Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under conditions of use without exceeding its temperature rating. Ambient temperature is a critical factor, as a cable's ability to dissipate heat generated by the Joule effect depends directly on the thermal difference between the conductor and the surrounding medium.

When the ambient temperature increases, the cooling capacity of the cable decreases. To avoid premature degradation of the insulation, correction factors (derating factors) must be applied. According to international regulations such as the NEC (National Electrical Code), the tabulated ampacity (generally calculated at 30°C or 40°C) must be multiplied by a factor less than 1.0 if the ambient temperature is higher.

It is essential to distinguish between operative thermal resistance and circuit integrity during a fire. Although both terms are often confused, they fulfill different safety functions and regulations.

Heat-Resistant Cables

These cables are designed to operate continuously in environments with elevated temperatures (from 150°C up to over 450°C). Their function is to maintain their mechanical and electrical properties without the insulation melting or becoming brittle. They are commonly used in furnaces, motors, and industrial machinery.

Fire-Resistant Cables

Unlike the previous ones, fire-resistant cables are designed to maintain circuit integrity during a fire for a specified period (e.g., 90 or 180 minutes). These cables ensure that critical systems such as alarms, fire pumps, and emergency lighting continue to function under direct fire. They often include mica tape or ceramic insulation that does not carbonize easily.

Flame-Retardant Cables

Flame-retardant cables have the property of not propagating a flame. If a short circuit or external ignition source occurs, the cable self-extinguishes and prevents the fire from spreading along the cable trays.

Wiring heater elements and heating bands requires conductors that support not only the electrical load but also conductive heat transfer from the heater itself. Using conventional PVC or XLPE cables in these applications would result in immediate failure.

Recommendations by Temperature Range

To select the appropriate cable, it is vital to identify the maximum temperature at the connection point (terminal):

1. Up to 180°C:​ The use of cables with Silicone insulation (SIAF) is recommended. They are highly flexible and suitable for humid environments, although they have lower mechanical resistance to abrasion.

2. Up to 250°C:​ The industry standard is the TGGT (Teflon-Glass-Glass-Teflon) cable. It uses a nickel-plated copper conductor to prevent oxidation and a multi-layer insulation of PTFE and fiberglass.

3. Up to 450°C - 550°C:​ For extreme applications like foundry furnaces or high-power heaters, MG (Mica-Glass) or MGT cables are used. These feature treated fiberglass braiding and mica reinforcements.

Conductor Considerations

In extreme heat applications, bare copper oxidizes rapidly, increasing resistance and causing failures. Therefore, the following are recommended:

• Tinned Copper:​ For moderate temperatures (up to 150°C).

• Nickel-Plated Copper:​ Essential for temperatures above 200°C.

• Pure Nickel:​ Used in environments exceeding 600°C.

Safety Note:​ When wiring heating bands, ensure that the terminals are properly protected with fiberglass sleeves (spaghetti) to avoid direct contact with the hot surface of the heater.