Electrical overload occurs when the current flowing through a circuit exceeds its rated capacity. In other words, it is an excessive energy demand that causes the current to be higher than the wiring or devices can handle. Unlike a short circuit (a fault between conductors that causes a sudden, intense current flow), an overload happens when too many high-power devices are connected at the same time.
This phenomenon can occur in both residential and industrial installations and is dangerous because it can damage equipment or start fires. For a better understanding of the differences, the Solera article on short circuits explains its origin and how it differs from an overload.
Common causes
- Connecting too many high-power devices on the same circuit (for example, multiple appliances or electronic devices on the same power strip). This scenario creates an outlet overload when the sum of the currents exceeds the circuit’s limit.
- Continuous and improper use of extension cords or power strips. When multiple devices are repeatedly plugged into the same extension, the single outlet can become overloaded.
- Old or defective equipment or installations. Old cables, defective fuses or circuit breakers may malfunction and fail to cut off the current properly. Worn wiring increases resistance and heat when asked for more current than it can safely dissipate.
- Poor circuit distribution. In older homes it is common for several outlets and lights to share a single circuit, so that a seemingly moderate load ends up saturating the entire system.
- Power supply variations. Voltage spikes or fluctuations on the electrical grid can suddenly trigger high currents that overload the installation.
Typical examples of electrical overload
- Outlet overload: Plugging a TV, a computer, several phone chargers and a lamp at the same time into the same power strip is a classic case. If the total current exceeds the outlet’s capacity, the outlet will overheat and the circuit breaker may trip.
- Lighting overload: When all the bulbs and fixtures on one lighting circuit are turned on together with other appliances connected on the same circuit, the lights may flicker or dim. This indicates that the demand exceeds what the circuit can supply.
- Simultaneous appliances: Turning on an electric oven, a dryer and a dishwasher at the same time can blow fuses. Each of these appliances consumes a lot of power, and together they exceed the capacity of the kitchen or laundry circuit.
- Overloaded extension cords: Using several chained power strips and plugging many devices (computers, printers, lamps) into a single outlet is dangerous. This kind of outlet overload heats the cables and can start a fire.
- Undersized heating or cooling systems: Installing a high-power heat pump on an older electrical network without upgrading it can make the thermal-magnetic breakers in the panel trip repeatedly, indicating that the circuit is overloaded.
Technical, economic and safety consequences
- Technical: Damage to equipment and installations. Excess current heats conductors to the point of melting insulation and wires. This can cause internal short circuits and permanent failures in motors, transformers and electronic devices. It can also force frequent blowing of fuses and tripping of circuit breakers, leaving an entire circuit out of service.
- Safety: Risk of fire and electric shock. Heating of cables and outlets can ignite fires if flames reach flammable materials. A burning smell or sizzling sounds are signs of imminent danger. In addition, an unstable installation increases the chance of electric shocks to people if any part becomes energized due to a fault.
- Economic: Losses from repairs and downtime. Replacing damaged equipment (computers, motors, pumps) and repairing melted wiring involves direct costs. In industrial or commercial environments, an overload causes interruptions in production, with unexpected stoppages that lead to losses from inactivity. In residential settings, an overloaded circuit can increase apparent power consumption and the electric bill, and requires investing in upgrading the installation.
How to avoid electrical overload
- Size each circuit correctly: Calculate the expected load (total power in watts) and choose the appropriate cable gauge and breaker rating. According to the Spanish Low Voltage Electrical Code (REBT), each circuit must have protection against overcurrents and short circuits. For example, homes commonly use 1.5 mm² cable with a 10–16 A breaker for lighting, and 2.5 mm² with a 20 A breaker for general-purpose outlets.
- Do not use all appliances simultaneously: Avoid turning on high-power devices (washer, oven, air conditioner) at the same time. Staggering or alternating their use reduces sharp demand peaks.
- Update old installations: Wiring or panels from 20–30 years ago probably cannot handle today’s load. Upgrading lines and panels with modern materials eliminates deficiencies and increases safety.
- Avoid excessive extension cords: Whenever possible, plug devices directly into wall outlets. If you use power strips or extension cords, make sure they are high quality and rated for the load. For example, install multiple wall outlets instead of chaining power strips. Among safe solutions are the special ION Series multi-outlet strips, designed to prevent internal overcurrent.
- Inspect cables and connections: Regularly check that there are no frayed wires, loose solder joints or loose connections in outlets and terminals. Ground-fault circuit interrupters (RCDs) should also be tested periodically. Warning signs such as sparks, buzzing or heat at outlets should never be ignored.
- Distribute the load across circuits: Do not concentrate multiple kitchen appliances or lighting loads on a single circuit. Creating independent circuits for the kitchen, air conditioning, or bathroom helps keep each circuit below its rated capacity.
- Install appropriate protective devices: Use thermal-magnetic breakers calibrated to the actual amperage of each line, and properly rated fuses in machinery. Protect critical loads with RCDs (as required by code), and install surge protectors at the panel to divert transient voltage spikes.
In the UK, the BS 7671 Wiring Regulations require that all final circuits be protected against overcurrent, in accordance with Section 433 (Protection against overload current) and Section 434 (Protection against fault current). This means that every circuit must be protected by a fuse or circuit breaker whose rated current does not exceed the current-carrying capacity of the conductors.
