basic wiring question, sorta...

frankson

I've got two sets of eight panels each (8 - 135 watt and 8 - 68 watt) where they are wired in series to 48 vdc in four sets of four each.

Each set of four comes into combiner box and it's own breaker, either 15 amp, one for each of the two 135 watt sets of four, or 10 amp, one for each of the two 68 watt sets of four.

There are two positive leads out of the combiner box, one for the two sets of 135 watt panels now combined and another for the two sets of 68 watt panels now combined.

They each go to two separate chargers, one for the now combined 8 135 watt panels and another for the now combined 8 68 watt panels.

My question is; Seeing as all the negatives for all 16 panels are all connected together in combiner box, can I have just one common negative come out of it go to both controllers AND can it be the same #8 gauge size as the two positive wires coming out of the combiner? There's only a couple feet between the combiner box and the two chargers.

Thank you for any insights here.

PS - Question might make a little more sense to add that between the combiner and two charge controllers wiring all goes through Mini Magnum Panel that controllers are wired into.

niel

Re: basic wiring question, sorta...

it is impossible for 1 wire to go to 2 controllers. if you did, it would be a seriesed arrangement and not be worth the hassle for a short wire run. furthermore, it can't be the same wire gauge as the 2 separate #8 wires because the current is now higher in a single #8 negative wire and will exhibit higher v drop losses so this would need to be the same equivalent gauge as 2 #8 wires or 1 #5 wire.

there may be other things you can do with your setup, but we lack all of the particulars of your present setup to be able to elaborate like are both going to a common battery bank? what is the battery bank voltage. what are the controllers used? why 2 separate systems? what is the run distance of the pvs to the combiners and the run distance from the combiners to the controllers? etc etc

SolaRevolution

Re: basic wiring question, sorta...

frankson wrote: »

My question is; Seeing as all the negatives for all 16 panels are all connected together in combiner box, can I have just one common negative come out of it go to both controllers AND can it be the same #8 gauge size as the two positive wires coming out of the combiner? There's only a couple feet between the combiner box and the two chargers.

It mostly depends on the charge controllers. Most (not all) MPPT charge controllers need to have the PV + and - wire runs isolated so they can get a good "read" on the I/V (current/voltage) curve. If you are using PWM charge controllers and if your combined neutral can handle the full current of all four strings it could work. Check and see if the CC's installation instructions specify that it is okay to wire it with only one neutral connection (to the battery - terminal) of the CC. If it does not specify that this is okay, you had better run two seperate neutrals.


On the other hand, as long as all of your wire (esp. the Neutral) is sized to handle the total current, and if your different sets of panels have very similar voltages, you could just parallel all of them at the combiner and run them all the way into one charge controller of any kind (provided it too has an adequate current rating for all 4 strings).

Alex Aragon

frankson

Re: basic wiring question, sorta...

Alex & Niel,
I have two Outback FM80's, one for the 8 135 panels and one for the 8 68 watt panels.

These panels are wired four together in series of four so that each of the four sets of four panels are at 48 vdc when their wires come into combiner box.

In combiner box the two sets of 135 watt panels are combined on positive side and, separately, the two sets of 68 watt panels have their positives combined.

Those two positives (8 gauge each) each then go to their respective and separate FM80's via Magnum mini panel.

Those two positives (8 gauge each) going out to their respective charge controllers are never connected to each other.

In the combiner box all the negatives for all the 16 panels are all common connected in there on one bus.

The negative inputs for both charge controllers are also already connected together inside the Magnum mini panel.

My question is; is above good/bad/ugly and also, if above OK, how big a gauge single negative input do I need to run between
the combiner box negative and mini panel negative bus where both the charger negative inputs are already connected together?

Make sense?

Cariboocoot

Re: basic wiring question, sorta...

On Outback controllers the PV IN (-) and BATT OUT (-) are common terminals; it is a "straight through" connection.

If you have two 8 AWG positive wires connected to two separate arrays & controllers they can not share one common 8 AWG negative wire for the reason Niel mentioned: the one 8 AWG negative is not sufficient to carry the current of the two 8 AWG positives. The negative cable would have to be increased in size to 4 AWG, which isn't going to fit in the OB terminals. Run two wires.

