# Adding panels to my setup

rake1
Solar Expert Posts:

**170**✭✭
I did a search but am not understanding completely. I have two panels in Parallel 125w vmp 17.8 voc 21.9 and a 135 w vmp 17.9 voc 22.38 I want to add another panel but want to make sure it will be compatible to what I have what vmp and voc range do i have to stay within so this will work?? I assume the wattage doesn't matter as long as my VMP and VOC are close. This is a 12 volt system with a 40 amp PWM controller.

Tagged:

## Comments

5,226✭✭✭✭170✭✭what happens if I go with say a 190 watt with a vmp of 18.8 and does the Voc matter it is 22.56 on the 190 watt panel. one other question

I was also told that if I add a 190 watt to my 135 and 125 watt that the 190w will only produce power up to the lowest panel i have. ex 190 will only produce 125 watts is that correct.

5,226✭✭✭✭170✭✭5,226✭✭✭✭Though we call them 'cells' each cell is a battery. Each lead acid cell is about 2.1 volts when fully charged. In series and fully charged a '12 volt battery' is actually @12.6 volts.

To charge a '12 volt' battery you must present voltage higher than the battery's current voltage by 15-20%. To reach a fully charged state with a good transfer of current. Charge controllers usually hold the system voltage to about 13.5-14.5 volts. When designing solar panels, 36 cells was chosen as a good balance. Each solar cell produces about 1/2 volt a string of 36 was chosen so that voltage losses during transmission in the wire and across the charge controller would allow to maintain high enough voltage to charge and equalize (an intentional over charging) a flooded lead acid battery bank.

There are more expensive Multi Power Point Tracking (MPPT) That convert most of the energy coming in to usable charging energy. But if you have the correct size panels, you really only lose a very minimal amount of potential, perhaps 10% as a max.

5,226✭✭✭✭170✭✭21✭✭170✭✭## MPPT Solar Charge Controller Tracer 4210A do you believe it will help me or am i just throwing away money.

30,647adminIn general, do several paper designs with different solar panels and charge controllers and see what is most cost effective for you.

Some general rules of thumbs...

upwards of 100 Vmp -- vs ~18 volts for PWM controller running a 12 volt battery bank..-Bill170✭✭4,478✭✭✭✭✭Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer

Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter

3,428✭✭✭✭✭You wire size is way to small even with the setup you currently have, the information on the Web, you're referring to, may be in reference to 120VAC, my rough calculation comes out at >9% volt drop, you want to be 3% or less, whilst it may work, it's not working efficiently. My suggestion is increase the conductor size, if cost is a consideration use aluminum, here is a calculator to determine the conductors required

http://www.calculator.net/voltage-drop-calculator.html

Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.

5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.

30,647admin- 420 Watts / 17.5 volt Vmp = 24 Amps Imp (estimate)
- 40 feet one way run
- copper cable
- 1% to 3% typical voltage drop

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=17.5&phase=dc&noofconductor=1&distance=40&distanceunit=feet&eres=24&x=65&y=2012 AWG cable (Note NEC specs max current of 20 amps for 12 AWG cable):

Voltage drop:

3.05Voltage drop percentage:

17.43%Voltage at the end:

14.45Now a 3% drop:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=17.5&phase=dc&noofconductor=1&distance=40&distanceunit=feet&eres=24&x=58&y=5

4 AWG

Voltage drop:

0.48Voltage drop percentage:

2.74%Voltage at the end:

17.02And a 1% drop:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.3224&voltage=17.5&phase=dc&noofconductor=1&distance=40&distanceunit=feet&eres=24&x=54&y=23

1/0 AWG

Voltage drop:

0.19Voltage drop percentage:

1.09%Voltage at the end:

17.31

Just to give you a rough idea--For every 3 AWG drop in wire size, the resistance (and current capability, and even wire costs) double.

So, if your cable run was ~20 feet instead, you could use +3 AWG lighter wiring (and save 1/2 the costs of cable).

Now lets say you use a "higher end" MPPT controller with a 150 VDC maximum voltage (typically works out to ~100 VDC Vmp-array STD max in colder weather) and 5x 17.5 volt Vmp panels in series for Vmp-array~87.5 volts Vmp nominal. 40 feet of cable run and Imp~4.8 amps (420 watt mythical array):

18 AWG wire:

Voltage drop:

2.45Voltage drop percentage:

2.80%Voltage at the end:

85.0512 AWG:

Voltage drop:

0.61Voltage drop percentage:

0.70%Voltage at the end:

86.89Also, regarding NEC max current for 12 AWG cable... For off grid solar array and DC inverter+battery bank wiring, I like to "derate" the wiring by 1.25x as even cable running at NEC limits for many hours a day (like solar/battery charging does), can make the wiring run hotter than I would like to see (things age faster as they get hot)... For example, a 24 amp circuit I would suggest:

From the basic NEC table:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm (note that there are different insulation types and temperature ranges, and for conduit on the roof, there are further temperature deratings in the full NEC manual).

That would be 10 AWG minimum (note that NEC derates 14/12/10 AWG cables to one size less current--I could never figure out why--but they do).

