Plug and Socket Wiring Diagram: 3-Pin Plug Connections

Wiring a plug or wall socket is one of the most common DIY electrical tasks, and also one where small mistakes -- a crossed terminal, a loose screw -- have serious consequences. This guide covers the wiring of a 3-pin plug and a wall outlet/socket: the terminal assignments, wire colors, and fuse ratings for both the UK (BS 1363) and US (NEC) conventions, since the terminology "plug socket" covers different hardware depending on where you are.

UK 3-Pin Plug Wiring (BS 1363)

The UK uses one of the safest plug standards in the world. The BS 1363 plug has three large rectangular pins: a longer top pin (earth) and two shorter bottom pins (live and neutral). Every BS 1363 plug contains a fuse -- this is unique to the UK system and provides per-appliance overcurrent protection at the plug itself.

UK Wire Colors (Post-2004)

Since March 2004, UK and European harmonized wiring uses:

Older UK wiring (pre-2004) used red for live and black for neutral -- if you are working on old wiring, treat any red wire as live and any black wire as neutral.

UK 3-Pin Plug Terminal Assignments

          [E -- top pin, longest]
         /
        /   [L -- bottom right pin]
        \   [N -- bottom left pin]
         \

Looking at the face of the plug with the earth pin at the top:

Terminal Wire Color Pin Position
E (Earth) Green/Yellow Top (longest pin)
L (Live) Brown Bottom right
N (Neutral) Blue Bottom left

Wiring a UK BS 1363 Plug -- Step-by-Step

Safety: Always disconnect from the mains before opening any plug or socket. Verify with a voltage tester.

  1. Unscrew the large central screw on the back of the plug to open it.
  2. Loosen the cable clamp screws at the bottom of the plug. The cable clamp must grip the outer insulation -- not the individual cores.
  3. Strip approximately 40 mm of outer sheath. Strip 8 mm of insulation from each core.
  4. Brown wire: Screw into the L terminal (bottom right). No loose strands.
  5. Blue wire: Screw into the N terminal (bottom left).
  6. Green/Yellow wire: Screw into the E terminal (top). The earth core should be slightly longer inside the plug so it is the last to disconnect if the cable is yanked.
  7. Check the fuse rating matches the appliance:
    • 3A fuse (red): Appliances up to 700W (table lamps, phone chargers, shavers)
    • 13A fuse (brown): Appliances 700W to 3000W (kettles, irons, vacuum cleaners, microwave ovens)
  8. Tighten the cable clamp firmly on the outer sheath.
  9. Close the plug and tighten the central screw.

UK Wall Socket Wiring

A UK 13A double socket (BS 1363) has the same terminal labels:

At the socket, the cable entry is from behind via a metal box (single gang, double gang). Standard single sockets serve one outlet; double sockets serve two. Each socket has its own L, N, and E terminals, though a ring main feeds both from a single cable loop.

Ring main wiring: In a UK ring circuit, two cables arrive at each socket -- one from the previous socket, one continuing to the next. Both live wires connect to the L terminal (using a connector block or by doubling under the same terminal screw), both neutrals to N, and both earths to E. The outer sheath of each cable must be identified with its cable number for future reference.

US 3-Prong Outlet Wiring (NEC)

The US uses NEMA 5-15 (15A, 120V) and NEMA 5-20 (20A, 120V) receptacles for standard residential and commercial outlets. Unlike UK plugs, US outlets do not contain a fuse -- overcurrent protection is at the circuit breaker in the panel.

US Wire Colors (NEC)

On 240V circuits: black is one hot, red is the second hot, white is neutral, green/bare is ground.

US Outlet Terminal Assignments

A standard duplex NEMA 5-15R receptacle has:

Terminal Color Wire
Brass (gold) screws Black Hot
Silver screws White Neutral
Green screw Green/Bare Ground

The narrow slot (hot) is on the right side of each outlet face. The wide slot (neutral) is on the left. The round hole at the bottom is the ground.

Wiring a US Duplex Outlet -- Step-by-Step

Safety: Turn off the circuit breaker and verify the outlet is dead with a non-contact voltage tester before touching any wires.

  1. Strip approximately 3/4 inch (19 mm) of insulation from each conductor.
  2. Black wire: Wrap clockwise under the brass (hot) screw. Tighten firmly.
  3. White wire: Wrap clockwise under the silver (neutral) screw. Tighten firmly.
  4. Bare or green wire: Connect to the green screw (ground).
  5. If the outlet has a break-off tab between the two brass screws (split-circuit outlet), verify whether it needs to stay intact or be removed for your application. A standard outlet keeps the tab in place -- both outlets share the same hot and neutral.
  6. Fold the wires carefully into the box. Push the outlet into the box and secure with the two mounting screws.
  7. Install the cover plate.
  8. Restore power and verify with a plug-in outlet tester (a 3-light tester is adequate).

GFCI Outlets

GFCI (Ground Fault Circuit Interrupter) outlets are required by NEC 210.8 in wet or damp locations: kitchens within 6 feet of a sink, bathrooms, garages, outdoors, crawl spaces, unfinished basements, and near pools. A GFCI outlet has LINE and LOAD terminals:

Wire colors are the same -- black to hot (LINE), white to neutral (LINE), ground to green. Do not reverse LINE and LOAD -- the GFCI will not function correctly.

Common Mistakes in Plug and Socket Wiring

Create Your Own Plug and Socket Wiring Diagram

Whether you are documenting a custom installation or teaching wiring conventions, having a clear diagram prevents terminal confusion. With CircuitDiagramMaker, you can:

Create your own plug and socket wiring diagram -- free

Code, Permits, and Wire Sizing (US NEC)

Replacing an existing outlet with an identical one is generally considered routine maintenance in most US jurisdictions and does not require a permit. Adding a new outlet, running a new circuit, or relocating a box typically does require a permit and inspection -- rules vary by city and county, so check with your local building department (AHJ, or authority having jurisdiction) before starting work that goes beyond a straight swap.

