How to Wire a 20A, NEMA 10-20 Non-Grounding Receptacle

Electrical Technology

How to Replace a 20A, 125/250V, NEMA 10-20 Receptacle with a 2P Breaker in Existing Installations

A NEMA 10-20R is a 20A, 125/250V dual voltage, straight-blade and non-grounding receptacle for high-power appliances. It is is no longer considered a safe or modern standard because it lacks a dedicated ground wire. The outdated 10-20 receptacle is unsafe for modern use, especially with high-consumption devices like dryers and ovens. It has 3 slots (for associated 3 pin pin/prongs) configuration, with the two tilted blade slots for Hot1 and Hot 2 and a center slot for the neutral wire.

It is a non-grounding receptacle, which means the third wire in 3-P is a neutral, not a ground, and can be replaced with a self-grounding feature.

The NEMA 10-20 (20A, 125/250V, 3-prong, non-grounding) receptacle is an old-style receptacle that was used before grounding was required in the NEC. It can only be used as a replacement in existing wiring systems where no equipment grounding conductor (EGC) is present.

It means, NEMA 10-20 is not for new installations. The NEC (since 1996) prohibits installing new NEMA 10-series receptacles and outlets (10-20, 10-30, 10-50) because they do not provide grounding protection. Safer alternatives today for new work are NEMA 6-20 for 240V-only appliances (with ground) or NEMA 14-20 for 120/240V appliances (with neutral and ground).

Since 2-wire non-grounding receptacles are no longer permitted for new installations, NEC 406.4(D)(2) and 250.130(C) allow their replacement only under the following conditions:

1. Replace with another 2-wire receptacle:

You may replace the existing non-grounding receptacle with a new one of the same type.

2. Replace with a GFCI receptacle:

A GFCI-type receptacle may be installed as a replacement. In this case, the receptacle must be clearly marked with the words “No Equipment Ground” and/or “GFCI Protected” to indicate that it is not connected to an equipment grounding conductor.

3. Replace with a grounding-type receptacle protected by a GFCI:

A grounding-type receptacle may be installed if it is protected by a GFCI device, such as a GFCI circuit breaker or a GFCI receptacle located at the first outlet on the circuit. In this configuration, the GFCI provides shock protection even though an equipment grounding conductor (EGC) is not present.

In the following wiring guide, we will show how to replace a damaged NEMA-10-20R in an existing installations with a 2-pole, 20A breaker or GFCI breaker.

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The NEMA 10-20 Receptacle

The NEMA 10-20R is a 20-amp, 125/250V, non-grounding receptacle. It is wired with two hot conductors and one neutral conductor, and it is designed to be used with a matching NEMA 10-20P plug. The “20” in 10-20R indicates the maximum current rating in amperes, while “R” stands for Receptacle. Similarly, the “P” in NEMA 10-20P stands for Plug.

The NEMA 10-20 has a 3-pole, 3-wire configuration (Hot 1, Hot 2, and Neutral). It is a non-grounding, straight-blade device that does not include a dedicated equipment grounding conductor. Its distinctive “crow’s foot” design features two angled blade slots for the hot conductors and one vertical slot in the center for the neutral conductor.

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Terminals

There are three terminals in a NEMA 10-20 receptacle:

X Terminal (Brass Screw): For HOT 1 (Line 1) – Black Wire
Y Terminal (Brass Screw): For HOT 2 (Line 2) – Red Wire
W Terminal (Silver Screw): For Natural – White Wire

Electrical Ratings & Specifications

NEMA: NEMA 10-20R – Straight-Blade Receptacle
Poles: 3-Pole, 3-Wire – Non-Grounding with Neutral
Wires: Three Wires – Hot 1, Hot 2 & Neutral
Voltage: 125/250V Single-Phase AC Supply – 60 Hz
Breaker: 20A
Current: 20A – 16A
Wattage: 4,800 W
Wire Size: #10 to #12 AWG (Copper Only)
Temp. Rating: -40°C to 75°C (-40°F to 167°F)
Dielectric-strength: Withstand 2kV Min.
Options: Residential and Industrial Grade
Mounting: Flush / Surface Mounting
Outdoor Box: Single or 2-gang outdoor box – NEMA 3R rainproof enclosure
Wiring: Hardwired / Dedicated Circuit

Wiring NEMA 10-20 Receptacle with a 2P Breaker

A NEMA 10-30 receptacle can be installed as a replacement for either a plug-in or hardwired connection in an existing electrical setup.

