Adding panels to my setup

rake1 · February 2018

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.

Photowhit · February 2018

Since you are using a PWM controller, pretty much any 36 cell panel with a vmp between 17.3 and 19.5

rake1 · February 2018

So am I right and saying any panel with a vmp between the 17.3 to 19.5 will work regardless of the wattage of the new panel?
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.

Photowhit · February 2018

While it would have a great ability to over power another panel if there was a short, there should be no problem. In parallel the amps will add, since a PWM controller doesn't try to use the power above the charging voltage anything above the charging voltage is lost. but the amperage will just add. You are likely looking at older Evergreen panel. Not a lot of high wattage 12 volt nominal panels out there.

rake1 · February 2018

Please explain anything above the charging voltage is lost I have a 12 volt system. are you saying that the vmp is 18 volts but all that is being used is 12 volt the other 6 volt is lost ? My panels are rated for about 15 amp combined and on a good day I have seen 14to 15 amps.Thanks for sticking with me.

Photowhit · February 2018

rake1 said:

Please explain anything above the charging voltage is lost I have a 12 volt system. are you saying that the vmp is 18 volts but all that is being used is 12 volt the other 6 volt is lost ?

A '12 volt battery' isn't, nothing true about a '12 volt battery'! What you have in what is called a '12 volt battery' is 6 batteries that are connected in series. They may be in 1 case which we call a 12 volt battery, or 2 - 6 volt cases or 6 - 2 volt cases.

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.

Photowhit · February 2018

FWIW - PWM = Pulse Width modified, works because it is an electro chemical reaction and doesn't happen instantly. So it switches the charging current on and off to maintain the correct voltage.

rake1 · February 2018

Photowhit thank you, in my case only two panels may add another an MPPT probably wouldn't make a lot of difference to me, by the sounds of it.

SupraLance · February 2018

If your sticking with 36cell panels then the advantage of mppt is mostly just to take advantage of low-light conditions. But a much bigger advantage of mppt is the ability to run cheaper 60cell or 72cell panels on a 12v system, and easily move up to a 24v system when you next replace the batteries. 36cell panels are really expensive per watt comparatively.

rake1 · February 2018

Thanks I am adding a 160 watt panel to my system that will give me 420 watts 12 volt system I am looking at a

MPPT Solar Charge Controller Tracer 4210A do you believe it will help me or am i just throwing away money.

BB. · February 2018

It is not an easy answer... (I don't know anything about "Tracer" controllers--Good or bad).

In 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...

Less than 400 Watt solar panels system work out nicely with PWM controllers.
More than 800 Watt systems usually work out better with MPPT controllers.
PWM systems are better for short wire runs between solar array and charge controllers.
MPPT controllers are better when you have longer wire runs (10's to 100's of feet) between solar array and charge controller (always keep controller to battery bank wiring short and heavy cable--keep voltage drops low). Long array wiring (depending on controller) can run upwards of 100 Vmp -- vs ~18 volts for PWM controller running a 12 volt battery bank.
PWM controllers are much less expensive. Vmp~18 volt solar panels are usually close to 1.5+x more expensive.
MPPT controllers are much more expensive (5-6x more expensive for high end controllers). However Vmp>=30 volt panels (typically >200 Watts) can be much cheaper than Vmp~18 volt panels (typically ~140 Watt maximum).
If you are in a very hot region--Vmp for 18 volt panels (36 cell) can drop by upwards of 20% (3.6 volts) on sunny days. It does reduce charging voltage/current/equalization voltage for battery banks somewhat. MPPT controller with Vmp>>Vbatt voltage can charge full current/battery charging voltage on any day.
In the grand scheme of things--Properly designed solar array+PWM or MPPT charge controllers are about the same efficiency (on average). I generally do not change the math based on PWM or MPPT controllers (both work out, close enough, to ~77% efficiency for solar panels+charge controllers).
Modern/higher end MPPT controllers usually have more "bells and whistles" (i.e, networking/communications) abilities and other settings that are helpful for larger battery systems. PWM controllers generally are more simple/stand alone type systems.

-Bill

rake1 · February 2018

Great info BB sounds like my PWM with 420 watt panels is doing a good enough job the only concern I have is I am running about 40 ft of 12 awg which is fine based on calculations I can get of the web, how do you feel about that size wire? I haven't added the new 160 watt panel yet so I am only at 260w and all is fine. thanks

Estragon · February 2018

IMHO, 12ga is too light to potentially carry 20+ amps (assuming you make the parallel connection near the array). Might be okay if you're running 3 pairs the 40', and combining at the controller.

mcgivor · February 2018

rake1 said:

Great info BB sounds like my PWM with 420 watt panels is doing a good enough job the only concern I have is I am running about 40 ft of 12 awg which is fine based on calculations I can get of the web, how do you feel about that size wire? I haven't added the new 160 watt panel yet so I am only at 260w and all is fine. thanks

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

BB. · February 2018

As an example for the voltage drop calcuations... Using a generic voltage drop calculator (keep changing wire AWG until you get the answer you need)...

