It depends on how many loads there are on the meter main besides the
feed-through. If the PV feed is coming from a point between the
distribution panel and the meter main, current will cancel out utility
power from the service drop.
If you had over 200 amps of branch circuit loads on the meter main, it
could be a problem with the feed coming from both directions.
---
On 2020-11-08 12:02, Jay wrote:
Hi Jason
My question is if it's 200a in from the main breaker and 100a from PV its 300 amps on the buss correct?
How is that ok with the 125% rule?
Or is this covered by some other rule as it's a feed through lug load center?
Jay
On Nov 8, 2020, at 9:46 AM, Jason Szumlanski
<[email protected]> wrote:
Maybe this is a better example (attached). I don't see how this would be a problem under NEC 2014, 2017, or 2020. No portion of the main bus, feeder conductors, or subpanel could possibly be subjected to overcurrent without an OCPD stopping it.
My point is that here we are, 3 code cycles in since feeder taps were addressed, and there is still no clarification of intent. AHJs are still struggling with this.
I'm not sure where residential meter/mains with feed through lugs are popular. I know they are in Florida and I have heard Hawaii. I'm curious what your jurisdictions think of this if you have this scenario. Most of the time the subpanel is main lug only, but adding a main circuit breaker is usually an easy and cost-effective fix to make this interconnection type work.
Jason Szumlanski
On Wed, Oct 28, 2020 at 5:15 PM Jason Szumlanski <[email protected]> wrote:
For further discussion, I don't see how my original scenario is any different from this attached scenario, which I think everyone would agree is allowed. All conductors and busbars are subject to the same potential loads and fault currents.
(Image attached).
On Wed, Oct 28, 2020 at 5:01 PM Jason Szumlanski <[email protected]> wrote:
Of course the feeder conductors and bus bar could be subjected to a fault. But we're not talking about faults here. Fault protection is the job of the PV OCPD And primary supply OCPD to protect the downstream busbar and feeders. If that wasn't the case, you would need a new OCPD on BOTH the load and line side of a solar connection as a feeder tap, not just the load side.
If your interpretation is correct regarding the location of the OCPD, that sounds like a sub-feed breaker is the only way to comply, and I haven't seen such an animal for a typical residential load center. You can get these for NQ panelboards and similar panelboards from other manufacturers of course. It doesn't say as close as practicable or anything like that. It says that a busbar connection is allowed when there are feeder CONDUCTORS connected to feed through LUGS. What does "overcurrent device .. _at_ the supply end" mean? I emphasize "at." It's unclear how you would implement this other than a sub-feed breaker I suppose, but that's not what it says. It refers to feeder conductors on lugs on busbars, not feeder conductors on load-side terminals of an overcurrent device.
My point is that 705.12 should have been wrapped up neatly in a bow, but the lack of clarity, still, is astonishing. Why add a section about feed-through lugs if it's going to be so vague?
Jason
On Wed, Oct 28, 2020 at 3:37 PM Brian Mehalic <[email protected]> wrote:
Hey Jason,
Here's the 2020 text:
6) Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors. The feed-through conductors shall be sized in accordance with 705.12(B)(1). Where an overcurrent device is installed at the supply end of the feed-through conductors, the busbar in the supplying panelboard shall be permitted to be sized in accordance with 705.12(B)(3)(1) through 705.12(B)(3)(3).
The OCPD on the supply end of the feed-through conductors would be in the form of a sub-feed breaker at the point of supply to those conductors, re-establishing overcurrent protection of the conductors (likely at the same ampacity as the main breaker in the supplying panel. The feed-through conductors are basically an extension of the busbar in the supplying panel; they can either be protected by the main, or in the presence of multiple sources of power in the supplying panel (such as a backfed PV system breaker) they can be protected based on (B)(3)(1) - "the 125% rule" - or they can be protected by a new overcurrent device at their point of supply, in which case current on them is limited based on that OCPD size; in this latter scenario the busbar in the supplying panel is allowed to be sized based on one of (B)(3)(1) - (3) because it is protected downstream at its end.
The theory is pretty much the same as 705.12(B)(1) for feeders - when not connecting at the end of the feeder, use the "125% rule" or re-establish overcurrent protection for that portion of the feeder subject to multiple power sources.
In your drawing the 200 A feeder conductors, as well as the busbar below the PV system breaker, could be subject to > 200 A in the event of a fault somewhere along those conductors.
Brian Mehalic
NABCEP Certified Solar PV Installation Professional(tm) R031508-59
National Electrical Code(R) CMP-4 Member
(520) 204-6639
Solar Energy International
http://www.solarenergy.org [1]
SEI Professional Services
http://www.seisolarpros.com [2]
On Wed, Oct 28, 2020 at 10:18 AM Jason Szumlanski <[email protected]> wrote:
Does anyone else think they botched the wording in this section? It's still not clear, and we have a ton of meter/main combos with feed-through lugs around here.
Where is it written, "where an overcurrent device is installed at the _supply end_ of the feed-through conductors," (emphasis added) are they referring to the solar backfed breaker on the busbar or another breaker somewhere along the feeder circuit? It goes on to state that the loads on the supplying busbar can comply with any method in 705.12(B)(3), which prescribes an OCPD at the load end of the feeder in 705.12(B)(3)(3), so they can't be talking about that. I have to assume it is the solar backfed breaker they are referencing.
See my interpretation of one scenario in the attached image.
We're a long way off from the 2020 code implementation here, but it can help sway plans examiners looking to clarify the intent of the 2014/2017 code cycles.
Jason Szumlanski
<Feed Through Lug Interconnection Option (2).pdf>
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