What Is The Hot Side On A Plug? | Narrow Blade Hot

Plugs aren’t symmetrical by accident. On polarized North American plugs, one flat blade is smaller than the other so the device always connects the same way. That single detail keeps switches, fuses, and internal parts on the correct conductor. If you wire a cord cap or swap a receptacle, getting the hot side right isn’t optional; it’s basic safety.

Plug Polarity Basics

Household AC circuits use three conductors: line (hot), neutral, and equipment ground. The hot conductor carries voltage from the source to the load. Neutral returns current to the source and is bonded to the system ground at the service. The equipment ground is a safety path that only carries current during faults. On a two-blade polarized plug, the narrow blade is hot and the wider blade is neutral. On a three-prong plug, the round pin is ground, and polarity is still enforced by blade width.

Two fast clues confirm polarity without a meter. First, look at the blades: smaller equals hot. Second, look at the terminal screws on the outlet or cord cap: brass means hot, silver means neutral, green means ground. These markings match the installation sheets from major device makers and reflect how safety standards label terminals.

Which Side Is Hot On A Plug? Practical Test Steps

Visual checks take you far, and a simple tester removes doubt. Here’s a quick method that keeps fingers away from live parts.

Use A Non-Contact Tester

Plug in a corded device, switch it on, and bring the tester near each blade. The tester beeps or lights near the blade tied to the hot conductor. Unplug the cord and confirm the narrow blade is the one that triggered the alert. If the device has a three-prong plug, the round pin won’t trip the tester.

Check Terminal Colors When Wiring

When replacing a receptacle or assembling a replacement plug, put the black conductor on the brass screw, the white conductor on the silver screw, and the green or bare conductor on the green screw. Manufacturers print this mapping on their instruction sheets. See the Hubbell GFCI guide for a clear color-to-terminal diagram.

Why Plug Polarity Exists

Polarization keeps the neutral side tied to the larger blade and the shell or threaded parts of certain fixtures. That way, when a switch opens, it interrupts the hot side and the exposed shell stays at or near ground potential. This reduces shock risk during bulb changes and service. Safety agencies and standards bodies require clear markings so anyone assembling a cord set matches blade width to slot width and lands conductors on the correct screws. UL’s own wording appears in luminaire guides as: “TO PREVENT ELECTRIC SHOCK, MATCH WIDE BLADE OF PLUG TO WIDE SLOT.” You can see that exact line in the UL marking guide.

How Blade Shapes And Screws Signal The Hot Side

Blade Width

Narrow equals hot. Wide equals neutral. This rule holds for two-blade polarized plugs and for three-blade versions with a ground pin. The wider neutral blade forces one orientation so the device internals keep the same reference every time you plug in.

Screw Colors

Device makers align screw color with function: brass for hot, silver for neutral, green for ground. If you find an unmarked replacement plug, the brass screw typically sits opposite the wider blade half so the narrow blade lines up with hot inside the cap. Many side-wire styles emboss “HOT” near the brass screw and “WHITE” near the silver screw.

Lamp Cord Texture

On flat two-wire lamp cord, the smooth jacketed side is hot and the ribbed or striped side is neutral. That lets you trace polarity from cord to plug at a glance. Connect the smooth side to the narrow blade terminal and the ribbed side to the wide blade terminal. Always tie the shell terminal of an Edison lamp holder to neutral so the outer metal stays at neutral potential.

Hot Side On A Plug: Narrow Or Wide?

Always narrow. If a plug has equal blade widths, it’s a non-polarized type and the device likely doesn’t depend on polarity. Phone chargers and many double-insulated electronics fall in that group. If the device depends on polarity and you see equal blades, the plug or cord is the wrong type and should be updated.

Code Notes, Labels, And Don’ts

Standards require polarized cord sets and clear markings so users don’t reverse line and neutral. UL 817 and related rules call for molded warnings on polarized plugs and extension cords telling users to match the wide blade to the wide slot. You can see that language repeated in the Federal Register notice on extension cords. A few habits keep you on track:

• Don’t shave a wide blade to fit an old two-slot outlet.
• Don’t break off a ground pin to make a three-prong plug fit a two-prong adapter.
• Use a listed adapter or, better yet, update the receptacle with a properly grounded, polarized type where permitted by local code.
• Keep white on silver, black on brass, and green or bare on green every single time.

