Under the 120% rule, 125% of the inverter output circuit current plus the rating of the breaker protecting the busbar must not exceed 120% of the busbar rating, with the sources at opposite ends of a busbar that contains loads. A 200 A busbar with a 200 A main leaves 40 A (an inverter up to 32 A). The same main on a 225 A busbar leaves 70 A. A 175 A main on the 200 A busbar leaves 65 A, if the Article 220 load calculation still supports the smaller main.
| Main | Busbar | 120% of busbar | Left for the source (125% of output) | Inverter up to |
|---|---|---|---|---|
| 100 A | 100 A | 120 A | 20 A | 16 A |
| 100 A | 125 A | 150 A | 50 A | 40 A |
| 200 A | 200 A | 240 A | 40 A | 32 A |
| 200 A | 225 A | 270 A | 70 A | 56 A |
The first four columns are the handbook table, Exhibit 705.2; the last column is 125% backed out. Your jurisdiction may enforce a different Code year; check which version applies.
The feeder ampacity is greater than or equal to 125 percent of the power-source output circuit current.
The sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the busbar ampere rating.
Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the busbar ampere rating. The busbar shall be sized for the loads connected in accordance with Article 220.
The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar.
WARNING: POWER SOURCE OUTPUT. DO NOT RELOCATE THIS OVERCURRENT DEVICE.
A 125 A panelboard feeding only a few circuits: two 40 A solar breakers and one 30 A load breaker add to 110 A, under the 125 A busbar, so it complies. The breaker protecting the busbar must not be rated above 125 A. The rule does not say where on the bus the breakers sit.
The required ampacity is based on the inverter output circuit current, rather than on the overcurrent device inside the inverter. Unlike service equipment, 705.12(B)(3) caps the total rating of the breakers on a panelboard.
The field guide: Solar on the Busbar, every 705.12 method worked through with field numbers.
Check your plans: a quick $195 written review of one single-line diagram for a load-side PV connection, before you submit.
The panel label, the inverter data sheet (output circuit current), and the Code book. Any hand tool = Klein Tools.
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