How close to failure is this MPPT+PV package and is there a way to protect against high voltage?

[Old_Skewler]

Strings comprised of panels facing different directions will be limited by the lowest current facing. 3S4P is a far better choice on your roof.

Thank you!

The midnight 250 is limited to 53A output.

I am not sure why this was brought up though... Also, for a 3S4P system I would go with a Victron 150/85:

The implications are pretty minimal - mostly higher losses from lower voltage and over-current protection. Unless you're making a very long run to the MPPT, the difference in losses should be minimal.

Understood, thanks.

I use a Midnite MNPV6 combiner box with midnight 150VDC breakers to join my 3S3P (eventually 3S6P) strings.

It just feels like every time you try to simplify something, you are forced to add another device... I guess it is the nature of the game.

And I thought a combiner box was required only above 3P.. did you add a combiner box because you knew you'd eventually expand to a 6P? Is this what happened? Would you still use a combiner box if you knew you'd stick with the 3S3P?

 

[sunshine_eggo]

Thank you!


I am not sure why this was brought up though... Also, for a 3S4P system I would go with a Victron 150/85:

Because I recommended a MPPT that addressed the 250V limit, but delivered a lower current.

It just feels like every time you try to simplify something, you are forced to add another device... I guess it is the nature of the game.

Yep... and code compliance doubles it...

And I thought a combiner box was required only above 3P.. did you add a combiner box because you knew you'd eventually expand to a 6P? Is this what happened? Would you still use a combiner box if you knew you'd stick with the 3S3P?

MC4 parallel connectors typically have a 30A limit. I don't think there's an arbitrary string count limit. Lower power panels could easily be 5S and still be within the MC4 connector limit. I've seen some that are designed for 4P in a single connector. Of course, you'd be limited to an in line MC4 fuse on each parallel string.

Once 30A is exceeded, you need a combiner box or other means of joining. My array was always planned as a 3S6P, I just haven't installed the last 3 strings. Lazy. 3kW has been more than enough even with a single RV A/C running.

 

[Old_Skewler]

MC4 parallel connectors typically have a 30A limit. I don't think there's an arbitrary string count limit. Lower power panels could easily be 5S and still be within the MC4 connector limit.

Yes, very clear.

I've seen some that are designed for 4P in a single connector.

Are you referring to a connector like this here:


They are still 30A.

Here is one possible ways I could wire a 4P in my situation, as I currently understand it:

Using a 4P Combiner Box
- run all (4) sets of PV wires on roof to Ironridge JBox,
- connect PV wire to THHN/THWN-2 inside Jbox using 8 Positions Dual Row Screw Terminal Strip Blocks (photo below)
- run (4) sets of THHN/THWN-2 inside 1" PVC conduit along roof and down the house to 4P Combiner Box
- run (1) set of THHN/THWN-2 to MPPT

Once 30A is exceeded, you need a combiner box or other means of joining.

Could I join the 4P strings inside the Ironridge Jaybox using this terminal strip block below?

 

[sunshine_eggo]

Are you referring to a connector like this here:


They are still 30A.

Yep.

Here is one possible ways I could wire a 4P in my situation, as I currently understand it:

Using a 4P Combiner Box
- run all (4) sets of PV wires on roof to Ironridge JBox,
- connect PV wire to THHN/THWN-2 inside Jbox using 8 Positions Dual Row Screw Terminal Strip Blocks (photo below)
- run (4) sets of THHN/THWN-2 inside 1" PVC conduit along roof and down the house to 4P Combiner Box
- run (1) set of THHN/THWN-2 to MPPT

Could I join the 4P strings inside the Ironridge Jaybox using this terminal strip block below?

If I understand you correctly, it sounds like it would work except I don't think the linked image would work due to the 25A rating... unless that's 25A per position.

 

[ShaneC]

You can build your own combiner box pretty cheaply and easily. Much cheaper than buying it prebuilt, here's one I put together in about an hour. It's only 2P but you can just add more fuse holders and adjust the breaker size to required.

 

[12VoltInstalls]

Correct, for those with budget constraints redundancy is a luxury.

Luxury is often way less expensive than emergencies.

 

[sunsurfer]

Luxury is often way less expensive than emergencies.

I agree. Also, I would bet most people that can't afford the better gear eat out allot at fast food, etc....
Most people can afford if they really wanted to make some simple sacrifices.

 

[Old_Skewler]

You can build your own combiner box pretty cheaply and easily. Much cheaper than buying it prebuilt, here's one I put together in about an hour. It's only 2P but you can just add more fuse holders and adjust the breaker size to required.
View attachment 157506
View attachment 157507

How are the breakers sizes determined? In this case of a 3S4P setup based on Isc 13.7A & Impp 12.9A, what over current protection rating should I have?

