Related reading: 70 Amp Wire Size: Breaker & Wiring Gauge Guide How Do You Size a Wire? If the circuit regularly draws more than this current, you should consider increasing the circuit breaker size if the wiring gauge is suitable. The maximum number of amps you should put through a 40-amp circuit breaker is 32 amps. Use the following formula to calculate the maximum amps you can put through a 40-amp breaker. It is okay if the amperage spikes periodically over 80% capacity as appliances start, but the circuit load must not consistently exceed this maximum. This principle means that the continuous current drawn through the circuit must not exceed 80% of the circuit breaker’s capacity for more than 3-hours. The NEC defines a “long time” as more than 3 hours continuously. The NEC or National Electrical Code requires that not more than 80% of the circuit breaker capacity be put through the circuit over a long time. In the case of a 40-amp circuit breaker, if more than 40 amps are drawn through the circuit, the breaker will trip. Related reading: 80 Amp Wire Size: Breaker & Wiring Gauge Guide How Many Amps Can You Put On a 40-Amp Breaker?Ī circuit breaker will trip or disconnect the circuit once its maximum ampacity has been reached. The wire gauging system works in reverse, so the smaller the wire gauge, the larger the diameter of the wire. The baseline standards used by the building codes are from the American Wire Gauge or AWG, which is a standard for measuring the thickness of a wire. Regulatory authorities have set standards regarding the minimum wire gauge suitable for specific amounts of current draw. The wiring can also melt completely through, breaking the circuit and requiring the wires to be re-installed in your home. The resulting short circuit can potentially cause lethal shock to people or start an electrical fire in your home. Once this happens, the wire can touch other cables or metal surfaces, causing a short circuit. The danger of pushing too much current through a thin wire is that the conductor will heat up to the point where it melts the protective insulator around it. Similarly, the larger the wire gauge, the larger the “pipe” that the electricity can pass through without the heat build-up of pushing too much current through a thin “pipe” or conductor. If you push more volume through the pipe, the pressure will increase, and the pipe can burst. A thin pipe will only allow a specific volume of water through the pipe. You can think of it in terms of a hosepipe. The thicker the wire gauge, the more space for current to flow through the wire, thus less resistance and less heat build-up in the conductor. The thickness of the wire conductor will determine how much current flow the conductor can handle before the heat build-up becomes dangerous. Thus, the circuit breaker will still protect the appliances connected to a 240v 40 amp wire size.Įlectrical current flowing through a copper wire encounters resistance from the wire, generating heat in the copper conductor. Many installers will use a larger gauge wire, such as a 6-AWG wire, which offers flexibility for future expansion, and still put a 40-amp breaker on this circuit, which is an acceptable practice.Įven though a 6-AWG wire is rated for 50 amps, putting a 40-amp circuit breaker on this wire gauge will cause the breaker to trip when its maximum current draw is exceeded, irrespective of the wire gauge. The minimum wire gauge for a 40-amp circuit breaker is 8-AWG solid core copper wire. Let’s explore which gauge of wire would be the best choice for this breaker.Ĭonclusion What Wire Size For a 40A 240V Circuit? The current that will pass through a 40-amp breaker is quite substantial, so using the correct 40 amp wire size and type of wire is vital. The unsafe circumstance is using a wire gauge that is too small for the breaker, but you can use a wire gauge with a rating of more than 40 amps. Using the correct size wire for a circuit and the circuit breaker is essential for safety in the electrical circuit. The 40 amp circuit breaker is only one component in the electrical circuit and is necessary to protect the wiring and the appliance. Always check the amp rating or wattage on the appliance nameplate for the correct wire size and breaker ratings. The wiring of the circuit must also be of an appropriate size to handle the current draw. The common breaker size for electric dryers is 30 amps, and for electric stoves is 50 amps. A 40-amp breaker is a heavy-duty breaker intended for use on a circuit connected by large appliances, such as an air conditioner, or appliances, such as electric cooktops.
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