Spotting Hot And Neutral In Common Scenarios

Swapping A Damaged Two-Blade Plug

Choose a polarized replacement cap. Strip the cord ends, keep equal lengths, and land the smooth conductor under the brass screw. Land the ribbed conductor under the silver screw. Tie the strain relief so the cord jacket, not the conductors, takes the pull. Check that the narrow blade aligns with the hot conductor before closing the cap.

Replacing A Lamp Holder

Follow the same cues: ribbed to the silver terminal, smooth to the brass terminal. The shell of the lamp holder must be neutral. The center contact must be hot. That way fingers near the shell during bulb changes touch neutral, not the live feed.

Wiring A New Duplex Receptacle

With power off, land the black conductor on the brass side, the white conductor on the silver side, and the equipment ground on the green screw. Leave the bonding tab intact unless you’re splitting the receptacle on a multi-wire branch circuit with a handle-tied breaker and a switched half. If your installation uses the LOAD terminals of a GFCI, follow the manufacturer’s diagram exactly.

Fast Polarity Checks With A Meter

Set the meter to AC volts. Insert one lead into the narrow slot and the other into the ground. You should read line voltage. Move the second lead to the wide slot. The reading to neutral should be similar or slightly lower. If the narrow slot reads zero to ground and the wide slot reads line voltage, the receptacle is reversed and needs correction.

Troubleshooting Reversed Polarity

Three-light testers help. Plug it in and read the legend. If it says “hot/neutral reversed,” power down the circuit at the breaker, pull receptacle, and move the conductors to the correct screws. Black belongs on brass and white belongs on silver. If it says “open ground,” look for a loose or missing equipment ground. Bond the metal box if present and land the ground on the green screw. If the home has no grounding conductor, install a GFCI receptacle and apply the “No Equipment Ground” label that ships with it, as required by listing. Retest after reassembly before restoring power. Label the cover plate if required. Stay patient.

About Tamper-Resistant And Weather-Resistant Receptacles

Tamper-resistant shutters don’t change polarity rules. The narrow slot remains the hot side and still pairs with the brass terminals. Weather-resistant devices add corrosion-resistant metals and gasketed faces, yet the markings stay the same. When you replace an outdoor outlet, use an in-use cover, verify the GFCI trips, and keep the same brass-to-hot, silver-to-neutral mapping. Leave room for the cover.

Common Mistakes And How To Avoid Them

Backwards Conductors On A Replacement Plug

Some caps rotate contacts 90 degrees relative to the blades. Always reference the internal diagram or the molded letters near the screws. If a cap lacks markings, fully assemble the blades into the body before landing conductors so you can map brass to narrow blade with certainty.

Using A Non-Polarized Adapter With A Polarized Plug

Adapters that accept a wide blade into two equal slots defeat polarization. Pick a listed adapter that preserves blade width or install a matching receptacle that enforces orientation. Marked adapters include a ground path and a slot set so the wide blade remains neutral.

Relying On Color Alone

Old cords fade, and imported cords don’t always follow familiar colors. When in doubt, use the blade width, screw color, and jacket texture method together. These cues are consistent and don’t depend on dye.

Regional Notes You Should Know

Polarity cues vary by region and plug type. North American Type A and Type B connectors enforce orientation through blade width, while some systems use round pins with no enforced neutral. Always follow the markings stamped into the plug body and the receptacle face for the region you’re in.

When Polarity Doesn’t Matter, And When It Does

Double-insulated Class II devices with two-pin supplies often don’t care about orientation. Their internal design isolates user-accessible parts from live conductors. Appliances that bond exposed metal parts to neutral, or that place a switch only on one conductor, do care. Those products carry polarized plugs and instructions that call out blade width alignment.

Safety Labels Worth Reading

You’ll see this line again and again on cord sets and luminaires: “match wide blade of plug to wide slot.” That sentence isn’t marketing; it’s there because safety standards demand clear polarity. If the label is missing or worn away, the blade width, screw color, and lamp-cord rib cues still get you to the right landing points.

Practical Takeaways

• Narrow blade equals hot; wide blade equals neutral; round pin equals ground.
• Black on brass, white on silver, green or bare on green.
• Smooth lamp-cord conductor goes to the narrow blade; ribbed goes to the wide blade.
• Don’t defeat polarization. Replace mismatched adapter