The Two Silent Killers in Electrical Installations: Undersized Cables & Oversized Switchgear

The Two Silent Killers in Electrical Installations: Undersized Cables & Oversized Switchgear

by Ohm Engineering Works | Jun 21, 2025 | Electric Company

In the world of electrical design and installation, small mistakes can lead to big consequences. Among all possible errors, two overlooked yet critical design flaws consistently emerge as root causes of electrical accidents: undersized cables and oversized switchgear.

They may seem like opposite problems, but when they occur together in a system, the result can be devastating—from equipment failure and panel overheating to fire and total system breakdown.

Let’s explore why these issues occur, what the risks are, and how you can prevent them through smarter engineering practices.

⚠️ 1. The Risk of Undersized Cables

It’s not uncommon for engineers and electricians to rely on thumb rules or manufacturer cable sizing charts without considering all influencing variables. While convenient, this method often ignores real-world conditions like:

• Ambient temperature

• Cable grouping (derating)

• Installation method (conduit, tray, buried)

• Harmonic distortion in non-linear loads

🔥 The Result?

• When cables are undersized:

• They overheat under load

• Insulation deteriorates prematurely

• Hotspots develop in concealed areas

• Fire hazards increase dramatically

• Tripping may not occur, masking the issue until it's too late

Undersized cables are time bombs, especially in commercial or industrial installations where continuous load and ambient factors vary significantly.

⚠️ 2. The Danger of Oversized Switchgear

On the other end of the spectrum is the misuse of high-rated breakers.

Many assume that choosing a higher-rated MCB, MCCB, or fuse ensures better performance or future-proofing. But this is a misguided belief.

What Really Happens?

• Let-through energy increases, which damages downstream equipment during faults

• Delayed tripping fails to protect the cables or connected loads

• Heat buildup occurs in panels due to sustained overcurrents

• The overload protection curve no longer aligns with actual load demand

Oversized breakers don’t trip when they should, and by the time they do, the damage is already done—especially when paired with an undersized cable.

💥 When Both Coexist: A Perfect Storm

Imagine a system where the cable is too small, and the breaker is too big:

• The cable carries more current than it's designed for

• The breaker doesn’t trip at the appropriate threshold

• Result: Unnoticed thermal degradation, fire, or catastrophic failure

Such mismatches are common in rushed projects or those designed without load analysis or thermal simulation. The installation may look fine, but hidden dangers build over time.

✅ The Solution: Embrace Data-Driven Electrical Design

We urge engineers, designers, and contractors to move away from outdated methods and adopt precise, standards-compliant engineering approaches:

• Use IS 732, IS 3043, or IEC standards for cable sizing

• Factor in installation conditions and environmental impacts

• Perform load flow analysis and protection coordination studies

• Align breaker trip curves with cable thermal limits

Smart engineering saves lives—and reduces project risk and maintenance costs.

Final Thought: It’s Time to Replace “Rule of Thumb” with “Rule of Data”

In electrical engineering, intuition and experience are valuable—but data and standards must drive decisions.

If you're still using one-size-fits-all cable charts or blindly choosing oversized breakers “just in case,” it’s time to rethink. Design safety doesn’t come from overengineering—it comes from right-sizing every element in your system.