Backfeeding a 10 kW PV System Into a 200 A Panel
An owner is adding a 10 kW ac rooftop PV system to an existing dwelling. The inverter is a 240 V, single-phase, non-islanding unit with a continuous rated output current of 40 A. The main service panel has a 200 A busbar with a 200 A main breaker, and it also supplies a detached subpanel through a 100 A feeder breaker. The installing contractor proposes to land the PV on a new 2-pole 50 A backfed breaker in the main panel, mounted at the opposite end from the main breaker, and argues that this is acceptable because the calculated load is under 200 A and the 100 A subpanel breaker is not a source.
1. Evaluate the proposed 50 A backfed breaker against the busbar interconnection rules.
Read the worked answer
Sizing the PV breaker
The inverter output is 40 A continuous. PV circuit conductors and the overcurrent device are sized at 125% of inverter output: 40 A × 1.25 = 50 A → a 50 A 2-pole breaker is the correct device size for the interconnection.
The 120% busbar rule
The busbar must be protected from all sources. The rule adds the main device rating to the backfed PV device rating and limits the sum to 120% of the busbar rating:
200 A busbar × 1.20 = 240 A maximum sum
240 A − 200 A main = 40 A maximum PV breaker
Result: 200 A + 50 A = 250 A > 240 A. The proposed 50 A breaker does not comply.
Points the contractor got right and wrong
- Correct: the 100 A subpanel feeder breaker does not count as a source in this calculation, and load breakers generally do not enter the 120% arithmetic.
- Wrong: mounting the backfed breaker at the opposite end of the busbar does not change the 120% math. Physical placement helps with heat and with the second busbar check, but it cannot buy you a breaker that exceeds the allowance.
- The 200 A calculated load is also the wrong test. The busbar interconnection rule is separate from, and additional to, the service load calculation.
Verdict: noncompliant as designed; the PV cannot land at that busbar with a 50 A device.
2. Would moving the PV breaker to the 100 A subpanel solve the problem? Show the analysis.
Read the worked answer
No - it makes it worse.
Apply the same busbar arithmetic to the subpanel:
- 100 A busbar × 1.20 = 120 A maximum sum
- 120 A − 100 A subpanel main = 20 A maximum PV breaker
A 20 A allowance cannot carry a 40 A continuous inverter output, which requires a 50 A device. A 50 A breaker in that panel yields 100 + 50 = 150 A > 120 A → noncompliant.
Additional reasons the subpanel is a poor choice
- The subpanel is a load-side connection fed through a 100 A feeder; the PV output would be exported back through that feeder, which must be evaluated as a feeder interconnection as well.
- Rapid shutdown, disconnecting means, and labeling must still be provided at the PV system and at the point of interconnection (690.12, 690.13, 690.56).
- Any future load growth in the detached building is now competing with the PV allowance.
Rule of thumb: smaller busbar plus proportionally sized main equals a smaller PV allowance. Moving PV to a subpanel rarely helps unless the subpanel main is substantially smaller than its busbar.
3. The main panel cannot accept a 50 A backfeed. Identify the compliant interconnection options and the key requirements for each.
Read the worked answer
Option 1 - Supply-side connection (705.12(A))
Connect ahead of the main service disconnecting means, between the utility and the main breaker.
- The sum of the ratings of all supply sources must be not less than the calculated load.
- The connection must be made with listed equipment and typically requires a service-rated disconnect or a listed tap box; often a separate fused disconnect or a service-rated PV combiner/disconnect.
- Utility coordination and AHJ approval are usually required because the service conductors and meter enclosure are being modified (230.82, 690.13).
- This bypasses the 120% busbar limit entirely because the PV never touches the load busbar.
Option 2 - Larger busbar
Replace the panel with a 225 A busbar panel with a 200 A main:
- 225 × 1.20 = 270 A; 270 − 200 = 70 A allowance ≥ 50 A → compliant.
- Cost and downtime, but it also future-proofs for load growth and a battery inverter later.
Option 3 - Reduce the PV system size
A 7.6 kW inverter with about 32 A output → 40 A device. 200 + 40 = 240 A = 120% exactly → compliant on the existing busbar. Trade-off: less production.
Do not rely on these:
- A line-side tap inside the meter enclosure without listed equipment and utility approval.
- A "load-side" connection at the service conductors using unlisted connectors.
- Simply installing a smaller breaker than the conductors and inverter require - the 125% factor for PV output is not negotiable.
Regardless of the option chosen: PV conductors sized at 125% of inverter output (6 AWG Cu at 75°C = 65 A for a 50 A circuit), a lockable PV disconnecting means per 690.13, rapid shutdown per 690.12, and plaque/labeling identifying the system and the location of the disconnects (690.56).