2020 NEC: 690.9(D) vs 705.30(C) - Power Transformers

[pvgreeze]

Don't these two sections completely contradict each other? Maybe I'm missing something extremely obvious here, but they're both one off sections that are pretty straight forward. Paraphrasing both, emphasizing points of confusion:

690.9(D): OCPD for a transformer with a source(s) on each side shall be provided per 450.3 considering first one side of the transformer then the other as the primary.

705.30(C): Transformer with sources on each side shall be provided OCPD per 450.3. The primary shall be the side connected to the largest source of fault current.

These seem to be saying the complete opposite of each other. Does 690 supersede 705 for PV specific installs given that 690 is exclusively for PV whereas 705 is general interconnected sources? What are your thoughts? For what its worth, I've always designed towards the interpretation of 705.30(C) - for PV, consider the utility side the primary, inverter side the secondary.

 

[jaggedben]

What code cycle are you referring to? Are both of those quotes from the same year's code? I could be mistaken but don't recall seeing that in 690.9 for a long time, maybe since 2011. The 705 reference looks like 2020 or 2023.

Don't have my books with me today.

 

[pvgreeze]

What code cycle are you referring to? Are both of those quotes from the same year's code? I could be mistaken but don't recall seeing that in 690.9 for a long time, maybe since 2011. The 705 reference looks like 2020 or 2023.

Don't have my books with me today.

2020 NEC, dropped it in the post title. Both 690.9 and 705.30 are from there. Haven't checked 2023 yet, but the main state I work with is on 2020.

 

[jaggedben]

Now that I look again the 690 language has been there all along.

They used to say the same thing, but in 2020 the Article 705 version was revised. Evidently everyone forgot about the section in 690. So at least for an interconnected system, 705 is what they want you to follow now.

 

[solarken]

The exception in 690.9(D) permits eliminating OCPD on the interactive inverter side if the Transformer has a current rating equal or greater than the inverter output. 705.30(B) looks to say pretty much the same thing. So I view it as a duplication of requirement more than conflicting.

 

[pv_n00b]

Don't these two sections completely contradict each other? Maybe I'm missing something extremely obvious here, but they're both one off sections that are pretty straight forward. Paraphrasing both, emphasizing points of confusion:

690.9(D): OCPD for a transformer with a source(s) on each side shall be provided per 450.3 considering first one side of the transformer then the other as the primary.

705.30(C): Transformer with sources on each side shall be provided OCPD per 450.3. The primary shall be the side connected to the largest source of fault current.

These seem to be saying the complete opposite of each other. Does 690 supersede 705 for PV specific installs given that 690 is exclusively for PV whereas 705 is general interconnected sources? What are your thoughts? For what its worth, I've always designed towards the interpretation of 705.30(C) - for PV, consider the utility side the primary, inverter side the secondary.

I don't see a conflict. 690.9(D) says to design the OCPD by assuming first one side is the primary and then the other. It does not define either side as the true primary in the final system. It's just using the language of 450.3 to provide guidance to specify the OCPD. In 705.30 it is specifying the true primary as the one facing the largest source of fault current. It does not say as a design exercise the designer can't swap them up to do a review of the OCPD.

 

[pvgreeze]

I just find it unusual that 690.9(D) specifically states that both sides are considered the "primary" as far as the implementation/OCPD sizing of 450.3 and then 705.30(C) says "actually one side is the primary and the other is the secondary" for decided OCPD sizing based off of 450.3. Again, 705.30(C) is much cleared in my opinion, and I'm unsure as to why 690.9(D) is there in the first place. Just a weird nuance I found while catching up on changes from 2017.

 

[jaggedben]

I just find it unusual that 690.9(D) specifically states that both sides are considered the "primary" as far as the implementation/OCPD sizing of 450.3 and then 705.30(C) says "actually one side is the primary and the other is the secondary" for decided OCPD sizing based off of 450.3. Again, 705.30(C) is much cleared in my opinion, and I'm unsure as to why 690.9(D) is there in the first place. Just a weird nuance I found while catching up on changes from 2017.

As I explained, for a long time they said the same thing. Then the 705 version was changed. I'm assuming that those who revised 705 simply forgot about the identical language in 690. The impetus for the 705 change is not documented in the 2020 revision information so there's nothing to suggest it was purposeful, or not, that the 690 section didn't get revised too.

 

[pv_n00b]

If it helps this was changed in the 2023 NEC to read:

Overcurrent protection for power transformers shall be installed in accordance with 705.30(F).