What is an electrical overload?

Glossary · Wiring, circuits and faults

How an overload works

Every cable can carry a limited current. Part 2 of BS 7671 defines an overload current as an overcurrent occurring in a circuit that is electrically sound. NICEIC adds that it may result from overworked equipment, or from a user connecting equipment so that the current exceeds what the circuit was designed to carry.^{[1]NICEIC, Overcurrent protection - overload}

The harm is heat. If an overload occurs where there is no overload protection, the temperature of the conductors is liable to rise significantly, which over time may damage insulation, joints and terminations and the surroundings.^{[1]NICEIC, Overcurrent protection - overload} The protective device, a circuit breaker or a fuse, is designed to disconnect the circuit automatically when an overload occurs.^{[1]NICEIC, Overcurrent protection - overload} Our guide to the circuit breaker shows how that works.

NICEIC’s consumer unit guide describes the same split: circuit breakers break the circuit when tripped by short-circuit issues or power overloads, and switch off faster than traditional fuses.^{[2]NICEIC, Consumer units and fuse boxes: installation and replacement}

Overload is slow, short circuit is fast

An overload builds gradually and can be modest. NICEIC gives a fuse example: a 16 A fuse can sustain 20 A for an hour before the element weakens and breaks, and at 25.6 A it will rupture within the hour.^{[1]NICEIC, Overcurrent protection - overload} That gradual character is the difference from a short circuit, which is a fault with a sudden large current.

Not every load can overload its own circuit. NICEIC says resistive loads such as electric showers, immersion heaters and convector heaters cannot draw more than their rating, so a device for overload protection need not be provided for them. But where such loads are connected to a circuit such as a ring final circuit, the circuit can still be overloaded.^{[1]NICEIC, Overcurrent protection - overload}

What you can check yourself

Electrical Safety First says overloading sockets and extension leads should not happen, and that you should take care not to overload extension leads, especially with larger devices such as TVs. It offers a socket calculator to help.^{[3]Electrical Safety First, Electrical Safety Advice for Living Rooms}

If a breaker has tripped, the IET describes a simple routine. Remove the plugs of all appliances on the affected circuit completely, not just switch them off. Reset the breaker or RCD. If it stays on, plug appliances back in one at a time to find the faulty one, and do not use it again until it is repaired.^{[4]IET, Consumer units - safe fuse-box upgrades for modern homes} The IET adds that resetting repeatedly on a fault can damage the device, so keep attempts to a minimum before consulting a registered electrician.^{[4]IET, Consumer units - safe fuse-box upgrades for modern homes}

Our guide to resetting a tripped consumer unit covers the steps in more detail.

When to call an engineer

Call a registered electrician if the breaker will not reset with everything unplugged, or keeps tripping. Electrical Safety First says tripping electrics can mean circuits are overloaded or that the wiring in the property is deteriorating.^{[5]Electrical Safety First, Haunted or Just Hazadous? Residents in Uk's Spookiest Cities Urged to Call an Electrician, Not a Ghostbuster This Halloween} Also call if you notice discoloured sockets, buzzing or warm plugs. Do not open the consumer unit. See why an RCD keeps tripping for the shock-protection side.

Example

Nadia in Kirkstall runs a heater, a kettle and a toaster from one kitchen circuit on a cold morning, and the breaker trips. She unplugs everything, resets once and the breaker holds. She plugs the kettle back in and then the toaster, and finds the circuit holds with both but trips when the heater joins them. She moves the heater to another circuit and mentions the pattern to her electrician at the next visit. She did not open the consumer unit or replace the breaker.

What people get wrong

Many people treat a tripping breaker as a nuisance and fit a higher-rated one, or keep resetting it. A breaker is rated to protect the cable behind it. Raising it removes that protection, and the cable can then overheat.

NICEIC’s rule is that the rating of the protective device must not exceed the lowest current-carrying capacity of the circuit’s conductors.^{[1]NICEIC, Overcurrent protection - overload} Another mistake is assuming overload and short circuit are the same. NICEIC’s definition makes overload a condition in a healthy circuit and not a fault.^{[1]NICEIC, Overcurrent protection - overload} The fix for an overload is less load or a better circuit. The fix for a fault is repair.

The bottom line

An overload is too much current in a circuit that is working normally, and the danger is heat in the cable. Breakers and fuses are there to disconnect it. If one trips, unplug everything, reset once and add appliances one at a time. If it trips again, or you see warning signs, ask a registered electrician and never fit a bigger breaker to stop the tripping.