Wire Size For PV Run from Combiner to FM80

nigtomdaw · October 2010

Hi one for the number crunchers.
I have a Wire Run from a PV Combiner (Midnite 6 Way) to a FM80 CC.
The 2 axis tracker array consists of 5 strings of 3 x 190wp panels each 36.6v vmp and 44.85v VOC with 5.2 Imp 15 panels in total 2850 wp

So I should see a working Vmp around 110v
With a max flow of 26 amps from combiner to FM80

The run is approx 40 meters about 130 ft.

I have a choice of 16mm2 approx AWG5 --- or--- 25mm2 AWG 3 approx for each positive and negative conductor.

The price difference is around 25% more for the 25mm2 AWG3 than the 16mm2 AWG5.

My client thinks Im going a little OTT on the 25mm2 AWG3 recommendation.

Your thoughts please am I a little over the top with AWG3

As always thanks for any assistance

BB. · October 2010

Re: Wire Size For PV Run from Combiner to FM80

The difference in voltage drop is around 3% drop vs less than 1.5% voltage drop... Either is certainly within normal practices...

If I assume 1.5% savings in power, an average of 5 hours of sun per day, the system will generate around 78 kWH per year extra in power with the heavier gauge wire (ignoring other losses).

That will work out to around USD $78 - $156 per year worth of electricity (assuming the system costs around $1-$2 per kWH).

Another different question for me is the 3x 36.6 volt Vmp panels in series... How cold does it get there? You are probably over 150 VDC Voc-cold at -10C (just guessing).

-Bill

niel · October 2010

Re: Wire Size For PV Run from Combiner to FM80

nigel,
another point might be if they ever expand the system that the wires would not accommodate it very well as the extra current would up the voltage drop and the percentage accordingly meaning new wires need put in. it is a tough call, but there is the option of using say the bigger wire on one of the polarities and use the smaller one on the other polarity. it would make the figures look well with the possibility of a bit of possible expansion in the future without needing to change the wires.
ultimately it's up to the customer, but at least i pointed out a 3rd option that may be a workable compromise.

nigtomdaw · October 2010

Re: Wire Size For PV Run from Combiner to FM80

Bill,

The Vmp max will be 109.8v at 25 degrees C
The VOC max will be 134.6v at 25 degrees C

They are in North Easter Spain near to a place called Horta de Sant Joan in the Teruel Region. They are quite high up 1505 ft, Lat 40.9500 Long 0.3000
The only records for low temps Ive checked for the Teruel region are -4.6 degrees C with an average low of 1.2 C in Febuary 2009

With these temps am I likely to get to 145 volts ? VOC

Im now questioning the 3 strings of five I thought I would be OK but I hadnt figured on there altitude
Nigel

nigtomdaw · October 2010

Re: Wire Size For PV Run from Combiner to FM80

Neil I posted b4 reading your reply, He wont be upgrading any further on this system as he is maxed out on the Tracker Sq Ft wise, he could fit one extra panel and that would give sixteen panels 8 strings of two. Its a nominal 48v system with a Outback Flexpower 1 system.

If he upgraded it would not be another tracked array just fixed so would need a separate controller anyway. But I doubt he will upgrade 3KW is very big by Euro Standards for a off grid home.

My advice was buy 1500w of more panels thats what he could have bought if he hadnt bought the tracker, giving him a 50% yield, above 35% yield of tracker, but he wanted a tracker and its his choice. His money

Nigel

BB. · October 2010

Re: Wire Size For PV Run from Combiner to FM80

Could be more accurate with Brand and Model number... I just use the Xantrex calculator (seems accurate).

Here is a link that lists lows for Spain:

http://www.aemet.es/en/elclima/datosclimatologicos/efemerides_extremos

[FONT=Fixedsys]Teruel

Altitude: 900 m  -  Latitude: 40º 21' 06'' N -  Longitude: 01º 07' 22'' O
Validity gap by variables.
Rainfall: 1986-2010  Temperature: 1986-2010  Wind: 1986-2010
Variable
    Annual
Absolute maximum temperature (ºC)     39.0 (03 Jul 1994)
Absolute minimum temperature (ºC)     -19.0 (26 Dec 2001)
Highest average temperature (ºC)     24.9 (Jul 1994)
Highest maximum average temperature (ºC)     35.1 (Jul 1994)
Highest monthly rainfall (l/m2 )     140.9 (Apr 2007)
Lowest average temperature (ºC)     -0.6 (Dec 2001)
Lowest minimum average temperature (ºC)     -8.4 (Dec 2001)
Lowest monthly rainfall (l/m2 )     IP (Jan 2005)
Maximum gust of wind: speed and direction (Km/h)     Vel 122, Dir 350 (14 Jul 2002 13:58)
Maximum number of raining days in the month     22 (May 2008)
Maximum number of snow days in the month     8 (Feb 2005)
Maximum number of storm days in the month     13 (Sep 1986)
Maximum rainfall in a day (l/m2 )     82.4 (02 Jul 1991)[/FONT]

Obviously, I don't know the local region...

3 panels in series get to ~149 volts at -6C for a similarly rated panel.

-Bill

azrc · October 2010

Re: Wire Size For PV Run from Combiner to FM80

No real-world experience here, but according to theAWG chart, 5 should be good to 47A for power transmission.

KeithWHare · October 2010

Re: Wire Size For PV Run from Combiner to FM80

azrc wrote: »

No real-world experience here, but according to theAWG chart, 5 should be good to 47A for power transmission.

You also have to look at voltage drop, which is a function of voltage and distance. If you don't pay careful attention, you can lose a large percentage of your power by using too small a wire.

Keith

Wire Size For PV Run from Combiner to FM80

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