diy solar

diy solar

PV Combiner box - help

T

totalconfusion

New Member
Joined
Dec 24, 2020
Messages
62
Hello wonderful people,

I’m making a PV combiner box and wanted to run some questions past you guys before I commit purchasing anything.

The combiner box will be used with a 2S4P PV set up.

Guilin Xinyou-xy190-72F-M5.jpg

Total number of PV panels
8.00​
Total Open circuit voltage (Voc)99.88V
Total Short circuit current (Isc)21.56A

From what I have gathered, a combiner box should consist of:

# Individual fuses for each STRING of panels
# SPD (Surge protection devices) also called TVSS (Transient Voltage Surge Suppressor)
# A switch

Are the following statements correct?

1. The individual fuses should be rated at 1.25x total short circuit current of each string – in this case, 26.95A

2. We only need one fuse per string as only the positive is being fused

3. The SPD will do nothing if it isn’t hooked up to an earth (grounding rod)


Questions:

1. Can the 2x individual fuses and the 2P switch be replaced with a single 4P circuit breaker? If so, are circuit breaker current ratings for each INDIVIDUAL input or is it a CUMULATIVE total

2. For the isolation switch, do we need to switch both positive and negative poles? Would we need a 2P unit or 4P

3. I can’t find the Maximum Series Fuse Rating on the panel spec sheet. Can we calculate it some other way?


Thanks a bunch people
 
Quote
"1. The individual fuses should be rated at 1.25x total short circuit current of each string – in this case, 26.95A"

In a series string the Isc of (one of) the panels is the Isc of the entire string. In this case 5.39 amps.


As to the series fuse rating it looks like 10 amps will be your most likely rating.
 
Last edited:
I only use fuses on the end of each series string of I have more than 2 strings in parallel to protect against fire in the event of the catastrophic failure of a solar panel. I don’t use any connections or fuses that are not necessary as every connection is an additional failure point and additional resistance. I use a double pole breaker rated well above my max dc voltage and current as a shutoff for maintenance purposes but not for protection as solar panels are power limited.
 
littleharbor2 said:
In a series string the Isc of (one of) the panels is the Isc of the entire string. In this case 5.39 amps.
Click to expand...
The set up I'm suggesting is 2S4P - Each of the two strings will have a total of 26.95A. Unless I'm getting my terminology incorrect I don't understand what you are saying.

As to the series fuse rating it looks like 10 amps will be your most likely rating.
Click to expand...
Is there any conventional way to calculate this? Is this just an educated guess?



Fingers said:
Why only fuse the positive?
Click to expand...
For a 2S4P PV set up, how many fuses would you suggest using? I assumed it would only be on the positive side and one fuse for each STRING, so two in total.

For that spd, you'll be looking for a un of around 130 to 150 volts and a uc of around 180 to 200.
Click to expand...
I'm unfamiliar with sizing up SPD's. What do these values mean and what are they based off?



Bobert said:
I only use fuses on the end of each series string of I have more than 2 strings in parallel to protect against fire in the event of the catastrophic failure of a solar panel. I don’t use any connections or fuses that are not necessary as every connection is an additional failure point and additional resistance. I use a double pole breaker rated well above my max dc voltage and current as a shutoff for maintenance purposes but not for protection as solar panels are power limited.
Click to expand...
That's a fair point regarding additional connections representing potential problems.
 
totalconfusion said:
For a 2S4P PV set up, how many fuses would you suggest using? I assumed it would only be on the positive side and one fuse for each STRING, so two in total.
Click to expand...
Fusing only positive is fine. One fuse per string.
2s4p is 4 strings of 2 panels. (2 panels in series x 4 parallel strings)
4 strings, means 4 fuses.
 
totalconfusion said:
Is there any conventional way to calculate this? Is this just an educated guess?
Click to expand...
Your panels are rated at 5.39 Isc. Hence your strings are the same, 5.39 amps, (regardless how many panels are in the string).
5.39 x 1.25 = 6.74 amps. The nearest common fuse is likely 10 amps. If you can find an 8 amp fuse use it.
The wire on your panels is either 12 or 10 ga. 12 ga is good for 20 amps and 10 ga. is good for 30 amps so clearly the wire can take normal current. This is why you need to fuse every string when you have 3 or more strings. The combined current of three or more strings is enough to cause a meltdown in the event of a short circuit in the combiner.
 
Last edited:
pv.drawio.png

timselectric said:
Fusing only positive is fine. One fuse per string.
2s4p is 4 strings of 2 panels. (2 panels in series x 4 parallel strings)
4 strings, means 4 fuses.
Click to expand...
Yeah I think I spaced out on the series parallel thing, 2S4P is definitely 4 strings of of 2 panels in series.
Will I need to use inline MC4 fuses, or can all the fusing be done inside the combiner box?


