Generator Set Circuit Breaker Sizing

How to choose a correct Circuit Breaker for our generator set? There are two typical user profiles: those with stable, ripple-free loads, and those who must start large motors with surge currents several times higher than normal. Breaker sizing must be tailored to the load profile – especially when the set has standby power rating. A prime 400kW / standby 440kW unit follows a completely different sizing logic than a pure prime-rated set. This guide provides a clear breakdown.

1. Breaker Types Overview

· ACB (Air Circuit Breaker) – Rated up to 6300A with adjustable electronic trips; the preferred main switch for sets above 200kW.

· MCCB (Molded Case Circuit Breaker) – Compact and economical; must be generator-duty rated to handle high DC components.

· GCB (Generator Circuit Breaker) – For medium/high voltage outputs; excellent breaking capacity.

2. Base Current Calculation for 400kW Generator Set

For a 400V/50Hz/PF0.8 unit:

Full-load current  I =400/1.732/0.4KV/0.8= 722A .

3. Scenario-Based Sizing – The Right Use of 1.1x Multiplier

Scenario A – Standard Users (stable loads, resistive or light inductive)

· Load characteristics: lighting, offices, data centers; starting current ≤1.1× rated, no momentary overload requirement.

· Sizing logic: For a prime 400kW / standby 440kW generator set operating without overload, the breaker can be sized at 1.1× the prime full-load current.

· Calculation: 722×1.1=794A → select 800A frame with C-curve trip (instantaneous 5~10×).

· Protection setting: Set long-time delay at 1.0~1.05× rated current to ensure any slight overload trips promptly.

· Application prerequisite: Load is absolutely constant, no surges, ambient temperature ≤40°C, and fully verified.

Scenario B – Heavy-Duty Users (momentary overload permitted, large motor direct starting)

· Load characteristics: compressors, crushers, pumps, cranes; starting current 3~5× rated.

· Sizing logic: 1.1x is completely inadequate – size according to peak starting current; multiplier depends on starting method.

· Calculation example: Single large motor DOL starting → 722×1.7≈1227A → select 1250A or 1600A frame.

· Mandatory trip curve: D-curve (instantaneous 10~14×) – otherwise nuisance tripping at every start.

· Protection setting: Short-time delay longer than motor start time (typically 0.2~0.4s) with thermal memory.

4. Consequences of Oversizing vs. Undersizing

· Undersized – In standard scenarios, trips at full load; in heavy-load scenarios, trips the moment a motor starts – production halts completely.

· Oversized – Fails to trip during overload; generator overheats until windings burn out. If breaking capacity is insufficient, contacts weld during short circuit, causing explosion.

5. Our Professional Commitment

We provide free load analysis and sizing calculations for every client – clearly distinguishing between ‘no-overload (1.1x)’and ‘momentary-overload permitted (upsize required)’ scenarios. We recommend the exact breaker frame and trip curve for optimal safety and cost-efficiency. Choose us for professional protection. Contact us today for your tailored proposal!