Trying to understand how multiple strings are connected to an inverter.

[HX_Guy]

If you have an inverter that has two string inputs but you have 3 or 4 strings, I understand that you need to do something called "series fusing" with a combiner box. What I don't understand is how does this work?

If the inverter you are using has a maximum input watts of 5250 per string, why does it work to have 2 strings (of say 21 x 250w panels) through a combiner box?

Looking at this diagram, it looks like a combiner box just takes each string and combines them all? Doesn't that then add up all the watts?

[HX_Guy]

Anyone? Inetdog?

[Mike90250]

Anyone? Inetdog?

Sheese, give us a bit of time to digest this. I've got a life, and maybe someone else does too.

Of course, there is always "Pay for Promptness", I take paypal.

[Mike90250]

OK, so that array is about 5Kw, if it's well aimed.

The voltages (340 - 450 VDC) are for a grid tie system, no batteries.

I'm a bit wobbly on the 3 fuses, they are nearly 2x the panel amps of a single string, and may not reliably blow if there is a fault on one of the 3 strings.

The fuses protect a bad string from the other live strings. The combiner fuses are not protecting the inverter or controller.

If one of the 3 strings goes bad somehow, and the other 2 feed power into it, 1 fuse is supposed to blow, and isolate the bad string from further damage. The inverter will limp along with only 3,000w and you will not have a fire on your roof.

Hope this helps

[HX_Guy]

Thank for the reply Mike. I think I was trying to find something else out.

This inverter I'm looking at from SolarEdge has two inputs for the solar panels, so you could feed two strings into it. Each string though can only be up to 5,250W even though the inverter can handle up to 12,400W (or 14,250 for the next size up inverter). I was told by SolarEdge that if you want to fed more power into the inverter, say the maximum 12,400, then you need to three strings and combine the strings using a combiner box. You can either combine two strings together and feed to one of the input and then the third string on it's own into the other input or combine all 3 strings through the combiner box and then input just one of the inputs (leaving the other input unused).

I'm trying to understand how that works. Why is it that going through a combiner box allows you to connect 3 strings to 1 input whereas if you didn't use a combiner box, you could only connect 1 string to 1 input.

[Mike90250]

Possibly some sort of power sharing arrangement, where it can handle more power with both inputs supplying power, than it can with only 1 input. Something with it's internal topography. Like my 6KW inverter can only handle 70% un-balanced load on its output, or full power with loads more equally split.

[bcroe]

Thank for the reply Mike. I think I was trying to find something else out.

This inverter I'm looking at from SolarEdge has two inputs for the solar panels, so you could feed two strings into it. Each string though can only be up to 5,250W even though the inverter can handle up to 12,400W (or 14,250 for the next size up inverter). I was told by SolarEdge that if you want to fed more power into the inverter, say the maximum 12,400, then you need to three strings and combine the strings using a combiner box. You can either combine two strings together and feed to one of the input and then the third string on it's own into the other input or combine all 3 strings through the combiner box and then input just one of the inputs (leaving the other input unused).

I'm trying to understand how that works. Why is it that going through a combiner box allows you to connect 3 strings to 1 input whereas if you didn't use a combiner box, you could only connect 1 string to 1 input.

My combiner box has 2 separate output circuits. Bruce Roecombiner.jpg

[Alaska Man]

If you have an inverter that has two string inputs but you have 3 or 4 strings, I understand that you need to do something called "series fusing" with a combiner box. What I don't understand is how does this work?

If the inverter you are using has a maximum input watts of 5250 per string, why does it work to have 2 strings (of say 21 x 250w panels) through a combiner box?

Looking at this diagram, it looks like a combiner box just takes each string and combines them all? Doesn't that then add up all the watts?

I'm new hear, but just went through something similar.

When you combine the "Series" wired Panels Volts mutiply. At the Combiner Box the connection is a "Parallel" one. So the Amps increase not Volts. If each Array String is 5KW at the combiner box what comes out the other end is 5KW, but your Amps are now 3 times higher.

[HX_Guy]

I see, so that's how that works! I was thinking...how can you combined two or three strings of 5kW and it not come out to 10kW or 15kW total?

So then to figure out how many strings you can have going to one input, you would need to know how many amps can be fed into that input, correct?
Also, that means I'd need to know how many amps are per panel...which is normally watts divided by volts, correct? On a 250W panel with max of 30Vmp, that's 8.3 amps? Is that right? Seems too high? Or do the amps stay the same when you wire the panels in series?

This is the spec sheet for the inverter...which figure here tells me how many amps can be input?

http://www.solaredge.us/files/pdfs/products/inverters/se-single-phase-us-inverter-datasheet.pdf

[Alaska Man]

I see, so that's how that works! I was thinking...how can you combined two or three strings of 5kW and it not come out to 10kW or 15kW total?

So then to figure out how many strings you can have going to one input, you would need to know how many amps can be fed into that input, correct?
Also, that means I'd need to know how many amps are per panel...which is normally watts divided by volts, correct? On a 250W panel with max of 30Vmp, that's 8.3 amps? Is that right? Seems too high? Or do the amps stay the same when you wire the panels in series?

Exactly, if each panel is 8.3 Amps wired in series then each string is 8.3Amps into the combiner box. That is why you can get away with running small wire over long distances.

8.3A x (3) strings = 24.9 amps going into your Charge Controller.

The MPPT Charge Controller then knocks down the Voltage to 12/24/48 etc. When it does your Amperage has to increase. 5,200W/48V = 115.55A

In your example you would need more than one MPPT Controller. They have limit of just how many Volts they can take.

[inetdog]

I'm new hear, but just went through something similar.

When you combine the "Series" wired Panels Volts mutiply. At the Combiner Box the connection is a "Parallel" one. So the Amps increase not Volts. If each Array String is 5KW at the combiner box what comes out the other end is 5KW, but your Amps are now 3 times higher.

What you mean to say is that if each string is X volts at Y amps the combined output will be X volts at 3Y amps. The power from one string would be XY watts and from the combination you get 3XY watts.

[HX_Guy]

So a Charge Controller is also needed, or is that built into the Inverter?

I edited my previous post to include the spec sheet of the Inverter, not sure if you saw it. Which number on the spec sheet tells me the max amp input?

[inetdog]

So a Charge Controller is also needed, or is that built into the Inverter?

I edited my previous post to include the spec sheet of the Inverter, not sure if you saw it. Which number on the spec sheet tells me the max amp input?

The illustration of the panels was put together to apply equally well to an on-grid or an off-grid system.

In a real system the panel output would go to a CC for an off-grid system with batteries and would go to a Grid Tie Inverter for an on-grid GTI system without batteries.

For a battery system the panels charging a battery via a CC, there would also have to be an inverter if the goal was to drive AC loads, but that inverter would be connected to the batteries not to the panels directly.

[Alaska Man]

So a Charge Controller is also needed, or is that built into the Inverter?

I edited my previous post to include the spec sheet of the Inverter, not sure if you saw it. Which number on the spec sheet tells me the max amp input?

I'm not familair with Grid-Tie inverters. So i'm not qualified to add my 2 cents on the Inverter, sorry.

[HX_Guy]

Inetdog, can you tell by looking at the specs of of the 12400 inverter how this would be wired up if there are 48 250W panels but there can't be more than 21 panels per string (5,250W)? Would you just do 3 strings of 16 panels, going to a combiner box, and then from the combiner box to one (of the two) inputs on the inverter?