What Is the Inrush Current and How Do You Limit It?

The inrush current is the maximum instantaneous input current drawn by an electrical device during the first few milliseconds after it is turned on. While a device might only pull 5 amps during normal operation, that initial startup transient can easily spike to 50 amps or more. This massive, brief surge dictates how we size branch circuit breakers, select fuses, and design front-end protection, because standard overcurrent devices must be chosen to tolerate the startup spike without nuisance-tripping while still protecting the wire from sustained overloads.

The Physics of the Startup Spike

To understand inrush, you have to look at how reactive components behave at t=0 (the exact moment voltage is applied). In capacitive loads like switching power supplies, an uncharged capacitor acts as a dead short circuit. Current rushes in to charge the bulk electrolytic capacitors until they reach their target voltage. In inductive loads like motors and transformers, the windings have very low DC resistance. Until the motor starts spinning (generating back-EMF) or the transformer core magnetizes, the only thing limiting the current is the tiny resistance of the copper wire.

Think of it like pushing a heavy, broken-down car. The initial shove requires massive physical force to overcome static inertia and get the tires rolling. But once the car is moving at a steady walking pace, you only need a light, continuous push to maintain that speed. The initial shove is your inrush current; the steady push is your continuous operating current.

What People Commonly Confuse It With

Makers and junior technicians frequently confuse inrush current with Locked Rotor Current (LRA) or Short Circuit Current. LRA is the steady-state current a motor draws if its shaft is physically jammed; it will persist indefinitely until a thermal overload trips. Short circuit current is a fault condition where line voltage meets near-zero impedance, resulting in thousands of amps. Inrush, by contrast, is a normal, expected transient that naturally decays in milliseconds to seconds as the device reaches its operating state.

Typical Inrush Multipliers by Load Type

The severity of the startup spike depends entirely on the load architecture. Below is a reference table of typical inrush multipliers compared to Full Load Amps (FLA) or steady-state current. Use these multipliers when sizing upstream protection or calculating I²t let-through energy.

Source data synthesized from Ametherm Inrush Current Guidelines and standard motor starting characteristics.

Worked Example: Limiting a 500W Switching Power Supply

Let’s look at a real bench build: powering a custom automated test rig using a Mean Well LRS-500-24 enclosed switching power supply. We need to ensure the startup spike doesn't trip the 15A branch circuit breaker when the rig is powered on alongside other equipment.

1. Calculate Steady-State and Peak Inrush

• Output Power: 500W
• Input Voltage: 115VAC nominal
• Efficiency: ~88%
• Power Factor: ~0.85

Steady-state RMS current = 500W / (115V × 0.88 × 0.85) ≈ 5.75A RMS.

According to the LRS-500-24 datasheet, the cold-start inrush current is 40A at 115VAC. If you have three of these supplies on a single 15A breaker and flip the switch simultaneously, you hit a combined 120A transient. While the thermal bimetallic strip in the breaker won't react fast enough, the magnetic trip mechanism (which typically trips instantly at 5x to 10x rated current, or 75A-150A) might catch that 120A spike and kill your power.

2. Sizing the NTC Thermistor

To limit this, we place a Negative Temperature Coefficient (NTC) thermistor in series with the AC line. When cold, it has high resistance to choke the inrush. As current flows, it self-heats, and its resistance drops to near-zero, minimizing steady-state power loss.

We select the Ametherm MS35 5R020. Its specs:

• Cold Resistance (R25): 5.0 Ω
• Max Steady-State Current: 9.0 A
• Hot Resistance (at max current): 0.08 Ω

3. Verifying the Math

The peak AC voltage of a 115VAC line is 115 × √2 = 162.6V peak.

With the 5Ω cold NTC in place, the absolute maximum instantaneous inrush is limited to:
I_peak = 162.6V / 5.0Ω = 32.5A.

We've successfully dropped the spike from 40A down to 32.5A, well below the magnetic trip threshold of a standard 15A breaker. Once the power supply stabilizes, the NTC heats up and drops to 0.08Ω. The steady-state power dissipated by the thermistor is I²R = (5.75A)² × 0.08Ω = 2.64W, which the MS35's massive epoxy body handles easily without overheating.

Where You Meet This in Practice

Inrush isn't just a datasheet footnote; it causes real-world failures across multiple disciplines if ignored.

• Home Lighting & Dimmers: Turning on a bank of 20 LED downlights can generate a combined inrush spike exceeding 300A for a few milliseconds. This micro-welds the contacts inside standard TRIAC dimmers, causing them to fail stuck-on, or causes C-curve MCBs to nuisance trip. Always check the dimmer's spec sheet for 'maximum LED inrush rating', not just wattage.
• Embedded Systems & USB: Plugging an ESP32 dev board with large bulk decoupling capacitors into an unpowered USB hub. The moment the hub powers up, the ESP32's caps draw a massive spike, tripping the hub's internal polyfuse or causing a voltage brownout that resets the microcontroller before it can boot. Adding a small series ferrite bead or a soft-start load switch (like the TI TPS2553) fixes this.
• Industrial Motor Starters: Starting a 10HP compressor Direct-on-Line (DOL) causes a massive voltage sag on the local grid due to the 6x-8x inrush. This is why industrial sites use Variable Frequency Drives (VFDs) or star-delta starters to ramp the voltage up over several seconds, intentionally stretching the inrush out to keep it under 150% of FLA.

FAQ: Breakers, Fuses, and Inrush