The standard boiling time for an egg in a kettle is approximately 8 to 10 minutes. This duration ensures that the egg is fully cooked, with the white set and the yolk firm but still slightly runny for a soft-boiled egg, or fully set for a hard-boiled egg. The exact time can vary slightly depending on the size of the egg and the temperature of the water when you add the egg.

The reason for this boiling time lies in the heat transfer process that occurs when an egg is submerged in boiling water. As the water reaches its boiling point, it begins to heat the egg from the outside in. The heat causes the proteins in the egg white to coagulate, forming a solid white, while the yolk remains liquid. The longer the egg is boiled, the more the proteins coagulate, leading to a firmer white and a more solid yolk.

According to the United States Department of Agriculture (USDA), eggs should be cooked to an internal temperature of 160°F (71°C) to ensure they are safe to eat. A hard-boiled egg will reach this temperature within the 8 to 10-minute range, while a soft-boiled egg will be closer to 145°F (63°C). The cooking time can be adjusted based on personal preference for the yolk's consistency. For example, if you prefer a runnier yolk, you might stop boiling the egg after 7 minutes, while for a firmer yolk, you might continue boiling for the full 10 minutes.

It's important to note that the initial temperature of the water also plays a role in the cooking time. If you add a cold egg to boiling water, it will take longer to reach the desired internal temperature than if you start with a room temperature egg. This is because the egg needs to heat up to the temperature of the water before it can start cooking. Therefore, it's often recommended to bring the water to a boil first, then add the egg, to ensure a consistent cooking time.

In summary, boiling an egg in a kettle for 8 to 10 minutes is the standard method to achieve a fully cooked, hard-boiled egg. This time frame is based on the heat transfer process and the need to reach a safe internal temperature. Adjustments can be made based on personal preference for the yolk's consistency and the initial temperature of the water.