2 Different Interconnections

JimmysLimeade

JimmysLimeade

EE Student & PV Design
Location
Utah
Occupation
PV Design
Hi, I have a project where our client wants to interconnect using a load side tap / feeder tap. My concern with this is that there is an existing PV system on a breaker in the MSP, up stream from where the feeder tap would be made. The new system would be 30A of backfeed (Load side tap), and the existing system is 30A of backfeed (PV Breaker). This is landing on a 200/200 MSP, which has a 100A breaker for the sub panel feeders, where the new system would be tapped onto.

They are under the 2020 NEC, and I can't figure out if that interconnection would be against any code in 705.12. Some help would be appreciated here, thanks!
 
You have a panel with a 200A busbar, 200A main breaker, 30A PV breaker, and 100A breaker supplying a feeder to which another 30A PV breaker is tapped. I will assume that the 30A PV breakers supply inverters with a maximum 24A continuous output current; if the current is less, adjust the following calculations accordingly:

You have 3 options under 2020 NEC 705.12(B)(3) for this panel's busbar. Option 1 is that the sum of the various sources add up to less than the busbar rating. This would require downsizing the main breaker to 125A, so that 125A + 125% * 24A + 125% * 24A = 185A <= 200A, the busbar rating.

Option 2 is the 120% rule. This would require downsizing the main breaker to 175A. As then 175A + 125% * 24A + 125% * 24A = 235A <= 240A, the busbar rating times 120%. This would also require that the 100A feeder breaker and 30A PV breaker both be located at the opposite end of the busbar from the main breaker. [Actually a literal reading of 2020 NEC 705.12(B)(3)(2) suggests that only one backfed breaker is allowed, in which case you'd have to move the 30A PV breaker to, say, also be on a tap on the 100A breaker. But I gather the section is not usually enforced this way.]

Option 3 is the "sum of all breakers" option. This would require that the other breakers in the panel add up (per leg) to at most 70A. As then 100A (feeder breaker) + 30A (PV breaker) + 70A (or less, all other breakers) <= 200A, the busbar rating.

So option (3) is the only possibility that could work without any changes, but that depends on not having many other loads in the panel.

Cheers, Wayne
 
wwhitney said:
You have a panel with a 200A busbar, 200A main breaker, 30A PV breaker, and 100A breaker supplying a feeder to which another 30A PV breaker is tapped. I will assume that the 30A PV breakers supply inverters with a maximum 24A continuous output current; if the current is less, adjust the following calculations accordingly:

You have 3 options under 2020 NEC 705.12(B)(3) for this panel's busbar. Option 1 is that the sum of the various sources add up to less than the busbar rating. This would require downsizing the main breaker to 125A, so that 125A + 125% * 24A + 125% * 24A = 185A <= 200A, the busbar rating.

Option 2 is the 120% rule. This would require downsizing the main breaker to 175A. As then 175A + 125% * 24A + 125% * 24A = 235A <= 240A, the busbar rating times 120%. This would also require that the 100A feeder breaker and 30A PV breaker both be located at the opposite end of the busbar from the main breaker. [Actually a literal reading of 2020 NEC 705.12(B)(3)(2) suggests that only one backfed breaker is allowed, in which case you'd have to move the 30A PV breaker to, say, also be on a tap on the 100A breaker. But I gather the section is not usually enforced this way.]

Option 3 is the "sum of all breakers" option. This would require that the other breakers in the panel add up (per leg) to at most 70A. As then 100A (feeder breaker) + 30A (PV breaker) + 70A (or less, all other breakers) <= 200A, the busbar rating.

So option (3) is the only possibility that could work without any changes, but that depends on not having many other loads in the panel.

Cheers, Wayne
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Thanks for the quick reply! I should have added an example, because I'm, bad at explaining. Here is what I am trying to describe:
Existing PV system is interconnected using 705.12(B)(3)(2) and the new system will be interconnected using 705.12(B)(1) & (2)
1743179568450.png
 
JimmysLimeade said:
Thanks for the quick reply! I should have added an example, because I'm, bad at explaining.
Click to expand...
Your diagram matches your textual description and my understanding of your first post. So my previous response stands unmodified.

The basic idea is that 705.12(B)(3) applies to any panelboard with more than one source, and applies with respect to all of the sources. Adding a PV connection to that 100A feeder makes the 100A feeder a 3rd source that can supply the main service panel.

Cheers, Wayne
 
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wwhitney said:
Your diagram matches your textual description and my understanding of your first post. So my previous response stands unmodified.

The basic idea is that 705.12(B)(3) applies to any panelboard with more than one source, and applies with respect to all of the sources. Adding a PV connection to that 100A feeder makes the 100A feeder a 3rd source that can supply the main service panel.

Cheers, Wayne
Click to expand...
That makes sense to me, thank you!

One last hypothetical, if the new system was connected to feeders on feed-thru lugs would that change the scenario that you noted above?
1743181933662.png
 
JimmysLimeade said:
One last hypothetical, if the new system was connected to feeders on feed-thru lugs would that change the scenario that you noted above?
Click to expand...
Certainly not physics wise, where the panel busbar can be overloaded doesn't depend on whether the source connection to the busbar is via lugs or a breaker.

Regulatorily wise, with feed-through lugs you need to comply with 2020 NEC 705.12(B)(3)(6). Its wording is a bit obtuse, but I don't see it as allowing any additional flexibility.

