Combiner box - Amps?

Sun1 · May 2020

Hi,

I have a basic question:

If I combine two strings into a combiner box and each string is producing 18 amps each with each string producing 150 volts. When I combine the two strings into a combiner box and then take the single output and connect it to the inverter, does the amps add up going to the converter and/or does the voltage add up?

BB. · May 2020

Adding power sources in parallel--The current adds.

Adding power sources in series--The voltage adds.

https://www.allaboutcircuits.com/textbook/direct-current/chpt-5/what-are-series-and-parallel-circuits/

Make sense? If not, please ask more questions. Getting the connections wrong can really ruin your day.

-Bill

Sun1 · May 2020

BB. said:

Adding power sources in parallel--The current adds.

Adding power sources in series--The voltage adds.

https://www.allaboutcircuits.com/textbook/direct-current/chpt-5/what-are-series-and-parallel-circuits/

Make sense? If not, please ask more questions. Getting the connections wrong can really ruin your day.

-Bill

Thanks. For each of the V panel strings, they are in series (150v, 18 amps for each string) running to a combiner box. With both strings going to the combiner box, then the single output of the combiner is going to the inverter.

I understand about the series - parallel theory of the v panels but when the strings are combined in the combiner box to a single output going to the inverter, is the single output now 300v at 36 amps or 300v at 18 amps? I guess the question is does the combiner box add the amps (from the two single strings at 18 amps each) to 36 amps going to the inverter?

BB. · May 2020

It really depends on how you wire the converter box... But the standard wiring is, if you have two (or more) "strings of panels", when you connect them to the combiner box, those strings are connected in parallel, so your combined output would be 150 Volts @ 36 Amps (2 parallel connections of 150 volts @ 18 amp strings).

But the "real question" is what is the voltage/current required by your inverter solar panel input (is this grid tied, or a hybrid system with a battery bank). You need to make sure that the solar array meets the inverter's input requirements.

-Bill

mike95490 · May 2020

Yow ! 150V is very close to the 150V threshold for killing most large charge controllers Be careful

From what you describe, two strings of 18A paralleled would produce 36A and you would need cable to handle that, likely 6ga.

Sun1 · May 2020

The original question was hypothetical.

I want to use a SMA SunnBoy 3000TL-22 (I will put the specs below). As far as The PV panels, I have two strings:

First String - Seven SolarWorld Sunmodule Plus SW 255 @ 250 Watts, Impp = 8.43 amps, Vmpp = 30.6v

Second String -Four - SolarWorld Sunmodule XL Mono 345 @ 340 Watts, Impp = 9.01 amps, Vmpp = 38.2v

I was given a brand new SMA Combo-Switch-10A (See image below). Looks like each MPP input on the inverter can do 18A max. If I use the Combiner/Disconnect, it looks like (going by what Bill stated about addition od amps of each string) that the combined amp strings would produce 17.44 amps total. Is 17.44 amps for the one MPP on the inverter pushing it?

Inverter Specs

Image: https://us.v-cdn.net/6024911/uploads/editor/uo/5k3msz63d0cx.png

Disconnect/Combiner Specs

Image: https://us.v-cdn.net/6024911/uploads/editor/77/nkgcoa7u22bv.jpg

BB. · June 2020

This is one of the issues of buying multiple different products and trying to integrate them together... Generally, you buy one type/model of solar panels and then connect them in series/parallel/array input configurations that will support your choosen GT inverter.&

I am making some guesses here--But I think this is what you can do with what you have...

