You've hit on a common observation and a very valid point! Here's the breakdown of why generators often have larger breakers than their calculated continuous output rating, and what safeguards are in place

Reasons for Oversized Breakers:
Motor Starting Loads (Inrush Current)

This is the primary reason. Generators need to handle the inrush current of motors starting. Motors draw significantly more current upon startup (typically 3-6 times their running current) than their steady-state running amperage. If the breaker were sized exactly to the continuous output, it would trip every time a large motor started (think air conditioners, well pumps, refrigerators).

NEMA Standards and "Duty Cycle"

Generator manufacturers sometimes use NEMA (National Electrical Manufacturers Association) standards and consider the generator's "duty cycle" when choosing a breaker. The duty cycle refers to how often the generator is expected to run at full load. Standby generators are typically designed for intermittent use. They are not expected to run at 100% load continuously for extended periods like a prime power generator.

Voltage Dip Tolerance

An oversized breaker allows for a larger temporary voltage dip during motor starting. A slight voltage dip is preferable to a breaker tripping and killing the generator output.

Voltage Regulation

An under-sized breaker could cause the generator to work even harder, possibly leading to poorer voltage regulation, especially under varying loads.

Component Tolerances and Design Margin

Manufacturers build in safety margins. A larger breaker provides some headroom for component tolerances and unforeseen load variations.

Salesman's Explanation (The "Next Size Up" is NOT the Whole Story)

While it's true that sometimes the exact breaker size needed isn't readily available, this is a weak explanation. The primary design factor is motor starting capability. The "next size up" is often chosen specifically because it accommodates the anticipated motor starting inrush.

Safeguards Against Overload (Besides the Breaker)

You're right to wonder about other safeguards. Generators DO have additional protection, though they vary in sophistication:

Overcurrent Protection within the Generator Control System (Electronic Governor/Voltage Regulator)

Modern generators, especially larger ones, often have electronic controls that monitor current and frequency. These systems can reduce fuel delivery to the engine if an overload is detected, limiting the generator's output and preventing damage. This is a crucial feature. It allows the generator to ride through temporary overloads (like motor starting) without tripping the main breaker, but it also prevents the generator from sustaining a prolonged overload that could cause damage. These devices will shutdown the engine, usually before damage occurs.

Overspeed/Underspeed Protection

The generator's electronic governor monitors the engine speed. If the load is excessive, the engine will start to slow down. The governor will try to compensate, but if the load is too great, the engine will bog down, causing the frequency (Hz) of the output voltage to drop. Underspeed protection will then shut down the generator to protect it from damage. An underspeed is a common indicator of an overloaded generator, or a generator that cannot properly respond to the load applied.

Overvoltage/Undervoltage Protection

Voltage regulators will also monitor the line voltages being produced, and will disconnect the load, or the generator itself, if the voltages are out of tolerances.

Thermal Overload Protection

Some generators, particularly smaller portable ones, have thermal overload protection. This uses a sensor to monitor the temperature of the generator's windings. If the windings overheat, the generator will shut down to prevent damage.

Voltage Regulation (AVR)

Automatic Voltage Regulators (AVR) maintain a relatively constant output voltage even as the load changes. While the AVR's primary function isn't overload protection, it does help to stabilize the output and prevent voltage fluctuations that could damage connected equipment.

Wiring to the Breaker Size

You are absolutely correct to wire the generator to the breaker size. This is essential for safety and code compliance. The breaker is there to protect the wiring from overheating due to excessive current.

The Danger of Added Loads

You've identified a very real risk: someone adding significant loads later. This is a common problem! Here's how to address it:

Load Calculation

Before connecting a generator, a proper load calculation is mandatory. This involves adding up the continuous loads and estimating the starting loads of all appliances and equipment that will be powered by the generator. This should be done by a qualified electrician or engineer.

Diversity Factor

The load calculation often incorporates a "diversity factor." This acknowledges that not all appliances will be running at the same time. However, the diversity factor should be used cautiously and conservatively.

Warning Labels

Clearly label the generator and the transfer switch with the generator's maximum output (kW or kVA) and the maximum allowable load in amps.

Education

Educate the homeowner or user about the generator's limitations. Emphasize the importance of not adding loads without first consulting an electrician to re-evaluate the load calculation.

Load Management Systems

For installations where load management is critical, consider installing a load management system. These systems can automatically shed non-essential loads (e.g., electric water heater, pool pump) if the generator is nearing its capacity. They can also be programmed to prevent specific appliances from running simultaneously.

In your 40kW example

A 40kW generator rated at 167 amps with a 200 amp breaker is not unusual. It allows for the anticipated inrush current of motors without nuisance tripping while still having protection against overcurrent in the conductors. Make sure the conductors feeding the generator are rated for 200A, and are properly sized for the environment they are installed in. Also verify the overcurrent rating for the conductors leaving the generator are also rated for 200A. It is standard for this breaker to be much larger than the actual rated amperage.

In Summary:

The larger breaker size in generators is primarily to accommodate motor starting loads. While the breaker is the first line of defense, modern generators have additional safeguards like electronic governors, overcurrent protection, and thermal overload protection. It is critical to perform a proper load calculation, educate the user about the generator's limitations, and consider load management systems to prevent overloading. Your concern about added loads is legitimate and should be addressed proactively. Flag for review