What ST19 Means on an Edison-Style Bulb—and Which Specs Still Need Checking

What the ST19 code actually describes

ST19 describes the bulb envelope—the outer shape usually associated with an elongated, vintage Edison-style bulb. “ST” means straight tubular. In practice, the outline is taller than a familiar pear-shaped household bulb, with relatively straight sides and a taper toward the base. The code does not describe the filament pattern, glass color, light source, or socket.

The number belongs to the traditional U.S. eighth-inch sizing system. Nineteen eighths equals 2 3/8 inches, so ST19 identifies a nominal diameter class of about 2.375 inches. Treat that as a naming convention rather than a guaranteed measured dimension. The applicable NEMA nomenclature standard defines envelope shapes independently of lamp bases, and manufacturers may publish dimensions that differ slightly from the nominal code.

For example, Feit Electric’s ST19100CL/950CA/2 is labeled ST19 but has a stated width of 2.5 inches and length of 5.4 inches. Its page separately identifies an E26 base, 120-volt operation, LED filament technology, and other performance details. That separation is the key to reading any ST19 listing correctly.

Base code and voltage are separate checks

ST19 does not mean E26. ST19 identifies the envelope, while E26 identifies a medium screw base. A listing that says “ST19 E26” is supplying two different pieces of information: the shape and size family of the glass, followed by the socket interface. Match the base code printed on the old bulb, fixture label, or fixture documentation rather than assuming it from the silhouette.

Voltage is another independent specification. A bulb with the correct E26 threads can still have the wrong electrical rating. For an ordinary U.S. line-voltage fixture, compare the lamp’s marked voltage or voltage range with the fixture and supply. Do not infer voltage from ST19, E26, the bulb’s appearance, or the word “Edison.”

Also check whether the fixture requires a particular lamp technology or type. The fixture label and model instructions take priority when they are more specific than the bulb package.

• ST19 describes the envelope.
• E26 describes a medium screw base.
• The voltage marking describes the intended electrical supply.

Use actual watts for the fixture limit and lumens for brightness

A package may show both actual wattage and an incandescent-replacement figure. They serve different purposes. Actual wattage—often labeled “Energy Used”—is the bulb’s electrical input. “60W replacement” or “100W equivalent” is a brightness comparison, not the power the LED draws.

When checking a fixture’s maximum-wattage label, use the bulb’s actual input wattage and obey any stated lamp-type restriction. The Consumer Product Safety Commission advises using the bulb type and maximum wattage identified by the fixture. A low-watt LED is not automatically approved for every fixture, because enclosure limits and other instructions remain separate.

For brightness, compare lumens rather than watts or the ST19 code. The Federal Trade Commission explains that lumens measure light output, while watts measure energy use. Its Lighting Facts guidance also points readers to light appearance, expected life, and energy use. In the Feit example, the distinct specifications are 15 actual watts, a 100-watt replacement claim, and 1,490 lumens.

“Dimmable” does not settle dimmer compatibility

If the fixture is switched only on and off, a dimmable bulb can generally be used without invoking its dimming function. On a dimmed circuit, first confirm that the bulb itself is marked dimmable. Then check the dimmer manufacturer’s compatibility information for the exact dimmer and lamp model or seek the lamp maker’s recommended dimmers.

LED drivers do not all respond identically to every dimmer. Lutron notes that compatibility affects dimming performance and maintains model-specific test information. A generic “dimmable” marking therefore does not promise a particular dimming range or freedom from flicker, dropout, ghosting, or audible noise with every control. Smart bulbs and sensor-equipped bulbs may also have control requirements, so follow their own instructions.

Enclosure rating and location rating answer different questions

An enclosure rating concerns heat. Fully enclosed fixtures trap heat, which the Department of Energy says can affect the performance and life of some LED lamps. Use an ST19 in a sealed globe or other enclosed fixture only when the exact bulb is approved for that application and the fixture documentation permits it.

A dry, damp, or wet location rating concerns moisture exposure instead. UL guidance distinguishes dry-only luminaires from equipment suitable for damp or wet locations. Damp-rated equipment may be used in damp or dry locations; wet-rated equipment may be used in wet, damp, or dry locations. A covered porch may still require a specific rating based on its actual exposure and the fixture instructions.

Neither rating substitutes for the other. A damp-rated bulb is not thereby enclosed-fixture rated, and an enclosed-fixture-rated bulb is not thereby suitable for rain. Feit’s example lists “Damp” and “Enclosed Fixture Rated: Yes” as two separate specifications, illustrating why both fields must be checked.

Measure the fixture instead of relying on ST19 alone

Physical clearance is the last independent check. Compare the manufacturer’s maximum overall length and maximum diameter with the available space. Check the shade or globe opening, distance from the socket to the cover, width near the bulb’s shoulder, and clearance around any harp, cage, retaining ring, or decorative sleeve.

The nominal 2 3/8-inch diameter suggested by “19” does not tell you the total length, the shape near the base, or the exact production dimensions. DOE guidance notes that LED replacement lamps can be larger near the base because of their integrated electronics, so a matching screw base does not guarantee that the bulb will seat fully or clear a small fixture.

With the power off and the old bulb cool, record its full markings and measure its length and widest point. Then compare those details with the candidate bulb’s specification sheet. If the fixture label or model documentation imposes a narrower shape, lower wattage, different rating, or other restriction, follow that information even when the candidate is labeled ST19.