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The process of calculating inrush current involves three steps, critical for protecting electrical equipment from the high surge of current when switched on. Inrush current, typically 2 to 3 times the steady state current of a device, can spike up to 30 times in devices like electric motors or transformers. Despite modern circuit systems’ efficiency and lower impedance, inrush current remains a concern, often leading to equipment overheating. Inrush Current Limiters (ICLs) are essential for controlling this surge and maintaining equipment safety.

To select an appropriate ICL, one must understand various parameters, including maximum output power, input voltage, reset time, single or three-phase system, filter or link capacitor value, scope trace of inrush current, and ratings of fuse, circuit breaker, and diode bridge if applicable.

Step 1: Calculating Minimum Resistance

This calculator is used to determine the minimum resistance required in a circuit to limit the inrush current to a specified maximum value. It’s important for designing circuits that can handle initial high currents without damage.

Formula: Resistance = Voltage Peak / Max Allowable Inrush Current
Where Voltage Peak (VDC) = Voltage RMS × √2
VAC
A
0.00 Ω
Example Calculation:

Given:
Voltage VAC = 230 V
Max Allowable Inrush Current = 5 A

Calcualtion:
Calculate Voltage Peak (VDC): 230 V × √2 ≈ 325.22 V
Resistance = 325.22 / 5
Resistance ≈ 65.044 ohms

Step 2: Calculating Steady State Current

This calculator is used to determine the steady-state current in an electrical circuit, based on the power and voltage of the system, and considering the system’s efficiency.

Formula: Current = (Power / Voltage) × Efficiency
W
V
%
0.00 A
Example Calculation:

Given:
Power = 500 W
Voltage = 220 V
Efficiency = 90%

Calculation:
Convert efficiency to a decimal: 90% = 0.9
Current = (500 / 220) × 0.9
Current = 2.2727 × 0.9
Current ≈ 2.0454 amperes

Step 3: Calculating Total Energy

This calculator determines the energy rating of an inrush current limiter. It’s particularly useful in electrical circuits to understand the energy dissipation characteristics of a limiter.

Formula: Energy = ½ × Capacitance × (Voltage)2
V
μF
0.00 J
Example Calculation:

Given:
Voltage = 230 V
Capacitance = 50 μF

Calculation:
Convert capacitance to farads: 50 μF / 1,000,000 = 0.00005 F
Energy = ½ × 0.00005 × (230)2
Energy ≈ 1.3225 joules

Based on the above example calculations, we would have recommended either the Ametherm SL03 22102 or the Ametherm SL05 4R003. This ICL has a resistance of up to 220Ω (at 25°C), a maximum steady state current of 2A (up to +65°C) and a maximum total energy of 2J. If you would like to discuss any of your Inrush Current Limiter requirements, please contact one of our engineers today.

Click here for our full range of Inrush Current Limiters

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The below table lists some of the inrush current limiter series available through us by Ametherm, along with their specifications:

Ametherm SL Series
SeriesMaximum Steady State CurrentResistance at +25°CMaximum Capacitance at 120VACMaximum Recommended Energy
SL031A to 2A10Ω to 220Ω100μF2J
SL050.25A to 3A4Ω to 120Ω200μF to 340μF3J to 12J
SL081.1A to 3A4Ω to 120Ω550μF to 697μF8J to 12J
SL101A to 5A10Ω to 30Ω700μF to 2000μF2J to 120J
SL121A to 10A1Ω to 220Ω1389μF to 2778μF10J to 60J
SL152A to 10A1Ω to 220Ω2778μF to 4167μF35J to 55J
SL183A to 12A0.7Ω to 47Ω4500μF to 8335μF65J to 120J
SL221A to 16A0.5Ω to 220Ω3473μF to 10000μF2J to 160J
SL3215A to 36A0.25Ω to 10Ω6946μF to 20839μF100J to 300J
Ametherm MS Series
SeriesMaximum Steady State CurrentResistance at +25°CMaximum Capacitance at 120VACMaximum Recommended Energy
MS122A to 2A150Ω to 150Ω7500μF to 7500μF110J to 110J
MS154.2A to 4.2A30Ω to 40Ω9300μF to 9300μF135J to 135J
MS222A to 8A0.5Ω to 220Ω5100μF to 16400μF75J to 240J
MS326A to 40A0.5Ω to 50Ω13500μF to 20700μF250J to 300J
MS3510A to 50A0.5Ω to 20Ω34000μF to 62000μF500J to 800J
Ametherm AS Series
SeriesMaximum Steady State CurrentResistance at +25°CMaximum Capacitance at 120VACMaximum Recommended Energy
AS326A to 36A0.5Ω to 50Ω17500µF to 20700µF250J to 300J
AS3510A to 50A0.5Ω to 20Ω–500J to 800J

Users are hereby informed that Rhopoint accepts no responsibility for any inaccuracies or issues that may arise from the use of our online calculators. All data and results should be independently verified prior to their application in any projects or calculations.