Controller displays F21 Generator Overload; output breaker may trip or controller may shed load.

Safety first: Open the generator output breaker and ATS before inspecting wiring. Use appropriate PPE when working on live circuits.

• Check load current: With a clamp meter, measure current on each phase when the generator is supplying load. Compare to the nameplate full-load current.
• Identify large loads: At the ATS or load panel, identify high-demand loads (HVAC, pumps, heaters). Temporarily switch these off and see if F21 clears under reduced load.
• Inspect for shorts: If overload occurs immediately on transfer, inspect downstream circuits for short circuits or ground faults. Check for tripped branch breakers.
• Verify CT configuration: In the Siemens controller, confirm that CT ratios and wiring are configured correctly. Incorrect scaling can cause false overload readings.
• Retest: After correcting load or wiring issues, clear F21 and test under normal operating conditions.