Which wire on a lamp cord is positive and negative?

Understanding which wire on a lamp cord is positive and which is negative is important for safely and properly wiring lamps and lighting fixtures. This guide will provide a quick overview of how to identify positive and negative wires on lamp cords, discuss why it matters, and give tips for proper wiring.

Quick Answer

For a typical two-wire lamp cord in the United States:

• The smooth, marked, or labeled wire is the positive wire
• The ribbed, unmarked, or unlabeled wire is the negative wire

The positive wire is often called the “hot” wire because it carries the electricity from the source to the light. The negative wire is the “neutral” or “return” wire back to the source.

Identifying the Wires

On a standard two-wire lamp cord, one wire will be smooth and one will be ribbed or striped running along its length. The smooth wire is positive, while the ribbed or striped wire is negative.

The wires may also have printing, writing, or tags on them to identify the polarity. The positive wire is often labeled with words like “Hot”, “Live”, or “POS” while the negative may be labeled “Neutral” or “NEG.”

Here are some examples of positive and negative wiring:

• Smooth/ribbed wires
• Black/white wires
• Brass/silver terminals
• Yella/black wires
• POS/NEG labels

In all cases, the smooth, marked, or brightly colored wire is positive while the ribbed, unmarked, or black/white wire is negative.

Why Positive and Negative Wires Matter

Understanding which wire is positive and negative is important for several reasons:

• Safety – Mixing up wires can lead to electric shocks, fires, or damage to the lamp.
• Proper circuit connections – Positive and negative wires must be connected properly to complete the electrical circuit.
• Avoiding short circuits – If positive and negative wires touch, they can short circuit which can lead to blown fuses, fires, or damage.
• Troubleshooting – Knowing the polarity helps diagnose connection issues or problems.

The bottom line is that positive and negative wires carry the electric current in a specific direction. Maintaining the correct polarity through the lamp ensures safe, proper function.

Tips for Wiring a Lamp

Follow these tips when wiring a lamp to ensure proper positive and negative connections:

• Clearly identify the positive and negative wires using the cord markings.
• Make sure the wires connect to matching positive and negative terminals on the socket and switch.
• On a metal lamp base or fixture, the negative wire should connect to the base.
• Keep positive and negative wires separated, especially around metal which can cause shorts.
• Use wire nuts, crimp connectors or solder when joining wires to ensure a proper connection.
• Insulate exposed wires with electrical tape or heat shrink tubing.
• Double check all connections to be sure the wires are securely fastened.

Testing Polarity with a Multimeter

You can use a digital multimeter to test and verify the polarity of lamp wires:

1. Unplug the lamp and expose the wire connections.
2. Set the multimeter to AC voltage mode.
3. Touch the black probe to the ribbed/neutral wire.
4. Touch the red probe to the smooth/hot wire.
5. The multimeter will display around 110-120V AC if the polarity is correct.
6. If reversed, it will show 0V or a very low reading.

This allows you to double check or troubleshoot wiring polarity issues with a lamp or fixture.

Reversing Polarity on a Lamp

If you discover a lamp has reversed polarity, the connections must be switched to correct it:

1. Unplug the lamp and access the wire connections.
2. Identify the smooth positive and ribbed negative wires.
3. Swap the connections so the smooth wire now connects to the positive terminal.
4. Connect the ribbed wire to the negative terminal.
5. Use a multimeter to retest and verify proper polarity.

Note that some lamps may have polarized plugs where one prong is larger than the other. Make sure these plugs are oriented correctly after reversing any switched wires.

Why a Lamp Cord May Read 0V When Tested

There a few reasons why a lamp cord may read 0V or a very low voltage when testing the wires with a multimeter:

• Reversed polarity – the wires are connected backwards.
• Open circuit – a broken wire connection.
• Bad switch – the inline switch has failed open.
• Blown fuse – a blown fuse in the lamp base or plug end.
• Bad plug – improper connection to the outlet.

So polarity issues can cause 0V readings, but other wiring faults can as well. Check all connections, switches, fuses/breakers, and outlet bonding to be sure.

International Lamp Wiring Standards

While lamp wiring standards are similar internationally, here are some key differences:

Country	Voltage	Positive Wire	Negative Wire
United States	120V	Smooth	Ribbed
Europe	230V	Brown	Blue
UK	230V	Brown	Green/Yellow Stripe
Australia	230V	Brown	Blue

So wire colors and voltages may vary around the world. Always verify local wiring standards when working with lamps and lighting in a different region.

What Is the Third Wire in a Lamp Cord?

Some lamp cords contain a third wire which serves a few possible functions:

• Ground Wire – Green or bare copper wire that connects to the metal body of a lamp for safety. Protects from electric shock if there is a short circuit.
• Second Hot Wire – Additional “hot” wire to provide extra voltage, like 240V instead of 120V. Allows the lamp to work on dual voltage.
• Switch Leg – Extra wire that connects to an inline switch on the cord. Allows the switch to turn the lamp on and off.

So the third wire can provide added safety, extra voltage capacity, or wiring for an inline switch. The function depends on the specific needs of the lamp.

Converting a Lamp from Hardwired to Plug-In

To convert a hardwired lamp to plug into an outlet, you need to:

1. Obtain a replacement inline plug with a built-in switch.
2. Cut off the existing hardwired connections.
3. Thread the wires through the plug base.
4. Connect the positive smooth wire to the brass screw.
5. Connect the negative ribbed wire to the silver screw.
6. Connect the ground wire to the green screw if present.
7. Tighten all connections securely and close up the plug base.
8. Test with a multimeter to verify correct polarity.

Be sure to get an inline switch plug rated for the wattage of the lamp. Test thoroughly before use to ensure proper function and polarity.

Summary

Identifying positive and negative wires is critical for safely wiring lamps and lighting fixtures. The smooth/marked wire is positive, while the ribbed/unlabeled wire is negative. Connecting the wires properly maintains polarity and ensures the lamp functions safely and as intended. Use labeling, wire colors, multimeter tests, and the cord markings to differentiate the two wires.