Overloaded Circuit: Signs, Dangers, and the Correct Solutions

An overloaded circuit occurs when the total current drawn by the appliances connected to a circuit exceeds the current-carrying capacity of the cable or the rating of the protective device (MCB or fuse). The excess current generates heat in the cable — beyond what the insulation is designed to withstand.

Every cable has a maximum current-carrying capacity (Iz) determined by the conductor size, insulation type, installation method, ambient temperature, and grouping with other cables. The protective device (MCB) must be rated at or below the cable's current-carrying capacity to ensure it trips before the cable overheats. When the current exceeds the MCB rating, the MCB should trip and disconnect the circuit.

Problems arise when the overload is sustained but not quite high enough to trip the MCB quickly. An MCB is designed to carry its rated current continuously without tripping. At 1.13 times the rated current (for example, 36A on a 32A MCB), it may take over an hour to trip. At 1.45 times the rated current (46A on a 32A MCB), it should trip within the specified time — but that could still be several minutes of overheating. During this period, the cable temperature rises, and repeated overloading degrades the insulation over time.

Understanding maximum demand and BS 7671 cable sizing requirements is essential for diagnosing and preventing circuit overload. Elec-Mate's maximum demand calculator helps you assess whether a circuit is properly sized for its load.

Recognising the signs of an overloaded circuit early can prevent damage, fire, and costly repairs:

If a customer reports any of these symptoms, an electrician should measure the actual load on the circuit using a clamp meter, compare it with the MCB rating and cable current-carrying capacity, and advise on the appropriate remedy — which is usually either redistributing the load or adding new circuits.

Maximum demand is the maximum current that a circuit or installation is expected to draw under normal operating conditions. Calculating maximum demand correctly is essential for determining whether a circuit is adequately sized and whether it is genuinely overloaded.

The basic calculation is straightforward: add up the power ratings (in watts) of all appliances that could be connected to the circuit, then divide by the voltage (230V) to get the current in amps. However, this gives the "total connected load" — the theoretical maximum if everything runs at full power simultaneously.

In practice, not all appliances run at the same time. This is where diversity comes in — a set of factors that reduce the calculated demand to reflect realistic usage patterns. BS 7671 (Table 3 of the On-Site Guide) and IET Guidance Note 1 provide standard diversity factors for different types of circuit and load.

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When an MCB trips due to overload (as opposed to a short circuit or earth fault), it is the thermal element within the MCB that responds. The thermal element is a bimetallic strip that bends when heated by the current flowing through it. When the current exceeds the MCB's rated value, the strip bends far enough to release the trip mechanism.

The key characteristic of thermal tripping is that it is time-dependent — the higher the overload, the faster the trip:

If a customer reports that their MCB trips after a few minutes of running certain appliances, this is consistent with a thermal (overload) trip. If the MCB trips instantly when an appliance is switched on, it is more likely a short circuit or earth fault in the appliance or the wiring.

Never replace an MCB with a higher rating to "fix" the tripping — this removes the cable protection and creates a fire risk. Instead, investigate the cause of the overload and add additional circuits if needed.

Circuit overloading is a leading cause of electrical fires in the UK. When current exceeds the cable's capacity, the consequences escalate progressively:

The EICR should record any evidence of overloading as a C2 (Potentially Dangerous) defect if the cable current-carrying capacity is being exceeded, or C3 (Improvement Recommended) if the circuit is close to its limit and likely to be overloaded as more appliances are added. Elec-Mate's defect code AI helps classify the severity correctly.

Diversity is the principle that not all connected loads operate simultaneously at full capacity. Without diversity, every circuit and every supply would need to be sized for the absolute maximum — which would be vastly over-engineered and prohibitively expensive.

BS 7671 and the IET On-Site Guide (Table 3) provide diversity factors for different types of circuit:

Diversity is applied to calculate the maximum demand of the whole installation (to size the main supply) and can also inform individual circuit sizing. However, diversity should be applied with caution — if there is any doubt, err on the side of higher demand. The consequences of undersizing are far worse than the cost of slight oversizing.

When a circuit is genuinely overloaded, the correct solution is almost always to add one or more new circuits to redistribute the load:

Adding a new circuit is notifiable work under Part P of the Building Regulations and requires either notification through a competent person scheme or a Building Control application. An Electrical Installation Certificate must be issued for the new circuit.

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Prevention is always better than dealing with the consequences of overloading. Electricians can advise customers on the following practical steps:

Elec-Mate's training courses cover circuit design, cable sizing, and maximum demand calculation in detail — essential knowledge for every electrician advising customers on safe circuit loading.