As shown in the wiring diagram, a 2-pole, 20A breaker in 120/240V main panel supply system can be used to re-wire the NEMA 10-20 receptacle as follow.

Connect Hot 1 (black wire) to the “X” terminal screw.
Connect Hot 2 (red wire) to the “Y” terminal screw.
connect the neutral (white wire) to the centered “W” terminal of the receptacle

For this configuration, use #12 AWG copper conductors for the 20A circuit and the corresponding NEMA 10-20R receptacle.

During the installation process, strip each conductor 3/4″ (25mm) or refer to the stripe gage on mounting plate. Similarly, Tighten terminal screws to approximately 12-14 lb-in (1.3 – 1.5 N·m).

Good to Know: Since the NEMA 10 series lacks a grounding conductor, compatibility issues can arise when used with a GFCI . In such cases, nuisance tripping may occur if the neutral comes into contact with the appliance frame.

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In indoor installations, the NEMA 10-20 are fixed in a single-gang box. For outdoor installations, a NEMA 3R weatherproof (rainproof) 2-gang enclosure should be used to protect the receptacle from moisture and environmental exposure.

Good to Know:

NEMA 10-20 receptacles are not intended for grounding use. They must be connected only to non-grounding circuits, except when used for ranges or clothes dryers as permitted by NEC 250.60.
According to NEC 210.8, GFCI protection is required for receptacles installed in garages, basements, outdoors, laundry areas, and other wet or damp locations. Specifically, NEC 210.8(A)(1) through (A)(11) mandates that all outdoor receptacles be installed downstream of GFCI protection, in compliance with Articles 426.28 and 427.22.
Be aware that nuisance tripping may occur when a GFCI breaker is used with a non-grounding receptacle, especially if the neutral is bonded to the appliance frame. In such cases, always follow the manufacturer’s recommendations to avoid overlapping protection issues.

FAQs:

How Many Amps Can a NEMA 10-20 Handle Safely?

A NEMA 10-20 receptacle is rated for 20 amps maximum. For continuous loads (lasting 3 hours or more), the NEC recommends using only 80% of the rating, which equals 16 amps continuous.

How Many Watts Can a NEMA 10-20 Receptacle Hold?

At 250 volts, a 20-amp NEMA 10-20 receptacle can supply up to 5,000 watts (20A × 250V). For continuous loads, the safe limit is 4,000 watts (16A × 250V).

Which Breaker Size is Suitable for a NEMA 10-20R?

The correct breaker size is 20 amps, double-pole (2P) to match the receptacle’s rating and protect the circuit wiring.

What is the Correct Wire Size and Cable Type to Use with NEMA 10-20?

Based on NEC Table – 310.16, the correct wire size for NEMA 10-20 is #12 AWG – Copper or #10 AWG – Aluminum.
Cable type should be NM-B, UF-B, or THHN/THWN in conduit, depending on the installation.

Can You Install NEMA 10-20 in New Installations?

No. NEMA 10 series (non-grounding) receptacles are no longer permitted in new installations under the NEC. They may only be replaced in existing installations. For new work, use a NEMA 6-20 (250V, grounding) or NEMA 14-20 (125/250V, grounding).

How Can I Replace a NEMA 10-20 with NEMA 6-20 Without Running an Additional Wire?

You cannot legally convert a NEMA 10-20 (2 hots + neutral, no ground) to a NEMA 6-20 (2 hots + ground, no neutral) without running a new grounding conductor. The NEC requires a dedicated equipment grounding conductor (EGC) for grounding-type receptacles.

Alternatively, you can remove the neutral wire from the NEMA 10-20 receptacle , re-identify it as a ground conductor, and connect it to the ground terminal of the 6-20 outlet. Since the neutral is bonded to the ground in the main service panel, this setup will function.

However, this is not considered good practice (and may only be legal in certain areas). If the neutral is repurposed as a ground, the conductor must be clearly marked with a permanent label or identifying note to avoid confusion and ensure safety.

Warning

Always disconnect the power supply by switching OFF the breaker in the main service panel before performing any electrical work.
If you are not confident or qualified, consult a licensed electrician to ensure the installation is performed safely and in compliance with local electrical codes.
The author assumes no responsibility for any losses, injuries, or damages resulting from the use or misuse of the information provided. Electricity is extremely dangerous! please exercise caution and never attempt wiring unless you fully understand the process.