Suggestions requested: Number of panels in series

[JWLV]

I'm about to rewire my array.

I have a EG4 6500EX which can accept up to 500V PV input and has 2 MPPTs.
The distance from the EG4 6500Ex to the panels is about 75 ft.
I have 24 panels. Voc is 39. Imp is 8.5A. These are used/old panels from Satan Solar. I'm seeing roughly 70% output from them. And even less in the summer due to extreme heat >110F. So the Imp current from each panel is closer to 5.5A. I'm in Las Vegas where the low temp might get to 32F once a year.

What I'm thinking:
Choice 1. Two strings. Each string will be 12 panels in series. Total Voc will be 468V. Current will be 8.5A.
Choice 2. Two strings. Each string will be 6S2P. Total Voc on each string will be 234V. Current will be 17A.
Choice 3. You suggest?

 

[timselectric]

32F gives you a temperature adjustment of 1.21. (Actually selected 31F for a safety margin)
468v x 1.12 = 524v = too high. 12 panels is too many.
Option #2 is safe.
For Option #3, I would go with 18 panels in 9s2p.

 

[SeaGal]

For Option #3, I would go with 18 panels in 9s2p.

Would love to understand why you say that?

In my less experienced mind... If you were only going to run 9 panels per string, why parallel the strings, rather than have one string per MPPT (= less current / losses in the wire). And why not go for 11s in 2 strings - that should just be OK voltage-wise?

 

[timselectric]

Would love to understand why you say that?

Edited (morning math)
9s gives you 393 VOC, at 31F.
10s gives you 437 VOC at 31F.
11s gives you 480 VOC at 31F.
11 is too close. 10 would probably be ok, but I like a little more safety margin.

2p gets you closer to what amperage the MPPT can accept.
One pair of conductors to install. A second MPPT can be used for future expansion.
I always try to get the most power for the least expense.

 

[timselectric]

Actually I just checked the specs for the MPPT.
I would probably go with 9s3p.

 

[SeaGal]

11 is too high.

468v x 1.21 = 524v = too high. 12 panels is too many.

Ah!.... I see what happened now. You said 12 was 524V, so I did 11/12th of 524V = 480V (hence my comment 'should be just OK voltage wise'). But 468V x 1.21 isn't 524V, it's 566V

Makes sense re the paralleling to save on wire cost. 9s3p sounds good if he can buy 3 more panels.

 

[timselectric]

But 468V x 1.21 isn't 524V, it's 566V

Good catch. Morning math isn't always accurate. lol

 

[OffGridForGood]

Makes sense re the paralleling to save on wire cost. 9s3p sounds good if he can buy 3 more panels.

If he can buy 3 more identical panels, and they will fit the roof with the same orientation as the others.
Otherwise, 8s 3P makes sense to me to match the 24 panels the OP has, and fit the roof area.

 

[timselectric]

Since the panels are 75' away. I assumed that they would be ground mounted.
But we all know were assumptions get us.

 

[OffGridForGood]

Since the panels are 75' away. I assumed that they would be ground mounted.
But we all know were assumptions get us.

Very True!
OP also noted 'used SanTan panels' - not sure if that means they can get three more, the same or not?
At least they have options!

 

[JWLV]

Yes the panels are ground mounted.

I don’t plan on getting any more panels from Santan. If I do get more panels it’ll be new ones from somewhere else.

Thanks for all the different suggestions and perspectives. It’s exactly what I need to help me make a final decision. I am leaning towards 8s3p right now.

Will 8s3p be too much current for the typical mc4 connectors? At maximum it could be as high as 25.5A.

 

[timselectric]

The better quality MC4 connectors are rated for 30a.
But I wouldn't want to push them that much.

 

[zanydroid]

Confused by why MC4 is relevant here for 3P. Wouldn't you want (need? fusing is needed) a fused combiner anyway? Then the MC4 only carries the load from 1 string to the combiner. 8.5A I_mpp before adjustment. Barely an inconvenience.

Unless you were considering inline MC4 fuses. Then it would be necessary to vet the MC4 after the combining.

 

[Zwy]

Actually I just checked the specs for the MPPT.
I would probably go with 9s3p.

Too many watts. 6500EX is 4000W per MPPT.

I have a EG4 6500EX which can accept up to 500V PV input and has 2 MPPTs.
The distance from the EG4 6500Ex to the panels is about 75 ft.
I have 24 panels. Voc is 39. Imp is 8.5A. These are used/old panels from Satan Solar. I'm seeing roughly 70% output from them. And even less in the summer due to extreme heat >110F. So the Imp current from each panel is closer to 5.5A. I'm in Las Vegas where the low temp might get to 32F once a year.

What I'm thinking:
Choice 1. Two strings. Each string will be 12 panels in series. Total Voc will be 468V. Current will be 8.5A.
Choice 2. Two strings. Each string will be 6S2P. Total Voc on each string will be 234V. Current will be 17A.
Choice 3. You suggest?

Choice 3: Add another MPPT

 

[timselectric]

Too many watts. 6500EX is 4000W per MPPT.

So, the 27a is the combined limit for both MPPT's ?
If so, the specs should be changed.

 

[Zwy]

So, the 27a is the combined limit for both MPPT's ?
If so, the specs should be changed.

It's 4Kw per MPPT, 8Kw combined . How you get there would depend on PV voltage. With 350V plus, you start moving towards 10A.

 

[timselectric]

It's 4Kw per MPPT, 8Kw combined . How you get there would depend on PV voltage. With 350V plus, you start moving towards 10A.

These must be some weird MPPT's.
I don't know of any others that have these odd limitations.

 

[42OhmsPA]

330W panels?

Are you facing all of them the same direction?...
How about 7s2p facing S (slightly over paneled) and 5s2p, facing SW and SE, to mppt 2?

Option 2 is most likely ideal.

 

[zanydroid]

These must be some weird MPPT's.
I don't know of any others that have these odd limitations.

I think it makes sense - 4kW in each of two physically independent MPPTs, totaling 8kW

If it was a more elegantly implemented hardware maybe it would allow the combining of two MPPTs, so you can have one tracker for 8kW.

 

[timselectric]

I think it makes sense - 4kW in each of two physically independent MPPTs, totaling 8kW

If it was a more elegantly implemented hardware maybe it would allow the combining of two MPPTs, so you can have one tracker for 8kW.

It doesn't sound like two individual MPPT's.
It's more like one MPPT with two inputs.
Or the specs are wrong. And 27a is the combined total. (13.5a per)