What size wires should I have connected to my Solar System?

Questions
#1

Hi All, could you please help me work out whether I have the right sized wiring connected to my solar panels. I am trying to work out my voltage drop. Thanks in advance.

It is a DC coupled system. My set up is 20 x 410w panels facing a tiny fraction East but mainly North so I will say north for this exercise. I also have 8 x 410w panels facing East but a tiny fraction South but will say East for this exercise. They are all attached to a Fronius Symo Gen 24 10kW Plus Inverter. It has 2 connections to MPPT1 25A total and MPPT2 has 1 connection with 12.5A total.

The 20 panels have been put in series then were paralleled:

(10 x 29.9V = 299V) and (2x 13.73A = 27.52A) (299V x 27.52A = 8.228kW)

They are connected to MPPT1 which is 25A so really 7.475kW (we have received 7.09kW at most)

The 8 panels have been put in series:

(8 x 29.9V = 239.2V) and (1 x 13.73A) (239.2V x 13.73A = 3.281kW)

They are connected to MPPT2 which is 12.5A so really 2.99kW (we have received 2.55kW at most)

We also have a BYD HVM 11kW Battery attached. The length of the wiring is 40 meters one way to the end of the panels. I am in a 2 story house. The 20 panels are on the top of the second story but the 8 are on top of a roof of the bottom part of the house.

I know you all could probably just work it out easy and tell me the answer but I would like to work it out by asking you some questions so I know what numbers to put in and work it out for myself. I will let you know what I think and you can all let me know if I am wrong or not.

  1. Do I use the Amps of the MPPT’s or the Amps of the Panels in my calculations?
  2. Do I add up the volts of both sets of panels together?
  3. How does the battery make a difference?
  4. Am I right in my understanding that the MPPT has to match up the voltage coming from the Panels to the Battery’s voltage - higher voltage gets drown to the lower voltage sort of thing… ?
  5. Am I right in thinking that the wires connecting the panels in series have to be the same size as the ones in parallel for the 20 panels? Not for the 8 panels right?
  6. To calculate the Resistance is it: a. R = 538.2V/ 37.5A (using MPPT’s Amps) or b. R = 538.2V/41.25A (using Panel’s Amps)? I have assumed the arrays get added; I hope I am correct!
#2

The battery is irrelevant, you want to know that the cable between the panels and MPPT is of sufficient size.

You need the voltage and amps for the panels on that string, not what the MPPT is rated at.

If I understand correctly your 20 panel array is arranged in 2 x10 panels in parallel. So the following is correct presuming your using the correct voltages and current.

When parallel the voltage stays the same and the current adds up.
When in series the current stays the same and the voltage adds up.

You’ve not mentioned the cable size used, or where you’ve connected the parallel arrays, presumably on the roof.

https://midsummerenergy.co.uk/voltage-drop-calculator

The above calculator uses a traffic light system to tell you if the result is acceptable.

When I did my roof mounted system, I had to run each string down to the garage on its own pair, and then parallel them in the garage, this meant 6mm cable was sufficient.

You need to check your using the correct voltages, and current though, 29.9v seems a little low for a 410w panel.

Hope that helps.

#3

Thank you for your reply ron-ski. All 28 panels are Sunpower SPR-P6-410-BLK.
I should have also said my system in on grid.
For the 20 panels for the series-parallel math I did this - 10 panels (10 x 29.9v = 299v) were added together in series and the other 10 were added together in series so another (10 x 29.9v = 299v) then both were added together in parallel so both being 299v in series x (13.73A x 2 = 27.46 A) = 8.2kW - I said 27.52 A earlier my apologies, not that is makes much difference lol. Good to know I used the correct math. :smiley:
The cable the installers used is 4mm2. I at first got the 25 amps ok on a clear sunny day, when they were put in the right MPPT, but not now.
The 20 parallel arrays are on the very top roof of my 2 story house but where the parallel cables are connected to the 20 panels I don’t know. I assume you are talking about where the negative on one end and the positive on the other end of the series strings are connected?
Are the following calculations correct; did I use the right numbers?:

Resistance = 299V/27.46A = 10.88r for the 20 Panels

Resistance = 239.2V/13.73A = 17.42r for the 8 Panels

Voltage Drop = 2 x 40L x 27.46A x 10.88r/1,000 = 23.9 V.D. for the 20 Panels

Voltage Drop = 2 x 40L x 13.73A x 17.42r/1,000 = 19.13 V.D. For the 8 Panels

I calculated the resistance using Ohm’s Law (Resistance = Voltage / Current)

Voltage Drop = 2 x L x I x R / 1,000, where L is the cable length in meters, I is the current, and R is the resistance per kilometer.

I calculated them separately because even though the wires go into the same Inverter, they go to different MPPTs or should I have added them together like this?:
Voltage Drop = 2 x 40L x 41.19A x 28.3r/1,000 = 93.25 V.D. because the MPPT1 and 2 are both connected to the same charge controller I assume in the inverter maybe!

The main reason I am asking about how to figure this out is that I really don’t want a fire and I have a strong feeling 6mm2 should have been used on our system when the remark from the installer was, “we use 4mm wire on all our big systems”. Shocked I asked, “4mm2 wire” and the answer was yes. I have already had one wire issue which the cause was not clear to neither my husband or myself nor were we ever shown that wire.

With the calculator I was given a link too (thank you btw :D) showed my voltage drop for the 8 panels was 1.95% and 20 panels was 3.12%. Our AU Standard say 3% at most. Now I reallllllllllyyy need to know whether these percentages need to be added or not!

This voltage drop calculator AC and DC Voltage Drop Calculator AS/NZS 3008 | jCalc.net showed my voltage drop for the 8 panels was 2.58% and 20 panels was 4.12%. I am a bit confused now! Does the calculation differ between countries?

#4

Each MPPT is a different circuit, so don’t add them together, they are individual circuits.

You don’t need to calculate the resistance, the calcutor I linked to does it all for you, the two calculators may be using slightly different wire resistance to come up with the results, hence the variation.

When I talk about the parallel connection of the two strings, it’s where the two strings are joined into one to make them parallel. If you only have a total of two pairs of cables coming down, one pair for each MPPT then they’ve joined them on the roof, which is the most likely scenario.

Most Mc4 connectors are rated at 30 amp, so the parallel connection point could be running close to maximum, although it will be rare it gets this high.

I think it’s clear they really should have used 6mm, especially for the 20 panel array which exceeds 3%, the installer really shouldn’t be cutting corners like this, they should know better.

How sure are you that 40 meters is the correct one way cable length, it can be very hard to measure? Mine came down from a two story house roof (ground floor and first floor), along the rear of the house to a detached garage, and I don’t think it was that long, I’m not at home so can’t check my spreadsheet at the moment.

If you want better qualified/more experienced advice I’d suggest posting on the DIY solar forum, you’ll get far more replies than here.