Last month, a Thai customer sent me a voice message on WhatsApp.

He sounded very anxious.

One of their factory's production lines uses 35 square millimeter copper cables with a load current of approximately 130A. According to the IEC 60364 standard, the current carrying capacity of 35 square millimeter copper cables laid in the air is approximately 160A.

“130A is far from 160A,” he said, “but this cable is so hot I can't touch it. Is it because your copper isn't pure?”

I didn't argue with him.

I first asked three questions:

1. How is this cable routed?

2. How many other cables are there around it?

3. What is the approximate ambient temperature?

His answers explained everything.

The “small print” that everyone overlooks

Everyone looks at the current carrying capacity chart. But 90% of people don't read the small print at the bottom of the chart.

The small print reads:

"Based on an ambient temperature of 30°C, air-laid, single cable, continuous load."

The actual conditions at the customer's site are:

Ambient temperature: 42°C (Thailand, under a factory's corrugated iron roof)

Laying method: Conduit buried in the wall (not in free air)

Number of cables: 4 in the same conduit

Each cable is "penalizing" its current carrying capacity.

There are three correction factors, and many people haven't multiplied any of them.

I won't go into the complicated formulas. I'll just explain how to calculate it in practice.

Correction factor one: Ambient temperature

The current carrying capacity in the IEC standard is based on 30°C. For every 10°C increase in temperature, the current carrying capacity decreases by approximately 5-8%.

42°C is 12°C higher than 30°C.

Rough estimate: Current carrying capacity × 0.92

Correction factor two: Laying method

Cables in free air dissipate heat best. After being laid in a conduit, heat cannot dissipate.

The correction factor for conduit laying is approximately between 0.7 and 0.8.

Correction Factor 3: Multiple Cables Side-by-Side

The more cables in the same conduit, the more severe the mutual heating.

The correction factor for 4 cables is approximately 0.75 to 0.8.

Multiplying them together makes it clear:

160A × 0.92 (temperature) × 0.75 (conduit) × 0.78 (4 cables side-by-side)

≈ 86A The customer's actual load is 130A.

The "true current carrying capacity" of 86A is only able to withstand 130A of current.

It would be strange if it didn't overheat.

This isn't a copper issue. It's a selection issue.

Here's a simple "three-step check method":

Next time you select cables, don't just look at the table. Follow this order:

Step 1: Check the basic current carrying capacity

Based on your cable specifications, insulation type, and copper/aluminum ratio, find the baseline value in the IEC or NEC table.

Step 2: Ask yourself three questions

Is the ambient temperature above 30°C? If so, reduce the current carrying capacity.

Is it in a conduit, cable tray, or directly buried? Different laying methods have different coefficients. How many cables are in the same bundle/conduit? Two cables will have an impact, three or more will be noticeably more significant.

Step 3: Multiply by a coefficient.

If your actual load is still within this value after multiplying, use it with confidence.

If it exceeds it—upgrade to a larger specification size, or change the laying method.

Changing the laying method is often more cost-effective than upgrading the specification.

What happened to that Thai customer later?

I wrote out the calculation process clearly for him and sent it to him.

He didn't believe it at first. He thought, "How could the standards be so different?"

Later, they did a field test on another line: under the same load, the cable that was originally very hot became normal in temperature after being replaced with a 50 square millimeter cable.

He messaged me:

"I never knew I had to multiply by these coefficients before. You are different from other suppliers."

Later, for the second phase project of that factory, they used us for the entire set of cables.

Three very important reminders:

1. Don't just look at the "large numbers" on the current carrying capacity meter.

Those are measured under ideal laboratory conditions. Your site is not a laboratory.

2. Overheating Doesn't Always Mean Impure Copper

We've seen far too many returns: customers complain of overheating cables, only to find the copper is fine upon inspection. The root cause turns out to be improper cable selection.

The cable manufacturer is wrongly accused, and the customer wastes time and money.

3. If You're Unsure, Don't Guess

Tell us your installation conditions:

→ Ambient Temperature

→ Conduit/Cable Tray/Direct Burial

→ Number of Cables Running Together

→ Load Current

We'll calculate it for you. Free of charge.

Because replacing a cable you installed incorrectly is much more expensive than choosing a larger size initially.

Frankly, in my ten-plus years in this industry, the most common reason for returns isn't material or manufacturing issues, but rather overlooking those three correction factors during selection.

Customers think they've chosen according to standards. In reality, they've chosen "laboratory standards," not "field standards."

If you're hesitant about whether a single cable is sufficient, spend 10 minutes writing down your site conditions and recalculating. The situation might be more critical than you think, or it might be more lenient. But at least—you won't receive another message asking "Why are your cables so hot?"