Application of 2020 NEC 250.25 and objectional current

sparkles97

Member
Location
Washington
Please excuse my messy drawing. I am trying to wrap my head around the requirements to satisfy 250.25 in the 2020 NEC. On a grid-tied microinverter solar system connected via supply side tap, 250.25 would have me treat the AC disconnect the same as a service as far as bonding requirements.

Question 1: Why bond the grounded conductor, earth, and EGCs in the AC disconnect when it is already done in the main panel already facilitating an effective ground fault path?

Question 2: Now that the grounded conductor, earth, and EGCs are bonded in the disconnect, the ground wire going back to the main panel would now be our GEC and subject to the requirements of a GEC?

Question 3: If the combiner box doesn't need a neutral for 120v control voltage, do we still need a neutral pulled to the AC disconnect to bond neutral and ground again?
 

Attachments

  • 20220111_081403.jpg
    20220111_081403.jpg
    729.9 KB · Views: 12

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The practice of having multiple main bonding jumpers and GEC taps when there are multiple service disconnect enclosures has been permitted and arguably encouraged by article 250 for decades. For that reason, we say that any current on the GECs is not considered objectionable current. Indeed, it is accepted that for multiple reasons GECs may carry current under normal conditions, which is the reason for the extra bonding requirements.

There is no duplication of bonding if you only bring a neutral from the tap point to the new solar disconnect. There is no need to bring an EGC as well if you have an MBJ in the disco, and if you have both (i.e. both an MBJ and a green wire that only meets EGC or SSBJ requirements) then you'd have objectionable current.

That leaves the GEC still to discuss. Read the section on GECs for multiple service disconnects. There are multiple options and possibilities, including running the new GEC outside of the enclosures directly to the electrode, running it outside and tapping it irreversibly to the exisiting GEC, or not needing a new GEC at all if the exisiting on is bonded to neutral at or upstream of the tap. Arguably these options are all better than running a green wire in conduit between the main panel and dolar disco, which may be permitted but requires additional bonding and may be more confusing to your AHJ.

I'll just add that this area of code was completely vague before being substantially clarified in the 2020 NEC, so your AHJ may not be up to speed or may have differing interpretations.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'll just add that I'm assuming that you have an otherwise code compliant option for your supply side connection. That is, you're not trying to modify a meter main combo or something. Also the practice of tapping inside an existing service panel, while I've done it a dozen times and it's not expressly prohibited, seems to be increasingly frowned upon as the code makers try to make service conductors more separated and less accessible with each code cycle. A junction box between the meter and the other enclosures would be a cleaner way. You could also reroute your GEC to such a box and avoid a lot of your original questions.
 

sparkles97

Member
Location
Washington
Yes, that's your effective ground fault path. See also 250.24(C).
Thank you for the lengthy in-depth reply. Very helpful. I see that reference, good to highlight that one.

I guess my follow up question to this would be: if we floated the neutral in this sense (now illegal with the clarification in 250.25) and ran an EGC back between the main and the disco, ground fault current would still make it home via the EGC to the MBJ to the grounded conductor in the main panel. What is the benefit of bonding again in the disconnect versus relying on the MBJ in the main panel? I know you mentioned above this has been the standard for decades. I would assume redundancy for the sake of worse case scenarios?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Thank you for the lengthy in-depth reply. Very helpful. I see that reference, good to highlight that one.

I guess my follow up question to this would be: if we floated the neutral in this sense (now illegal with the clarification in 250.25) and ran an EGC back between the main and the disco, ground fault current would still make it home via the EGC to the MBJ to the grounded conductor in the main panel. What is the benefit of bonding again in the disconnect versus relying on the MBJ in the main panel? I know you mentioned above this has been the standard for decades. I would assume redundancy for the sake of worse case scenarios?
Physically it really doesn't make a difference if the ground fault path is colored white, green, or bare. It's just that 250.24(C) essentially says it should be white. Either way you're bringing a wire to the disco and bonding it there

If there is any neutral current at all, it should be on a white wire.
 

pv_n00b

Senior Member
Location
CA, USA
Please excuse my messy drawing. I am trying to wrap my head around the requirements to satisfy 250.25 in the 2020 NEC. On a grid-tied microinverter solar system connected via supply side tap, 250.25 would have me treat the AC disconnect the same as a service as far as bonding requirements.

