Misc questions on tips for rooftop circuit runs for AC/DC compatibility

[zanydroid]

I'm planning out a microinverter installation for an expansion of my residential rooftop system. I may want to convert part or all of this to a DC system in the future. Total power going into a single conduit would be 5.2kW-DC.

Planning out how microinverters should be cabled up seems pretty trivial. But I'm less confident about DC system.

What are some rules of thumb you like for planning out the conduit?

According to conduit calculator, I can fit 8 #10 + 1 #8 AWG into 3/4 EMT (4 circuits and 1 #8 ground, I want the redundant EGC path, #8 upsized in case the #10 needs to be derated, which would force the EGC to be derated too along with it) with a very very snug just below 40% conduit fill. This is way more than I need in ampacity, only 28A wire is needed to carry 5.3kW @ 240V, and allows me to split an array into up to 4 strings.

What are the nice ways to handle bends across roof planes? I was thinking flexible liquidtight (either metallic or non-metallic, depending on whether it's below the roof, and whether it would make bonding complicated for EMT conduit). I'd rather use standard angle bend fittings over using a pipe bender for roof work.

What are the pros/cons for stubbing into the roof immediately? Professional installation I used ran some really long EMT on top of the roof all the way to a single roof penetration on another roof plane. Like 80 feet... It looked pretty bad.

After stubbing into the attic, do you like multiple MC circuits or a shared metallic conduit? I can see some arguments either way (conduit looks better, MC doesn't need as much derating calculations and lower chance of mixing up strings).

Other thoughts

Part of the reason for 3/4 is that I already have a bender for 3/4 EMT, but I guess I don't need one if I use fittings / flex elbows, and I don't mind buying another size EMT cutter, easy to find space for that.

Previous rooftop installation used 3/4 EMT for an AC system (overkill?) on the roof. SCH40 would work above the roof, however it might look worse with stretching and may exceed the allowed ambient temp up there.

 

[Bobert]

Calculate how much your wire needs to be derated before you do anything. 3 current carrying conductors or more must be derated if they are bundled together in conduit this is in addition to derating the wires for the high heat conditions normally experienced on a roof. I am not a licensed electrician ( no such license exists in my area for residential wiring) and in residential applications I have only had to do this calculation once. It was quite an eye opener and if I recall correctly I ended up figuring out a different way to run my wire as the loss was that significant.

 

[zanydroid]

Calculate how much your wire needs to be derated before you do anything. 3 current carrying conductors or more must be derated if they are bundled together in conduit this is in addition to derating the wires for the high heat conditions normally experienced on a roof. I am not a licensed electrician ( no such license exists in my area for residential wiring) and in residential applications I have only had to do this calculation once. It was quite an eye opener and if I recall correctly I ended up figuring out a different way to run my wire as the loss was that significant.

Yes I will do that, I have the derating calcs for my roof already with the wiring method used by my previous installer. Mainly that would be the roof derating table that I am not familiar with for NEC. They had used 4 CCC in their design.

Compared with what I posted going with 6 instead of 8 current carrying conductors would allow more ampacity to go in those cables, as well as avoid filling the conduit to 40% (should be 30% instead)

For #10 and smaller conductors, they are subject to the small conductor rule which means they anyway can’t be used above 60C ampacities. Conversely conduit fill derating hurts you more with bigger wire, which can be a strong argument to avoid running big wire long distances in shared conduit if it can be avoided. This is also an argument for going with MC or multiple smaller conduits in the attic.