Array voltage drop with pwm controller

jimbuck · November 2019

Hi there, its been a while. I have a question regarding acceptable voltage drop from a pv array to a pwm controller. My understanding is that the pv array will be dragged down to just above Vbatt when charging.

When calculating voltage drop from my array to controller, Do i calculate for worst case voltage eg, low battery Soc 12.x volts or at pannel vmp of 16-17 volts?

Im just upgrading my cable run. Its an 8 meter round trip 4m each way and im currently running 200w through 6mm2 (10awg) cable. Im thinking of going to 25mm2 (4awg) for an upgrade to 400w of solar.

Thanks in advance for any advice.
Jim

Ps, im currently away from my setup otherwise i would take some measurements.

mcgivor · November 2019

Rather than simply answering I'll give information to allow you to answer the question for yourself. Current is inversely proportional to voltage, which means as the voltage rises the current will drop and vice versa. This can be calculated using Ohm's law, V=I×R, the resistance of the conductors are however dependent on temperature, the higher the voltage drop the greater the temperature rise of the conductors which will result in an increase in resistance thus resulting in increased voltage drop.

For basic calculations let's assume conductor temperature is fixed, therefore voltage drop would be in direct proportion to resistance, as the cross sectional area increases the resistance decreases, where would you calculate the greatest potential loss? At the maximum voltage or the maximum current flow?

jonr · November 2019

Solar panels are almost constant current devices. So a reasonable, safe value is the short circuit current (Isc) of the panels. With a PWM controller, don't take the watts, choose a voltage and then try to calculate amps. PWM systems operate at a voltage well below Vmp - so Imp isn't accurate.

BB. · November 2019

Here is a nice IV curve for solar panels... You will see that in Constant Current mode (less than Vmp/Imp point), that the current increase below Vmp is quite small:

BB. said:
I did find a copy of the orginal PDF that I used for discussion, and here are the IV curves:

http://www.solarsourcepower.com/STP175S_24Ab-1BLK.pdf

You can "rescale" this drawings to any crystalline solar panel/cell and be close enough for modeling overall behavior.

-Bill

And, for our calculations, you can use Vmp/Imp as your calculation points for the voltage drop... Although, using Vbatt+1 or 2 volts and Imp is probably more accurate for PWM type charge controllers.

Choosing Vmp/Vbatt=14.75+1 volts/Vbatt=12.0+1 volt/etc. does not change the behaviour of the wire resistance (still X.X volts drop based on Imp/Isc)--Just the percentage (Vdrop/Varray-your-choice * 100%)... The 1% through 3% drop of Varray voltage is just a rough suggestion... 1.0 vs 1.3% drop -- Close enough for solar.

-Bill

jimbuck · November 2019

Hi everyone, Thank You for your answers.

@mcgivor
I would say maximum current flow. This would be at vbatt or just above. Using the lowest voltage i expect to draw my bank down to hence, the highest charging current.

@BB
Im aiming at somewhere between 1 and 2% (cost effective) drop from the pv array to cc. I studdied the graphs, they were both a challenge for me to understand. From the rest of your reply, i figure that i should calculate for maximum isc current at just above the voltage of a depleted battery bank.

@jonr
That makes sense, Isc at vbatt for a pwm system.

I would conclude that for my eventual upgrade to 400watts (4x 100w) of pannels that the Isc of arround 5.1A each (20.3A) total and a Vbatt of 12.3v @ 50% Soc and a cable run of 10m round trip, i would need 35mm2 cable?

Am i on track?

Jim

BB. · November 2019

Hi Jim,

Just to make things clear, there are actually two different wire sizing steps here... The first, is sizing the wire to carry the current safely (wire will not overheat/breakers trip), and the second based on the length of the wire run and current rating.

