The Type B plug is the grounded version of Type A. Two flat pins for live and neutral plus a round earth pin below. Defined under NEMA 5-15 in the US, Type B is the standard for any North American or Japanese appliance that needs grounding for safety. This guide covers the design, how it differs from Type A, and where you'll meet it.
What Type B is
The Type B plug has three pins:
- Two flat parallel live and neutral pins, 12.7 mm apart, identical to Type A
- One round earth pin, 6.35 mm diameter, located 11.9 mm below and centered between the flat pins
- The earth pin is slightly longer than the flat pins (typically 1-2 mm longer) so the ground connection is made first during insertion
The socket has matching slots: two parallel slots for the flat pins plus a round slot for the earth. Modern Type B sockets are polarized, with the neutral slot wider than the live slot, so polarized Type A or Type B plugs only fit in one orientation.
Type B vs Type A
Type A and Type B are closely related but serve different purposes:
| Feature | Type A | Type B |
|---|---|---|
| Pin count | 2 | 3 |
| Earth pin | None | Yes |
| Polarized | Modern versions yes | Yes |
| Used for | Double-insulated devices | Grounded appliances |
| Socket compatibility | Fits Type A and Type B sockets | Fits Type B sockets only |
The directional compatibility is the practical point: a Type A plug fits both Type A and Type B sockets, but a Type B plug requires a Type B socket. If you're traveling with three-prong appliances and your accommodation has only Type A sockets, you need a "cheater" adapter that converts three-prong to two-prong.
Why grounded plugs exist
Resistive heating, motor windings, and exposed metal cases all create scenarios where a fault could put live voltage on a surface a person might touch. Without a ground connection, that surface stays at line voltage until something completes the circuit, often a human hand.
The earth pin provides a low-resistance path to ground for fault current. When a fault occurs:
- Live voltage tries to flow through the appliance's metal case
- Instead of through the user, the current flows through the earth pin
- The high fault current trips the circuit breaker (or blows a fuse) within milliseconds
- The user touches a now-de-energized surface
For double-insulated devices (plastic-bodied chargers, lamps with no exposed metal), the second layer of insulation makes the earth redundant. Type A is acceptable for these.
For appliances with exposed metal, grounding is essential. The US National Electrical Code requires grounded outlets in any room where they're likely to be needed (kitchens, bathrooms, laundry rooms, garages, outdoor spaces).
Why the earth pin is longer than the live pins
The Type B earth pin is intentionally 1-2 mm longer than the live and neutral pins. This means:
- When inserting the plug, the earth makes contact first
- When removing the plug, the earth disconnects last
- Any fault current during insertion or removal has a path to ground
This is the same principle behind the UK Type G's longer earth pin (which is more dramatic, 4 mm longer) and the recessed-socket design of Schuko.
Where Type B is used
Type B is the standard grounded outlet in:
- United States and Canada (NEMA 5-15)
- Mexico (same NEMA spec)
- Japan (modern construction; older buildings have only Type A)
- Taiwan (mixed Type A and B)
- Philippines (mixed Type A, B, and C)
- Parts of Central America
- Parts of South America (where US electrical influence dominates)
Type B is also used in some countries with non-US plug standards as a secondary outlet type for US-imported appliances. Older Japanese hotel suites, for example, sometimes have one Type B socket alongside the standard Type A sockets specifically for US business travelers.
Type B's amperage and voltage limits
NEMA 5-15 (the standard Type B residential outlet) is rated:
- 15 A continuous current
- 125 V maximum voltage
- 1,875 W maximum power delivery
For higher-amperage circuits, the related NEMA 5-20 standard exists, with one pin rotated 90 degrees to indicate the higher current rating. NEMA 5-20 plugs don't fit NEMA 5-15 sockets but NEMA 5-15 plugs do fit NEMA 5-20 sockets.
For travel purposes, NEMA 5-15 covers essentially every device a traveler might use. Industrial and large-appliance circuits use entirely different plug shapes (NEMA 6-15, 6-20, L5-30, etc.).
Type B in Japan
Japan uses Type A as the dominant standard but has been adding Type B sockets in newer construction since the 1980s. As of 2026:
- New hotels and modern Japanese homes often have at least one Type B socket per room
- Older buildings (especially traditional ryokan and family-run hotels) usually have only Type A
- Public charging stations and most airports have a mix
The Japanese version of Type B is electrically identical to the US version but operates at 100 V instead of 120 V. The plug shape is the same; the voltage spec is different.
Practical implications for travelers
If you have a three-prong Type B device:
- Works directly in Type B sockets (US, Canada, Mexico, modern Japan)
- Doesn't fit Type A sockets without a cheater adapter
- Doesn't fit anywhere outside North America and Japan without a country-specific adapter
If you have a two-prong Type A device:
- Works in Type A and Type B sockets directly
- Needs a regional adapter for Europe, UK, Australia, and most of the world
For most modern travel electronics (laptop chargers, phone chargers, USB-C wall bricks), Type A is sufficient because the device is double-insulated. Grounded Type B is more relevant for kitchen appliances, power tools, and older electronics.
How Type B compares to grounded plugs in other regions
| Plug | Region | Earth approach |
|---|---|---|
| Type B | US/Canada/Mexico/Japan | Round earth pin below flat pins |
| Type G | UK/Ireland/Singapore | Long rectangular earth pin above |
| Schuko (Type F) | Continental Europe | Side earth clips |
| Type E | France/Belgium | Female hole on plug, male pin on wall |
| Type I | Australia/NZ/China/Argentina | Vertical earth pin below angled live/neutral |
Each approach solves the grounding problem differently. Type B is the simplest (just an extra pin), Type G is the safest (longest pin, opens socket shutters), Schuko is the most space-efficient (no dedicated earth pin).
The bottom line
Type B is the grounded version of Type A. The earth pin makes the design safer for appliances with exposed metal but doesn't change the underlying US/Japanese electrical standard. A Type A plug fits Type B sockets, so most consumer electronics that come with Type A plugs work in Type B outlets without thinking.
For travelers visiting the US, Canada, Mexico, or Japan: Type B awareness matters if you're bringing grounded three-prong appliances. For most travelers carrying double-insulated devices, Type A vs Type B is invisible.