Also for long outdoor cable runs, I would suggest that 14 or 12 AWG cable be the minimum run AWG (less chance of mechanical damage breaking the cable when pulling for installation).

-Bill

170✭✭4,478✭✭✭✭✭Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer

Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter

30,647adminFor your initial system, instead of 24 amps of charging current into your <80% state of charge battery bank on a hot day, you may only got ~12 amps. Remember besides voltage drop in the wiring first example:

- 420 Watts / 17.5 volt Vmp = 24 Amps Imp (estimate)
- 40 feet one way run
- copper cable
- 1% to 3% typical voltage drop

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=17.5&phase=dc&noofconductor=1&distance=40&distanceunit=feet&eres=24&x=65&y=2012 AWG cable (Note NEC specs max current of 20 amps for 12 AWG cable):

Voltage drop:

3.05Voltage drop percentage:

17.43%Voltage at the end:

14.45

A flooded cell lead acid battery can be charged at ~14.75 volts (at room temperature) very nicely. The "available" voltage is -3.05 volts or ~14.45 volts at the battery... And, things like fuses/circuit breakers and charge controllers will also have their own voltage drops too... So the 24 amps available at the battery is possibly another 0.5 to 1.5 volts or so additional drop (MPPT controllers need >2.0 volt drop at the controller to "operate" correctly--And really "prefer a bit more (minimum)" drop).when you get down to 13.8 volts or so, that is no longer a "fast charging" current but a "float charge" (or maintenance charge). And your battery will not charge quickly or fully under these conditions.

You can take a DC current clamp DMM (digital multi-meter like this one) and see how much current you get middle of the day with a discharged battery bank (and/or a DC load >24 Amps DC) and see what your system is outputting now.

To have current "move" through wiring and devices, you need a "voltage drop" (i.e., pressure) to move the electrons. As the pressure is reduced (resistance), there is less reason for electrons to move (i.e., 0 volts across a charge controller, no current wants to flow).

PWM charge controllers with Vmp~17.5 volt panels on a "Hot" day with "cool" batteries can have their current flow limited by wiring resistance and device voltage drop--MPPT controllers where you run Vmp-array>~30 VDC, do not have the "hot weather" limitations with voltage drop (into a 12 volt battery bank in this example).It is usually not a "big" problem... In hot/sunny weather, most systems generate more energy than needed by the loads--So less than "perfect" output is not noticed during these times.

-Bill170✭✭WILL IT BE SAFEwith the extra amps?? As mentioned can i wire the new panel using the 14 awg I have now and leave the other two panels as they are? Or am i better of buying an MPPT and wire all three together with the 12awg i have now. what MPPT do you suggest? for a small system like mine.One other thing this is at my camp and at the most i am there 3 days and gone for 4 days so batteries are always full when i come back. I am not your typical solar guy, because when i leave the camp I always charge my batteries for a minimum of 1 hour with my gen set just to give them a good start. Thanks again this is great info.4,478✭✭✭✭✭Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer

Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter

5,226✭✭✭✭Be aware that even Fluke clamp meter are not all 'DC' clamp meters. It's likely they only make 1 DC clamp meter and a couple different AC clamp meters.

The problem with a MPPT type charge controller is that you need to have the same amperage from each panel to run them in a single string. If yo add the higher amperage panel it will only be able to produce the amount of amperage as the lowest amperage panel.

If you run them all in parallel, you won't have high enough voltage for the MPPT type charge controller. It need about 30% higher voltage than the charging voltage to work properly.

170✭✭safetywith 23 amps running through 12awg wire, but no one has addressed this question could someone please give me their thought on this.30,647adminhttps://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

It assumes lots of things that are not true in house wiring (not in conduit, wiring not bundled, high temperature insulation, etc.):

105°C insulation rating:All Ancor wire uses 105°C insulation rating. Lower temperature insulation cannot handle as much currentAWG wire sizes, not SAE:All Ancor wire uses AWG wire sizes. SAE wire sizes are 6%–12% smaller, carry proportionally less current, and have greater resistanceWires are not run in engine spaces:Maximum current is 15% less in engine spaces, which are assumed to be 20°C hotter than non-engine spaces (50°C vs. 30°C).Conductors are not bundled:If three conductors are bundled, reduce maximum amperage by 30%. If 4–6 conductors are bundled, reduce maximum amperage by 40%. If 7–24 conductors are bundled, reduce amperage by 50%.If you look at the NEC chart--Technically, 12 AWG wire is rated for 25 amps minimum--But there is a little note at the bottom that 14, 12, and 10 AWG cables are all derated by 5=10 amps by code--Why, I do not know and could never find a reason (Google search).

For one apartment unit I changed out 14 AWG and 15 amp breaker to 10 AWG cable and 30 amp breaker because, in one unit, the old breakers were failing after 5-10 years (started tripping more and more often--down to every few days) as they aged. Never had any signs of overheating in the wiring (in conduit) itself.

Remember, one of the power equations is Power=Current^2 * Resistance... If you double the current in a circuit, the heating goes up by 2^2=4x ...

-Bill "your mileage may vary" B.

170✭✭