In other words, the protective device must always disconnect the supply before the conductors are damaged. By applying these principles—selecting appropriate MCBs or fuses based on cable size, load demand, and installation conditions—most overload and short-circuit risks are eliminated. This is essential for ensuring long-term safety and compliance with UK standards, whether in residential, commercial or industrial installations.
Recommendations according to UK Wiring Regulations (BS 7671)
In the United Kingdom, the design and protection of electrical installations are governed by the BS 7671 IET Wiring Regulations. These regulations require that every circuit be properly protected against overcurrent (overload and short circuit) and earth faults, using devices such as MCBs, fuses, and RCDs. For example:
- MCBs (Miniature Circuit Breakers) are used to protect against overloads and short circuits in final circuits.
- RCDs (Residual Current Devices) are mandatory for socket outlets rated at 32 A or below, and for circuits supplying bathrooms, kitchens, and outdoor equipment.
In practice, this means calculating the circuit’s design current using the formula I = P / V, and selecting protective devices and cable sizes accordingly. UK standards also account for diversity factors and voltage drop limits to ensure that circuits can operate safely even under peak demand. Typical minimum requirements include:
- 1.5 mm² cable protected by a 10–16 A breaker for lighting circuits.
- 2.5 mm² cable with a 20 A breaker for ring or radial socket circuits.
Additional considerations such as correction factors (for grouping, insulation, ambient temperature, etc.) are applied to ensure compliance with installation conditions and long-term safety.
Best practices in solar PV installations
In solar photovoltaic systems, additional precautions must be taken to prevent overloads of the installation’s components. Each string of panels (series of modules) must be sized so that the generated current does not exceed the capacity of the charge controller or inverter. According to Solera, the protective distribution panel is essential: “it ensures that the installation is protected against overloads, short circuits, and electrical discharges.”
For this reason, it is advisable to follow the five key elements of a solar installation (modules, controller, inverter, batteries and protective panel) described in the relevant article. In particular, the batteries must have charge/discharge protections to avoid overcharging, and the main distribution panel must include circuit breakers matched to the system’s voltage and current. The design of conduits and enclosures should allow heat to dissipate and facilitate inspection. In summary, implementing proper sizing, protection and maintenance practices in solar plants minimizes the risk of overload throughout the system.
Load sizing and recommended protections
To properly size an electrical installation, calculate the total power demand and add a safety margin. From this, determine the nominal current of each circuit and choose the cable section that can handle that current (according to UNE 20460-5-523). For example, in a single-phase AC home installation, it is common to assign 20 A (2.5 mm²) to general-purpose outlets and 10–16 A (1.5 mm²) to lighting circuits.
Once the cables and conduits are defined, select fuses or breakers to protect the cable: the nominal current of the protection device should not exceed the allowable current of the conductor, preventing overheating. In practice, this means installing 20 A breakers on 2.5 mm² lines and 16 A breakers on 1.5 mm² lines (or following the manufacturer’s recommendation). In three-phase or industrial systems, similar calculations are applied to each phase. It is always advisable to follow the REBT and the supplementary technical instructions (ITC) so that each circuit is consistent in terms of load, gauge and protection.
Protective devices
- Fuse: A single-use device containing an internal metal wire. When the current exceeds its rated value, the fuse melts and opens the circuit. It is common in appliances, transformers or specific subcircuits. For more information on how it works, see the article “What is a fuse and what is its function?”.
- Circuit breaker: Combines thermal and magnetic trip mechanisms. It protects against overloads (slow thermal trip by heat) and short circuits (instant magnetic trip). It is installed in the panel and can be reset after tripping.
- Residual-current device (RCD): Detects leakage currents to ground. Its function is to protect people; it does not act on overloads. RCDs are mandatory in bathroom, kitchen and outdoor circuits.
- Surge protector: A device (surge arrester) that diverts high transient voltage pulses (from lightning or switching events) to ground, thereby protecting electronic equipment. Surge protectors are installed in the main panel or at critical data/electronic outlets.
- Special multi-outlet strip: A power strip with built-in protection (fuse or breaker) or with a reinforced design to prevent overloads. For example, the special ION Series multi-outlet strips include mechanisms to cut off the current if the nominal capacity is exceeded. They are useful in offices or laboratory environments where many connections are concentrated.
Comparative table of electrical protection devices
| Type | Function | Application |
|---|---|---|
| Fuse | Interrupts the circuit when it melts due to overcurrent. | Basic protection for low-current equipment and circuits (small transformers, motors). |
| Thermal-magnetic circuit breaker | Cuts off the current on overload (thermal element) or short circuit (magnetic trip). | Comprehensive protection in electrical panels of homes, offices and industries. |
| Residual-current device (RCD) | Disconnects on ground leakage, protecting people. | Circuits with risk of direct contact: bathrooms, kitchens and outdoors (per regulations). |
| Surge protector | Diverts high transient voltage pulses to ground. | Electrical panels and outlets of sensitive equipment (computers, electronics). |
| Protected power strip | Includes a fuse or internal cutoff to prevent overload on the strip. | Series-connected devices in homes and offices; prevents strip overheating and fires. |
In summary, preventing an electrical overload requires planning the installation with technical criteria and following regulatory recommendations. With proper load calculation, correctly rated protection devices, and good connection practices, the risks of damage and electrical accidents are drastically reduced. Remember: it is better to have a few extra outlets or circuits than to push a system to its limit.