On job sites and in panel shops, there’s a common mindset among electricians and even some engineers: “If the current is high, just pick a thicker cable. Bigger means safer.”
It’s an understandable reflex. Undersized cables overheat, trip breakers, and cause fires. But here’s the hard truth: cables are not like shoes—you can’t just size up and expect everything to work perfectly. Blindly jumping to a larger cross‑section can actually create a whole new set of problems that are easy to overlook until something fails.
The “Bigger is Safer” Trap
Many projects fall into the habit of “rather thick than thin.” If 16 mm² is calculated to be sufficient, they’ll install 25 mm². If 35 mm² is enough, they’ll pull 50 mm². The extra copper feels like cheap insurance. In reality, it’s a hidden cost—and a hidden risk.
Let’s break down the five major issues that come with oversized cables.
1. Terminal Crimping: The First Pitfall
Inside a distribution board, every component—breakers, contactors, terminals—has a rated wire‑size range.
Take a circuit breaker whose terminal is designed for a maximum of 25 mm². If you force a 35 mm² cable into it, the wire won’t fit. The usual “solution” on site? Electricians cut off a few strands of copper to make it squeeze in.
The problem: Even if the screw feels tight, the effective contact area is now reduced. That means higher resistance, localized heating, and eventually loose connections or arcing.
Code reality: A terminal may allow two wires of the same size, but it must never be used with a conductor that exceeds its maximum rated diameter. Oversizing here directly violates good practice.
2. Bending Radius: Installation Nightmares
The thicker the cable, the larger its minimum bending radius. In a cramped control cabinet, a tight cable trench corner, or a conduit run, a fat cable becomes a rigid bar.
Try to bend it too sharply and you might keep the outer jacket intact, but the internal copper strands suffer invisible damage—micro‑cracks that don’t show until months later when the conductor breaks or develops hot spots.
3. The Domino Effect on Cost
Oversizing doesn’t just make the cable more expensive. It forces a cascade of upgrades:
- Conduits must be larger to pull the bigger cable through.
- Cable trays need to be wider to accommodate the extra volume.
- Panel cutouts and gland sizes must increase.
- Installation labor goes up because heavier cables are harder to handle.
What started as a “little extra copper” becomes a significant budget overrun—money that could have been spent on better terminals, proper derating, or actual safety devices.
4. The Skin Effect in AC Systems
In AC power circuits, current doesn’t flow evenly through the entire cross‑section of a large conductor. It migrates toward the surface—a phenomenon called the skin effect.
As the cable gets very large, the effective current‑carrying capacity stops scaling linearly with area. You keep adding copper, but the ampacity gains diminish. Beyond a certain point, you’re paying double for a marginal improvement that might never be needed.
5. Thermal Cycling and Loose Connections
Copper expands when it heats up and contracts when it cools. With a large‑section cable, this thermal movement is more pronounced. Over thousands of power cycles, the repeated expansion and contraction can loosen the terminal bolts.
In vibrating environments—think factories, near heavy machinery, or on moving equipment—this effect is amplified. The result? A higher failure rate of connections, requiring frequent retightening and inspection.
The 4‑Step Cable Sizing Method (Do It Right)
To avoid both undersizing and oversizing, follow this professional workflow:
- Select by Current‑Carrying Capacity
Consult the cable ampacity tables. Apply correction factors for ambient temperature, grouping, and installation method (in air, in conduit, direct burial, etc.). - Verify Voltage Drop
For runs longer than 100 m, calculate the voltage drop. Keep it within 5 % (or the limit specified by your local code) to ensure equipment receives proper voltage. - Coordinate with Protection Devices
The cable’s continuous ampacity must exceed the circuit breaker’s trip setting. The breaker should protect the cable, not the other way around. - Check Terminal Compatibility
This is the most overlooked step. Before finalizing the size, confirm that the chosen cable fits the terminals of the breakers, contactors, and panel lugs. If it doesn’t fit, don’t force it—re‑evaluate the design.
Quick Reference: Common Motor & Distribution Circuits
(Assumes 35 °C ambient, free air, standard copper conductors. Adjust for your conditions.)
Motor Circuits (Typical 3‑phase, 380‑415 V)
|
Motor Power |
Rated Current (Approx.) |
Recommended Cross‑Section |
|---|---|---|
|
5.5 kW |
11 A |
2.5 mm² |
|
7.5 kW |
15 A |
4 mm² |
|
11 kW |
22 A |
6 mm² |
|
15 kW |
30 A |
10 mm² |
|
18.5 kW |
37 A |
16 mm² |
|
22 kW |
44 A |
16 mm² (25 mm² for long runs or high temp) |
Lighting & Socket Distribution
|
Breaker Rating |
Recommended Cross‑Section |
|---|---|
|
16 A |
2.5 mm² |
|
25 A |
4 mm² |
|
32 A |
6 mm² |
|
40 A |
6 mm² or 10 mm² (check terminals & conditions) |
|
63 A |
16 mm² |
Note: For long distances or multiple cables grouped together, you may need to upsize. Always run the four‑step check.
The Bottom Line
A cable that’s too small will overheat and fail. A cable that’s too large will strain terminals, break bending rules, bloat your budget, and still cause failures. The right size is the one that meets the electrical, mechanical, and economic requirements of the installation.
Don’t let “bigger is safer” become an excuse for lazy design. Do the math, check the terminals, and specify with confidence.
“The best cable is not the thickest—it’s the one that fits the job perfectly.”
What’s your go‑to rule of thumb for cable sizing? Have you ever seen a project where oversizing caused more trouble than it solved? Share your field stories in the comments below!
