Modules per String

[shortcircuit2]

Using the new SMA 3000TL inverter (Specs http://www.sma-america.com/en_US/pr...-3000tl-us-3800tl-us-4000tl-us-5000tl-us.html) with LG280N1C-G3 modules with a Voc of 38.87volts, can we put 12 or 13 modules on a string where the recorded low temp is -19C degrees?

 

[GoldDigger]

Using the new SMA 3000TL inverter (Specs http://www.sma-america.com/en_US/pr...-3000tl-us-3800tl-us-4000tl-us-5000tl-us.html) with LG280N1C-G3 modules with a Voc of 38.87volts, can we put 12 or 13 modules on a string where the recorded low temp is -19C degrees?

Have you tried using the SMA string sizing guide? It is available as part of the downloaded Sunny Design software or via a web tool at [B][U]http://www.sunnydesignweb.com[/U][/B]
Registration is required, but the web tool is free.
It does a lot of other design-related work for you too!

I would call 13 too close for comfort, depending on the exact temperature coefficient for those panels and remembering that in early morning in a clear sky the panels may actually be colder than the air temperature.

 

[jaggedben]

The exact inverter model is not very relevant. Max DC voltage is 600V for just about every UL listed inverter on the market these days, so you can simply say max voltage 600V. It could be 1000V if applicable to the system, but not in this case..

In order to answer the question, we need to know the Voc temperature coefficient for the modules. It is generally expressed as a negative percentage in C, e.g. -0.37%/C.

To find the max voltage, you take the (number of modules)*(Voc of modules)*((1+(temp coefficient/100*record low-25C)

Or to simplify in your case, 13*38.87*((1+(??/100*-44).
If this number exceeds 600 then you can't do it.
(Note that you must input the temp coefficient as a negative number.)

If there is no listed temp coefficient, replace the last part with 1.18 per table 690.7.

 

[shortcircuit2]

Voc temp coefficient is -.31%/K

 

[shortcircuit2]

In this LG chart it lists that only 12 modules can be connected in series though. Anyone know why this is?

 

[jaggedben]

In this LG chart it lists that only 12 modules can be connected in series though. Anyone know why this is?

That's very strange. Why don't you ask them?

 

[Zee]

Given the strict NEC 1.18 factor, 13 panels gets you 596 V. Fine.
Since coeff. is given, it must be used instead of above factor:
Thus your de facto "factor" is actually 1.1364
(for a 44 deg C drop from 25 C to -19C), much more lenient.)
Thus,
13 panels at 38.87 VOC at 1.1364 = 574 V.
Looks fine, so what DO they mean? I agree, it IS strange....am curious for their response.

 

[GoldDigger]

In this LG chart it lists that only 12 modules can be connected in series though. Anyone know why this is?

SWAG:
In the temperature graph they characterize the panels down to -40C (which is also -40F). At that temperature 13 panels will go above 600V.
So instead of qualifying the maximum number per string based on a specific temperature, they give the number which will be safe all the way down to -40.

 

[shortcircuit2]

SWAG:
In the temperature graph they characterize the panels down to -40C (which is also -40F). At that temperature 13 panels will go above 600V.
So instead of qualifying the maximum number per string based on a specific temperature, they give the number which will be safe all the way down to -40.

I don't follow you. Which temperature graph? Table 690.7?

 

[shortcircuit2]

Given the strict NEC 1.18 factor, 13 panels gets you 596 V. Fine.
Since coeff. is given, it must be used instead of above factor:
Thus your de facto "factor" is actually 1.1364
(for a 44 deg C drop from 25 C to -19C), much more lenient.)
Thus,
13 panels at 38.87 VOC at 1.1364 = 574 V.
Looks fine, so what DO they mean? I agree, it IS strange....am curious for their response.

Thanks Zee. In reviewing the instructions they don't include the temp coeff. in the instructions. I pulled the coeff. off the LG web site under Tech Specs. 690.7 requires the temp coeff. be in instructions with product. So we wouldn't be able to apply the temp coeff.?

Here is a link to product. The instructions are under manuals and downloads...
http://www.lg.com/us/commercial/solar-panels/lg-LG280N1C-G3

Of interest on page 4 are instructions that's say "125% factor not explained in instructions" and "Do not exceed 80% of system voltage"

Also they say..."Maximum parallel strings without proper measures,
e. g. fuse 20A : 1 string"

So even single string layouts require fusing?

 

[shortcircuit2]

That's very strange. Why don't you ask them?

I'll try to contact them.

 

[shortcircuit2]

Have you tried using the SMA string sizing guide? It is available as part of the downloaded Sunny Design software or via a web tool at [B][U]http://www.sunnydesignweb.com[/U][/B]
Registration is required, but the web tool is free.
It does a lot of other design-related work for you too![/B]

I tried the Sunny Design and it allows 13 per string? The disclaimer warns me that the software is only for reference. Very good program though.

