size][/size]

My panels are in 2 arrays -3s7p and 3s8p. They are combined near thearray (and doubles as quick disconnect - required in US) and then run thru 6AWG (13.3mm2)wiring between 100 and 150feet - and they get a little warm carrying 50a in conduit (e.g. max rating for this wire size). Size your wire to carry the amps properly (per code) and perhaps 1 size larger if you are worried then, personallyI wouldn't worry about it.

You might be talking a fraction of a percent? ... undetectable. Significant (detectable) losses to think about are ambient heat (10% or more), dust/smoke-dust (5%? clean the panels), snow (50% - clean the panels), and... theinverter!

I'm running 14% loss at my inverters... If you want to focus on something efficiencywise, I'd go for a 96% efficient inverter (such as Magnum claims to be) instead of cheaper AIMS with 88% peak efficiency. You could save as much as 8% in just this one area... e.g. an order of magnitude more significant than properly sized wiring:)
 
OffGridInTheCity said:
>I used above calculator and i am in doubt whether i need to type in 15m or 30m (round trip). What's correct ?
Thisisn't that significant if you use proper size wire for the amps. You can go with 30m - never hurts to have thicker wire if you can afford it.

So the issue here is not that the cable cannot handle the amperage, that's not the issue at all, the whole issue is how to lower the cable loss in the cable, i mean don't go to big and don't go to low so the energy loss it to much, it's never my intention to go as low as it get's dangerous because the cable can't handle it.

>In the calculator, for the 3x4 array i have 12 panels which is 310W x 12 = 3720W but when put the panels in series i get a voltage of 120V and then in parallel i get 39,52A so 120V * 39,52A = 4725W which is not 3720W, why ?
Assuming you mean 39.52a / 4 (in parallel) then that's 9.88a per panel. 310w/9.88a = 31.4v. 31.4v * 3 in series = 94.2v (not 120v). 94.2v * 39.5a = 3720w. I'm guessing you did 4 * 31.4v instead of 3 * 31.4v - and that's why your off.

I have been using 40V since tha't what's stated as Voc in the datasheet so i did 3 * 40 = 120V

[size=small][size=small]>[/size][/size][size=small][size=small]Option 2 has less cable loss per cable because i have lower amperage but is it worth running all the cables like that, i mean if you run all cables and have a little loss per cable whenyou count that up compared running just one set cable isn't it going to be the same? What would you do in this case ? Go for option 1 or 2 ?[/size][/size]

My panels are in 2 arrays -3s7p and 3s8p. They are combined near thearray (and doubles as quick disconnect - required in US) and then run thru 6AWG (13.3mm2)wiring between 100 and 150feet - and they get a little warm carrying 50a in conduit (e.g. max rating for this wire size). Size your wire to carry the amps properly (per code) and perhaps 1 size larger if you are worried then, personallyI wouldn't worry about it.

You might be talking a fraction of a percent? ... undetectable. Significant (detectable) losses to think about are ambient heat (10% or more), dust/smoke-dust (5%? clean the panels), snow (50% - clean the panels), and... theinverter!

I'm running 14% loss at my inverters... If you want to focus on something efficiencywise, I'd go for a 96% efficient inverter (such as Magnum claims to be) instead of cheaper AIMS with 88% peak efficiency. You could save as much as 8% in just this one area... e.g. an order of magnitude more significant than properly sized wiring:)

Sure you are saying that cables loss might be the least of a problem, i will invest in Victron equipment and they are known for good efficiency but if my calculations are right then the cable loss is as stated before 3.5% but only if i use the 15m length that i have and not round trip and i still don't have an answer on this, this should be pretty easy for one who does these calculations to answer, i just don't know the correct answer yet.
Click to expand...
 
Not sure if this helps, but...
1) In most places, solar PVs rarely output 100% power. So while the max cable loss calculation may be 3% on a super sunny day at noon, the average loss will be much much lower.
2) On super sunny days with 100% PV output, my batteries will be nearly full just after noon. Losses of 3% or even 10% aretherefore of no practical consequence. Of course your situation might be different.
So overspending on cables might not be worth that much in practice.


donnib said:
  • 21 panels x 310W, Voc 40V, Isc 9,88A (datasheet attached)
  1. In the calculator, for the 3x4 array i have 12 panels which is 310W x 12 = 3720W but when put the panels in series i get a voltage of 120V and then in parallel i get 39,52A so
Click to expand...

I think you're using the wrong values for the calculations:

Voc = Voltage when Open Circuit. Ie. the maximum voltage possible(*1)when nothing is connected to the panel leads
Isc = Current when Short Circuit. Ie. the maximum current possible(*1) when the panels are short circuited with nothing inbetween
These two values are just used for safety calculations. They cannever occur at the same time.