Whatever the scope of the job, the wire gauge has to match the breaker rating protecting the circuit:

Never connect 14 AWG wire to a 20A breaker -- the breaker will not trip fast enough to protect the smaller wire from overheating in a fault condition. It is fine to run 12 AWG wire on a 15A breaker, just not the reverse.

Other code basics that apply to most residential plug and socket work:

Call a licensed electrician instead of doing the work yourself if: the panel itself needs to be opened, the home has aluminum branch wiring (it needs special connectors and technique), an outlet box has no ground wire present (common in homes built before the 1960s), or you are not confident identifying which breaker actually controls the circuit before you start.

Troubleshooting a Plug or Socket

Symptom Likely Cause Fix
Socket has no power Tripped breaker or GFCI, loose wire at a terminal, dead circuit Check the breaker panel and any upstream GFCI outlet first; if power is restored elsewhere, re-check terminal connections at the dead outlet
Socket sparks when plugging in Normal arcing under load (common with heaters, vacuums), or worn/loose socket contacts A small spark when unplugging a loaded appliance is usually normal; frequent sparking, scorch marks, or a socket that feels loose means the receptacle should be replaced
Plug feels loose in socket Worn-out spring contacts inside the socket, or a plug/socket mismatch Replace the receptacle -- a loose fit causes intermittent contact and heat buildup
Socket is warm to the touch Circuit overloaded near its rated capacity, or a loose terminal connection creating resistance heat Reduce the load on that circuit; if the outlet stays warm with normal load, turn off the breaker and re-check or replace the terminal connections
Breaker/fuse trips repeatedly on that circuit Circuit overloaded, a short circuit, or a ground fault Unplug devices to check for overload first; if it trips with nothing plugged in, there is likely a wiring fault -- stop and call an electrician

Testing a Plug or Socket

Confirm the circuit is dead before touching any wires, then confirm it is wired correctly once you are done.

Before working:

  1. Turn off the breaker (or remove the fuse) feeding the circuit.
  2. Test a non-contact voltage tester on a known-live outlet first to confirm the tester itself works.
  3. Hold the voltage tester near both slots of the outlet you are working on -- it should not indicate any voltage.
  4. For extra certainty, use a multimeter set to AC voltage across the hot and neutral terminals, and again between hot and ground. Both readings should be at or near 0V before you proceed.

After installation:

  1. Restore power at the breaker.
  2. Use a plug-in outlet tester (US) to confirm correct wiring -- the tester's light pattern will flag reversed polarity, an open ground, or an open neutral.
  3. With a multimeter, check the voltage between hot and neutral -- it should read close to the circuit's nominal voltage (around 120V on a standard US outlet, around 230V on a UK/EU socket).
  4. Check hot to ground -- the reading should be close to the hot-to-neutral reading, confirming the ground path is intact and not floating.
  5. If any reading is off, or a tester flags reversed polarity, shut the breaker back off and re-check the terminal connections before using the outlet.

Key Takeaways

3 Pin Plug Socket Diagram — circuit diagram showing component connectionsLNPELPENPlug / Source EndLPENReceptacle / Load EndMains Plug Continuity (L/N/PE)Verify terminal markings and rating against the exact device datasheet before installation.
3 Pin Plug Socket Diagram — open the interactive version of this diagram to customise and export it.

Frequently asked questions

Which wire is hot on a plug or socket?

On a US plug or outlet, the black wire is hot and connects to the brass screw or narrow slot. On a UK plug, the brown wire is live and connects to the L terminal. Never assume color -- verify with a voltage tester before touching any wire, since old wiring or a miswired outlet can put voltage on an unexpected conductor.

Can I use a 2-prong plug on a 3-prong socket?

A 2-prong plug fits into a 3-prong socket, but the appliance has no ground connection either way. Using an adapter to force a 3-prong plug into a 2-prong outlet is not a substitute for a real ground path and is not recommended for appliances with metal housings. If the outlet has no ground wire, a GFCI outlet labeled "No Equipment Ground" is the safer, code-compliant option.

What happens if hot and neutral are reversed at an outlet?

Many appliances still function with reversed polarity, but it is unsafe. On a US outlet it means the neutral path carries what should be the hot voltage, so a lamp's metal socket shell or an appliance's exposed metal parts can become live even when a switch is off. A polarized plug tester will detect this immediately after wiring.

How do I know if a socket is wired correctly?

Use a plug-in outlet tester (US) after restoring power -- its light pattern confirms correct hot, neutral, and ground wiring and flags reversed polarity or an open ground. You can also check with a multimeter: hot to neutral should read close to the circuit's nominal voltage, and hot to ground should read almost the same value.

Is it safe to overload a socket with an extension cord or power strip?

Plugging too many high-draw devices into one socket through an extension cord or power strip can pull more current than the circuit's wire and breaker are rated for, causing heat buildup at the connections and, in some cases, a fire. Check the total wattage of everything plugged in against the circuit's breaker rating, and avoid daisy-chaining multiple power strips together.

Why does a socket feel warm even when nothing seems wrong?

A warm socket usually means either the circuit is running near its rated load, or a terminal connection has loosened and is generating resistance heat. A socket that stays warm without a heavy load plugged in should be treated as a fault -- turn off the breaker and inspect or replace the outlet rather than continuing to use it.

Interactive diagrams for this guide

Related guides