Why is the Long Prong Neutral Instead of the Narrow Prong?

Electrical Technology

Why is the Wide Prong and Slot Designated as Neutral Instead of the Narrow Prong and Slot in a Plug and Outlet?

Engineers typically avoid making arbitrary choices when designing systems. However, in some cases, they do make standardized decisions that may seem arbitrary. For instance, selecting standard voltages like 120V, 230V, or 240V, standard frequencies of 50Hz or 60Hz, the direction of current flow, HV transmission levels, and the structure of power distribution networks.

One such design choice is the use of wider prongs and slots in polarized plugs and outlets/receptacles as a safety measure. But why is the wider prong assigned to the neutral wire? Why not use the narrow prong for neutral and the wider one for hot?

While NEMA (National Electrical Manufacturers Association) standards govern these designs, the wider prong is likely dedicated to the neutral wire for the following reasons: 

Most wall outlets are easily accessible to children playing nearby. If a child picks up a butter knife, paperclip, or other metallic object and tries to insert it into the outlet, they are more likely (psychologically) to choose the larger slot rather than the narrower one, as it appears more accommodating.

Based on this assumption, designers chose to designate the wider slot for the neutral connection. Since the neutral wire is electrically bonded to the ground at the main panel, the risk of electric shock is significantly lower compared to the hot prong, which carries 120V or 240V.

Why is the Long Prong Neutral Instead of the Narrow Prong

Why is One Slot or Prong Wider than the Other?

In North America, the wider prong on an electrical plug is the neutral prong, not the narrow prong. This is used as a safety measure to avoid accidental contact of Hot to Neutral and vice versa . For instance, a polarized plug with a wide neutral prong only fits one way into the receptacle.

This way, a three pin plug inserted into a receptacle / outlet makes a connection in such a way where:

If the hot prong were wide too, users could potentially reverse polarity by inserting the plug upside-down into older or non-polarized outlets, creating a shock hazard or damage to the appliances.

The wider and narrower prongs are designed to ensure correct polarity in the electrical circuit. The hot wire (smaller prong) should connect to the hot side of the appliance, and the neutral wire (wider prong) to the neutral side.

By making the neutral prong wider, the plug can only be inserted in the correct orientation. This prevents accidental reverse insertion, which could lead to safety hazards. This is why the Neutral prong or slot wider on a plug or outlet.

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