It also sounds as though your panels are not properly fused against overload by adjacent panels. Each string of four panels needs to have circuit protection according to its Isc. Those 68 Watt panels are probably only capable of 4 Amps, so even a 10 Amp breaker is too large to protect in case of a short.

frankson

Re: basic wiring question, sorta...

Cariboocoot wrote: »

If you have two 8 AWG positive wires connected to two separate arrays & controllers they can not share one common 8 AWG negative wire for the reason Niel mentioned: the one 8 AWG negative is not sufficient to carry the current of the two 8 AWG positives. The negative cable would have to be increased in size to 4 AWG, which isn't going to fit in the OB terminals. Run two wires.

Doing that now, many thanks!

vtmaps

Re: basic wiring question, sorta...

Cariboocoot wrote: »

It also sounds as though your panels are not properly fused against overload by adjacent panels. Each string of four panels needs to have circuit protection according to its Isc. Those 68 Watt panels are probably only capable of 4 Amps, so even a 10 Amp breaker is too large to protect in case of a short.

As I read the OP's wiring description (am I missing something?) he does not need fusing in each string of panels. He has two strings of 135 watt panels going to one fm80 and two strings of 68 watt panels going to another fm80.

--vtMaps

Cariboocoot

Re: basic wiring question, sorta...

vtmaps wrote: »

As I read the OP's wiring description (am I missing something?) he does not need fusing in each string of panels. He has two strings of 135 watt panels going to one fm80 and two strings of 68 watt panels going to another fm80.

--vtMaps

No, you're not missing something; I read it wrong.
As you describe it will work fine.

frankson

Re: basic wiring question, sorta...

vtmaps wrote: »

As I read the OP's wiring description (am I missing something?) he does not need fusing in each string of panels. He has two strings of 135 watt panels going to one fm80 and two strings of 68 watt panels going to another fm80.

--vtMaps

Actually, just to add some more detail, I do have in the combiner box a 15 amp breaker on each of the
two sets of four 135 watt panels and a 10 amp breaker on each of the two sets of four 68 watt panels.

Again, these are all coming in at 48vdc.

Cariboocoot

Re: basic wiring question, sorta...

Like vtMaps said, the two strings per controller don't really need fusing. That's because there's only two in parallel, and one can not force too much current into the other because they both have the same current potential.

Otherwise, the 135's are probably capable of 8 Amps maximum, so a 15 Amp breaker could never trip. The 68 Watt panels are probably about 4 Amps each so again the 10 Amp breakers could never trip.

Unless I'm getting confused again, the total 1080 Watt array on the first controller would be 16 Amps @ Isc all together and could probably run through a single 15 Amp breaker. Likewise with the other panels & controller and one 10 Amp breaker. It depends on what the exact panel specs are.

And if I am confused again I can only say that answering 20+ questions a day (on average) tends to blend all the systems into one. Understandably, people tend to ask very similar questions.

frankson

Re: basic wiring question, sorta...

Cariboocoot wrote: »

Unless I'm getting confused again, the total 1080 Watt array on the first controller would be 16 Amps @ Isc all together and could probably run through a single 15 Amp breaker. Likewise with the other panels & controller and one 10 Amp breaker. It depends on what the exact panel specs are.

Here are the STC numbers if you want to glance them over...

The eight 135 watt panels are Kyocera KD-135-SX-UPU, 135 watt 12vdc
Pmp 135 W
Vmp 17.7 V
Imp 7.63 A
Isc 8.37
Voc 22.1 V
Series Fuse Rating 15 A

The eight 68 watt panels are Uni-Solar PVL-68, 68 watt 12vdc
Pmax: 68 W
Vmp: 16.5 V
Imp: 4.1 A
Isc: 5.1 A
Voc: 23.1 V
Maximum Series Fuse Rating: 8 A

Does above fuse ratings tell us all we need to know in choosing breakers inside combiner box?

Again, for both, wired four together in series for 48vdc

Cariboocoot

Re: basic wiring question, sorta...

Generally you can't go wrong using the manufacturer's recommended series fuse rating. So you're good on the Kyoceras, but a little high on the others. I wouldn't worry about it though as there are only the two parallel connections on each controller.

frankson

Re: basic wiring question, sorta...

Thank you, Cariboocoot, much appreciated here!