120% rule and a sub panel

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I plan on connecting 12 Enphase M215 micro inverters to a sub panel with a 2 pole 15 amp breaker. The sub is MLO and fed by a 100 amp breaker in a 200 amp main all-in-one. The 200 amp main is in the center of the bus.
The AHJ says the 120% rule does not apply because the main is in the center of the bus, I can't find anywhere in the NEC that prohibits using the 120% rule in this situation. I have read the California 2010 Electrical Code section, 692.65 and found no reference to center bus mains.
The AHJ told me I needed to change the 200 amp main to 150 amp and the 100 amp feeder to the sub to a 50 amp.
Please give me your opinions in regard to this dilemma.
12 x 215w = 2580w
2580w x 125% = 3225w
3225w / 240v = 13.4375 amps
13.5 amps maximum inverter output circuit, I should be well within the set parameters without doing any downgrading.
Thank you, Bill
 

Smart $

Esteemed Member
Location
Ohio
...
The AHJ told me I needed to change the 200 amp main to 150 amp and the 100 amp feeder to the sub to a 50 amp.
...
All that woud be required is to change the 200A main to 175. The most that can be backfed through the 100A feeder ocp is 15A. So in the main, 175A POCO + 15A PVS = 190A, which is less than main panel rating of 200A. The preceding assumes the 15A ocpd in the subpanel is at the load end of the bus, and the sub-panel feeder is rated for the full 100A [i.e. ocpd not upsized per 240.4(B)]. That is, the subpanel meets the 120% rule requirements [705.12(D)(2)].

PS: my earlier posted reference should have been 705.12(D)(7).
 

BillK-AZ

Senior Member
Location
Mesa Arizona
The draft for the 2014 NEC rewords the 125% allowance but adds an exception: "Exception: Equipment with multiple ampacity busbars or
center fed panelboards are not addressed by this provision."
It is not in the earlier 2011 NEC.

Most of these center fed panelboards actually have a four-pole breaker that feeds each end of the busbar with one-half of the nominal current. In your case this is 100A busbars fed by 100A + 15A breakers (you consider only the 15A breaker in the subpanel), and complies with the 125% concept. I have drawn the 1-line and 3-line wiring diagrams showing this detail and convinced the AHJ to evaluate only the affected busbars.

BTW: you need to base your ampacity calculation on the maximum AC output current of the inverters (0.9A at 240V), not the power. Then apply the 125%.

0.9A x 12 x 125% = 13.5A

Same answer in this case.
 

Smart $

Esteemed Member
Location
Ohio
...

Most of these center fed panelboards actually have a four-pole breaker that feeds each end of the busbar with one-half of the nominal current. In your case this is 100A busbars fed by 100A + 15A breakers (you consider only the 15A breaker in the subpanel), and complies with the 125% concept. I have drawn the 1-line and 3-line wiring diagrams showing this detail and convinced the AHJ to evaluate only the affected busbars.

...
Is this center fed bus actually two 100A busses, each fed by 100A ocpd?

If so, the 100A subpanel ocpd would have to be at either end to comply with the 120% rule (not 125% concept), right? I find it hard to believe the meter-main manufacturer would permit a 100A breaker at the far end of a bus that is only rated 100A...???
 
705.12 (D) (7). Inverter Output Connection. Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording

This section does not address a main OCPD being in the center of the bus. I could install the breaker feeding the sub at the furthest spot from the main and that would be in complance with 705.12 (D) (7). Where does it state otherwise?
Again, The 120% rule should apply and not downgrading of breakers should be required. Now, if there is a section which states,"If the main OCPD is located in the center of the bus, then the 120% rule does not apply," then I would agree. I am unable to locate such wording.
 

ToolHound

Senior Member
From 2011 Code

From 2011 Code

Related Code, from 2011 NEC:
Chapter 7 Special Conditions
ARTICLE 705 Interconnected Electric Power Production Sources
I. General
705.12 Point of Connection.


705.12(D):

(D)
Utility-Interactive Inverters. The output of a utility-interactive inverter shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises. Where distribution equipment including switchboards and panelboards is fed simultaneously by a primary source(s) of electricity and one or more utility-interactive inverters, and where this distribution equipment is capable of supplying multiple branch circuits or feeders or both, the interconnecting provisions for the utility-interactive inverter(s) shall comply with (D)(1) through (D)(7)

(1) Dedicated Overcurrent and Disconnect. Each source interconnection shall be made at a dedicated circuit breaker or fusible disconnecting means.