 

[ggunn]

Also they say..."Maximum parallel strings without proper measures,
e. g. fuse 20A : 1 string"

So even single string layouts require fusing?

I read that as saying that they want you to fuse double string layouts but not single string ones. That's overkill, but if they require it for their warranty you'd probably best do it.

 

[GoldDigger]

I tried the Sunny Design and it allows 13 per string? The disclaimer warns me that the software is only for reference. Very good program though.

IMHO the difference is that Sunny Design allows you to enter your actual minimum temperature while the LG document does not.
The LG document I was referring to is their spec which gives a graph of voltage, current, and power versus temperature. The lowest temp on the graph is -40C.

Sent from my XT1080 using Tapatalk

 

[jaggedben]

I don't follow you. Which temperature graph? Table 690.7?

I think he means LG's temperature spec. Basically Goldigger's theory is that they went with the number that is okay for 600V all the way down to the module's lowest rated operating temp. Makes sense to me.

...

Of interest on page 4 are instructions that's say "125% factor not explained in instructions" and "Do not exceed 80% of system voltage"

Also they say..."Maximum parallel strings without proper measures,
e. g. fuse 20A : 1 string"

So even single string layouts require fusing?

You wish they'd keep it simple with something like 'Installer must follow all applicable local building codes.' My question to them might be something like 'If I install your modules in a 100% code compliant installation according to the NEC, may I assume your warranty will be honored?".

BTW I've use LG modules on one installation so far, but I think they're very nice modules.

 

[Zee]

Thanks, Zee. In reviewing the instructions they don't include the temp coeff. in the instructions. I pulled the coeff. off the LG web site under Tech Specs. 690.7 requires the temp coeff. be in instructions with product. So we wouldn't be able to apply the temp coeff.?

Here is a link to product. The instructions are under manuals and downloads...
http://www.lg.com/us/commercial/solar-panels/lg-LG280N1C-G3

Of interest on page 4 are instructions that's say "125% factor not explained in instructions" and "Do not exceed 80% of system voltage"

Also they say..."Maximum parallel strings without proper measures,
e. g. fuse 20A : 1 string"

So even single string layouts require fusing?

The quickest way to get confused in this field is to (painfully try to) read a module installation manual. Spec sheet OK....but the booklet, good lord. Seriously...who writes those anyways? Either someone in China or Japan or Germany with a different understanding of Engrish...or engineers who have never installed one per NEC.

None of the confusing sentences you quoted seem to call out anything other than basic NEC requirements we already all follow. Except for not exceeding 80% of system V????? I don't even understand it, let alone agree with it. What does system v mean? It is what you have designed maybe..... but if you have it how can you not exceed 80%??

As for your Q, i don't know if the V/T coeff. has to be "published" in paper - or online is acceptable? I see a grey area on this one...... What is the literal wording of the relevant code section?

Otherwise use the one that suits your interest: the lower one from LG!

 

[ggunn]

The quickest way to get confused in this field is to (painfully try to) read a module installation manual. Spec sheet OK....but the booklet, good lord. Seriously...who writes those anyways? Either someone in China or Japan or Germany with a different understanding of Engrish...or engineers who have never installed one per NEC.

None of the confusing sentences you quoted seem to call out anything other than basic NEC requirements we already all follow. Except for not exceeding 80% of system V????? I don't even understand it, let alone agree with it. What does system v mean? It is what you have designed maybe..... but if you have it how can you not exceed 80%??

As for your Q, i don't know if the V/T coeff. has to be "published" in paper - or online is acceptable? I see a grey area on this one...... What is the literal wording of the relevant code section?

Otherwise use the one that suits your interest: the lower one from LG!

I concur. I once was trying to determine where to specify the mid and end clamp positions on modules for a roof mounted array design, so I consulted the module installation manual. It specified three different clamping ranges for three different ranges of loading, which is understandable, and it said that the ranges were downward compatible (you could clamp for more loading than was necessary), which is also understandable, but the clamping range for midrange loading was more restrictive than that for heavy loading.

Huh?

 

[Zee]

I concur. I once was trying to determine where to specify the mid and end clamp positions on modules for a roof mounted array design, so I consulted the module installation manual. It specified three different clamping ranges for three different ranges of loading, which is understandable, and it said that the ranges were downward compatible (you could clamp for more loading than was necessary), which is also understandable, but the clamping range for midrange loading was more restrictive than that for heavy loading.

Huh?

:dunce: Yeah i know, i know (tears) :weeping:
.....and let's not get started on the grounding instructions
...inadequate.... confusing...ineffective...unsafe....impractical.
Which would all be fine if they were guidelines.
Unfortunately they are required to be followed by the NEC.