The flicker that means the compressor just started
Stand in a kitchen on a hot afternoon and watch the ceiling light. When the air conditioner's compressor kicks on, the light dips — a quick, shallow sag, then it recovers. Most people never notice it. Electricians notice it constantly, because that flicker is a motor announcing the single most misunderstood moment in its working life: the instant it starts.
A motor running steadily might draw twenty amps. For a fraction of a second at startup, that same motor can pull well over a hundred. Nothing is broken. Nothing is wrong. This surge — called locked rotor current, or inrush — is baked into the physics of how a motor works, and understanding it is the difference between correctly sizing a breaker and chasing nuisance trips for a week.
A stopped motor is a short circuit wearing a disguise
The most common motor on any job is the AC induction motor, and the cleanest way to understand it is to stop thinking of it as a motor at all. At the instant of startup, an induction motor is a transformer with its secondary shorted.
Here is why. The stator — the stationary windings you connect your wires to — is the primary. When you energize it, it sets up a rotating magnetic field. The rotor sitting inside it is the secondary: a cage of conductor bars, and those bars are shorted together at each end by solid rings. That's the whole rotor. No brushes, no connections, just a shorted cage.
When the rotor is standing still, the rotating field sweeps past those bars at full speed, inducing the maximum possible voltage in them. Because the bars are shorted, that induced voltage drives a very large current. And a large current in the rotor demands a large matching current in the stator — the same way a shorted secondary drags enormous current through a transformer's primary. The only thing holding it back is the winding's own resistance and leakage reactance, which is small. So the motor gulps current: typically five to seven times its normal running draw.
Why the surge collapses on its own
If that current stayed high, motors would cook themselves and trip every breaker in the building. It doesn't, and the reason is motion.
As the rotor begins to spin, it starts chasing the rotating field. The relative speed between the field and the bars — electricians call the gap slip — begins to shrink. The field is no longer sweeping past the rotor at full tilt; it's only slightly outrunning it. Less relative motion means less voltage induced in the rotor, which means less rotor current, which means the stator current drops right along with it.
Within a second or two, the rotor settles just shy of the field's speed, drawing only enough current to carry its mechanical load. The transformer-with-a-shorted-secondary has quietly become an efficient motor. In a DC or universal motor the same story is told with a different word — counter-EMF, the voltage a spinning armature generates against its own supply — but the principle is identical: a motor at rest has nothing opposing the incoming current, and a motor at speed generates its own opposition.
That is the flicker. For the second the compressor's rotor is accelerating, it pulls a slug of current large enough to sag the voltage across the whole panel, and every light on that service dims a hair until the surge passes.
The number is stamped right on the motor
Manufacturers don't leave you guessing at how big the surge will be. Look at the nameplate and you'll find a single letter — the NEC code letter, from Table 430.7(B). That letter tells you the motor's locked-rotor draw expressed as kilovolt-amperes per horsepower. A code letter G, for instance, means roughly 5.6 to 6.3 kVA per horsepower at startup; a code letter J means more. From that one character you can calculate the actual inrush amps before the motor ever spins.
There's a second, older shorthand still worth knowing: service factor and the design letter tell you about sustained overload capacity, but for inrush the code letter is the one that matters. It exists precisely because the starting surge is predictable enough to be tabulated — this is not a mysterious spike, it's an engineered, documented characteristic of the machine.
Why the breaker has to ignore the surge
Here's where the physics collides with the panel. If you protected a motor with a breaker sized to its running current, that breaker would trip every single time the motor started. The inrush would look exactly like a fault, because electrically, for that first instant, it nearly is one.
So motor protection is deliberately split into two jobs. The overload — often a separate heater or electronic relay in the starter — watches for sustained current a little above normal running amps, the slow kind of overload that means a jammed load or a failing bearing. It's designed to be patient and let the brief startup surge pass untouched.
The breaker or fuse ahead of it handles short circuits, and it is sized generously on purpose. This is why an inverse-time breaker protecting a motor is allowed by NEC 430.52 to be rated well above the wire's normal ampacity — up to 250 percent of the motor's full-load current for a standard breaker, and higher still for instantaneous-trip types. To an electrician wiring a lighting circuit, a breaker rated at two and a half times the load current looks insane. For a motor, it's the only way to let the machine start without the breaker mistaking a healthy startup for a catastrophe. The wire is still protected against genuine faults; it's just the overload protection, not the breaker, that guards the conductor's long-term thermal limit.
What this changes on the job
Once you can see the surge, a lot of field problems stop being mysteries. A motor that trips its breaker only on startup — never while running — isn't overloaded; its instantaneous trip is set too low for its inrush, or the code letter was never checked. A well pump on a long, undersized feeder that hums but won't quite start is starving during inrush, because the voltage drop that's merely annoying at running current becomes crippling when the motor is demanding six times that current. Lights across a shop that dip hard every time the big compressor cycles are telling you the service is soft relative to that motor's locked-rotor draw.
Every one of those calls is the same calculation underneath: what does this motor actually pull in the second before it's up to speed, and does the wire, the breaker, and the voltage all survive that second? That draws on the code letter, the full-load amps, the conductor's voltage drop under surge, and the breaker allowance in 430.52 — four different tables that all have to agree.
That's the kind of cross-referenced calculation Voltly was built to make disappear. Punch in the motor's horsepower and code letter and it works the locked-rotor current, checks it against voltage drop on your actual run length, and shows you the breaker sizing 430.52 allows — the ampacity tables, the motor tables, and the NEC math in one place, offline, in the panel room where there's no signal. The physics of inrush will always be six times the running current. Knowing what to do about it shouldn't take six trips to the truck. Try Voltly.