 

[sunshine_eggo]

How are the breakers sizes determined? In this case of a 3S4P setup based on Isc 13.7A & Impp 12.9A, what over current protection rating should I have?

Each fuse/breaker is only handling the current from one string. The panels have a maximum fuse value on their label. Use that size fuse.

 

[Old_Skewler]

Each fuse/breaker is only handling the current from one string. The panels have a maximum fuse value on their label. Use that size fuse.

Panel spec says "25A Max Fuse Rating".

So I need to find/make a 25A 4P Combiner Box? Or greater than 25A? (most combiner boxes on Amazon seem to be 10A or 15A)

Any protection on the combined line going from the combiner box to the MPPT? That could potentially run 4x13.7=54.8A.

I am trying to close the specs for the combiner box so that I can find one that fits the project.

 

[sunshine_eggo]

Panel spec says "25A Max Fuse Rating".

This indicates a fuse no larger than 25A.

So I need to find/make a 25A 4P Combiner Box? Or greater than 25A? (most combiner boxes on Amazon seem to be 10A or 15A)

15A and 20A would be fine provided they're 25% higher than Isc.

Any protection on the combined line going from the combiner box to the MPPT? That could potentially run 4x13.7=54.8A.

Yes.

 

[ShaneC]

I used 20A fuses, my panels max fuse is also 25A but their Isc is around 13. The protection for the combined current is from the central breaker which I've sized at 32a. I'm using 6mm cable so it should be safe.

 

[12VoltInstalls]

I agree. Also, I would bet most people that can't afford the better gear eat out allot at fast food, etc....
Most people can afford if they really wanted to make some simple sacrifices.

If you want something bad enough you’ll find a way. -Mike Tyson

 

[Old_Skewler]

If you want something bad enough you’ll find a way. -Mike Tyson

Enlightening and life changing... I just magically free up another $15K from this last Tesla stock run and I am really considering now purchasing an extra MPPT charger just so that I can join the exciting Redundancy Club. Or maybe not.. To be seen.. Like a very wise (and maybe drunk) man once told me... it is a decision …you are putting your income where you choose to - duh!

 

[12VoltInstalls]

Enlightening and life changing... I just magically free up another $15K from this last Tesla stock run and I am really considering now purchasing an extra MPPT charger just so that I can join the exciting Redundancy Club. Or maybe not.. To be seen.. Like a very wise (and maybe drunk) man once told me... it is a decision …you are putting your income where you choose to - duh!

Not everyone is positioned at that scale. Or fortunate: I thought Tesla was tanking a bit?

For many, it is more of a struggle to exceed their situation. For others, they think in terms of ‘can I afford the payment?’ Still others aren’t willing to sacrifice some things today to position themselves better down the road.
For myself- and I suspect many others- I try to spend wisely and frugally but not specifically cheap. I may buy certain items cheaply, or shop the lower-shelf products, but only if the backup/serviceability makes longer-term economic sense.

“Known” cheap products with good service life reports sort of require backups if one depends on them in my mind. And redundancy does buy you a bit of time to recover and keep you from being completely dead while you recover the failure. I’d feel similarly even with Victron- if something dies what’s the recovery time?
Off grid in my geography (Vermont ) it is cold, long winters. If I’m not charging or I lose an inverter or whatever and have nothing on hand or parallel it could be catastrophic!

So you do what’s necessary to maintain uptime 100%. I want that dependability so even early in when it was a struggle I found a way because I wanted my goals bad enough. Though I’m not drunk I agree; it is a decision to configure, sacrifice, and configure one’s life to reach a goal.

 

[efficientPV]

Each fuse/breaker is only handling the current from one string. The panels have a maximum fuse value on their label. Use that size fuse.

Let's use the example of #20 copper wire. Wire that size will handle 50A. Is that something you want to do? So the traces in the solar panel will handle 25A before the panel bursts into flames. 25A is a safety rating. If the panel experiences a lighting event, it might short out several bypass diodes. I experienced that exact thing. I replaced the diodes and the panels for the last couple years have been fine so far. Shorted diodes reduce the reverse voltage capability. The other panel strings are are now feeding current into the damaged string. Even if you didn't want to fix the one or two panels that are bad, The other ones in the string are cooking and will be damaged. The panels can tolerate a little above the Isc current without damage. Fusing should be very close to Isc current.

 

[Old_Skewler]

Moving forward with the recommended 3S4P configuration, planning to wire it as follows and could use some blessings from the group:



- (2) Strings combined via 50A MC4 Branch Connector (Staubli) protected with 15A inline fuses

Then combining these two lines via a 2-Parallel Combiner Box with 32A fuse + 63A circuit breaker (Ebay find)



How does this look?