For power generation, the values you need are:
Vmp = Voltage Max Power, typically around 80% of Voc
Imp = Current Max Power, typically around 90% of Isc
multiplying Vmp * Imp should equal the rated output of 310W

The actual Vmp and Imp will vary depending on a lot of factors, particularly the temperature. A MPPT (Maximum Power Point Tracking) charger will try to figure out those 2 values in real timeby continuously trying out slightly lower and then slightly higher voltages.

(*1) Temperature can affect those values by a few percent - look at "Temp. Coefficient" values in the datasheet.
 
Note the power output you should expect is based on the NOCT parameters not STC ones.
+1 to ajw22's comments, use Vmp & Imp (at NOCT) for typical losses calc's & operating power.
Use Isc at STC for fault current safety (ie fuse/breaker + wire max current rating) calc's.

I'd suggest NOCT power output is about 75% for the STC power ratings used for 310W panel numbers.
NOCT Imp current is approx 80% of STC Isc current.
So approx operating Imp at NOTC for you would be 9.88Ax 0.80 = 7.9A per string.

If it was my system, assuming 120V Voc is allowed in your area, I would do the following:
Each array (ie the 3x4 & 3x3) should feed separate MPPT charge controllers.
Assuming 6mm2 XLPE (not PVC) insulated cables in conduit, I get a rated current of 47A max (safety rating)
I would split the 3x4 array into 3x2 + 3x2 & run each set on its own 6mm cables back to the MPPT indoors.
Similarly, you could split the 3x3 array into 3x2 + 3x1 & run each set on its own 6mm cables back to the other MPPT indoors.

So if you're putting 3 panels in series & 2 of these strings in parallel (ie 3x2), current per cable would be approx
2 strings per cable = NOCT operating amps approx = 15.8A (ie well under safety rating)
6mm cable I used had 4.06mV per Amp per m loss
Your loss length is 15m (one way) x 2 = 30m
So 0.00406V/A x 15.8A x 30m = ~1.92V voltage drop loss.
Ploss = V x A, so 1.92V x 15.8A = ~30.4W with two strings of panels per cable pair.
Total losses would be:
a) 3x2 = 30.4W
b) 3x2 = 30.4W
c) 3x2 = 30.4W
d) 3x1 = 15.2W
So total cable losses = ~106W from a "name plate" array of 21 x 310W = "6.5kW" (STC). Not bad.

Inverter efficiency (good unit vs a cheapie) will eat any cable losses based on above!
 
Redpacket said:
Note the power output you should expect is based on the NOCT parameters not STC ones.
+1 to ajw22's comments, use Vmp & Imp (at NOCT) for typical losses calc's & operating power.
Use Isc at STC for fault current safety (ie fuse/breaker + wire max current rating) calc's.

I'd suggest NOCT power output is about 75% for the STC power ratings used for 310W panel numbers.
NOCT Imp current is approx 80% of STC Isc current.
So approx operating Imp at NOTC for you would be 9.88Ax 0.80 = 7.9A per string.

If it was my system, assuming 120V Voc is allowed in your area, I would do the following:
Each array (ie the 3x4 & 3x3) should feed separate MPPT charge controllers.
Assuming 6mm2 XLPE (not PVC) insulated cables in conduit, I get a rated current of 47A max (safety rating)
I would split the 3x4 array into 3x2 + 3x2 & run each set on its own 6mm cables back to the MPPT indoors.
Similarly, you could split the 3x3 array into 3x2 + 3x1 & run each set on its own 6mm cables back to the other MPPT indoors.

So if you're putting 3 panels in series & 2 of these strings in parallel (ie 3x2), current per cable would be approx
2 strings per cable = NOCT operating amps approx = 15.8A (ie well under safety rating)
6mm cable I used had 4.06mV per Amp per m loss
Your loss length is 15m (one way) x 2 = 30m
So 0.00406V/A x 15.8A x 30m = ~1.92V voltage drop loss.
Ploss = V x A, so 1.92V x 15.8A = ~30.4W with two strings of panels per cable pair.
Total losses would be:
a) 3x2 = 30.4W
b) 3x2 = 30.4W
c) 3x2 = 30.4W
d) 3x1 = 15.2W
So total cable losses = ~106W from a "name plate" array of 21 x 310W = "6.5kW" (STC). Not bad.

Inverter efficiency (good unit vs a cheapie) will eat any cable losses based on above!
Click to expand...

