Why does an electrical cable get hot?

A hot plug on an otherwise cool lead and a cable that warms along its whole length suggest different problems. The first may involve a poor contact; the second may involve loading, conductor size or restricted cooling. Replacing the cable with a thicker one will not necessarily fix a damaged socket, and tightening a terminal will not correct an overloaded circuit.

In brief: Current flowing through electrical resistance produces heat. Excessive cable temperature can result from too much current, excessive resistance at a connection, damaged conductors or inadequate heat dissipation. Finding where the heat is concentrated helps determine what needs investigating.

Start with the location of the heating

The following patterns are clues, not a diagnosis. Do not feel along suspect cables or open energised equipment to locate a hot spot. Several faults can produce similar temperature patterns.

Why a poor connection can overheat without tripping the breaker

A conventional circuit breaker responds to current according to its trip characteristic. It does not measure the temperature of every connection downstream. A deteriorated contact can therefore become dangerously hot while the circuit current remains below the level needed to operate the breaker.

The distinction becomes clearer with a simple calculation. Resistive heating follows P = I² × R, using current in amperes and resistance in ohms. At a hypothetical contact resistance of 0.1 ohm, 5 A produces 2.5 W of heat; 10 A produces 10 W. That power is concentrated at the contact rather than spread over the entire cable.

For unchanged resistance, doubling current produces four times the heating power. It does not mean four times the temperature. The final temperature also depends on how heat leaves the connection. The example is an illustration, not an acceptable contact-loss specification. See Ohm’s law for the underlying electrical relationships.

This is why the presence of a fuse or overcurrent protective device is not proof that a hot connection is safe. Installing a higher-rated protective device will not repair that connection or increase the cable’s capacity.

Some warming is expected, but touch is not a safety test

Real conductors have resistance, so some temperature rise under load is normal. Whether the rise is acceptable depends on the cable, its terminations and the conditions of use. A blanket statement such as “warm is fine” cannot establish that an installation is safe.

The conductor temperature specified in a cable’s technical data is not a safe-touch limit for its outer sheath. Surface temperature also cannot reveal every hidden hot spot. Increasing temperature, unusual smells, softening insulation or discolouration deserve investigation rather than a hand test.

Conductor size is only part of the thermal rating

The same cable can carry different permissible currents in different surroundings. Open-air installation, enclosed conduit, grouping with other loaded circuits and thermal insulation all affect heat dissipation. A hot environment leaves less room for the conductor’s temperature to rise before reaching its limit.

This is why cable sizing involves more than matching an appliance’s wattage to a conductor area. Familiar sizes such as 1.5 mm², 2.5 mm², 4 mm² and 6 mm² do not have one universal current rating.

External diameter can also mislead. A thick jacket does not guarantee a large metal conductor. Material matters too: copper and aluminium do not have equal resistance at the same cross-section and length.

Connection quality can defeat an otherwise suitable cable

Loose contacts, corrosion, damaged strands and incorrectly crimped lugs can all restrict the current path. The conductor may be adequately sized while its termination is not. Mechanical damage near a plug or repeated bending at a cable entry can also affect the conductor beneath an apparently intact outer sheath.

Terminals must suit the conductor material, size and construction. The requirements for solid and stranded conductors are not automatically interchangeable. Using a ferrule or cable lug helps only when the terminal permits it and the termination is correctly selected and formed.

More tightening is not always the answer. Excessive torque can damage a terminal or conductor, while heat-damaged components may need replacement. Repairs should follow the manufacturer’s connection requirements after safe isolation.

Why reels and bundled leads are especially vulnerable

A loaded cable needs to release heat to its surroundings. Closely wound layers or tightly packed leads make that harder. A reel can therefore overheat at a load that the same product is rated to carry when fully unwound.

Check the manufacturer’s separate wound and unwound ratings. Unwinding improves cooling; it does not change the conductor cross-section. A thermal cut-out is a protective backstop, not permission to run the reel until it trips repeatedly.

Multiple outlets do not multiply a lead’s total capacity. The supply cable and plug carry the combined load of the connected appliances. The distinction is explained in extension-lead wattage limits. For portable leads specifically, see why extension leads get hot.

Does cable length make every part hotter?

For the same material and cross-section, a longer cable has greater total resistance. At the same current, total resistive loss and voltage drop increase. However, heating per metre does not increase merely because extra metres have been added, provided current, conductor properties and conditions remain the same.

The load’s response to the lower supply voltage also matters. A resistive heater, a motor and a regulated electronic power supply can respond differently. It is incorrect to assume that every appliance draws more current whenever voltage falls.

Will an RCD or earthing prevent cable overheating?

An RCD detects residual current; it is not a general temperature monitor. Heating that develops without a sufficient residual current may not operate it. An RCCB does not provide the overcurrent protection included in an RCBO. See the differences between an MCB, an RCD and an RCBO.

Protective earthing serves another essential purpose, but it does not remove excessive load current or correct a high-resistance joint. These protections must work alongside suitable cable selection and sound connections.

What does a useful inspection establish?

A competent investigation checks actual loading, conductor size, routing, connections and the protective arrangement. A suitable clamp meter can help establish current, while voltage-drop measurements and thermal inspection provide different pieces of evidence.

A thermal image is useful only with context. Load level, ambient conditions and comparable connections affect its interpretation. A circuit photographed with little current flowing can look unremarkable even though a fault becomes apparent under normal operating load. Energised inspection and access to enclosed conductors require appropriate competence and precautions.

Technical references: Fluke — electrical heating and thermal inspection; Fluke — loose or corroded electrical connections; Schneider Electric — general cable-sizing method; Brennenstuhl — why cable reels should be unwound; WAGO — ferrule compatibility and termination requirements; Schneider Electric — RCCB and RCBO protection.