5KW System Design Help Requested - Array Disconnect location

[Scratchbuilt]

So getting back to my original question. Does the 200A disconnect need to be fused if there is only one output circuit? Also, SolarPro, three combiners are not necessary. We installed a Midnite Solar MNPV16 combiner box which has fuse accommodations up to 16 strings.

BTW, thanks for everyone's help and responses. I will be coming here a lot more in the future. There seems to be a pretty good well of knowledge on this Forum.

 

[SolarPro]

It will pass inspection! It may be costly but it will pass!

And let's be honest, anyone buying a system with these particular BIPV modules isn't afraid of costly solutions.

It will look great either way.

 

[SolarPro]

So getting back to my original question. Does the 200A disconnect need to be fused if there is only one output circuit?

Logic says no. There is no parallel source for potential fault currents.

The code, unfortunately, is less clear.

690.9 Overcurrent Protection.
(A) Circuits and Equipment. Photovoltaic source circuit, photovoltaic output circuit, inverter output circuit, and storage battery circuit conductors and equipment shall be protected in accordance with the requirements of Article 240. Circuits connected to more than one electrical source shall have overcurrent devices located so as to provide overcurrent protection from all sources.

Exception: 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:

(a) There are no external sources such as parallel-connected source circuits, batteries, or backfeed from inverters.

(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 nameplate.

Note that the Exception does not specifically mention PV output circuits.

 

[SolarPro]

Also, SolarPro, three combiners are not necessary. We installed a Midnite Solar MNPV16 combiner box which has fuse accommodations up to 16 strings.

Correct. I was brainstorming ways to design around the 200 A disconnect. Suffice it to say that there are Code-compliant alternatives. But since the existing design is approved by the AHJ, you'll want to adhere to that design.

 

[jaggedben]

And this is why I am here asking people with more experience. I thought all of this was strange from the get go but i am glad to get some other opinions that back up my original thinking.

I think a lot of us here with plenty of solar experience have never installed solar shingles.

It will pass inspection! It may be costly but it will pass!

More power to you.

 

[Scratchbuilt]

So here is what I have never quite understood about fusing the PV Output circuit when only one is installed from the combiner to the inverter. If the amperage of the system is calculated from the short circuit current, and then a 125% irradiance factor is tacked on, which is saying that the strings "could" produce that amount of current, but probably won't, and then you add an additional 125% on top of the increased ampacity ( the 156% rule, I think right?) for conductor ampacity. Why is the fuse even necessary in a single source output circuit? When is that fuse ever going to blow and what would ever cause it to blow if the maximum amount of current is dictated by the short circuit current. The strings can never achieve an ampacity greater than the short circuit current. It's not like we have 65K of fault current and are trying to limit it with a current limiting fuse to achieve acceptable AIC ratings (22K, 10K) for the panels and circuit breakers.

Just rambling but I have never quite understood the logic behind it

 

[Scratchbuilt]

Correct. I was brainstorming ways to design around the 200 A disconnect. Suffice it to say that there are Code-compliant alternatives. But since the existing design is approved by the AHJ, you'll want to adhere to that design.

I got ya. Smart idea of getting around using a 200A disco. Two combiner boxes feeding two 60A rated output circuits into a 60A disco. Hmmm, that has me thinking now......

 

[jaggedben]

So here is what I have never quite understood about fusing the PV Output circuit when only one is installed from the combiner to the inverter. If the amperage of the system is calculated from the short circuit current, and then a 125% irradiance factor is tacked on, which is saying that the strings "could" produce that amount of current, but probably won't, and then you add an additional 125% on top of the increased ampacity ( the 156% rule, I think right?) for conductor ampacity. Why is the fuse even necessary in a single source output circuit? When is that fuse ever going to blow and what would ever cause it to blow if the maximum amount of current is dictated by the short circuit current. The strings can never achieve an ampacity greater than the short circuit current. It's not like we have 65K of fault current and are trying to limit it with a current limiting fuse to achieve acceptable AIC ratings (22K, 10K) for the panels and circuit breakers.

Just rambling but I have never quite understood the logic behind it

The fuse is not necessary with a single output circuit. The is part (b) of the Exception to 690.9(A). You don't need the fuse if your conductors are rated for the total short-circuit as calculated in 690.8. And 690.8 requires that they must be rated as such for a combined output circuit.

You just need fuses on all your source circuits at your combiner, that takes care of your overcurrent protection.

 

[SolarPro]

That has always been my understanding as well. Just be aware that in NEC 2011, the 690.9(A) Exception only mentions PV modules and PV source circuits. An AHJ could take that to mean that the exception does not apply to PV output circuits.

Logically, I don't know why the exception wouldn't apply beyond the PV source circuits, for the reasons mentioned above. But the language in the exception was modified as part of the 2011 cycle of revisions such that it is more specific than in previous Code cycles. Maybe the RFP and RFC documents for NEC 2011 provide some insight as to why this change was made?

I'm only aware of this change because someone somewhere was taken to task by an AHJ in situation like the one described above. Not sure if this came up here or on the RE Wrenches list. Probably doesn't come up often. Most AHJs don't know a PV source circuit from a PV output circuit, and those that do probably follow why a fuse won't provide any protection in a system w/ a single output circuit.

 

[jaggedben]

That has always been my understanding as well. Just be aware that in NEC 2011, the 690.9(A) Exception only mentions PV modules and PV source circuits. An AHJ could take that to mean that the exception does not apply to PV output circuits.

Even with that language, in my opinion the output circuit is protected from 'all sources', as required in the language, so long as each source circuit has an appropriate OCPD. Of course this is a finer point that I wouldn't want to have to argue with an AHJ.

 

[Zee]

My conductors will be # 2 THHN for the output circuit.

Use THWN-2. 90 degrees rated in wet conditions.
THHN = 90 degrees rated in dry conditions.
THWN = 75 degrees rated if wet.
I believe.
Most wire is rated both THHN and THWN-2, but not all.

 

[GoldDigger]

Use THWN-2. 90 degrees rated in wet conditions.
THHN = 90 degrees rated in dry conditions.
THWN = 75 degrees rated if wet.
I believe.
Most wire is rated both THHN and THWN-2, but not all.

Especially not anything you find on the shelves rather than the elevator at Big Orange.

 

[Zee]

and, my understanding is that conduit outdoors is "wet conditions" per NEC.

Condensation will collect and flow down----> standing water inside conduit is typical.