First, the Combi switch appears to be a combiner with a disconnect. 4x 10 amp circuits each capable of turning off 600 VDC @ 10 amp circuits and a single 40 amp output.

https://www.ecodirect.com/SMA-CS-p/sma-cs.htm

Looking at your solar panels:

First String - Seven SolarWorld Sunmodule Plus SW 255 @ 250 Watts, Impp = 8.43 amps, Vmpp = 30.6v
Vmp-array = 7 * 30.6 volts = 214.2 volts

Second String -Four - SolarWorld Sunmodule XL Mono 345 @ 340 Watts, Impp = 9.01 amps, Vmpp = 38.2v
Vmp-array = 4 * 38.2 volts = 152.8 volts

First string would run one of the MPPT inputs on your inverter of 175 to 480 VDC

Second string is too low of Vmp-array voltage to reliably start the GT inverter.

Another option would be to connect 1st and 2nd strings in series for ~367 VDC @ ~8.43 amps

You can connect panels that have their Imp in parallel within 10%--And yours are.

That would allow you to use 1 input on the Combi switch (10 amps is "close"--Normally, I would like fuse+wiring to be at least 1.25x Imp of array--If you look at the data sheets, you should find a series fuse/breaker specification (probably around 15 amps for those panels).

You would not need the combi switch for this configuration as the array current is right at its maximum specifications (you may blow fuses over time--weeks/months/years by operating so close to their 10 amp rating). And you cannot put larger fuses in this unit (it seems). And just as an FYI--You cannot (should not) parallel two differently rated Vmp-strings in parallel... Their Vmp-strings, if paralleled, need to match within 10% or better. Like putting a 12 volt battery in parallel with a 6 volt battery (and blocking diodes). The 6 volt battery will do nothing useful.

You can replace the Combi box with a simple properly rated knife/disconnect switch (one circuit) if you need one (your code requirements).

The single array is OK for a 3,000 Watt GT inverter... Roughly, this array would output:

367 Volts Vmp * 8.43 Amps Imp = 3,094 Watt estimated array output
3,094 Watt array * 0.77 panel+controller deratings = 2,382 Watt derated output (typical maximum, seen on a few days for a short period of time on cool/clear days around solar noon).

So, based on what you have (and the very quick look at the various specifications), that is what I would probably do (all panels in series, a solar array disconnect if needed by your code).

-Bill

Sun1 · June 2020

Thanks for the information. I think based on your information I probably should make use of the two MPP inputs on the SunnyBoy and place the disconnects as needed by code.

I do have a question: do you see any problem using the SMA Combi Switch 10A just as a disconnect for one of the strings?

Thank You!!

BB. · June 2020

I would avoid using the Combi Box for these panels... That 10 amp max fuse rating is really too low. Check the panel ratings. You will probably see that they use either 15 or 20 amp series protection fuses/breakers. Running a 10 amp fuse with 9 amp input, the fuse will probably false blow on you over time. I guess you could parallel 2 (or more) circuits--But that is a bit of a hack and may be hard to explain to an inspector.

You can only use two of the MPPT inputs if you add, at least one more, series panels to String 2... And bring its Vmp-array voltage up to, at least, 175 volts (according your your spec screen shot) for reliable operation.

That would be an ideal solution (another panel on your 3 kW GT inverter is fine, not placing "mixed" Imp panels in series--A bit more output from the higher Imp string, and you do not need any combiner boxes--Just simple disconnects per string).

-Bill

BB. · June 2020

I should add, you do not need series protection fuses for single strings... Generally, per string fuse/breaker, are only needed when ~3 or more strings are paralleled to one circuit.

-Bill

Sun1 · June 2020

Do you know any disconnects I could purchase that have fusing? The reason I ask is I am in a high lightning area (rocky mountains) and would like some kind of safety for near lightning strikes. I believe for the SMA inverter there is no ground wire needed but the disconnect needs to be able to cut the power to both positive and negative wires from the array. I have found anything yet that does the disconnect of both wires as well as has fusing.

