voltage drop

System · December 2006

I'm upgrading my offgrid system and moving components out of the house and into a seperate building. This is a multiyear project. I have setup a new array that is about 180 feet from my batteries. When the new building is built, the array will be about 70 feet away from the batteries and the building about 110 feet from the house. What I need to do is run a temporary line along the 180 feet so that I can use the array through winter and then resume the project in the spring.

The new array is 8 suntech 170's on an mx60.

My system without the new array is 1000ah, 24v, SW4024, 600w PV on SB50.

I'm wondering if I should worry too much about voltage drop in the temporary setup? In the end, I plan on wiring the new array at 48v and using OO. Should I consider wiring it at 96v and use smaller cable? maybe even outdoor 10AWG such as used for well pumps? I could resuse some of the OO in the final install but there is no garrantee I would have enough from a 180 length to go panels to batteries and from inverter to house given it would be buried, etc., I already have enough 10AWG due to another project.

crewzer · December 2006

Re: voltage drop

The “24 V” Suntech 170 PV module voltage specs are 35.2 Vmp and 43.8 Voc. You can’t wire four modules in series to make “96 V” because the series string’s Vmp and/or Voc will regularly be >140 V, which would cause the MX60 to suspend operation. Also, Outback says to not exceed 150 V, and, since the MX logs records its highest input voltage, that might cause warranty issues.

The specs for wiring three modules in series to make a “72 V” string would be 105.6 Vmp and 131.4 Voc. NEC table 690.7 indicates that the 106% correction factor for ambient temperatures down to 50 F would keep the system below the 140 V threshold, and that the 113% correction factor for ambient temperatures down to 14 F would keep the system below the 150 V threshold.

Winter temperature records for your area will help assess this option. However, eight modules aren’t easily wired into strings of three…

So, you may be stuck with the “48 V” option. When calculating the voltage drop, don’t forget that a 180 foot run means 360 feet of wire.

HTH,
Jim / crewzer

BB. · December 2006

Re: voltage drop

I guess you are planning on using the new array with the MX60 Controller plus the 8x Suntech 170's?

If so, you will have to watch the maximum voltage for the MX60... Its maximum never to exceed voltage is 150 Vdc--If the maximum cold temperature for your area is 5 degrees F or below, 3x ST 170 panels in series (assuming that is your definition of a 96 Vdc system) would be 150 Vdc or above--violating the warranty for the MX60 (and the MX60 will permanently log the maximum voltage).

The maximum 135 Vdc operational voltage of the array will be reached around 72 degrees F (maximum open circuit voltage of 45 Vdc)... I don't know how the MX-60 operates, but it shuts down if the voltage is above 135 Vdc--and so it might not startup until the panels reach 72 F (like a thunder cloud passing in front of the sun)... It is also possible that the MX60 will start first thing in the morning when there is little direct sun (Vmp=35.2 F), so assuming the same -0.155V/C temperature coefficient, then the controller will shut down at:

Shutdown temp = ((135 Vdcmax/3 - 35.2 Vmp) / -0.155V/C / 1.8F/C) + 77Fstc = -36F (temperature at which panels will exceed controller's operating point of 135 Vdc with three Suntech 170 panels).

--So, the big issue is the 5 degree Fahrenheit maximum cold temperature for the panels at which three Suntech 170's will exceed the safe non-operational voltage of the MX60--Depending on where you live, your record low winter temperature may limit your ability to use three panels in series.

-Bill

PS: What Jim said--he types faster. :-)

System2 · December 2006

Re: voltage drop

Thanks for the help.

Its Vermont. It will get below -5F but probably not hit -36F.

My thought was that the voltage drop over the wire might prevent me from reaching the mx60 limits. I'm just trying to save a few bucks and not have too much cable waste since this is just a temporary wiring.

Looks like as far as the 180 feet go, I'm good with using OO at "48v". I believe 48v strings would put max amps at about 20. So thats 290ft on the MX60 chart for a 1.5% loss.

If I use 10AWG, that looks like ~15% loss. 35.2vmp3=105.6*.85%=89.76v and 43.8Voc*3=131.4*.85%=111.69v which seems like it would fall within the limits...with two panels not connected.

mike95490 · December 2006

Re: voltage drop

BB wrote:

If so, you will have to watch the maximum voltage for the MX60... Its maximum never to exceed voltage is 150 Vdc--If the maximum cold temperature for your area is 5 degrees F or below, 3x ST 170 panels in series (assuming that is your definition of a 96 Vdc system) would be 150 Vdc or above--violating the warranty for the MX60 (and the MX60 will permanently log the maximum voltage).

But - if in winter, and you don't optimize your panels to the sun angle, you may get away with it. You could try a couple of panels in series now, and see what they do in cold weather at the angle you will be using.
What are alternatives if you can't use the MX60 ? buy a few more panels to make another string ? next step up in controller??

BB. · December 2006

Re: voltage drop

No, I would not recommend trying to use 3x suntech 170 panels in series--at least without talking with Outback...

The maximum open circuit voltage is with no current flowing... Since you are charging batteries, there are times when the batteries are going to be about full--or you disconnect via switch/fuse, or you have a cloud (or even bird or kite) that could cause a dropout/reconnect and it will easily reach damaging (or at least warranty violating) over voltage conditions (and would probably be against NEC/Fire Code) in cold weather.

