Power connectors are primarily designed to ensure a safe and reliable connection of devices to AC mains power, although they are generally specified for AC and DC voltages. AC receptacles and plugs come in various configurations with different blade widths, shapes, positions, and dimensions, designed to make them safe to use and non-interchangeable with different combinations of voltage, current capacity, and groundings. This FAQ begins with an overview of non-locking and locking AC power connectors meeting National Electrical Manufacturers Association (NEMA) standards in the US and IEC 60309 standards in Europe and closes with a review of the requirements for hospital-grade AC connectors in North America.

Typical ratings for AC power connectors meeting ANSI/NEMA WD 6 Wiring Device specifications in the US are 15 to 60 A, up to 600 V, and up to 400 Hz. In Europe, the maximum current rating of IEC 60309 connectors is 800 A, voltage ratings are up to 1000 V, and the maximum frequency is 500 Hz. There are 24 common non-locking NEMA connector configurations that fall into four voltage classes (Table 1).

Single-phase sockets are classified as two-pole (2P) and provide a single line contact and a neutral contact. In addition, a protective earth, or ground, contact may be present, in which case the socket is classified as two pole and earth (2P+E).

NEMA designations are divided into three elements, separated by a dash (-): the first number is the class and indicates the voltage, pole count, and whether it’s grounded; the second number is the amp rating of the connector, and; the third element is a letter indicating whether it’s a plug (P) or receptacle (R). As described below, the NEMA 7-15P plug is a grounded, two-pole plug rated for 277V and 15A. The following is a brief overview of the 24 common NEMA connector classes:

NEMA 1 has two flat prongs for two poles, two-wire non-grounded connections rated for 125 V. NEMA 1 is not used in new designs; it has been replaced by grounded configurations.

NEMA 2 is also obsolete, it’s two-wire and rated for 250 V.

NEMA 3 was a planned series rated for 277 V with two wires.

NEMA 4 was a planned series rated for 600 V with two wires

NEMA 5 has two poles, three wires with two blades, and a grounding pin. It is rated for 120 V. (Figure 2).

NEMA 6 has three-wire grounding used for 208 and 240 V commercial or industrial devices.

NEMA 7 are two-pole, grounded connectors rated for 277 V. The NEMA 7-15 plug and receptacle have current-carrying pins at angles to each other (Figure 3).

Figure 3: The NEMA 7-15P plug is a grounded, two-pole plug rated for 277V and 15A. (Image: World Cord Sets)

NEMA 8 is a planned series that will have three wires, two poles and be grounded with a rating of 480 V.

NEMA 9 is planned to be similar to NEMA 8 with a rating of 600 V.

NEMA 10 is obsolete and has been replaced by NEMA 14. NEMA 10 are 125/250 V non-grounding (hot-hot-neutral), and were designed to be used in a manner that indirectly grounds the appliance frame to the neutral, which was allowed before the National Electrical Code added the requirement of a separate safety ground. Safe operation relied on the neutral conductor being connected to the system ground at the circuit breaker. If the neutral conductor breaks, disconnects, or develops high resistance, the appliance frame could become energized with dangerous voltage levels.

NEMA 11 is a series of three-wire, three-pole, and ungrounded for three-phase 250 V.

NEMA 12 is a planned series with three-wire, three-pole, and ungrounded for three-phase and 480 V.

NEMA 13 is a planned series similar to NEMA 12, rated for 600 V.

NEMA 14 are four-wire and grounded rated for 250 V.

NEMA 15 is similar to NEMA 14 but is three-phase without a neutral conductor.

NEMA 16 is a planned series with four-wire, three-pole, grounded, and three-phase with a 480 V rating.

NEMA 17 will be similar to NEMA 16 with a 600 V rating.

NEMA 18 is 120Y/208V three-phase ungrounded.

NEMA 19 is a planned series with 277/480Y four-wire, three-pole, ungrounded.

NEMA 20 will be similar to NEMA 19 with 247/600Y ungrounded.

NEMA 21 is a planned series with 120/208Y, five-wire, three-pole with neutral, three-phase, and grounded.

NEMA 22 will be similar to NEMA 21 for 277/480Y usage.

NEMA 23 will also be similar to NEMA 21 for 347/600Y usage.