littleharbor2 said:
Your panels are rated at 5.39 Isc. Hence your strings are the same, 5.39 amps, (regardless how many panels are in the string).
5.39 x 1.25 = 6.74 amps. The nearest common fuse is likely 10 amps. If you can find an 8 amp fuse use it.
The wire on your panels is either 12 or 10 ga. 12 ga is good for 20 amps and 10 ga. is good for 30 amps so clearly the wire can take normal current. This is why you need to fuse every string when you have 3 or more strings. The combined current of three or more strings is enough to cause a meltdown in the event of a short circuit in the combiner.
Click to expand...
Will I need to fuse the strings BEFORE they go in to the branch connectors?
 
totalconfusion said:
Will I need to fuse the strings BEFORE they go in to the branch connectors?
Click to expand...
The 4-way branch connectors are a cheap way of combining the individual strings. If you decide to go with them you will need to use inline MC-4 fuses. There's a problem in that you need to increase the wire size to accept the high current of the combined strings. You may find it hard to find MC-4 connectors that will accept larger wire. A cleaner, simpler method would be to use a real combiner. Pricewise you might pay a little bit more for the combiner box but you end up with a better product.

The way you have drawn the wiring with 4-way harnesses, the combiner box has nothing to combine. If you can locate the combiner box at the array it will be a cleaner wiring strategy as well and the logical place to increase the wire size before heading to the charge controller.
 
Last edited:
timselectric said:
Either is acceptable.
Click to expand...

Thinking about it, it seems like I should put FOUR 10A in-line MC4 fuses on the positive connections BEFORE the positive branch connector
and put a 27A (32A more likely) 2P circuit breaker in the combiner box.

Does this updated wiring diagram look any more sane?
pv_and_combiner_box.drawio.png



Littleharbor, your post just came through as I was typing
littleharbor2 said:
The 4-way branch connectors are a cheap way of combining the individual strings. If you decide to go with them you will need to use inline MC-4 fuses. There's a problem in that you need to increase the wire size to accept the high current of the combined strings. You may find it hard to find MC-4 connectors that will accept larger wire. A cleaner, simpler method would be to use a real combiner. Pricewise you might pay a little bit more for the combiner box but you end up with a better product.
Click to expand...
Yes, I think I'm only now appreciating that the branch connectors are maybe more trouble than they are worth. I should probably just run each 2-panel string directly to the combiner box and fuse them there. Not screw around in-line fuses and also don't have to deal with 21.56A going through a 4mm-6mm2 wire
 
totalconfusion said:
Thinking about it, it seems like I should put FOUR 10A in-line MC4 fuses on the positive connections BEFORE the positive branch connector
and put a 27A (32A more likely) 2P circuit breaker in the combiner box.
Click to expand...
You don't need both, branch connectors and a combiner box.
They both serve the same purpose. (Combining strings)
Pick one or the other.
 
littleharbor2 said:
totalconfusion
My apologies, I was editing my post while your post came in. I added one more comment.
Click to expand...
timselectric said:
Yes
Click to expand...

I've finished my design and would love some input especially regarding the wiring and wire sizing.
I don't know how to tag you guys without quoting you so apologies for that.

The design has been upgraded to a 2S4P PV set up with no branch connectors as per the new uploaded image.

Thanks a million for all your help. This forum is an oasis of knowledge.

The LiFePO4 batteries are rated for 0.5C so I acknowledge they will only be good for a continuous discharge of 50A, whereas the whole system should be good to serve 100A, this is because I intend to upgrade the batteries with another 100AH in parallel in the near future. I should fuse the load output to 50A until it's upgraded to avoid any unwanted damage to the batteries.

Datasheets:
MPPT

Battery Protect

PV Panels:
https://imgur.com/a/t58CjL7

Power distribution board:
www.ebay.com.au

12 Way Blade Fuse Box Block Holder Marine LED Light 12V 32V Auto Car Accessories | eBay

Find many great new & used options and get the best deals for 12 Way Blade Fuse Box Block Holder Marine LED Light 12V 32V Auto Car Accessories at the best online prices at eBay!
www.ebay.com.au

Batteries:
https://imgur.com/a/NcL31ki
  • Nominal voltage- 12.8V
  • Discharge Cut-Off Voltage- 10V
  • Charge Cut-Off Voltage- 14.8V
  • Standard Discharge- 50A-0.5C
  • Standard Charge- 50A-0.5C
  • Fast Charge- 0.5C
  • Fast Discharge- 1C
  • Maximum Charge Current- 1C
  • Maximum peak discharge current- 2C
  • Weight- 2.25kg per cell approx 9kg for set of 4
  • Cycle life- >2000+ times. Cell will still have 80% available


final.drawio.png
 

Attachments

  • final.drawio.png
    final.drawio.png
    329.4 KB · Views: 2
Last edited:
You just have to make sure that their clamping voltage is above the normal expected circuit voltage. So that they are only activated by a surge.
 
You must log in or register to reply here.

Similar threads

Kleazy
Combiner Box > Why fuse the negative?
Replies
14
Views
517
Kleazy
Kleazy
T
pv combiner box
Replies
1
Views
248
cdsolar
C
JWLV
PV combiner box
Replies
8
Views
663
AmpD
A
D
Combiner box
Replies
4
Views
390
Dennis 1
D
WattAboutThat
Combiner Box at PV Array or IMO Disconnect???
Replies
10
Views
921
SilverbackMP
S
Back
Top