When we look at a building, especially through the eyes of a resident, there are certain parts of it that we think of as permanent. Anything inside the walls or tightly fastened is often glossed over. But nothing is truly permanent, with heat stress and age slowly reducing the working capacity and efficiency of infrastructure. To top that all off, older buildings weren’t built with our modern habits in mind!
Electricians wiring buildings built in the 1970s and 1980s had no idea what was on the horizon. A bedroom had a 20A circuit breaker – sometimes shared across multiple rooms! – just enough for a heater in winter, and maybe a radio. Now if you look at a bedroom, you will find a television, possibly a home theatre setup and several outlets full of charging devices. Phones, tablets, laptops, and more all require energy, possibly much more than those old circuit breakers can handle.
When picking a circuit breaker, it is best to start with what the circuit is being used for. It doesn’t matter if you are replacing an old circuit breaker or adding a new room to your distribution board, you should consider how many devices are going to be used on that circuit at the same time.
For many situations, a simple rule applies: A circuit breaker should only be used for 80% of its rated amperage. For example, the regular, continuous usage of a 20A circuit breaker is 16 amps. Continuous usage is defined as three hours or more.
Some people use the same rule, just calculated slightly differently: A circuit breaker should be 125% of the load, meaning 25% extra capacity. Using the same example as before, a room with a 16 amp load would require a circuit breaker that provides 125% of 16, which is 20.
Knowing both of these rules means you can easily determine the correct usage for a circuit, based on if you know the load of the circuit or the amp rating of the circuit breaker.
This rule also applies to three-phase power systems. When calculating the current, you want the circuit breaker to be 125% of the load. The formula ends up looking like this: (P / V x √3) x 1.25.
In some cases, you will want to add more than 25% extra capacity. While 125% works well for lights, stoves, water heaters and motors, some devices have a significant starting energy requirement known as a non-continuous load. Hermetically sealed motors and heat pumps often get a 175% circuit breaker, while big welding equipment uses up to 200%.
In most situations, it is best to have these devices on a separate circuit. Otherwise, you need a circuit breaker that can handle both the continuous load as well as the non-continuous load at the same time. For example, a circuit breaker that handles 28A of continuous load, as well as 30A of non-continuous, will need to be (1.25 x 28A ) + (30A) = 75A
It might be tempting to just add large circuit breakers on every circuit instead of working out the specific requirements of a circuit. Besides being against safety codes (and probably voiding your insurance claim), an oversized circuit breaker runs the risk of not switching off the current when something goes wrong. This could result in damage to your appliances, a fire, or being shocked.
Other important factors
Circuit breakers were built with specific environments in mind, and South Africa has a few areas that are considered “atypical” by these manufacturers. Places with high humidity, high ambient temperatures (above 40 degrees) and high altitudes (1,800m above sea level and higher) all affect electricity and electrical systems. It is worth talking to a professional about these factors, as your circuit breakers might have to be derated for safety reasons.
Quality is key
Considering how important circuit breakers are to both the electrical appliances and the safety of inhabitants of a building, you definitely do not want to skimp on quality to save a few bucks. Cheaper circuit breakers might not detect a fault, or be able to break the flow of electricity. They might take longer to trip, and every millisecond is critical. Be sure to check the ampere interrupting capacity (AIC) of the breaker, as this is the current they can safely interrupt.
We carry an extensive range of circuit breakers from leading brands. Ask in-store to see our Gewiss, C&S or ACDC Dynamics circuit breakers.