Can You Use a Cheater Plug for Halloween Decorations?

What a cheater plug actually does

A three-prong plug has two flat blades that carry the power and a round third pin that is the equipment ground. A cheater plug is an adapter that takes those three and hands you two flat blades, so a grounded cord will fit a two-slot outlet. On the back it has a small metal tab or a green pigtail wire. That tab is meant to be screwed to the cover-plate screw of a grounded metal box, which is how the adapter is supposed to pass the ground through. Two things go wrong in real life. First, the outlets that still have only two slots are almost always on old wiring with no ground in the box at all, so the cover screw is not grounded and the tab connects to nothing. Second, almost nobody attaches the tab anyway. Either way you end up running a three-prong decoration with its ground pin going nowhere.

What the third prong is for

The ground pin gives fault current a safe path home. If a hot wire inside a decoration works loose, gets wet, or chafes through and touches the metal frame, the ground pin carries that fault straight back to the panel and trips the breaker. The prop goes dead and safe. Take that path away with a cheater plug and a fault has nowhere to go. The metal frame of a lit-up prop, or a damp cord connection lying in the yard, can sit there energized and waiting, and the first thing to complete the circuit might be a child reaching for the decoration. Nothing trips, because from the breaker's point of view nothing is wrong. That is the hazard the third prong exists to prevent, and it is exactly the hazard a cheater plug reintroduces.

Outdoors, GFCI is the protection, not the ground pin

Here is the part that surprises people. Outside, the ground pin is not your main shock protection. A GFCI is. A ground-fault interrupter watches the current going out on the hot and coming back on the neutral, and the instant they differ by about 5 milliamps, one two-hundredth of an amp and well below the level of shock that injures, it shuts the outlet off in milliseconds. It does this whether or not an equipment ground is present, because it is measuring leakage, not relying on a ground wire. That is why the code answer for an old two-prong outlet is a GFCI:

Under NEC 210.8(A), the receptacles that feed anything outdoors at a home have to be GFCI protected, adapter or no adapter. And where an outlet has no ground to begin with, NEC 406.4(D)(2) lets you replace it with a GFCI receptacle (or feed ordinary outlets from one), marked "No Equipment Ground," precisely so an ungrounded location can still be made safe. A cheater plug does none of that. It gives you a two-slot fit and zero protection. A GFCI gives you protection with no ground wire needed. One of those is the code-sanctioned fix and the other is the thing the code was written to replace.

Two-prong decorations are a different thing

Do not confuse a cheater plug with a decoration that simply came with a two-prong plug. Many lights and small props are double insulated, which means the whole thing is built with two layers of insulation and no metal to become live, so it needs no ground and ships with a two-blade plug on purpose. Those are fine in a two-slot outlet exactly as they are, and fine in a grounded outlet too. The problem is only when you take a decoration that was built with a three-prong grounded plug and force it onto a two-slot outlet with an adapter. The manufacturer put that ground pin there because that prop needs it. Removing it with an adapter, or worse, snapping the pin off, throws away protection the design counts on. If a prop has a third pin, it wants a real ground path or, outdoors, a GFCI in front of it.

What to do instead

• Run a cord from a good outlet. The simplest fix is an outdoor-rated cord back to a modern grounded GFCI receptacle, sized for the distance so the prop still gets full voltage.
• Use a portable GFCI. A plug-in GFCI adapter gives you ground-fault protection at an outlet that lacks it. That protects the person, which is the whole point outdoors.
• Have the outlet replaced with a GFCI. For an old two-prong receptacle you keep coming back to, the lasting fix under NEC 406.4(D)(2) is a GFCI, and that is an electrician's half-hour, not a rewire.
• Never cut off the ground pin. Snapping the third prong to make a plug fit is the same defeat as a cheater plug, permanent and worse, and it ruins the cord.
• No good outlet within reach at all? A battery power station skips the outlet question entirely. See can you run Halloween decorations on a portable power station.

NEC reference

NEC 2020. Outdoor receptacles at a dwelling must be GFCI protected under NEC 210.8(A). Where a two-wire, non-grounding receptacle has no equipment ground in the box, NEC 406.4(D)(2) permits replacing it with another non-grounding receptacle, a GFCI receptacle marked "No Equipment Ground," or grounding-type receptacles fed through that GFCI and marked "GFCI Protected, No Equipment Ground." The equipment ground itself, the third prong's job, is defined in Article 100 and its function follows Article 250. The 5-milliamp trip point is the listing standard for a Class A GFCI, not an NEC table value. None of this is a substitute for the manufacturer's instructions on a given decoration.

Related

• Holiday Circuit Map and Load Worksheet (free PDF): a fill-in circuit map for your own house plus six one-page seasonal safety cards, with every load already converted to amps.
• Can You Run Halloween Decorations on a Portable Power Station?: the no-outlet answer that beats forcing a bad outlet with an adapter.
• Do Outdoor Halloween Decorations Need a GFCI Outlet?: the shock-protection rule that stands whether or not a ground is present.
• Weatherproof Cover for an Outdoor Outlet: keeping the outlet itself dry once you are feeding it correctly.
• Extension Cord Voltage Drop Calculator: size the cord from a grounded GFCI outlet to the prop.