I am sorry i though i had attached the datasheet of the panels, according to the panels they have Vmp 32,8V and Imp 9,46A

So i understand your suggestion like this :

The 3x3 array make it into :

3s2p ~ (3 * 32,8V) * (2 * 9,46A) = 1861W -> combine them at the array then run one set of cable 6mm2 from there
3s1p ~(3*32,8V)* 9,46A= 930W -> as above

The 3x4 array make it into :

[size=medium]3s2p ~(3*32,8V)* (2 *9,46A)= 1861W -> as above[/size]
[size=medium][size=medium][size=medium]3s2p ~[/size][size=medium]([/size][size=medium]3[/size][size=medium]*[/size][size=medium]32,8V)[/size][size=medium]* (2 *[/size][size=medium]9,46A)[/size][size=medium]= 186[/size][size=medium]1W -> as above[/size][/size][/size]

I get the cable loss calculated using the Vmp andImp to following (i used the calculator above):

3s2p ~ 64,5W
3s2p ~ 64,5W
[size=medium]3s2p ~ 64,5W[/size]
[size=medium]3s1p ~ 16W[/size]

[size=medium]So total = 210W[/size]

[size=medium]That's double of your calculation, how come ?[/size]
 
donnib said:
I am sorry i though i had attached the datasheet of the panels, according to the panels they have Vmp 32,8V and Imp 9,46A
Click to expand...
You can't post pdf's to this forum, only images.
Maybe post a link to the file online or a screenshot of the specs...?

Are you sure you are using NOCT specs?

donnib said:
So i understand your suggestion like this :

The 3x3 array make it into :

3s2p ~ (3 * 32,8V) * (2 * 9,46A) = 1861W -> combine them at the array then run one set of cable 6mm2 from there
3s1p ~(3*32,8V)* 9,46A= 930W -> as above

The 3x4 array make it into :

3s2p ~(3*32,8V)* (2 *9,46A)= 1861W -> as above
3s2p ~(3*32,8V)* (2 *9,46A)= 1861W -> as above
Click to expand...

yes, that's it.

donnib said:
I get the cable loss calculated using the Vmp andImp to following (i used the calculator above):
3s2p ~ 64,5W
3s2p ~ 64,5W
3s2p ~ 64,5W
3s1p ~ 16W
So total = 210W
That's double of your calculation, how come ?
Click to expand...

Looking at their formulas & wording on the cals site, they say "distance from source to appliance", not "round trip cable distance", so I think you're doubling the actual cable run.
There's one cable run of 15m each way total 30m for you.
 
There is actually a way to attach pdf's so it's done, have a look at the first post at the bottom of it where the pdf is attached and see if i am using the right values for the 310W panel.

Yes so regarding the length tha't why i was asking so that would explain the double up in energy loss.

UPDATE:

Oh i see it's says STC and NOCT is further down however no values for 310W panel but i found it here :

https://www.enfsolar.com/pv/panel-datasheet/crystalline/41593

And now i see Normal Operating Cell Temperature and i understand why that's more important than STC.

So the values are Vmp 30V and Imp 7,67A
 
I'd "print to pdf" from the site & save those figures for your later reference....
Never mind, saw the pdf you did manage to attach incl the 310W column NOCT numbers :)
 
Se bearing in mind the values NOCT Vmp 30V and Imp 7,67A i can calculate following again :

3s2p ~ (3 * 30V) * (2 * 7,67A) = 1381W
3s1p ~ (3 * 30V) * 7,67A = 690W

The 3x4 array make it into :

3s2p ~ (3 * 30V) * (2 * 7,67A) = 1381W
3s2p ~ (3 * 30V) * (2 * 7,67A) = 1381W

I get the cable loss :

3s2p ~ 21W
3s2p ~ 21W
3s2p ~ 21W
3s1p ~ 5,3W

Total 69W

If i combine all the cables and pull just two sets one for each array the loss is :

3s3p ~ 47,7W
3s4p ~ 84,81W

Total 132,5W so by splitting and pulling 4 sets of cable i gain 50%.

Now i just need to find out whether i want to combine using Y, MC4 connector the strings before running the cable to the MPPT chargeror combine things in a terminal block :-/
 
With the 3s4p array, I wouldn't be happy with the 4x 7.67A = ~30.7A in 6mm2 cable, it's going to run pretty warm.
If you get a cold clear good sun day you can get more than the NOCT current sustained. Good idea to have some margin.

donnib said:
Now i just need to find out whether i want to combine using Y, MC4 connector the strings before running the cable to the MPPT chargeror combine things in a terminal block :-/
Click to expand...

I'd use terminal blocks in good UV rated boxes with proper cable glands. Water gets into everything.
Also MC4 connectors are only rated for max 20A.
https://en.wikipedia.org/wiki/MC4_connector
 
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