Wire Size, Ampacity, and Circuit Breaker Current Ratings

Wire Size, Ampacity and Circuit Breaker Current Ratings

Whether you’re building a new house or renovating your current home, electrical wiring will always be part of the project. Unfortunately, many people get consumed by other design aspects that they overlook crucial electrical requirements.

Among the most important factors in any wiring or rewiring project are electric cable sizes, ampacity, and the current ratings of the circuit breakers. As you would notice, the wire size used to supply power to your cooker is different from that used in other ordinary socket outlets. So, what are the main considerations in such cases? This article aims to answer all your questions about cable and circuit breaker amperage.

Wire Sizes

From single core to stranded wires, the electrical market has a variety of products specifically designed for your needs. With every type of wire you choose, there is a variety of sizes depending on the intended purpose. Therefore, it’s imperative that you understand what size cable for power points to avoid any electrical damages that might occur due to the wrong choice of wire. The sizing criterion follows the American Wire Gauge (AWG), the British Imperial Standard Wire Gauge (SWG), and the International Electrotechnical Commission (IEC 60228) systems.

SWG and AWG are standard in the U.K. and U.S. but aren’t universal. IEC 60228, however, is quite common and is used across most parts of the world. This system uses the conductor’s cross-sectional area (ranging from 0.5 to 2500 millimeter-square) to determine its size and ampere capacity. The most common wire sizes are 0.5, 1.0, 1.5, 2.5, 4, 6, and 10 mm².

What Is Ampacity?

Wire ampacity is basically the maximum amount of current that a conductor can safely handle at any given time. The larger the cable size, the more current, in amperes, it will be able to conduct. This is an important aspect of any wiring system because it determines the maximum load that can be connected to the circuit.

For instance, a 1.5mm² wire has a standard current capacity rating of 10 amps (but it can carry up to 20 amps). It is, therefore, used primarily in lighting systems because most of them demand up to 7 amps, which falls within the “safe zone.”

Of course, the same wire can work in a circuit fused at 13 amperes, but that’s dangerous because it’s right on the edge. Therefore, it’s recommended that you use a 2.5 mm square wire for such circuits because it has a current rating of 13.5 amps, though it can handle up to 25 amps. As you’ve noticed, a wire can still carry more current than its rated ampacity but overloading it is risky.

Circuit Breaker Current Ratings

Wire ampacity and fuse ratings go hand in hand. The work of a circuit breaker is to protect the circuit and all the connected loads from damages in case of excess current. It acts as an automatic switch that interrupts the flow of current whenever there is an overload. As such, it’s imperative that you install a circuit breaker that fits your system’s amperage.

For optimum safety, it’s recommended that the operating current of the protective device used in any circuit should be at most 125% the circuit current. In other words, the rated value of the circuit breaker shouldn’t exceed 125% of the total current drawn by your appliances. Failure to follow this rule could lead to fire accidents because the current will continue flowing even after a short-circuit. (1)

How To Determine Wire Ampacity And Circuit Breaker Ratings

So, how will you know which circuit breaker and electrical cable size to use for your set-up? First, you’ll need to determine the total expected amperage of the circuit. This can be done by dividing the total wattage by the circuit’s voltage. Then add the power of all appliances in the circuit.

For instance, if it’s a lighting system, you’ll need to calculate the total power consumed by all the bulbs and fluorescent tubes connected to that particular electrical line. In a socket line, find out the appliances that will be using all the outlets. Although you can have as many socket outlets as you want, it’s recommended that you connect a maximum of eight outlets in a 15-amp circuit.

Once you have the total power in watts, you can go ahead and divide it by the voltage. For instance, if all the lights consume a total of 800 watts in a 240-volt circuit, then the current drawn will be 3.3 amps (800/240). As earlier mentioned, a lighting system generally uses a 1.5 mm sq. (16 gauge) wire, and that will work perfectly in such a circuit because it has an ampacity of 10 amps.

As for the breaker, it should be 125% of 3.3 amps, which is 4.4 amperes. Therefore, a circuit breaker rated 5 or 6 amps should work perfectly in such a system. Remember, the fuse rating should never be larger than the wire’s ampacity, otherwise, your safety will be compromised. (2)

Conclusion

Wire and circuit breaker ampacities are important aspects when wiring or rewiring your home. Knowing which size of these items to use in any given circuit will save you a lot of unnecessary electrical-related damages. In an ideal system, the circuit breaker capacity should always be slightly larger than the total circuit current, but smaller than the wire’s ampacity.

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