Nominal Voltage of System Panels

[ejanderson]

Hello All,

I posted a question last week, and after re-reading it, I decided I didn't word it very well. My apologies.

During a solar class, we were posed with a wiring "quiz." We were to configure a 48V system using specific lengths of distance between each component: PV Array to CC and BB, BB to DC Loads, BB to Inverter, and determine which size wire we would need to run between each component accounting for NEC and Vdrop. To start, we were to figure out what configuration our panels would need to be configured in to provide the 48V system requirement; however, unlike previous examples we had discussed, this "problem" gave us NO Vnom of each individual panel, but instead gave the Open Circuit Voltage, Max Power Voltage, Isc, and Max Power Current. No where was there listed the nominal voltage of each panel. The quantity of panels to be used was 8. We just needed to figure out how they were to be wired. Without getting this section right, the rest of the "problem" became pointless in trying to configure.

Needless to say, that particular quiz did not go over well. Because this system was to be a 48V system, the choices for nominal voltage was either 12Vnom, or 24Vnom. I just don't know how, from the information we were given, that I could have determined the nominal voltage for these individual panels???

Any help would be greatly appreciated!

ejanderson

[daz]

...however, unlike previous examples we had discussed, this "problem" gave us NO Vnom of each individual panel, but instead gave the Open Circuit Voltage, Max Power Voltage, Isc, and Max Power Current. No where was there listed the nominal voltage of each panel.

You get a Vmp (voltage at maximum power)...not sure about a nominal voltage though? The normal rating is voltage at maximum power and current at maximum power.
If you are using MPPT controllers, Voc is all you need to know (that and Imp + nominal system voltage)!

The quantity of panels to be used was 8. We just needed to figure out how they were to be wired. Without getting this section right, the rest of the "problem" became pointless in trying to configure.

If you know Voc, Imp, and number of panels, you can calculate wiring for the system. Imp and Voc need to remain within the limits of your controller, which will require you finding the optimum wiring for the 8 solar panels (with regards to series parallel).


So it is pretty easy the assignment that you have to do! It just requires a bit of thinking!

[Sunking]

You are making thing to complicated. You break it down into sub-systems.

First sub systems is the panels to the CC. Now you stated a MPPT Charge controller so you automatically know the panels will be operating at Vmp. So lets say this is a 4000 watt panel system on a 48 volt battery system, using say 20-200 watt panels with a panel Vmp of 27 volts and Imp = 7.4 volts. One way wire distance is 40 feet, 2% Voltage drop, using a MidniteSolar Classic 150 controller.

OK can configure the panels with this controller 5 panels in series with 4 parallel strings. By doing that we know the Vmp = 5 x 27 volts = 135 volts, and Isc = 4 x 7.4 amps = 29.6 amps. So now we are ready to find the cable size to limit voltage drop to 2% or less. So .02 x 135 volts = 2.5 volts. So no wwe now the total loop resistance has to be 2.5 volts / 29.6 amps = .085 Ohms. Now it is just a mattery of going to a set of table like the one provide in NEC Chapter 9 Table 8 and find a conduct where 80 feet of wire = .085 Ohm's or less. To get started we know from NEC 310.17 we much use a minimum of #10 AWG to be safe, but does 80 feet of 10 AWG have .085 Ohm's or less. We see 10 AWG stranded has a resistance of 1.24 Ohms /Kft. so 80 feet = .08 x 1.24 = .0992 Ohms which is too high. What about # 8 AWG with a resistance of .778 Ohm's /Kft? .08 x .778 Ohms = .062 Ohms. Bingo!

If we use # 8 AWG stranded copper with 29.6 amps flowing will develop .062 Ohms x 29.6 amps = 1.83 volts. At 1.83 volts on 135 volts is [1.83 / 135] x 100 = 1.35% voltage drop which is less than the 2% maximum tolerance...

The next sub system is the wire from the Charge Controller to Batteries. The battery nominal voltage is 48 volts. No figure it out using a 1% voltage drop with 5 feet one-way. Tell me what you get.

[Naptown]

They gave you all the info you needed to solve that problem. Other than temperature coefficient (you could just use the NEC version of that) all the panel info was there.

[Sunking]

Other than temperature coefficient (you could just use the NEC version of that) all the panel info was there.

Multiply the spec VOC x 1.25

[Naptown]

Multiply the spec VOC x 1.25

If the system is in Minnesota yes Florida No
To get to the 1.25 multiplier it has to be really cold.

[Sunking]

If the system is in Minnesota yes Florida No
To get to the 1.25 multiplier it has to be really cold.

Very true, but me like the NEC prefer to error on the side of caution. I can sleep at night knowing I have a bit of overkill in a design.

[inetdog]

You get a Vmp (voltage at maximum power)...not sure about a nominal voltage though? The normal rating is voltage at maximum power and current at maximum power.

The term nominal voltage in the case generally refers to the voltage of a panel which will be used with a PWM type charge controller directly to the batteries.
For a 12 volt battery, which requires upward of 14 volts for some charging stages, the rule of thumb is that the Vmp of the panel should be as close as possible to 18 volts.

So in that specific sense, a panel with 36 silicon cells and a Vmp of 18 volts would be called a nominal 12 volt panel. A 72 cell, 36 volt panel would be nominally 24 volts.

In typical practice, PWM panels arrays for 24 volts and higher would be built out of series strings of "12 volt" panels rather than higher voltage panels. For 48 volts, that would definitely happen.