How to Size AC Distribution Panels for Hybrid Inverters

AC Combiner & Distribution Panels · Managing AC conductors safely

Correct AC distribution panel sizing keeps hybrid inverters safe, efficient, and expandable. You will see clear formulas, a worked example, and a practical table for quick checks. The approach fits backup, grid‑interactive, and off‑grid systems. It also aligns with common code principles used in NEC and IEC frameworks. This content is general information, not legal advice. Always confirm with local codes and a licensed electrician.

Why sizing AC panels for hybrid inverters is different

Two AC sources, multiple modes

Hybrid inverters can feed loads from the grid and the inverter output. That creates backfeed paths and fault scenarios you must address in the AC combiner or subpanel. Grid‑connected PV always requires an inverter stage, as noted by the International Energy Agency in Solar Energy Perspectives. Your AC distribution panel must coordinate both sources in grid and island modes.

Storage and inverter performance matter

Round‑trip and conversion efficiency shape currents and thermal load in the panel. As summarized in Ultimate Reference: Solar Storage Performance, LiFePO4 systems often achieve about 90–95% round‑trip efficiency, while hybrid inverter AC efficiency commonly sits around 94–97%. Many hybrid units can deliver short surge power (often up to 2× rated for a few seconds). Account for those factors in breaker selection and heat in the enclosure.

Grid coordination and protection

Coordinated protection and flexibility are core themes in IEA work on integration. See System Integration of Renewables and Getting Wind and Solar onto the Grid. Distributed solar keeps growing, per the U.S. Energy Information Administration’s Solar explained. Hybrid inverter AC panels must therefore meet interrupt ratings and labeling that withstand utility fault levels and ensure safe backfeed control.

Core sizing math for AC distribution panel sizing

Step 1 — Define operating mode and loads

• Grid‑interactive with backup: service panel plus a backed‑up subpanel or whole‑home transfer.
• Off‑grid: inverter output is the only source; generator may be present.
• Parallel inverters: sum per‑phase currents; keep phase balance and rotation correct.

Calculate continuous and non‑continuous loads. For continuous loads (running 3 hours or more), size conductors and overcurrent protective devices (OCPDs) at 125% of the continuous current per common code practice.

Step 2 — Translate power to current

• Single‑phase: I_ac = P_out / (V_ll × η_ac × PF)
• Three‑phase: I_ac = P_out / (√3 × V_ll × η_ac × PF)

Use PF close to 1 for most hybrid inverters in steady operation. For η_ac, a conservative 0.95 aligns with values summarized in the storage performance reference.

Step 3 — Choose feeder breaker and conductors

• Feeder OCPD ≥ 125% of continuous inverter output current.
• Conductor ampacity ≥ feeder OCPD rating, adjusted for ambient temperature, conduit fill, and grouping per NEC 310 or IEC 60364.
• Consider enclosure heat. Higher inverter utilization raises internal temperature and may require a larger enclosure or derating.

Step 4 — Size the panel busbar

Panel busbar rating should exceed the upstream feeder OCPD and the expected total load current, with headroom for future strings or additional inverters. Many designs target 125% headroom for continuous duty.

Step 5 — Check available fault current and AIC

Interrupting capacity must match or exceed the maximum prospective fault current at the panel location. Residential service in many regions uses 5–10 kAIC equipment, but urban services can exceed that. Hybrid inverters typically contribute limited AC fault current (often 2–3× rated current for a few cycles), while the utility dominates in grid mode. Confirm the utility fault level and the inverter’s short‑circuit contribution, then select breakers and panel AIC accordingly.

Protection, RCD/RCBO types, and residual currents

Transformerless inverters can produce DC residual components. Many jurisdictions require Type A or Type B RCDs; some manufacturers specify Type B for full coverage. Verify the inverter manual and local code. Include Type 2 surge protective devices (SPDs) at the AC combiner or subpanel feeding the backed‑up loads.

Wiring schemes affect current and cost. The U.S. Department of Energy highlights in a success story that parallel DC wiring can need thicker conductors and added devices, improving performance but raising complexity (EERE: new converter and wiring setup). While that example sits on the DC side, the same thinking applies on AC: actual currents and duty drive conductor size, enclosure heat, and breaker selection.

Worked example: single‑phase hybrid inverter feeding a backup subpanel

Inputs

• Inverter rating: 8 kW, 230 V single‑phase
• Assume PF = 1, η_ac = 0.95
• Continuous output fraction: 100% (worst‑case backup)
• Ambient 40 °C inside enclosure (apply moderate derating as needed)

Calculations

• I_ac = 8000 W / (230 V × 0.95) ≈ 36.7 A
• Feeder OCPD ≥ 1.25 × 36.7 ≈ 45.9 A → select 50 A breaker (next standard size)
• Conductor: choose copper with ampacity ≥ 50 A after derating. A typical choice is 10 mm² Cu (IEC) or AWG 6 Cu (NEC 75 °C ~65 A), adjusted for local rules.
• Panel busbar: at least 63 A; 100 A gives growth headroom.
• AIC: verify utility value; target 6–10 kAIC minimum for residential. Confirm inverter short‑circuit contribution (often limited).

