This is a common point of confusion with VFD installations. Here's a breakdown of the likely reasons and how the circuit breaker rating impacts your cable size calculation

Why the Over-Sized Circuit Breaker?

Several factors contribute to ABB recommending a 250A circuit breaker for a VFD controlling a 100A motor:1. Inrush Current

VFDs, especially during startup, draw significant inrush current. This is due to the charging of the DC bus capacitors within the VFD. While the motor's full-load amps (FLA) are 100A, the inrush current to the VFD can be several times higher, albeit for a very short duration. A smaller breaker could trip nuisance tripping during startup. The 250A breaker is sized to handle this inrush without tripping.2.

Short-Circuit Current Rating (SCCR) & VFD Protection

VFDs are sensitive to short-circuit currents. The circuit breaker's primary purpose is to protect the wiring and the VFD itself from damage in the event of a fault (short circuit). The VFD likely has internal protection mechanisms (e.g., fuses, current limiting) to handle moderate overloads, but a dead short needs a breaker with sufficient interrupting capacity to act quickly. The 250A breaker provides the necessary interrupting capacity for potential short-circuit faults within the VFD. It is sized to let the VFD internally deal with smaller overloads, but to protect the components from serious damage with short circuit currents.3.

VFD Internal Components and Design

The VFD contains power electronics (IGBTs, diodes, etc.) that have limited short-circuit withstand capability. A properly sized circuit breaker limits the duration and magnitude of fault currents, protecting these sensitive components. The manufacturer designs the VFD and its internal protection around a specific upstream breaker characteristic.4.

Manufacturer's Recommendations and Certification

ABB's recommendation isn't arbitrary. They've likely tested and certified the ACS880 with that specific breaker rating to meet safety standards and ensure proper operation under various fault conditions. Deviation from their recommendations can void warranties and potentially compromise safety.

How the Circuit Breaker Rating Affects Cable Size Calculations

The fundamental reason the breaker rating impacts cable size in your calculations is because the cable must be rated to safely carry the maximum current the breaker will allow before tripping under overload or short-circuit conditions. Here's why:

Overcurrent Protection

The cable's primary protection comes from the circuit breaker. If a fault current exceeds the cable's ampacity but is below the breaker's trip curve, the cable will overheat and potentially fail before the breaker trips.

Software Calculation Logic

When you enter the 250A breaker into the software, it uses this information to ensure the cable is sized to handle at least 125% of continuous non-linear loads and handle 250A until the breaker trips. The software looks at:

Ampacity

The cable's continuous current carrying capacity based on installation method (conduit, free air, etc.), ambient temperature, and number of conductors bundled together. It has to be high enough that 250A would not heat the cable above its rated operating temperature before the breaker would trip.

Short-Circuit Withstand

The cable needs to withstand the let-through energy (I²t) of a short-circuit current until the breaker clears the fault. Larger cables have a greater ability to withstand this energy.

Voltage Drop

While less relevant in this specific case, voltage drop is considered.

Why the Large Jump in Cable Size?

Going from 150mm² to 240mm² is significant. This usually happens when the 150mm² cable cannot safely handle 250A until the breaker trips according to the time current characteristics. The breaker trip curve dictates how long it will take the breaker to trip at different overload levels. A larger cable (240mm²) is needed to handle that higher possible current without overheating or being damaged during the time before the breaker trips.

What to Do Next

1.

Strictly Adhere to ABB's Recommendations

Follow ABB's recommendations for the circuit breaker and cable size. This is crucial for warranty, safety, and proper VFD operation. Deviating from their guidelines without thorough analysis is strongly discouraged.2.

Consult with ABB Directly

If you're unsure, contact ABB's technical support. Explain your situation, calculations, and concerns. They can provide specific guidance and clarification.3.

Review Calculation Assumptions

Double-check all the assumptions in your cable sizing software:

Installation Method

How the cable is installed (e.g., conduit, open air) has a significant impact on its ampacity.

Ambient Temperature

Higher ambient temperatures reduce cable ampacity.

Grouping of Conductors

Bundling multiple conductors derates their ampacity.

Voltage Drop

Verify the software settings for maximum allowable voltage drop. While less critical for short-circuit protection, excessive voltage drop can affect motor performance.

Cable Material

Copper vs. aluminum significantly affects ampacity.4.

Consider Alternative Breaker Technology (with ABB Approval)

Some advanced circuit breaker technologies (e.g., electronic trip units) offer more precise overload and short-circuit protection. However,

never substitute a different breaker without ABB's explicit approval. A different breaker's trip characteristics might not align with the VFD's internal protection mechanisms, potentially leading to VFD damage.

In summary, the seemingly oversized circuit breaker is there to protect the VFD from fault currents, not just to protect the motor from overloads. The cable size is determined by the breaker rating because the cable must be able to safely handle the maximum current the breaker will pass until it trips under fault conditions. Adhering to the manufacturer's recommendations is paramount. Flag for review