Stop Overheating: Thermal Derating for AC Busbars Explained

Topic: AC Combiner & Distribution Panels — Managing AC conductors safely.

Thermal derating is the practice of reducing the allowable current of AC busbars as temperature rises. In AC combiner panels and distribution panels, ignoring derating leads to overheated enclosures, nuisance trips, and reduced equipment life. This page gives clear math, practical steps, and a data table to keep temperatures under control. You can apply the same logic to microgrids, residential ESS, and commercial switchboards.

Why thermal derating matters in AC combiners and distribution panels

Distribution gear runs closer to ambient conditions than substation equipment, yet it sees frequent load swings from PV inverters, storage, and HVAC. That mix makes heat build-up more likely inside enclosures.

• Distribution networks carry the vast majority of line length. An IEA assessment notes that distribution systems account for over 90% of total line length, and they increasingly host residential PV and other DERs (IEA: Critical Minerals and Clean Energy Transitions).
• As variable renewables connect, panels must handle dynamic current and thermal cycles. Best-practice material from the IEA highlights operational flexibility for DER integration, which includes robust protection and thermal design (IEA: System Integration of Renewables).
• Policy and deployment momentum continue. U.S. resources from Energy.gov: Solar Energy point to growing PV adoption, pushing installers to size conductors and busbars with enough temperature headroom.

Storage adds another layer. A technical resource that compiles storage performance patterns shows how temperature, depth of discharge, and inverter behavior change usable energy and current in the AC panel. That affects continuous and 10‑minute currents during peak output (Ultimate Reference: Solar Storage Performance).

What drives busbar temperature rise

I²R losses in conductors and joints

Heat scales with current squared. Even modest resistance at bolted joints can dominate total heat. Contact resistance rises if surfaces are oxidized, misaligned, or under‑torqued. High-frequency components from inverters can increase skin and proximity effects, raising effective resistance in bars that are closely stacked.

Enclosure effects

Panel temperature is not ambient. Sun exposure, poor ventilation, and high breaker density reduce cooling. Altitude also matters: lower air density degrades convection, which effectively cuts the rating further. Many manufacturers base ratings on 40 °C ambient, still air, and sea level.

Load diversity and harmonics

Multiple inverters and motor loads rarely peak together. Yet short alignment windows do occur at midday or during grid events. Harmonics from non‑linear loads increase RMS current and copper loss. Keeping harmonic current low reduces heat and improves protective device accuracy.

Reading and applying thermal derating curves

Derating curves convert ambient temperature to an allowable current factor. If a busbar is rated 250 A at 40 °C, and the factor at 55 °C is 0.82, the allowable current is 205 A. The table below shows typical factors often seen in data sheets for copper busbars in enclosed panels. Always confirm with the specific curve for your product.

Notes for the table: factors represent common ranges derived from vendor curves for enclosed copper busbars referenced to 40 °C; the loss column assumes a 300 mm² copper bar with resistance about 5.7e‑5 Ω per meter; joint losses are not included. Even though allowable current drops at higher ambient, internal temperatures still rise because available cooling is lower.

Why this matters for planning: IEA energy balances material makes clear that power systems track losses carefully at every stage, highlighting that heat and auxiliary loads influence delivered energy (IEA: Energy Balances Overview). In panels, that same accounting mindset keeps you within temperature limits under summer peaks.

Fast sizing flow for AC busbars in ESS projects

1) Define currents across operating states

• List continuous current, 10‑minute peak, and fault withstand. Use inverter nameplate and EMS setpoints. A storage performance resource shows that usable AC output tapers with state of charge and temperature; size for the highest current window the EMS allows (storage performance reference).
• Account for diversity. Avoid assuming that all inverters peak at the same second unless the EMS purposely aligns them.

2) Pick a base rating then apply ambient and enclosure factors

• Start with a bar rated at or above the continuous current at 40 °C.
• Apply the temperature factor from the curve. If the enclosure is sealed, add a safety margin.
• If altitude exceeds 2000 m, consult the manufacturer for additional reduction.

3) Control contact resistance

• Use tin‑plated or silver‑plated interfaces for aluminum‑to‑copper transitions.
• Follow torque specs, and re‑torque after thermal cycling if specified. Use calibrated tools.
• Space bars to reduce proximity effect and improve airflow.