(2) Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

Exception: Where the photovoltaic system has an energy storage device to allow stand-alone operation of loads, the value used in the calculation of bus or conductor loading shall be 125 percent of the rated utility-interactive current from the inverter instead of the rating of the overcurrent device between the inverter and the bus or conductor.

HANDBOOK COMMENTARY:
The exception to 705.12(D)(2) permits an alternative to determining the load supplied by the bus or conductor. For example, because the inverter is capable of supplying only 30 amps of utility-interactive current to the panel and to the utility grid, it is safe to allow 125 percent of the continuous inverter backfeed current, which is 37.5 amps (1.25 ? 30), to be used in the 705.12(D)(2) calculation rather than the rating of the required load breaker determined by the main requirement.

(3) Ground-Fault Protection. The interconnection point shall be on the line side of all ground-fault protection equipment.

Exception: Connection shall be permitted to be made to the load side of ground-fault protection, provided that there is ground-fault protection for equipment from all ground-fault current sources. Ground-fault protection devices used with supplies connected to the load-side terminals shall be identified and listed as suitable for backfeeding.

(4) Marking. Equipment containing overcurrent devices in circuits supplying power to a busbar or conductor supplied from multiple sources shall be marked to indicate the presence of all sources.

(5)
Suitable for Backfeed. Circuit breakers, if backfed, shall be suitable for such operation.

Informational Note: Circuit breakers that are marked “Line” and “Load” have been evaluated only in the direction marked. Circuit breakers without “Line” and “Load” have been evaluated in both directions.

(6) Fastening. Listed plug-in-type circuit breakers backfed from utility-interactive inverters that are listed and identified as interactive shall be permitted to omit the additional fastener normally required by 408.36(D) for such applications.

(7) Inverter Output Connection. Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording:

WARNING
INVERTER OUTPUT CONNECTION
DO NOT RELOCATE THIS OVERCURRENT DEVICE​


 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
705.12 (D) (7). Inverter Output Connection. Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it...
Could that possibly be worded more confusingly? I remember the first time I encountered that article - I read it over and over trying to figure out what it was trying to say, which is just (I'm pretty sure) that if the sum of the breakers feeding the busbar is less than the busbar rating, you don't have to put them at opposite ends.
 
Just got back from plan check.

The AHJ quoted section 705.12 (d) (7), a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location.
Center of bus still is not addressed and I still believe my interpretation is correct,but, as you all know the AHJ's interpretation could be flawed, but, it is the one you need to follow: Degrade 200 amp main to 150 amp and feeder from 100 amp to 70 amp.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
as you all know the AHJ's interpretation could be flawed, but, it is the one you need to follow: Degrade 200 amp main to 150 amp and feeder from 100 amp to 70 amp.

Fighting them on the centerfed thing is hard. They can just say 'solar and utility aren't at opposite end, as the language requires.'
(See also: this thread.)

Now on the numbers they are just flat out wrong and you should be able to fight them. The main breaker can be downsized to anything below 179A, so a 175A breaker would do. The subpanel breaker does not need to be downsized at all.
 
That is what I figured. I doubt a 185amp breaker is even made for that panel, a 175 will be hard to find.
I'll put in a 150 amp, lick my wounds and move on to the next one.
Thank you for your replies, it is good to have someone to bounce your thoughts off of, especially
when that someone is as knowledgeable as you guys. Thank you, Bill
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
That is what I figured. I doubt a 185amp breaker is even made for that panel, a 175 will be hard to find.
I'll put in a 150 amp, lick my wounds and move on to the next one.
Thank you for your replies, it is good to have someone to bounce your thoughts off of, especially
when that someone is as knowledgeable as you guys. Thank you, Bill
One thing to take a moment to look at, before you bite the bullet is what the bus rating in the panel actually is. Just because it is a 200A panel does not mean that it does not use a higher amp rated bus. Some manufacturers do this routinely, using a 225A bus. (There are several threads in the forum on that topic, including examples of some panels that do this.)
A 225A bus would give you an additional 30 amp leeway. (25 +5)
 
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