You have probably heard it before. A bulb that has been fine for months gives a small, bright flash and goes dark. It happens the instant you hit the switch, never at 2 a.m. while you sleep. It feels like bad luck, but it isn't.

The bulb was already dying. Turning it on is the hardest thing you ask of it, and that is the moment the weakness shows.

The wire that changes while you watch

Most materials we wire with get more resistive as they heat up. Tungsten does this dramatically.

An incandescent filament runs at roughly 2,500 to 3,000°C when lit. At room temperature it is a different conductor. Its resistance is commonly cited as around a tenth or less of its hot value. That is a rough figure and varies by lamp, but the direction matters more than the exact number: a cold filament is a very low-resistance path.

Ohm's law does the rest. A 100-watt, 120-volt lamp draws a bit under an amp when lit, which puts its hot resistance near 144 ohms. At the instant of switch-on, the resistance is a small fraction of that. The current, set by the line voltage and the cold resistance, is many times the running current. Then the filament heats in a fraction of a second, its resistance climbs, and the current falls to its normal value.

That opening spike is inrush current. It lasts well under a second, but the filament feels all of it.

Why the filament is the one that suffers

A filament never ages evenly. Over hundreds of hours, tungsten slowly evaporates from the hottest spots. Those spots thin out. A thinner section has higher resistance, so it dissipates more power and runs hotter, which makes it evaporate faster. It is a slow runaway process that leaves tiny narrow necks in a coil that still looks intact.

While the bulb stays on, the filament is in thermal equilibrium and carries a steady current. The weak spot is simply the hottest point, and the bulb keeps working.

Then you switch it on. Current surges through the whole coil at once, and the filament heats and expands within moments. Heating is not uniform, and the thinned section takes the worst of both the electrical and the mechanical shock. If the neck is thin enough, it melts or fractures. Often the broken ends arc briefly across the gap, and that arc is the flash you see.

So the switch doesn't kill the bulb. It reveals a failure that has been building for a long time.

Why it matters beyond the bulb

This is not a trivia point. Inrush shapes how electrical equipment is rated and installed.

Switches are rated for loads, not just amps. A switch that handles 15 amps of resistive heating may not survive 15 amps' worth of tungsten lamps, because the inrush stresses its contacts through repeated surges and arcing. That is why some switches carry a T rating for tungsten-filament loads. The NEC addresses switch ratings in Article 404, and 404.14 covers how general-use snap switches may be applied to different load types. Check the specific provision and the marking on the device rather than assuming amps alone settle it.

Breakers see it too. A thermal-magnetic breaker has a magnetic trip that responds to sudden high current. A circuit with many lamps or other surge-prone equipment switched on together can produce an inrush large enough to nudge that trip, even though the steady load is well under the rating. It is the same family of problem as motor starting, but the cause is a cold conductor, not a stalled rotor.

Modern loads have their own version. LED drivers and other electronic power supplies often have capacitors that charge at turn-on, which can produce a brief, sharp current spike. The spike is short, but a large bank of them on one switch or relay can add up. Manufacturers publish inrush figures for exactly this reason, and they are worth reading when you gang many fixtures on one control.

Why dimmers and soft-starts help

If inrush kills filaments, then reducing it should extend their life, and it does. Dimmers bring the lamp up gradually, so the filament warms before it carries full current. Installers sometimes call this a soft start. It is one reason lamps on dimmers often outlast the same lamps on plain switches.

The same principle explains why some designs use a small series element, such as an inrush-limiting thermistor, that starts with higher resistance and then drops as it warms. It is the filament's behavior in reverse, borrowed on purpose.

What this teaches about reading a load

The deeper idea is that resistance is not always a fixed number printed on a nameplate. For many real loads it depends on temperature, and temperature depends on what the circuit has been doing. The same fixture has two electrical personalities: the one at the instant of energizing and the one in steady state.

When a circuit behaves oddly only at the moment of switching, such as a breaker that trips on energizing but holds fine afterward, a switch that pits and sticks, or a bulb that pops, suspect the transient before you suspect the steady load. Steady-state math is correct and still tells only half the story.

Your next moves

  • Note when the bulb fails. Next time a lamp dies, notice whether it was at switch-on. If most of your failures happen at that moment, you are seeing end-of-life behavior, not a wiring fault, and you can stop hunting for one.
  • Check the switch marking. Pull a switch plate on a circuit with a lot of incandescent or halogen lighting and read the markings on the device. Look for the amp rating and any tungsten or T designation, and compare them to the lamp wattage on that circuit.
  • Add up the lamps. Total the wattage on a single switch and divide by the voltage to get the steady current. Then remember the first instant is several times higher and ask whether the switch and breaker were chosen with that in mind.
  • Try a dimmer for hard-to-reach fixtures. For high ceilings or fixtures that are painful to re-lamp, a compatible dimmer softens the start and can cut how often you climb the ladder. Match its wattage rating and lamp type to the load.
  • Read the datasheet for LED gear. If you are ganging many LED drivers on one switch or relay, look up the manufacturer's stated inrush current and maximum number of fixtures per circuit before you wire them together.

Where a field reference helps

Standing in a doorway with a dead lamp, it is easy to feel that electricity is simple: on, off, done. The cold filament is a small reminder that every load has a first moment before it settles. Keeping the steady-state numbers at hand, such as ampacity, breaker sizing, and wire size, makes it much easier to ask the right second question about what happens when the circuit first closes. Voltly puts those calculators and NEC references in your pocket, offline, so you can check them on the job: voltly.lumenlabs.works.