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&amperes=24&x=65&y=20
12 AWG cable (Note NEC specs max current of 20 amps for 12 AWG cable):
Voltage drop: 3.05
Voltage drop percentage: 17.43%
Voltage at the end: 14.45

Now 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&amperes=24&x=58&y=5
4 AWG
Voltage drop: 0.48
Voltage drop percentage: 2.74%
Voltage at the end: 17.02

And 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&amperes=24&x=54&y=23
1/0 AWG
Voltage drop: 0.19
Voltage 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: 2.45
Voltage drop percentage: 2.80%
Voltage at the end: 85.05

12 AWG:
Voltage drop: 0.61
Voltage drop percentage: 0.70%
Voltage at the end: 86.89

Also, 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:

24 amps * 1.25 NEC derating = 30 Amp rated branch circuit (wiring, circuit breaker if used, etc.)

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).

1 The load current rating and the overcurrent protection for conductor Types shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

References: To buy NEC National Electric Code books go to World Wide Web "nfpa.org"

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

rake1 · February 2018

A lot of info BB what is the effect of the voltage drop? does it just mean that I am not getting the power from the panels I could be hence it is charging my Batteries slower? What about satey is it safe to do what I plan with the additional 160 watt panel? Also I do have another 40 ft of 14 awg I could use for the new panel and leave the other two panels as is with the 12 awg.

Estragon · February 2018

Depending on the size of the voltage drop, and the temperature of the battery, there may not be enough voltage to charge at all.

BB. · February 2018

Bascially, "excessive voltage drop" can dramatically reduce the charging current available to the battery bank.

For 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&amperes=24&x=65&y=20
12 AWG cable (Note NEC specs max current of 20 amps for 12 AWG cable):
Voltage drop: 3.05
Voltage 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.

-Bill

rake1 · February 2018

I do have a Fluke clamp meter I will check the next time I am at my camp, I also have a display on my system that does show amps and the voltage but I believe the voltage is just the state of charge of the batteries. An example of the voltage going to the batteries I believe would be when I equalize them they will go as high as 16.2 volts using the solar array I have now. With my two panels i have now I have seen 15 amp on my gauge on a good day and batteries at about 60 % charged. I am concerned about safety with the 12awg when I add the new 160 watt panel WILL IT BE SAFE with 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.

Estragon · February 2018

Using the 14ga is safe, provided you put a fuse/breaker on the wire to prevent the combined current of the other two panels feeding a short fault in the new panel. You will still be left with a ~10% voltage drop though. Replacing the controller with a decent MPPT one is likely to cost more than rewiring with 4-6ga to reduce the voltage drop (~$50-60 for 40' of 6ga 2conductor copper).

Photowhit · February 2018

It will be hard to measure any better than the display. The problem is that it's rare to have the full current (amperage) running and you won't 'see' the higher voltage drop without it. Since you will only experience problems with the full potential needed, that would be in bulk with full sun on the panels. When lower current is passing there won't be as much voltage drop.

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.

rake1 · February 2018

Thanks guy I was leaning towards an Mppt but maybe changing the cable is the way to go. But as I mentioned it is at my camp so I am not as concerned about voltage drop as I would be if I was using it every day at home. I am concerned though about safety with 23 amps running through 12awg wire, but no one has addressed this question could someone please give me their thought on this.

BB. · February 2018

Here is a marine wiring chart...

https://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 current
AWG 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 resistance
Wires 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%.

In an old apartment building, have 4x electric water heaters running for 60+ years at 19.8 amps @ 240 VAC and 14 AWG wiring with 15 amp breakers (--I think, I don't remember for sure at the moment).

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).

The load current rating and the overcurrent protection for conductor Types shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

Will your place burn down if you run 23 amps through 12 AWG wiring. No. Will it degrade/have issues decades down the road. Maybe.

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.

rake1 · February 2018

Ok I think instead of changing the controller I will drop the wire size I will check my controller next trip to the camp and see what size wire it will handle. Thanks to all for the advice.

Adding panels to my setup

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MPPT Solar Charge Controller Tracer 4210A do you believe it will help me or am i just throwing away money.

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