Dear Dave,
Some of my appliance plugs only have two prongs but all the outlets have three slots. Is there a correct way to plug these in? —Kevin, 2024 Grand Design
Dear Kevin,
Your three-slot outlets are 120-volt AC and have a hot, neutral, and a semi-round or “U”-shaped hole for the ground. This ground helps prevent dangerous electrical shocks or fires. If an appliance develops a loose hot wire internally, it could touch a metal component and become energized. Touching the energized component could result in a deadly shock hazard.
Notice the two vertical slots are different sized, as the smaller slot is the hot wire and the larger slot is the neutral. The positioning of the ground hole has been discussed many times. Some position the ground hole at the top so if a metal object were to fall on the terminals of the plug, it could not arch across the hot to neutral. This is common in hospitals and medical offices. Check out the article regarding outlet orientation here.
Electricity, or electrons, stream in a closed path or circuit, from the power source through the distribution center, and flows from hot to neutral. When an appliance is plugged into the outlet, it completes the circuit and electricity safely flows from the distribution center to the appliance.
Two-prong outlets and appliances
According to the National Electrical Code (NEC), two-prong outlets were used in residential and other applications prior to a code change somewhere around 1962. The two slots were the same size and were hot and neutral, no grounding slot. They were safe as long as everything is wired correctly and tight.
A two-prong plug can be used in a three-prong outlet. However, a three-prong appliance and plug cannot be used in a two-prong outlet without an adapter.
Most two-prong appliances such as toasters and hair dryers have a larger plug, and can only be inserted in one direction.
However, there are some plugs that are the same size, such as cell phone chargers. These are classified as non-polarized plugs and can be plugged into an outlet in either direction. These typically do not have a metal case or enclosure that could be an electrical hazard if a wire is compromised.
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What you did not mention Dave is the mentioned 3 prong plug to 2 prong outlet adapter is meant to be grounded by the user. There is either a short green wire or a metal tab to be attached to the screw that is (hopefully) grounded inside the outlet.
Since AC alternates positive and negative, it has no actual polarity so prongs and outlets are not polarized but rather biased to neutral.
The larger lug/slot intentionally denies potential on the backside of a switch or fuse. By directing potential (Load) down a specific wire that flows through a fuse or switch, it prevents everything after the fuse/switch from being energized.
Imagine if it wasn’t wired this way and you blow a fuse. Everything in the circuit would be hot except for the small part of the circuit between the fuse and Load.
Prongs without a large lug are typically unswitched circuits such as coils where neutral bias doesn’t matter.