PV conductor transition from PV cable to THWN-2

Designer69

Senior Member
I am reviewing this project and have a question about the cables transitioning from PV cable to THWN-2.

they show "quantity 3" PV cables to transition JB but "quantity 1" THWN-2 cables from JB to inverter.

Shouldn't it also be "quantity 3" THWN-2 cables from JB to inverter?

Please see pic for better info and let me know how you interpret it..

pic: https://ibb.co/tMYJMt9

Thank You
 

tkb

Senior Member
I have found that the solar engineering leaves a lot to be desired.
I guess anyone can become a solar engineer.
 

ggunn

PE (Electrical), NABCEP certified
I have found that the solar engineering leaves a lot to be desired.
I guess anyone can become a solar engineer.
I don't know what you mean by the first comment, but I agree with the second one. Anyone can become an engineer; all you have to do is go to school and earn an engineering degree.
 

jaggedben

Senior Member
Seems to me they should both say 6.

PV design is super competitive in many areas now, people are churning them out and don't necessarily make the time to proofread.
 

beanland

Senior Member
DC

DC

I am reviewing this project and have a question about the cables transitioning from PV cable to THWN-2.
they show "quantity 3" PV cables to transition JB but "quantity 1" THWN-2 cables from JB to inverter.
Shouldn't it also be "quantity 3" THWN-2 cables from JB to inverter?
Please see pic for better info and let me know how you interpret it..
Thank You
I assume this is a DC combiner? It is not unusal to have several strings using PV Wire in air come to a DC combiner and leave as a single circuit using THWN in EMT or PVC.

You cannot just parallel strings of modules without overcurrent protection so 3:1 is not just a simple splice.

For those who responded without addressing the question, thanks for nothing!
 

jaggedben

Senior Member
I assume this is a DC combiner? It is not unusal to have several strings using PV Wire in air come to a DC combiner and leave as a single circuit using THWN in EMT or PVC.

You cannot just parallel strings of modules without overcurrent protection so 3:1 is not just a simple splice.

For those who responded without addressing the question, thanks for nothing!
They are showing a Solaredge inverter with 3 strings landed on a common bus. No combiner. Solaredge is fine with this but provides an alternative version with fuses in case the AHJ insists on it.
 

Designer69

Senior Member
You cannot just parallel strings of modules without overcurrent protection so 3:1 is not just a simple splice.
Each string must have it's own individual ocpd? So the combiner box has a fuse for each string? Is this this a code requirement somewhere?


I understand there is only one DC ocpd at the solaredge inverter.
 

Designer69

Senior Member
They are showing a Solaredge inverter with 3 strings landed on a common bus. No combiner. Solaredge is fine with this but provides an alternative version with fuses in case the AHJ insists on it.

the riser seems to show the 3 strings landed on a common bus right at the inverter (which would indicate to me actually 6 thwn-2 cables from JB to inverter)

however, the schedule says "1" for thwn-2 cables from the JB to the inverter which makes me think it is actually a combiner box and the common bus tie-in point.
 

jaggedben

Senior Member
the riser seems to show the 3 strings landed on a common bus right at the inverter (which would indicate to me actually 6 thwn-2 cables from JB to inverter)

however, the schedule says "1" for thwn-2 cables from the JB to the inverter which makes me think it is actually a combiner box and the common bus tie-in point.
First of all the plan you posted shows single conductors in raceways, which is not 'cables'. Hence they should show '6' conductors for 3 strings (3 positive, 3 negative), as this is what is relevant for any derating calculations if a ground is not included. Or '7' for fill, if ground is included.

As far as your logic about combining, that's a reasonable supposition except that it's contradicted by the following:
-The line schematic doesn't show combining in the j-box, it shows it in the inverter.
-No fuses are shown
-One who is familiar with installing the actual inverter (SE10000H) will know that the 'combining' is typically done in the inverter via the terminal block provided, which does not have fuses. This also matches the drawing.

So I would go back to whomever made the plan and request clarification. The table does not match the drawing, simply put.
 

jaggedben

Senior Member
Each string must have it's own individual ocpd? So the combiner box has a fuse for each string? Is this this a code requirement somewhere?


I understand there is only one DC ocpd at the solaredge inverter.
With traditional PV strings (modules in series) fuses were required by 690.9 when you had more than two strings (essentially, though not precisely). With optimizers the same logic does not exactly apply and code interpretation is a bit of a grey area. SolarEdge has white paper explaining why fuses are not necessarily required but the code isn't clear cut enough that all AHJs will necessarily accept it.

http://www.solaredge.com/files/pdfs/string_fusing_requirements.pdf

Note: The drawing you posted shows no fuses.
 

ggunn

PE (Electrical), NABCEP certified
Each string must have it's own individual ocpd? So the combiner box has a fuse for each string? Is this this a code requirement somewhere?


I understand there is only one DC ocpd at the solaredge inverter.
In the 2014 NEC, see 690.9(A) Exception (b). With normal DC strings this usually means that when combining three or more PV source circuits (strings), each string must be individually fused. This is to protect a faulted string from being backfed by the other unfaulted strings. With optimizers, it's a bit different, and SolarEdge has released white paper reports that state that combining more than two strings does not necessitate fusing them, but some AHJ's will not accept this as proof since the papers were not written by an independent testing agency.

SolarEdge inverters with a single DC input (or two) have no DC OCPD at all, only a switch.
 

pv_n00b

Senior Member
Whoever designed it made a simple error. Bounce it back with a comment that both sides should be six current carrying conductors and an EGC. No need for everyone to get all wound up over this.

Now for real fun let's debate if the PV conductors are really in "free air" in the array as it states. There has never been a general definition in the NEC for what it means for a conductor to be in free air, although it looks like the 2020 NEC will have it.
 
I am reviewing this project and have a question about the cables transitioning from PV cable to THWN-2.

they show "quantity 3" PV cables to transition JB but "quantity 1" THWN-2 cables from JB to inverter.

Shouldn't it also be "quantity 3" THWN-2 cables from JB to inverter?

Please see pic for better info and let me know how you interpret it..

pic: https://ibb.co/tMYJMt9

Thank You
To me it looks like the "Qty 1" is for number of conduits. 1 conduit from JB to inverter that has 6 current carrying conductors running in it (5th column in the table says 6 for # of current carrying conductors).

Thank you
 

jaggedben

Senior Member
To me it looks like the "Qty 1" is for number of conduits. 1 conduit from JB to inverter that has 6 current carrying conductors running in it (5th column in the table says 6 for # of current carrying conductors).

Thank you
Lol you're right. Although, the '3' still doesn't make compete sense for the PV wire, as that's the number of circuits not conduits. They probably should have put NA there.
 

pv_n00b

Senior Member
Lol you're right. Although, the '3' still doesn't make compete sense for the PV wire, as that's the number of circuits not conduits. They probably should have put NA there.
I would still bounce it with a comment. It's poorly documented and two pages of comments here on the subject show it's unclear.
 
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