Bottom line, to a 200A busbar panel with 200A main breaker, the only way to connect more than 32A of PV inverter output current (which you can do via the 120% rule, 705.12(B)(3)(2)) is to limit the total rating of the load breakers installed (under 705.12(B)(3)(3). But if you can do that, you could instead downsize the main breaker without any loss of functionality, and then the 120% rule would give my more flexibility.

So if the 30A PV breaker sizes are fixed (and minimum sized, so the inverter output current is over 20A in each case), and the 200A panel bubar rating is fixed, the only viable compliance route for a load-side interconnection is changing the main breaker to 175A.

Cheers, Wayne
 
I didn't go through all this but the bottom line is that you must comply with 705.12 in the main panel for all the PV that goes through it irrespective of how it gets there. Feeder tap, subpanel, backfed breaker... it doesn't matter, it all counts toward the 705.12 calculation.
 
wwhitney said:
Bottom line, to a 200A busbar panel with 200A main breaker, the only way to connect more than 32A of PV inverter output current (which you can do via the 120% rule, 705.12(B)(3)(2)) is to limit the total rating of the load breakers installed (under 705.12(B)(3)(3). But if you can do that, you could instead downsize the main breaker without any loss of functionality, and then the 120% rule would give my more flexibility.
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ggunn said:
I didn't go through all this but the bottom line is that you must comply with 705.12 in the main panel for all the PV that goes through it irrespective of how it gets there. Feeder tap, subpanel, backfed breaker... it doesn't matter, it all counts toward the 705.12 calculation.
Click to expand...
That makes sense to me now, thank you!

Another question: Does this change if the AHJ is under the 2023 NEC? It has different grouping under 705.12(A) for feeder taps, and 705.12(B) for busbars. The wording in 705.12 seems like they can be calculated and applied separately.

Im sorry to keep asking questions, but the rules for load side taps have been confusing to me for a while, so I might as well get them out now.
 
JimmysLimeade said:
That makes sense to me now, thank you!

Another question: Does this change if the AHJ is under the 2023 NEC? It has different grouping under 705.12(A) for feeder taps, and 705.12(B) for busbars. The wording in 705.12 seems like they can be calculated and applied separately.

Im sorry to keep asking questions, but the rules for load side taps have been confusing to me for a while, so I might as well get them out now.
Click to expand...
No, it doesn't make any difference in the main panel. Of course, if there is a subpanel and/or tap involved, 705.12 applies to them as well. 705.12 (A) for taps and 705.12(B) for panels. They are calculated separately but they all must be applied to everything that PV current goes through. If you have 20A through a backfed breaker, 30A through a feeder tap, and 40A through a subpanel, all into the main panel, the main sees 90A and that is what you need to qualify the bus for under 705.12(B).
 
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wwhitney said:
No. If current can get from the PV inverter to the busbar in question, it's a source for the busbar as far as complying with 705.12(B).
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To be slightly more precise, it's a source whose connection to the busbar must be dealt with as far as complying with 705.12(B). Your 100A subpanel, for example, can receive current from 3 different sources (2 PV 1 utility), but only via one connection, namely the load end of the 100A feeder. Because one of those sources connects to that 100A feeder after the 100A OCPD at the supply end of the feeder, the load end of that 100A feeder requires a 100A OCPD to protect that 100A busbar, per Article 408. [The feeder tap interconnection rules also require that 100A OCPD if the feeder between the point of interconnection and the 100A panel are only rated for 100A.]

Cheers, Wayne
 
I cannot add anything to Wayne's succint yet detailed response.

I have 2 practical notes:

1. A long shot, but very worth it , if true: maybe it is in fact a 225A busbar?
Many Eaton panels are rated 225A, even when they don't seem to be.
If so, you now have 120% of 225A as your limit of breakers feeding the bus = 270A total = 200A main + 70A solar limit. You would be home free.

2. I always end up downsizing to a 175A main breaker. Usually the simplest solutuon. Never had a home owner complain once that they needed 176A of power!
 
Zee brings up a good point. If you haven't already done so, read all of the tiny writing on the panel sticker and see if it has a 225A bus. If so, you are home free. If not, do a load calc and if it doesn't exceed 175A, downsize the main to 175A.

Another option to consider is 705.13, since you are under the 2023 NEC. Lots of inverters now have the capability to implement a PCS solution that will dynamically limit the current to the main panel busbar. Send your schematic to an applications engineer at the inverter manufacturer and see if they have a PCS solution. This might be tricky as you are backfeeding at two locations on the distribution system, but it certainly can't hurt to ask.
 
It's odd🤔 how many plan drawings show an existing 175 amp breaker 😏 in a 200 amp panel. What a coincidence!.... and so fortunate as no load calcs needed.
Let's face it unless you've got multiple Teslas and heat pumps and a pool and a hot tub you're unlikely to even exceed much more than 100 amps of use.
 
Zee said:
It's odd🤔 how many plan drawings show an existing 175 amp breaker 😏 in a 200 amp panel. What a coincidence!.... and so fortunate as no load calcs needed.
Let's face it unless you've got multiple Teslas and heat pumps and a pool and a hot tub you're unlikely to even exceed much more than 100 amps of use.
Click to expand...
I'd love to see a graph showing 175A breaker sales from say 1990 to current.
 
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