Do all combiner boxes need a disconnect?

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sketchy

Senior Member
Location
MN
I plan on installing a combiner box to combine 5 strings into 1 than runs to the inverter. Is a disconnect required?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I plan on installing a combiner box to combine 5 strings into 1 than runs to the inverter. Is a disconnect required?
In most cases, no, as long as there is a DC disconnecting means at or in the inverter.
 

sketchy

Senior Member
Location
MN
But if the combiner box fuses the strings, wouldn't a disconnect be required per 690.16(A)?
 

sketchy

Senior Member
Location
MN
In my installation it goes from service - meter - disco - sub panel - inverter (inside building) - combiner box (maybe combiner/disco) - array. The AHJ said a disco wasn't needed, hence my confusion.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
In my installation it goes from service - meter - disco - sub panel - inverter (inside building) - combiner box (maybe combiner/disco) - array. The AHJ said a disco wasn't needed, hence my confusion.

Since the inverter is inside the building, you need at a minimum a DC disco at the inverter end (which may be integrated with the inverter in some cases).
And under code you need either a disco just before the wires enter the building from the roof or, using the exception that allows the disco to be at the inverter end instead, you need a disco within sight of the panel array. The MC4 or other connectors coming off each panel or off the end of each string can fill this role.
For servicing and troubleshooting purposes, integrating that disco function into the combiner box along with the OCPD is attractive.
 

sketchy

Senior Member
Location
MN
If I install a combiner that fuses 5 strings with the finger safe fuses, then disconnect those 5 strings, all I'm doing is disconnecting power form the combiner to the inverter. In my case, that will be a pair of #6 wires. What purpose does that serve? I already have the fuses in the combiner box.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
But most of the time cannot not serve as a load-breaking disconnect for DC.
Therefore use depends on what the purpose of the disconnect is seen to be.
The code does not specify that the fuse isolating disconnecting means must be load break rated. If you have a load break rated DC disco in or at the inverter and touch safe fuse holders in the DC combiner, in most situations that is all you need. If the DC goes inside a building, then other rules may apply.
 

sketchy

Senior Member
Location
MN
The inverter has an integrated DC disco. It is a ground mounted array, with the DC conductors running on the outside of the building. I am leaning toward just a combiner box.
 

Garrison

Member
Location
Chicago, IL
If you are on the 2011 NEC, and you are installing source circuit fuses, then 690.16(B) requires a disconnecting means to be installed on the PV output circuit. In my opinion a touch safe fuse holder/fuse on the source circuit would not meet this requirement. An inverter integrated disconnect will meet the requirement, but then, ?a directory showing the location of each disconnect shall be installed at the overcurrent device location.? A sticker may be cheaper than a disconnect, but the disconnect is more useful.

My $.02
 

sketchy

Senior Member
Location
MN
So after watching Mike Holt's dvd on solar, I think that not only do I need a combiner/disconnect, but it has to be fused as well. Am I seeing this install clearly? Because I am fusing the strings but after they are combined into a larger wire, there is no fusing.
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
So after watching Mike Holt's dvd on solar, I think that not only do I need a combiner/disconnect, but it has to be fused as well. Am I seeing this install clearly?

Any time you have more then two strings in parallel, you need one OCPD per string at or below the maximum series fuse value listed for the panel. The easiest way to furnish this is to use a combiner containing either fuses or CBs.
 

sketchy

Senior Member
Location
MN
Any time you have more then two strings in parallel, you need one OCPD per string at or below the maximum series fuse value listed for the panel. The easiest way to furnish this is to use a combiner containing either fuses or CBs.

I will have that fusing via the combiner box. But after it leaves the combiner box, those circuits are not fused or are you saying the string fuses satisfy the fusing for the larger wire.
 

sketchy

Senior Member
Location
MN
Or does 690.9 (A) exception apply? An overcurrent device shall not be required for PV modules or PV source circuit conductors sized in accordance with 690.8(B) where one of the following applies:(b) the short-circuit currents from all sources do not exceed the ampacity of the conductors or the maximum overcurrent protective device size specified on the PV module template
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Or does 690.9 (A) exception apply? An overcurrent device shall not be required for PV modules or PV source circuit conductors sized in accordance with 690.8(B) where one of the following applies:(b) the short-circuit currents from all sources do not exceed the ampacity of the conductors or the maximum overcurrent protective device size specified on the PV module template

That applies, but if you have three or more modules or strings in parallel, it is very probable that the current produced by two of the strings will be greater than the max OCPD specified. With four strings in parallel it is a certainty.
With only two strings, it is equally certain that the single string cannot produce more current than the other string and therefore the current must be less than the max OCPD.
As for the wire ampacity, that varies with your installation practice.
In this case the OR in the code section means that it cannot exceed either of them, so both conditions must be met to get the exception.
 

sketchy

Senior Member
Location
MN
So I guess the question I have now is does the output current from the combiner to the inverter need to be fused? I will be combining 6 strings, 7.9 Isc, which equals 73.9 amp. That 73.9 amp load is where I am having confusion.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
So I guess the question I have now is does the output current from the combiner to the inverter need to be fused? I will be combining 6 strings, 7.9 Isc, which equals 73.9 amp. That 73.9 amp load is where I am having confusion.

The code says that the conductors from the combiner to the inverter need to be protected.
If the sum of the individual fuses or breakers is greater than the wire ampacity, then you will be better off to also use a main breaker, even though the panels cannot actually deliver that current. There may be an alternate calculation where you apply a safety factor to the actual panel output current instead.
And in some specific cases you can put a disconnect and the OCPD protecting the wiring at the inverter end, even though the wiring goes through the building.
The details are complicated, and I will not try to explain it all here. (Because I will probably get it wrong the first few times. :))
 
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