NEMA 24 are two-pole and ground with a 347 V rating, mainly used in Canada.

Locking

Locking connectors are designed to ensure that the connection is secure and cannot be accidentally disconnected. These connectors have plugs with curved blades that can be turned when inserted into the receptacle. Locking connectors are designated with “L” placed before the series number as in NEMA L6, NEMA L7, or NEMA L21.

Most locking connectors have corresponding non-locking connectors. NEMA 21 is an exception. NEMA 21 straight-blade devices are “reserved for future configurations,” and no designs for this series currently exist. There are, however, NEMA L21 series locking connectors.

Locking connectors are found in commercial and industrial systems, as well as higher-power residential appliances. There are also midget or miniature locking connectors identified by “ML”, designed to be used when standard locking connectors are too large.

IEC 60309 connectors

In Europe, most connectors conform to IEC 60309 (formerly IEC 309) and various standards based on it (including BS 4343 and BS EN 60309-2). These are often referred to in the UK as CEE industrial, CEEform, or CEE connectors. Most three-phase AC connectors have an earth connection but may not include a neutral since some loads like motors don’t need the neutral connection. These connectors have four connections, three phases, and earth, designated as 3P+E.

Common IEC 320 connectors are available in P+N+E (unbalanced single phase with neutral), 2P+E (balanced single phase), 3P+E (three phase without neutral), and 3P+N+E (three phase with neutral) with current ratings up to 200 A. Connectors with different current ratings have different overall sizes, making it impossible to connect incompatible configurations. Like NEMA, there are many varieties of IEC 320 connectors. A few examples include:

IEC 320 C13/C14 connectors are common in the PC and AV industry. The mating connector for the C13 socket is the C14 plug, which is often mounted into a recessed panel or chassis on computer power supplies or power transformers.

IEC 320 C5 is a polarized power socket often found on the power supplies used by notebook computers.

IEC 320 C7 (Non-Polarized) is a 2-prong connector with two round prongs side-by-side used mostly with consumer electronics.

IEC 320 C7 (Polarized) has a square shape of the overmold on the one end of the socket to provide the neutral connection.

CEE 7/7 connectors have a round shape with two rounded pins and a socket that accepts the grounding pin from a ‘Type F’ European wall outlet.

Hospital-grade in North America

In North America, hospital-grade AC mains connectors are subject to general medical equipment standards as well as specific standards related to mains connection and attachment. General medical equipment standards include UL 60601-1 sections 57.2 and 57.3 that require hospital-grade AC mains connectors with ‘patient care equipment’ used in the ‘patient vicinity,’ and CAN/CSA C22.2 no. 21. Connector-specific requirements include:

Attachment standards—UL 498 and CAN/CSA C22.2 no. 42 and UL 60601 include requirements for abrupt plug removal, ground pin retention, fault current, ground contact temperature and resistance, assembly security, cord grip strain relief and cord pull, terminal strength, and durability and impact tests of the material.

Power supply connector standards— UL 817 and CAN/CSA C22.2 no. 21 and NEMA WD-6 requirements include: The blades must be solid instead of folded brass; The blades can be nickel-plated; The plug must include an internal cable retention device or strain relief to prevent any stress to the plug’s internal connections, and; The connector must include a ‘green dot’ indicating they meet the requirements for hospital-grade power cords and cord sets (Figure 5).

The UL 498 and 817 standards only allow the NEMA 5-15, 5-20, 6- 15, and 6-20 straight blade devices to be marked hospital-grade. CSA will allow the NEMA 1-15 to be classified as hospital-grade if it is double insulated and meets all other requirements.

Summary

Power connectors are designed to provide a safe and reliable connection to mains power. NEMA, IEC, and other standards bodies have established numerous detailed standards to ensure that AC power connectors from various makers are compatible with each other and with the electrical distribution infrastructure. Dozens of connector formats have been developed for single- and three-phase power distribution systems with varying combinations of poles, neutral and ground connections. In addition to meeting all of the general industry standards, several additional standards must be met by hospital-grade AC mains connectors.

 

References

Industrial and multiphase power plugs and sockets, Wikipedia
NEMA Connectors, Wikipedia
NEMA Reference Guide, World Cord Sets
Power Connector Guide, C2G Legrand