Making some guesses here:

400 Watt array (4x panels)?
P=V*I, V=P/I, Imp=Pmp/Vmp = 100 Watts / 17.5 volts Vmp = 5.71 Amps Imp
Isc ~ 1.25 * Imp = 1.25 * 5.71 Amps = 7.14 amps Isc
Normally, these size panels are rated 10-15 Amps series protection fuse (or breaker)
You should use a fuse (or breaker) in each + wire from each panel (4 panels in parallel, 4 fuses). This is to prevent a shorted panel from being set on fire by the other 3 parallel connected panels. Your fuse/breaker rating should be around 10-15 amps (check panel documentation).

So, using 1.25 * Imp or Isc current rating, assuming one pair of cables from combiner box (fuse box) at array to charge controller in house/battery shed:

4 * Isc = 4 * 7.14 amps = 28.56 Amps Isc-array

In the USA/North America, that would be 10 AWG cable minimum (note, I like to take the max continuous current * 1.25 to "up rate" the wiring and branch circuit breakers. Helps keep wire/breakers/fuse cooler, and prevents false trips by fuses/breakers if running max Imp current for a few hours (say charging your depleted battery bank).

Now:

Isc-array = 28.56 Amps
Varray ~ 12.0 volts (discharged battery)
4 meter or 13.2 feet (one way run for this voltage drop calculator)
Start with 10 AWG cable

Simple Voltage Drop calculator (sorry, in US units):
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=12&phase=dc&noofconductor=1&distance=13.2&distanceunit=feet&amperes=28.56&x=61&y=13

Result

Voltage drop: 0.75
Voltage drop percentage: 6.28%
Voltage at the end: 11.25

If you want 2% drop, play with the AWG sizing (note dropping 3 AWG units is the same as using wire twice as heavy): Roughly 3x heavier wire (6.28%/2%). Try 10-3 awg - 2 awg or 5 AWG cable:

5 AWG cable:

Voltage drop: 0.24
Voltage drop percentage: 1.97%
Voltage at the end: 11.76

Or your 2% number. In the US, our "smaller" AWG nujbers are available in "even numbes"... So we would have a choice of 6 AWG or 4 AWG--You can figure out which mm^2 number is available in your region of the world:

https://www.rapidtables.com/calc/wire/awg-to-mm.html

AWG #	Diameter (mm)	Diameter (inch)	Area (mm2)
0000 (4/0)	11.6840	0.4600	107.2193
000 (3/0)	10.4049	0.4096	85.0288
00 (2/0)	9.2658	0.3648	67.4309
0 (1/0)	8.2515	0.3249	53.4751
1	7.3481	0.2893	42.4077
2	6.5437	0.2576	33.6308
3	5.8273	0.2294	26.6705
4	5.1894	0.2043	21.1506
5	4.6213	0.1819	16.7732
6	4.1154	0.1620	13.3018
7	3.6649	0.1443	10.5488
8	3.2636	0.1285	8.3656
9	2.9064	0.1144	6.6342
10	2.5882	0.1019	5.2612
11	2.3048	0.0907	4.1723
12	2.0525	0.0808	3.3088
13	1.8278	0.0720	2.6240
14	1.6277	0.0641	2.0809

Going heavier AWG copper cabling is not usually a safety issue--But you may find it difficult to fit thicker cabling into your existing hardware (wire will not fit in wire holes/clamps without cutting strands or making a short adapter cable).

-Bill

jimbuck · November 2019

Hi Bill, Apologies for the delay (traveling home from hollidays).
I understand the two wire sizing steps, current carrying capacity and voltage drop over length. I have plugged a few figures in to the calculator, based on 4 metres pv to cc i have decided to use 5 awg (16mm2). From experience although the calculator gave a 1.5% drop, i never see full potential fgom my array due to location, so i expect real world figures to be 1% or less.

I appreciate the time taken to explain the maths behind working out the maximum Isc of the array and the 1.25 uprating. I have gone over your figures several times and saved a screenshot for future reference. Were well on our way into winter here in the uk so the extra 200w of pannels will be much welcomed. I went pwm due to space restrictions on my caravan roof.

Many Thanks once again to everyone who replied.

Jim

BB. · November 2019

Our pleasure.

Bill

Array voltage drop with pwm controller

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