BB. · June 2020

Ganged circuit breakers (2 pole) are what you are asking for... Midnite has a lot of 2 pole breaker options... Although, you have to check the maximum switching voltage. Your 367 VDC (higher for Voc) is not trivial:

http://www.midnitesolar.com/products.php?menuItem=products&productCat_ID=16&productCatName=Breakers

This is (similar) to what was used on my home for the 240 VAC GT Solar disconnect (mine is unfused and just a huge pair of knife switches--Circuit breakers are in main panel):

https://www.homedepot.com/p/GE-30-Amp-240-Volt-Fusible-Outdoor-General-Duty-Safety-Switch-TG4321R/202978635

You probably have to call SMA or a company such as our host (NAWS) about the proper disconnect for your installation. High voltage DC switches are just not very common. NEC (National Electric Code) has been changing quite a bit over the last couple of decades--Makes keeping up with the requirements a bit of work--Plus different cities/counties may use different NEC versions or other codes).

From my understanding, no circuit breaker/fuse is going to be fast enough (or have enough of a "gap") to actually do any real protection against lightning.

The typical suggestion is to put good quality surge suppressors just before the equipment (or where the array wiring enters the home), and use those to shunt high voltage surge to earth ground.

https://www.solar-electric.com/search/?q=surge+suppressors

Midnite uses MOV (Metal Oxide Varistors) to short out the high voltage surges (line to line or line to earth ground--typically).

Many other surge suppressors just use a simple spark gap and sand/ceramic powder to quench the arc.

-Bill

Sun1 · June 2020

Thanks Bill for all the information

juicyjay · August 13

BB. said:

Adding power sources in parallel--The current adds.

Adding power sources in series--The voltage adds.

https://www.allaboutcircuits.com/textbook/direct-current/chpt-5/what-are-series-and-parallel-circuits/

Make sense? If not, please ask more questions. Getting the connections wrong can really ruin your day.

-Bill

Hi Bill
So in saying that a parrellel connection dose not add up voltage in a pv combiner box consisting of (3)strings of x4 12v solar panels creating 48v 6a with a 80v 6a max output for each of the (3)strings.

will that mean the total solar input volts to the charger will not exceede the max rating of 150v for the charger ?

As each string is only 80v 6a max dose this mean the pv box will only deliver 80v 6a to the solar charger despite having (3)stings of 80v 6a max ?

Dave Angelini · August 13

Jump in for Bill. You do have to wire all of it correctly but yes each string will add current and the voltage will remain the same. You should use a voltmeter and measure Voltage before you connect to the solar charger.

Also you should really start a new thread as this one is pretty old. Not hard to do either. Good Luck!

BB. · August 13

Welcome to the forum JJ,
Can you tell me the Vmp/Imp (and the Voc/Isc too) of the panels?
Basically... If you have (for example) Vmp~19 volt panels and Imp~6 amps.. .You have P=V*I=114Watt panels.
The series connected string of 4 panels will have:
4 series * 19 Volts Vmp = 76 Volts Vmp-std (standard conditions) per string at 6 amps
then 3 parallel strings:
3 parallel * 6 amps = 18 amps Imp-array @ 76 Volts Vmp-std
I think you live in a relatively warm/hot area in Australia(?). So you probably do not get very cold (freezing?). This is a concern because Voc (voltage open circuit) and Vmp (voltage maximum power) rise as the panels get hot... If you where in a high mountain area with harsh winters (below 40C sometimes), Voc-array can approach 120 Volts Voc-array-very-cold... (just using rules of thumbs). You can "do the math" for your panels/area or use something like this handy array calculator:
https://www.midnitesolar.com/sizingTool/index.php

Using roughly accurate numbers (SWAG) for your array, your Voc-array-0C would be around 95.2 Volts max for your 4s*3p array...
So out of your combiner box, your Vmp-array-std is ~78 volts / 95.2 volts max @ 0C, and Imp~18 amps. Classic series + parallel connected array. Just like adding batteries in series (such as a 12 volt car battery with 6*2 volt cells) and then placing 3 in parallel for 3*Amp*Hour capacity.... (excluding temperature changes for solar panels).
Is this making sense?
-Bill

Combiner box - Amps?

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