2 PV Strings

[rnehrboss]

Given the cost of wiring these days, can I run 2 strings of panel over one 10/3 wire? Kind of like you do on 240 split leg.
So I'd tie the negatives from each string to one wire (like the neutral) and then one string's positive to hot-red, and the other string's positve to hot-black.

Would that work? Am I then limited to 15 amps (since two are shareing the negative) instead of 30 amps?

Thanks

 

[timselectric]

No
They have a be isolated from each other.

 

[rnehrboss]

Thanks

 

[Hedges]

But you can parallel two strings, if current is within MPPT limits.
If the strings are oriented differently, current peaks at different times so they take turns making use of ampacity.

Besides, by "10/3" do you mean Romex? Where would use that for PV?
(UF could make sense.)

 

[millsan1]

definitely not

 

[zanydroid]

Would it work if you used half the voltage rating of two (unicorn) transformer isolated MPPTs?

Not that I would recommend it. Thought experiment for frankensteining two 250V strings together. Going higher would violate 600V limits in code and common equipment.

I believe 99% of stuff is non isolated now. Except for microinverters. Most I looked at are isolatef

 

[rnehrboss]

But you can parallel two strings, if current is within MPPT limits.
If the strings are oriented differently, current peaks at different times so they take turns making use of ampacity.

Besides, by "10/3" do you mean Romex? Where would use that for PV?
(UF could make sense.)

Yeah.. was planning UF for direct burial.. open to other/better suggestions

 

[zanydroid]

Maxing out the working voltage is another idea. EG push it to 500VDC and see what approaches pop out there.

How many panels are we talking about, what is their Voc/Vmpp, and what are the orientations? That would help with suggestions.

 

[rnehrboss]

Voc 36.9 Vmp 30.9
14 panels.. 2 strings of 7.. LuxPower 6000 inverter (same as EG4 6000xp)

 

[zanydroid]

Why 7s2p instead of 14s or 13s?

EDIT oops it’s 480Voc, adjust the math down accordingly.

 

[zanydroid]

Another possibility, purely addressing the issue with maxing out your value from a 10/2. is put a weather resistant grid tie inverter out at the array. You will boost the working voltage to 240VAC for lower loss versus your 7s. Code allows you to derate with 1.25 instead of 1.56 for this type of solar circuit. You can put as many separate MPPTs as you want out there and they will merge into one backhaul.

It increases costs in other places obviously. I’m just throwing it out as a pedagogical tool.

 

[rnehrboss]

Why 7s2p instead of 14s or 13s?

EDIT oops it’s 480Voc, adjust the math down accordingly.

2 mppt on inverter, so i assume it's better to run 2 strings? Am I off on that?

 

[zanydroid]

2 mppt on inverter, so i assume it's better to run 2 strings? Am I off on that?

You need to look at the full system design.

If the circuit wiring is the most important / bottleneck resource as (an exaggerated) interpretation of this thread would say, then maybe not.

If one MPPT is not enough capacity, then you need to use two.

If the hardware allows MPPT to be sync’d (I’m not sure 6000XP can), yet another design node to math out

 

[zanydroid]

It’s often better to spam out the full situation in a clean to understand format, then people can design it for you. Instead of doing a tiny peephole at a time

 

[timselectric]

2 mppt on inverter, so i assume it's better to run 2 strings? Am I off on that?

7s2p
One circuit to one MPPT.
Save the other MPPT for later. I'm sure that you will probably need it.

 

[Hedges]

Would it work if you used half the voltage rating of two (unicorn) transformer isolated MPPTs?

Not that I would recommend it. Thought experiment for frankensteining two 250V strings together. Going higher would violate 600V limits in code and common equipment.

I believe 99% of stuff is non isolated now. Except for microinverters. Most I looked at are isolatef

Maybe if one was negative ground and the other positive ground.
Until the GFCI fuse blows due to PV(most negative) shorting and PV(most positive) gets raised to 2x Voc.

It’s often better to spam out the full situation in a clean to understand format, then people can design it for you. Instead of doing a tiny peephole at a time

Yes, inverter and PV panel part numbers an link to specs. And wire run lengths. We can suggest what array and wiring works well.