 

[sunshine_eggo]

IMHO 13.68A Isc * 1.25 = 17.A

The general rule for resistive components is to operate at no more than 80% of rated current continuous.

I would use 20A inline fuses.

13.68Isc * 2 * 1.25 = 34.2A for each combined string INTO the combiner box. Assuming the 32A fuse links are intended to protect that circuit, 32A is too small IMHO.

 

[Old_Skewler]

IMHO 13.68A Isc * 1.25 = 17.A

The general rule for resistive components is to operate at no more than 80% of rated current continuous.

I would use 20A inline fuses.

Okay, I could upgrade to 20A. Thank you and appreciated.

13.68Isc * 2 * 1.25 = 34.2A for each combined string INTO the combiner box. Assuming the 32A fuse links are intended to protect that circuit, 32A is too small IMHO.

I understand the math behind but I just don't get how people are using combiner boxes when the largest one I was able to find for a 2-parallel string (32A & 63A) does not work with a 4kW 3S4P string. Amazon does not even have anything above 15A a breaker.

I would expect a range of combiner boxes on the market to satisfy these configurations but everything I see either tells me I am chasing the impossible rainbow OR that my string configuration 3S4P for these panels is out of whack.

I can't find a larger 32A combiner box on the market.

 

[sunshine_eggo]

Consider that you're trying to use a combiner box for two strings instead of a combiner box for 4 strings. You're doubling the current into the box, so it's not unexpected that you're hitting a limit.

I mentioned I have a MNPV6:

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

You then install your own 150VDC breakers, which are available in 5, 10, 15, 20, 25, 30, 40, 50 and 60A.

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

Here's a 4P combiner box:

https://www.amazon.com/Combiner-Lightning-Arreste-Breakers-Resistant/dp/B09WYJWF2G

The included fuses are 15A, but it looks like the holders are rated for 30A/1000V. Could simply replace with 20A fuses.

 

[Old_Skewler]

Consider that you're trying to use a combiner box for two strings instead of a combiner box for 4 strings. You're doubling the current into the box, so it's not unexpected that you're hitting a limit.

I mentioned I have a MNPV6:

You then install your own 150VDC breakers, which are available in 5, 10, 15, 20, 25, 30, 40, 50 and 60A.

I tried to go that route but the final product was up of $350

Here's a 4P combiner box:

https://www.amazon.com/Combiner-Lightning-Arreste-Breakers-Resistant/dp/B09WYJWF2G

The included fuses are 15A, but it looks like the holders are rated for 30A/1000V. Could simply replace with 20A fuses.

I saw this one but it says "2880W in 48V systems" limitation so even if I upgrade the fuses i'd still be over the rating for the box.

 

[sunshine_eggo]

I tried to go that route but the final product was up of $350



I saw this one but it says "2880W in 48V systems" limitation so even if I upgrade the fuses i'd still be over the rating for the box.

Yep. I couldn't see the rating on the combined breaker... 63A.

You may have locked yourself into a higher end solution with those high current panels.

If you can come in with a 4X 20A fused PV wires and then join via a suitably rated bus bar (like the 25A one you mentioned before, but higher rating) with 4awg output wire that is then fused/breakered downstream might work.

 

[Old_Skewler]

Yep. I couldn't see the rating on the combined breaker... 63A.

You may have locked yourself into a higher end solution with those high current panels.

If you can come in with a 4X 20A fused PV wires and then join via a suitably rated bus bar (like the 25A one you mentioned before, but higher rating) with 4awg output wire that is then fused/breakered downstream might work.

Yep, I am (slowly) getting to the same realization.

Thanks for all the help up to here.... back to design board...

 

[Hedges]

- more balanced sun exposure?

Multiple orientations has the advantage of reducing peak current while extending hours, might fit a lower amperage SCC.
You can also accept clipping on best days/seasons, especially if they would be producing more power than you need.

Once 30A is exceeded, you need a combiner box or other means of joining. My array was always planned as a 3S6P, I just haven't installed the last 3 strings. Lazy. 3kW has been more than enough even with a single RV A/C running.

4 PV strings, each with MC4 fuse. Into two, 2:1 Y cables. Either run two home-run wires to SCC or join in an outlet box to a home-run wire. This avoids a combiner box.

In-line MC4 fuse holders are touch-safe only while closed. Disconnect MC4 connector from both sides before opening to access fuse.

A DIN rail in a box with four touch-safe fuse holders would be convenient. I've bought stuff used on eBay.

Neither kind of fuse holder is suitable to make/break under load, so shut off somewhere else.

 

[timselectric]

Just make your own.
A din rail box, breakers, and an SPD.
Easy and cheap.