One of the most common questions we get from installers and system designers is:
“Why does the recommended MCB size in the FoxESS manual seem higher than the inverter’s rated output current?”
At first glance, it seems counterintuitive. If an inverter outputs 16 amps, why would we recommend a 20A or even 25A miniature circuit breaker (MCB)? The answer lies in how inverters operate in real-world conditions — and particularly how FoxESS systems handle backup (EPS) functionality.
Understanding MCB Ratings vs. Inverter Output
The key point to understand is that MCBs are there to protect the cable, not the inverter. While an inverter might be software-limited to output 16A, this doesn’t mean the cable or breaker should be rated for exactly 16A. Here’s why:
1. Avoiding Nuisance Tripping
Inverters don’t always output a constant current — they respond to dynamic load demands. Even with a max rated output, brief current surges can occur, especially with inductive loads or during rapid changes in demand. If an MCB is rated too closely to the inverter’s nominal output, it could trip unnecessarily.
2. Breaker Curve Types
A slightly oversized MCB (e.g. 25A on a 16A circuit) offers better tolerance for these surges while maintaining overall circuit protection.
3. Cable Protection and Voltage Drop
Larger MCBs often correspond with larger cable sizes, which:
- Reduce voltage drop over long runs
- Handle higher current without overheating
- Ensure safety during EPS operation (more on this below)
Matching the MCB size to the cable rating — rather than the inverter output — is common practice and aligns with the requirements of BS 7671 (IET Wiring Regulations).
EPS Mode: Why MCB Sizing May Vary
FoxESS inverters with EPS (Emergency Power Supply) functionality have another layer of complexity. These systems can:
- Charge the battery from the grid via the normal AC input
- Simultaneously supply power to critical loads via the EPS output
- In bypass mode, the inverter can route AC input directly to the EPS circuits
This means there may be two simultaneous power flows:
- Grid-to-inverter (charging batteries)
- Grid-to-EPS loads (via bypass)
To safely accommodate both, we often recommend:
- Upsizing the AC cable and MCB on the grid feed to the inverter
- Specifying different MCB ratings depending on whether EPS is connected
This ensures that both charging and load supply can occur concurrently without exceeding the capacity of the upstream protection. It also prevents nuisance tripping in EPS bypass scenarios, where the inverter is passing through full-load current to the EPS output.
Summary
FoxESS inverter MCB recommendations are based on practical operating conditions, surge tolerance, and safety requirements — not just nominal output current. These recommendations:
- Help avoid unnecessary tripping
- Match the expected operating environment (especially with EPS)
- Ensure the protection of the entire circuit, not just the inverter
For accurate installation, always refer to the product-specific manual, which outlines the correct MCB sizes depending on whether EPS is used. If in doubt, consult a qualified electrician or reach out to our technical support team.