4) Confirm protection and heat under fault and overload

• Check short‑time withstand for bars and supports.
• Verify breaker AIC and let‑through energy. Keep bus supports and ties rated for mechanical forces.

These steps align with the integration focus highlighted by the IEA for reliable DER operation in distribution assets (IEA: System Integration of Renewables) and with practical deployment context from EIA and IRENA.

Worked example: midday heat inside an AC combiner

Setup: three hybrid inverters feed a 3‑phase AC combiner. Each unit supplies 35 A per phase at peak. The combiner carries about 105 A per phase continuous during midday charge and house load support. The busbar is 250 A at 40 °C in a painted steel NEMA‑type enclosure mounted in partial sun. Internal ambient measured with a probe reaches 55 °C.

• Temperature factor: about 0.82 at 55 °C. Allowable current becomes ~205 A. The 105 A load is within limit, but the margin shrinks for any future inverter addition.
• Loss estimate: with 300 mm² copper bars, per‑meter copper loss at 105 A is about 0.63 W/m. Joint losses can equal or exceed that if surfaces are not ideal, so focus on joints.
• Risk flags: breaker clusters reduce airflow; direct sun adds a few degrees; neutral and ground bars carry unbalanced loads and can add heat near the terminal blocks.

Mitigations applied:

• Added perforated side vents and a sunshield. Internal ambient dropped from 55 °C to 48 °C in summer, increasing the factor from 0.82 to ~0.88.
• Reworked bar spacing and added a second bar in parallel on the highest‑load phase to reduce proximity effects and current density.
• Cleaned and reassembled joints with conductive joint compound and correct torque. IR scan confirmed a 10–15 °C reduction at the previously warm joint under 100 A.

These practical steps align with the broader push for reliable distribution assets cited by IEA materials on network expansion and with the deployment insights curated at Energy.gov.

Heat sources checklist for AC panels

• Conductor copper loss (I²R) in busbars and feeder cables
• Contact resistance at bolted joints and breaker stabs
• Core and contact losses inside breakers and switches
• Solar gain on the enclosure door or sidewalls
• Restricted airflow from dense wiring or backplates

Keep the checklist close during commissioning. It shortens the time from a hot spot on an IR camera to a fix on the wrench.

Standards, documentation, and field care

Choose equipment listed or tested to relevant panelboard and switchgear standards, and follow the specific thermal derating guidance in each product data sheet. Track as‑built currents and ambient conditions in the O&M file. Repeat IR scans at seasonal peaks. The IEA energy balances overview reminds engineers that auxiliary losses and heat matter from plant to panel; keep that discipline inside your enclosure (IEA: Energy Balances Overview).

Note: code compliance and safety decisions rest with the responsible engineer and local authority. This material is technical information, not legal advice.

How storage behavior ties into AC derating

Battery temperature and inverter control modes change current on the AC bus. A consolidated storage performance resource shows charts where discharge profile, temperature, and inverter limits shape available AC power through the day. That changes diversity factors and heat inside the combiner (Ultimate Reference: Solar Storage Performance). Use those curves to set a worst‑case 10‑minute current for your derating math.

Takeaway

Thermal derating for AC busbars is a simple calculation with big payback: scale allowable current by ambient, cut joint resistance, improve airflow, and confirm with field measurements. Link the math to storage and inverter behavior so the combiner and distribution panel stay cool on the hottest day while leaving headroom for upgrades.

FAQ

How do I pick a starting busbar rating?

Sum the realistic continuous currents at 40 °C, include diversity, and choose a rating with upgrade headroom. Then apply the temperature factor from the vendor curve.

Do aluminum bars need different derating?

Yes. Aluminum has higher resistivity and different thermal properties. Use the manufacturer curve for aluminum bars and pay extra attention to joint preparation.

What if my enclosure runs at 60 °C in summer?

Use the 60 °C factor from the curve. Consider vents, fans, a sunshield, or parallel bars. Re‑check breaker spacing and joint torque to cut hot spots.

How often should I scan with IR?

At least once after commissioning under peak load, then during the first summer and annually. Scan again after any major maintenance or upgrades.

Which public sources back the need for careful thermal design?

See IEA materials on distribution networks and DER integration (IEA networks overview, DER integration practices), Energy.gov: Solar Energy for deployment context, and market data at EIA.