A 40-amp circuit is usually used for stoves or heat pumps. Because of the high power requirements of these appliances, choosing the correct wire gauge is very important. Choosing too small a gauge will result in excessive heating of the electrical wires, which could cause a fire. Choosing too large a wire will result in excessive cost.
National Electric Code Requirement
The National Electric Code (NEC) requires that the gauge wire used for a circuit must be able to handle 125 percent of the load's continuous current plus the load's noncontinuous current. A 40-amp circuit means that the maximum continuous current plus the noncontinuous current cannot be more than 40 amps. The NEC recommends 8-gauge copper wire, depending on the length of the circuit.
The length of the circuit will affect the voltage drop at the load. As the length increases, so will the voltage drop. The National Electric Code states that the voltage drop should not exceed 5 percent. For a 120-volt circuit, this equates to 6 volts. This means that the minimum voltage at the load is 120 volts minus 6 volts or 114 volts. To address this problem, the wire size is increased when the voltage drop becomes too high. This reduces the wire resistance per foot. An 8-gauge copper wire can carry 40 amps a maximum of 96 feet. After that, a 6-gauge copper wire is needed due to excessive voltage drop. A 6-gauge copper wire will carry 40 amps a maximum of 184 feet before another gauge change is needed.
The type of material the conductors are made of also affects the wire gauge. Different materials, such as copper, gold and aluminum, all have different resistances per foot. This affects their ability to carry current. As previously stated, an 8-gauge copper wire can carry 40 amps a maximum of 96 feet. An 8-gauge aluminum wire can only carry 40 amps a maximum of 49 feet before needing a gauge change.
Temperature can also affect the gauge of wire used. The National Electric Code lowers the allowable current flowing through a wire when the wire is in a high-temperature environment. The objective is to keep the wire from overheating.
The type of load used on a 40-amp circuit will affect the gauge cable used. This is a big factor when dealing with electric motors and their excessive start-up current. The motor may have a steady state or continuous current of 33 amps, which is well within the limits of an 8-gauge copper wire. However, if the motor's start-up current is 60 amps, then this exceeds the limits of an 8-gauge copper wire. A 6-gauge copper wire should be used because a 6-gauge can handle the start-up current without overloading the wire.