In residential electrical systems in the United States, the black wire is typically the "hot" wire, while the blue wire is usually the "neutral" wire. This standardization is in accordance with the National Electrical Code (NEC), which provides guidelines for electrical installations and safety. The black wire is designated as the hot wire because it carries the ungrounded current from the electrical panel to the various outlets and fixtures in a home.

The NEC specifies that the hot wire should be black or red, and the neutral wire should be white or gray. The color coding is consistent across different regions and is intended to ensure that electricians and homeowners can easily identify the purpose of each wire. The use of black for the hot wire is a long-standing convention that has been adopted for its simplicity and clarity.

The choice of black for the hot wire is not arbitrary. It is based on practical considerations and safety standards. For instance, black is a highly visible color, making it easier to distinguish from other wires. Additionally, black is a color that is not commonly used for other purposes in electrical installations, reducing the likelihood of confusion. The use of black for the hot wire also aligns with the color coding of circuit breakers and fuses, where the "hot" position is typically marked with a black or red indicator.

According to the NEC, the color coding of electrical wires is crucial for safety. Proper identification of wires helps prevent electrical accidents and ensures that the electrical system operates correctly. The consistent use of black for the hot wire and blue for the neutral wire across the United States contributes to the overall safety and reliability of residential electrical systems.