Electrical materials
An installation is, materially, a selection of materials: copper, insulation, sheet metal, plastic and iron. Choosing any of them badly is not noticed on the first day; it is noticed two years later, when something heats up, cracks or fills with water.
01Conductors
| Material | Resistivity at 20 °C | When it is used |
|---|---|---|
| Copper | 0.0172 Ω·mm²/m | Practically everything: installations, boards, motors. The best common conductor, ductile and easy to connect. |
| Aluminum | 0.0282 Ω·mm²/m | Overhead lines and large-cross-section feeders: it weighs a third as much and costs less. It requires specific terminals and pastes. |
| Silver | 0.0159 Ω·mm²/m | Only in contacts: it is the best conductor, but its price rules it out as a line conductor. |
A single-phase circuit of 2.5 mm² supplies a 16 A load at 28 m from the distribution board, at 220 V.
ΔU = 2 × 0.0172 × 28 × 16 / 2.5 = 6.2 V, that is 2.8 %. Just inside the allowable 3 %, but with no margin.
With 4 mm²: ΔU = 3.85 V, or 1.8 %. The current-carrying capacity was already fine with 2.5 mm²; what forces the move to a larger cross-section is the distance. It is the most frequent calculation mistake on long runs: the current is checked and the drop is forgotten.
| Insulation | Maximum temperature | Characteristics |
|---|---|---|
| PVC | 70 °C | The standard in home installations. Inexpensive. Gives off dense, corrosive smoke when burning. |
| Halogen-free (LSZH) | 70 to 90 °C | Mandatory in places with many people: schools, hospitals, subways. It gives off no toxic or corrosive gases. |
| XLPE / EPR | 90 °C | Cross-linked polyethylene and ethylene-propylene rubber. They carry more current at the same cross-section and withstand short circuits better. |
| Silicone | 180 °C | Ovens, boilers, motors. Very flexible and very expensive. |
It depends on the cross-section, the insulation, and above all on how the cable is installed: in free air, embedded in conduit, buried, or bundled with others. The same 2.5 mm² conductor carries much less current in a conduit next to five others. The regulation tables include those correction factors, and skipping them is what produces warm cables inside walls.
02Insulators and enclosures
- Thermoplastics (PVC, polyethylene, polyamide): they soften with heat and are easy to mold. Switch bodies, boxes, raceways.
- Thermosets (Bakelite, epoxy resins): they do not soften, and resist temperature and arcing. Lamp holder bases, insulators, encapsulated windings.
- Ceramics and glass: line insulators, fuse holders, power resistors. They withstand weather and temperature for decades.
- Impregnated paper and varnishes: insulation between turns of transformers and motors.
- Dielectric strength: how many kV per millimeter it withstands before breaking down.
- Operating temperature: above it, it ages quickly and becomes brittle.
- Insulation resistance: measured with a megohmmeter, and the basic acceptance test of an installation.
- Fire behavior: whether it spreads flame and what gases it emits.
| Rating | Where it applies |
|---|---|
| IP20 | Boards in dry indoor locations. Protects against fingers, not water. |
| IP44 | Bathrooms, laundry rooms, covered porches: splashing. |
| IP55 | Dusty industry with hose-down cleaning. |
| IP65 | Outdoors, water jets. Dust-tight. |
| IP67 / IP68 | Temporary or continuous immersion: manholes, submersible pumps. |
An IP65 enclosure that gets a hole drilled with a screwdriver to pass a cable is no longer IP65. The rating is maintained only with cable glands, blanking plugs and gaskets in good condition. It is by far the most common cause of “sealed” enclosures full of water.
03Ferromagnetic materials
Everything that transforms or converts energy —transformers, motors, contactors, reactors— needs a magnetic circuit, and that circuit is as important as the electrical one.
| Material | Use and reason |
|---|---|
| Silicon-iron sheet | Cores of transformers and motors. Silicon increases resistivity and reduces eddy currents. |
| Grain-oriented sheet | Power transformers: the crystals are aligned with the direction of the flux and losses drop considerably. |
| Ferrite | High frequency: switching power supplies, filters, pulse transformers. Extremely high resistivity, almost no eddy currents. |
| Permanent magnets | Ferrite for low cost; neodymium for compact, high-efficiency motors. |
The varying flux induces currents in the iron itself —eddy currents, or Foucault currents— that close in circles and heat the core without doing any work. By cutting the core into thin laminations insulated from one another, those circuits are interrupted and the losses fall drastically. On top of that come the hysteresis losses, which depend on the loop area of the material and on the frequency.
Both are studied in more detail in transformers and in magnetism and electromagnetism.