I could not find the charts for SunTech, but solar panels develop current in proportion to amount of solar energy and the output voltage is relatively fixed (assuming that you are operating at Imaxpower or less) regardless of sunlight levels.

http://solar.sharpusa.com/files/sol_dow_208U1F_ss.pdf

For example, from this 208 watt panel, the difference between Voc at 1,000w/M^2 and 600W/M^2 is ~37 volts vs ~34.5 volts... And if you have snow on the ground, reflections from the snow are not insignificant.

My own system which runs around 300 volts at 10 amps, can develop 240+ volts at 0.1-0.2 amps surprisingly late into the evening with no direct sun. It is enough to generate high voltages, but not enough for the 3kW GT inverter to develop any power for the grid.

-Bill

crewzer · December 2006

Re: voltage drop

But - if in winter, and you don't optimize your panels to the sun angle, you may get away with it. You could try a couple of panels in series now, and see what they do in cold weather at the angle you will be using.

Not so, I'm afraid. The "off" angle will reduce output current, but, everything else being equal, there'll be little effect on the module voltage. It's much like in the early morning when the azimuth error is large: the voltage is there, but there's little current.

Here's a link to the Suntech's spec sheet: http://www.mrsolar.com/pdf/suntech/Suntech170.pdf

HTH,
Jim / crewzer

niel · December 2006

Re: voltage drop

this may also help you calculate those losses. it uses excel or a clone excel program.
http://www.wind-sun.com/smf/index.php?topic=1477.0

System2 · December 2006

Re: voltage drop

Thanks.

Looks like its going to have to be OO.

I wired them up in series pairs and at about 3:00PM at 30 degrees F, I was reading VOC at about 117 volts (about 59V per panel), so clearly 3 in series will not work..even with substantial losses in the wire.

Now I'm kinda wondering what VOC will be when it gets down to -10F.

BB. · December 2006

Re: voltage drop

For Suntech STP170S-24/Ac panels, it is -0.155 V/°C or -0.0861 V/°F (ref temp is 77°F--IIRC).

From your numbers, Voc of 117v @ 30°F would be Voc=30°F-(-10°F)*-0.0861 V/°F + 117v = 120.44 volts @ -30°F

-Bill

See below... Forgot to double V/°C because of two panels in series instead of one.

crewzer · December 2006

Re: voltage drop

Bill,

Yikes! He's got two panels in series... need new numbers!

Thx,
Jim / crewzer

BB. · December 2006

Re: voltage drop

Thank Jim--typing without reading closely--not good.

From above:

For Suntech STP170S-24/Ac panels, it is -0.155 V/°C or -0.0.0861 V/°F (per panel ref temp is 77°F--IIRC).

From your numbers, Voc of 117v @ 30°F would be Voc=(-10°F)-(30°F)*2 panels*-0.0861 V/°F + 117v = 123.9 volts @ -30°F (for two panels in series)

-Bill

Also fix the equation so that the "+/-" signs add up correctly for the temperature ranges

And here I am all over my daughter about making her equations work out correctly with signs... Sad example.

Just to make sure that this all makes sense... The

Voc = 2 * (43.8v + -0.0861 V/°F * (-10°F-77°F)) = 102 volts at -10°F (for two panels).

So, I am not quite sure where you are getting 117 volts at 30°F (~59 volts per panel)... If I have the right specs., you should be getting about 47.7 volts at 32°F--not the 59 volts...

Either I have the wrong Voc, the wrong panel part number, or there is a problem with your, temperature, voltmeter or setup somewhere.

-Bill

System2 · December 2006

Re: voltage drop

It sounded odd to me too. I have another meter and another thermometer and will try it again. All the panels measured the same or nearly the same, so I don't think its a case of one being odd. I also measured them individually and each was 58-59v or so. The only thing I could think of doing was reversing Pos and Neg for one or more pairs at the combiner box, but I was really careful with regard to that and I did measure panels individually and got the 59v. I measured each string at the common neg and the input side of the breaker with the breaker off and all measured +117V. I also flipped each breaker on and measured across the main pos and neg lugs and got +117V.

I guess what I'll do is remove each string from the common neg and measure with different tools.

According to my calcs it should be -99F for a VOC of 59v. Something is not right.

Is there anything that could happen to a panel to make VOC actually higher than advertised? I would guess damage or manufacturing problem would cause the opposite.

These are the panels and thats the right VOC, etc.

http://www.affordable-solar.com/SunTech.Power.STP.24V.170W.PV.Module.htm

BB. · December 2006

Re: voltage drop

My first guess is that the voltmeter is not reading correctly... We had an earlier thread where a nice meter was damaged by over current. It seemed to work OK just playing with the meter, but the readings did not make sense if the system was working correctly.

In general, the maximum voltage and current of the solar panels are based on the physics of the materials involved. It is unlikely that they would be so far off. If the manufacturers could build one panel that well, they would try and build the rest consistently at the higher ratings and charge more too. You might see +/- 5-10% changes in power between panels--for new panels--but you should not see 20%+ over nameplate ratings.

-Bill

System2 · December 2006

Re: voltage drop

Yeah, it was a bad meter.

Using a newer and better meter I read 91VOC at 26F. The math says it whould be about 96V, But the temp measurement was shaded air temp and panels were probably somewhat warmer, so 91VOC seems right.

voltage drop

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