Result

• Feeder breaker: 50 A
• Backup subpanel busbar: 100 A
• Conductors: 10 mm² Cu (IEC) or AWG 6 Cu (NEC), adjusted for ambient and grouping
• Protection: 2‑pole main breaker, Type A or B RCD per inverter manual, Type 2 SPD

Quick reference: AC panel capacity calculation outcomes

Notes: Values assume PF=1 and η=0.95. Adjust for temperature, grouping, conduit fill, and local code tables. Always confirm standard breaker sizes and conductor ampacities per your region.

Neutral‑ground strategy and transfer switching

Keep a single neutral‑to‑ground bond in grid mode, typically at the service disconnect. In island mode, many hybrid inverters switch in an internal bond to maintain fault clearing. Your AC distribution panel must support this logic without creating parallel bonds. Use listed transfer equipment and follow the inverter instructions. Proper labeling reduces troubleshooting time and prevents nuisance trips.

Thermal, enclosure, and future capacity

Higher efficiency reduces heat and allows tighter panels. As noted in the IEA’s Solar Energy Perspectives, every grid‑tied PV path passes through power electronics, so thermal planning is fundamental. Consider enclosure size, ventilation, and sun exposure. Leave busbar headroom for added inverters or phases. Modular layouts can cut rework costs later, echoing system integration guidance in IEA integration reports.

Checklist for hybrid inverter AC combiner panel sizing

• Confirm inverter AC rating, PF, η_ac (see storage performance reference)
• Compute I_ac and apply 125% for continuous duty
• Select feeder breaker and conductor with derating
• Verify panel busbar rating with growth headroom
• Check available fault current; set AIC for worst case
• Choose RCD/RCBO type per residual current behavior
• Set neutral‑ground logic for grid and island modes
• Add Type 2 SPD and clear labels
• Document settings, torque values, and test results

Field tips tied to research

• Parallel inverters: balance phases and use a panel with adequate AIC, as coordination remains a priority in IEA’s grid integration work.
• Service fault current can exceed expectations in dense areas. Validate with the utility. If needed, select higher AIC breakers or a current‑limiting main device.
• Battery and inverter performance vary by temperature and state of charge. The storage performance overview shows how efficiency and surge behavior shift with conditions. Size with margin.
• Growing distributed PV (see EIA Solar explained) means more hybrid panels in the field. Standardize labels and documentation to reduce maintenance time and error rates.

Final checks and takeaways

Start with clear operating modes, then compute current with realistic efficiency. Apply the 125% rule for continuous duty, select conductors after derating, and set busbar headroom for growth. Verify AIC against utility data and inverter contribution. Choose the right RCD type and get neutral‑ground bonding right in both grid and island modes. These steps keep hybrid inverter AC panels safe, scalable, and easier to service.

References and further reading: IEA: Solar Energy Perspectives; IEA: System Integration of Renewables; IEA: Getting Wind and Solar onto the Grid; EIA: Solar explained; Energy.gov EERE success story; Ultimate Reference: Solar Storage Performance.

Disclaimer: This is not legal, engineering, or electrical advice. Always follow NEC or IEC and local regulations, and use a licensed electrician.

FAQ

How do I size the feeder breaker for a hybrid inverter?

Compute inverter output current from power, voltage, and efficiency. Multiply continuous current by 125%. Choose the next standard breaker size. Then pick conductors with sufficient ampacity after temperature and grouping adjustments.

What AIC rating should I target for hybrid inverter AC panels?

Match or exceed the maximum prospective fault current at the panel. Many residential sites use 6–10 kAIC equipment, but urban services can be higher. Confirm with the utility and the inverter’s short‑circuit contribution. Select breakers and panels that meet the worst case.

Do I need a Type B RCD for a hybrid inverter?

Some transformerless designs require Type B due to DC residual components. Others accept Type A. Follow the inverter manual and local rules. When in doubt, consult a qualified electrician.

How do parallel inverters change AC panel sizing?

Sum per‑phase currents, keep phase rotation correct, and verify the panel busbar and AIC. Use coordinated breakers to avoid nuisance trips. Leave space for future units and label each feeder.

How does storage performance affect AC panel choices?

Higher efficiency lowers heat and current stress. Surge capability can drive short‑term current peaks. The storage performance reference lists typical efficiency and surge behavior; use those values to set margin.