Question 1: Why bond the grounded conductor, earth, and EGCs in the AC disconnect when it is already done in the main panel already facilitating an effective ground fault path?
There are two options for supply side interconnections that are usually accepted. The NEC does not specify one over the other.
  1. Use the N-G bond in the existing service and don't add a second in the PV disconnect. In this case, no GEC is needed and you need a Supply Side Bonding Jumper between the switch ground and the ground in the existing service.
  2. Add a new N-G bond in the PV switch, run a GEC from that to the existing grounding electrode system. Still need a Supply Side Bonding Jumper between the switch ground and the ground in the existing service even though it is in parallel with the GEC. Different conductor names, different purposes.
Some AHJs will only allow one or the other of these.
Question 2: Now that the grounded conductor, earth, and EGCs are bonded in the disconnect, the ground wire going back to the main panel would now be our GEC and subject to the requirements of a GEC?
The GEC and ground wire (Supply Side Bonding Jumper) have different purposes so even though they are in parallel there need to be separate conductors.
Question 3: If the combiner box doesn't need a neutral for 120v control voltage, do we still need a neutral pulled to the AC disconnect to bond neutral and ground again?
I can't remember where it is but I remember an NEC requirement that if there is a grounded conductor from the utility it must be brought to the first disconnect, the PV disconnect in this case. But you don't need to run it from the disconnect to the inverters if it is not needed.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
[*]Add a new N-G bond in the PV switch, run a GEC from that to the existing grounding electrode system. Still need a Supply Side Bonding Jumper between the switch ground and the ground in the existing service even though it is in parallel with the GEC.
Sorry, why is a SSBJ required? How is this any different than two services?

Cheers, Wayne
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Pv_noob I think you need to look at 250.25 as referenced in the thread title. New in the 2020 NEC. Your option 1 above isn't really allowed anymore, as I see it. The requirement for the grounded conductor to be brought to the disco is 250.24(C) as mentioned above.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Sorry, why is a SSBJ required? How is this any different than two services?

Cheers, Wayne
Yes, it would be considered the grounded conductor if there's one from the utility. It would only be an SSBJ on an ungrounded service, extremely rare. That said, there is essentially no difference except whether it's white or green respectively.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Yes, it would be considered the grounded conductor if there's one from the utility. It would only be an SSBJ on an ungrounded service, extremely rare. That said, there is essentially no difference except whether it's white or green respectively.
Agree with the above, but since the quoted option (2) included "Add a new N-G bond," I took the SSBJ to be in addition to the service neutral conductor. Which would be weird.

Cheers, Wayne
 

pv_n00b

Senior Member
Location
CA, USA
Pv_noob I think you need to look at 250.25 as referenced in the thread title. New in the 2020 NEC. Your option 1 above isn't really allowed anymore, as I see it. The requirement for the grounded conductor to be brought to the disco is 250.24(C) as mentioned above.
I agree, 2020 250.25 does seem to change things and requires the N-G bond in the PV disconnect in addition to the one in the existing service equipment. Would the removal of this bond be allowed under 250.6(B) to stop objectionable current? If so Option 1 is still in play.
 
Last edited:

pv_n00b

Senior Member
Location
CA, USA
Yes, it would be considered the grounded conductor if there's one from the utility. It would only be an SSBJ on an ungrounded service, extremely rare. That said, there is essentially no difference except whether it's white or green respectively.
I was thinking of separately derived systems. But a separately derived system can have a grounded conductor and still require an SSBJ. The grounded conductor does not replace it except in very specific setups outlined in the exceptions.
 
Top