04Losses, efficiency and thermography
- In the copper: I²R. It grows with the square of the current, so doubling the load quadruples the loss.
- In the iron: hysteresis and eddy currents. They are almost constant: they exist even if the equipment is unloaded.
- In the contacts: a loose or corroded terminal behaves like a series resistance that dissipates power and heats up.
- Mechanical: friction and ventilation in rotating machines.
An infrared camera shows where there is heat without touching anything and with the equipment in service. It is the most efficient diagnostic tool in distribution boards: a loose terminal shows up as a hot spot long before it fails.
The temperature of the point is always compared with that of an equivalent point —the same phase at another terminal— and with the ambient temperature. The difference is what matters, not the absolute value.
A terminal with 10 mΩ of contact resistance carrying 30 A dissipates P = I²R = 900 × 0.01 = 9 W at a point the size of a fingernail. Those 9 W concentrated raise the temperature of the terminal by tens of degrees, the heat degrades the insulation, the resistance increases, and the process feeds on itself until it ends in fire. That is why periodic re-tightening of terminals is not a detail: it is essential maintenance.
05In the lab
With a micro-ohmmeter or the four-point method, measure the resistance of known lengths of conductor of different cross-sections and calculate the resistivity. Compare copper with aluminum and with the table. Verify the increase with temperature by heating a sample.
Build a circuit with 30 m of 1.5 mm² cable and a resistive load. Measure the voltage at the source and at the load with different currents, and compare with the calculation. Repeat with 2.5 mm²: the difference shows on the instrument and on the thermometer.
Measure with a megohmmeter the insulation resistance of new cables, of used cables and of a cable deliberately damaged or wetted. Record the values and establish an acceptance criterion.
With a thermal camera —or a phone attachment— survey a board in service. Identify the hot spots, compare with the corresponding terminal, re-tighten and measure again. Document the before and the after: it is a real maintenance report.
06Common mistakes
| Mistake | Consequence |
|---|---|
| Choosing the cross-section only by current | On long runs the voltage drop goes above 3 % and the equipment works at reduced voltage. |
| Ignoring grouping factors | Six cables in one conduit do not carry the same as a single one: the whole bundle heats up. |
| Joining copper and aluminum directly | Galvanic couple: the contact corrodes, the resistance increases and it ends up overheating. Bimetallic terminals exist for this. |
| Drilling a sealed enclosure without a cable gland | The IP rating is lost and the enclosure fills with water or dust. |
| Using PVC where halogen-free is required | In a fire, the smoke is toxic and corrosive exactly where people are being evacuated. |
| Solid core instead of laminated | Huge eddy currents: the iron heats up and the efficiency collapses. |
| Not re-tightening terminals | Contact resistance grows, the point heats up and ends in a fault or a fire. |
07Self-assessment
Why is aluminum used in overhead lines if it conducts worse than copper?
Because it weighs about a third as much and costs less. For the same current more cross-section is needed, but the whole is still lighter and cheaper, and in an overhead line weight is decisive.
Calculate the voltage drop over 20 m of 1.5 mm² with 10 A, single-phase.
ΔU = 2 × 0.0172 × 20 × 10 / 1.5 = 4.6 V, or 2.1 % of 220 V. Acceptable, but at 30 m it would already exceed the limit.
What does the current-carrying capacity of a conductor depend on?
On the cross-section, the type of insulation and the installation conditions: in free air, in conduit, buried, bundled with others, and at what ambient temperature.
What does IP44 mean and where does it apply?
Protection against solid objects larger than 1 mm and against water splashing from any direction. It applies in bathrooms, laundry rooms and covered porches.
When is halogen-free cable mandatory?
In places where people gather or must be evacuated: schools, hospitals, cinemas, subways. In a fire, PVC gives off dense, toxic and corrosive smoke.
Why is a transformer core made of insulated laminations?
To interrupt the eddy currents that would be induced in a solid core and heat it without producing useful work.
What is the difference between copper losses and iron losses?
Copper losses are I²R and depend on the load —they grow with the square of the current—; iron losses are hysteresis and eddy currents, and are practically constant even if the equipment is unloaded.
What is compared in a thermographic inspection?
The temperature of the suspect point with that of an equivalent point —the same phase at another terminal— and with the ambient temperature. What matters is the difference.
What happens when copper is joined directly to aluminum?
A galvanic couple forms: the contact corrodes, the resistance increases and the point heats up. It is solved with bimetallic terminals and specific pastes.
How much power does a 5 mΩ terminal dissipate at 40 A?
P = I²R = 1,600 × 0.005 = 8 W concentrated in a tiny spot. Enough to degrade the insulation and make the problem get worse by itself.