Design temperatures for PV circuits in attic space

[ggunn]

Puts a high premium on keeping the trunk cable well above the roof surface and keeping it in free air.
It would be great for 2014 users if trunk cable with X... type conductors were available. You could still combine two trunk cables into one feeder circuit using better/larger wire.

Can you combine two AC circuits with no OCPD? I didn't think you could.

 

[GoldDigger]

Can you combine two AC circuits with no OCPD? I didn't think you could.

I do not see any reason (other than manufacturer instructions) to prevent joining two runs of trunk cable together. But you would have to make sure that the combined OCPD protects both runs individually.
The theoretical problem is this:
You can limit the normal in each segment to the derated ampacity of the cable section by limiting how many units you connect to it. And the OCPD can be sized according to manufacturer max values.
But you cannot limit the potential reverse fault current info a magic fault which is not a short circuit.
I cannot guess how AHJs would react to that situation.
The concept is vaguely similar to tap conductors only in reverse.

 

[Carultch]

Can you combine two AC circuits with no OCPD? I didn't think you could.

With the special exception of listed cables for doing precisely that, every inverter AC output circuit needs to connect to a dedicated OCPD.

 

[jaggedben]

Can you combine two AC circuits with no OCPD? I didn't think you could.

As long as your OCPD is not exceeding the capacity requirements of any piece of the trunk cable, you can parrallel different sections as much as you want. Enphase max OCPD instructions are based entirely on the trunk cable (although they don't seem to believe that any temperature deratings are required). They actually push 'centerfeeing' because it's advantageous for voltage rise/drop considerations.

 

[iwire]

Measure the attic temperature over the course of some hot summer days?

Do this in a few different attics and develop a 'rule of thumb' for a temperature adder over ambient outside temp?

I'm just trying to make some suggestions. I'm not aware of anything in the NEC that addresses this.

I agree.

Edit, never mind, responded to year old post.

 

[shortcircuit2]

But you cannot limit the potential reverse fault current info a magic fault which is not a short circuit.
I cannot guess how AHJs would react to that situation.
The concept is vaguely similar to tap conductors only in reverse.

What do you mean here?

 

[GoldDigger]

What do you mean here?

Using some actual hypothetical numbers:
Assume a trunk cable rated for 20A and a 20A breaker. This can support 16A of total module output (125% for continuous current).
Now assume that temperature correction at rooftop temperatures reduce the ampacity of the trunk cable to only 12A.
If I place 8A with of modules on each of two runs of trunk cable combined (without additional OCPD) into one 20A branch circuit there is no way that the inverter outputs can overload either trunk cable segment.
But a resistive short circuit at one of the modules (or a skinned cable in a water filled conduit) could theoretically draw 20A of fault current through the breaker indefinitely, overloading one 12A segment.
However if you are willing to accept this argument against this configuration you would also have to disallow 16A of modules on a 20A circuit with no derating because a hypothetical resistive fault could pull 20A from the breaker and 16A from the modules, causing a current of 36A near the fault.