Sample Single Line Diagram
Context: The existing meter-main (service disconnect) feeds a Microgrid Controller (MGC)—e.g., Tesla Gateway 3, Enphase IQ Controller, etc. PV and ESS outputs land inside the MGC. The existing main-breaker load center becomes a protected/backup (or whole-home) panel downstream of the MGC.
We’re trying to choose the correct NEC 705 interconnection method for this topology. We see two plausible readings:
Interpretation:
The power-source output connections (PV/ESS inverter outputs) land on the MGC’s internal bus via dedicated breakers. Therefore, evaluate that bus under 705.12(B)(3). The common path is (B)(3)(3): the sum of the ampere ratings of all OCPDs mounted on that bus (supply + load devices), excluding the OCPD that protects that bus, shall not exceed the bus rating.
Why this seems right to us:
705.12 creates distinct methods for busbars vs feeders/taps. In this topology, the DER outputs don’t splice into feeder conductors; they terminate on the MGC bus. Thus the busbar method applies at the equipment where sources parallel the EPS.
Interpretation:
Define the Point of Interconnection (POI) as the feeder landing at the MGC—i.e., “where the new equipment (the microgrid system) connects to the home's electrical system” Treat the microgrid system (MGC + PV + ESS) as a composite source exporting to the premises at that point. Because the POI is on the feeder, apply feeder/tap rules: ampacity ≥ 125% of source output current, or protect the downstream feeder segment with OCPD at the connection per the subsection options, etc.
Why this seems plausible to others:
Verbiage like “Where the power-source output connection is made to a feeder…” could be read as including a microgrid controller interface at the feeder as the “power-source output connection,” since the microgrid as a system exports at that point—even though the individual PV/ESS outputs terminate on an internal bus.
Main Question
For applying 705.12, should “power-source output connection” be understood as the DER output(s) landing point (i.e., the MGC bus), or can the microgrid controller’s feeder interface itself be considered the “power-source output connection” for feeder methods?
Context: The existing meter-main (service disconnect) feeds a Microgrid Controller (MGC)—e.g., Tesla Gateway 3, Enphase IQ Controller, etc. PV and ESS outputs land inside the MGC. The existing main-breaker load center becomes a protected/backup (or whole-home) panel downstream of the MGC.
We’re trying to choose the correct NEC 705 interconnection method for this topology. We see two plausible readings:
Option 1 — Busbar method: 705.12(B)(3), typically (B)(3)(3) “sum of all OCPDs”
Interpretation:
The power-source output connections (PV/ESS inverter outputs) land on the MGC’s internal bus via dedicated breakers. Therefore, evaluate that bus under 705.12(B)(3). The common path is (B)(3)(3): the sum of the ampere ratings of all OCPDs mounted on that bus (supply + load devices), excluding the OCPD that protects that bus, shall not exceed the bus rating.
- If the MGC is lugged-out (no load/output breaker on the MGC bus): the sum is typically just PV + ESS breakers (often well ≤ 200 A), so (B)(3)(3) passes.
- If the MGC does have a load/output breaker on the same bus, that breaker counts in the sum. In some whole-home cases, PV + ESS + load breaker can exceed the 200 A bus rating → (B)(3)(3) fails.
- PCS path: When the controller is listed as a PCS and configured to limit current, we would instead claim 705.13 (PCS) compliance—treating the PCS setting as the “power-source output current” for 705.12 calcs. That’s a separate compliance path (not making (B)(3)(3) true).
Why this seems right to us:
705.12 creates distinct methods for busbars vs feeders/taps. In this topology, the DER outputs don’t splice into feeder conductors; they terminate on the MGC bus. Thus the busbar method applies at the equipment where sources parallel the EPS.
Option 2 — Feeder / Feeder-Tap method: 2023 705.12(A)
Interpretation:
Define the Point of Interconnection (POI) as the feeder landing at the MGC—i.e., “where the new equipment (the microgrid system) connects to the home's electrical system” Treat the microgrid system (MGC + PV + ESS) as a composite source exporting to the premises at that point. Because the POI is on the feeder, apply feeder/tap rules: ampacity ≥ 125% of source output current, or protect the downstream feeder segment with OCPD at the connection per the subsection options, etc.
Why this seems plausible to others:
Verbiage like “Where the power-source output connection is made to a feeder…” could be read as including a microgrid controller interface at the feeder as the “power-source output connection,” since the microgrid as a system exports at that point—even though the individual PV/ESS outputs terminate on an internal bus.
Main Question
For applying 705.12, should “power-source output connection” be understood as the DER output(s) landing point (i.e., the MGC bus), or can the microgrid controller’s feeder interface itself be considered the “power-source output connection” for feeder methods?