The Busbar Nobody Checked
Permit-approved design: 10.4 kW array, 10 kW inverter, 60 A 2-pole PV breaker backfed into an existing 200 A residential panel. The array is mounted and the DC is landed; the crew is ready to make the AC connection. On site the service turns out to be a 200 A meter-main combination device with a 200 A main breaker, a 200 A busbar, and 2/0 Cu service conductors; the panel has no open spaces and there is no wall space beside it for a subpanel. The utility will not allow a supply-side (line-side) tap on this meter-main type and requires a lockable visible-blade AC disconnect within 10 ft of the meter. The plan reviewer approved the drawing without checking the busbar rating. Inverter output is 240 V single-phase.
1. Check the backfeed against the busbar rule and state the compliance finding.
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Compute both numbers the plan reviewer skipped.
- Inverter output current: 10,000 W ÷ 240 V = 41.7 A. Continuous-duty OCPD sizing = 41.7 × 1.25 = 52.1 A → 60 A breaker, matching the plan.
- Busbar limit: 200 A × 1.2 = 240 A allowable total. Available for PV = 240 − 200 (main) = 40 A.
Finding: 200 A main + 60 A PV = 260 A > 240 A. The design is non-compliant, and the PV breaker cannot simply be reduced - the inverter requires a 60 A OCPD. The AHJ approval on paper does not make it compliant; the installer is the last check and is the party who owns the violation.
Do not energize. Stop the AC connection, document the finding with the calculation, and notify the AHJ and the customer before any wall is opened or the disconnect is mounted.
2. List the realistic remedies and choose one for this site, with reasoning.
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Options, cheapest to most disruptive:
- Reduce the PV system so a 40 A breaker works. 40 A ÷ 1.25 = 32 A → 32 × 240 = 7.68 kW inverter maximum. This is a 7.6 kW inverter, i.e., about 3 kW of array and revenue lost. Viable only if the customer accepts the downsizing.
- Replace the 200 A main with a 175 A main. Then 175 + 60 = 235 ≤ 240 ✓. Permitted only if the load calculation shows the service load is ≤ 175 A - with an existing dwelling, NEC 220.87 (12 months of utility demand data) is usually the fastest defensible route, and it requires utility records, not a guess. A supporting point: 2/0 Cu at 75 °C is 175 A, so a 175 A main still protects the service conductors.
- Supply-side tap. Electrically cleanest, but the utility has ruled it out for this meter-main.
- New meter-main or panel with a 225 A+ bus. 225 × 1.2 = 270; 270 − 200 = 70 A available, so the 60 A breaker fits. This is compliant and future-proof but needs a permit revision, a utility disconnect/reconnect, and a day of outage.
Choice for this site: pursue option 2 with a documented load calculation. It is the only option that keeps the contracted system size without a utility service change. If the demand data shows > 175 A, fall back to option 4 and re-quote - do not default to shrinking the array without the customer's sign-off.
3. How do you manage the customer, the schedule, and the paperwork from here?
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Treat it as a documented change, not an apology.
- Disclose immediately and in writing, with the two-line calculation (260 A vs. 240 A) and photographs of the existing panel nameplate. Customers accept engineering facts; they do not accept finding out at final inspection.
- Separate who pays and why. The error originated in the design/plan review, so the company absorbs the main-breaker swap and the permit revision. If the customer elects the larger panel upgrade for future expansion, that portion is a quoted option they approve.
- Give a schedule impact with a date, not a range: permit revision turnaround, utility disconnect request lead time, and the new inspection date.
- Sequence the work. Complete and inspect everything that does not depend on the panel (DC, racking, grounding, labels, rapid-shutdown test) so only the AC terminations and final inspection remain.
- Do not energize or backfeed and do not let the crew "temporarily" land the breaker; an energized non-compliant service is an injury and liability exposure, not a scheduling convenience.
- File the as-built and the corrected calculation in the job record so the next technician inherits the reason for the 175 A main.