Calculator
The Air Circuit Breaker (ACB) Sizing Calculator is a online calculator tool designed to assist electrical engineers, panel designers and technical personnel in sizing Air Circuit Breakers (ACBs) for low voltage (LV) power distribution systems.
⚡ ACB Sizing Calculator
Air Circuit Breaker Sizing | IEC 60947 / NEC / IS 13947
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The calculator supports three international standards:
- IEC 60947: International Electrotechnical Commission (global)
- NEC / ANSI: National Electrical Code (North America)
- IS 13947: Bureau of Indian Standards (India)
It accounts for full load current, system voltage, power factor, efficiency, demand factor, temperature and altitude derating, harmonic distortion, and short-circuit current requirements to get at a properly sized ACB with appropriate protection settings.
Input Parameters
The calculator is organised into 3 input sections.
System Configuration
System Type
Select 3-Phase (Three-Phase AC) (or) 1-Phase (Single-Phase AC). This determines the FLC formula used.
System Voltage (V)
Line-to-line voltage in Volts (e.g. 415 V for Indian/European systems, 480 V for USA).
System Frequency
50 Hz (India/Europe/Asia) (or) 60 Hz (USA/Canada).
Design Standard
The applicable standard governs naming conventions and recommendations in the output report.
Load Parameters
Connected Load / Power (kW)
Total rated load power in kilowatts.
Power Factor (PF)
Ratio of real to apparent power (0.1–1.0). Typical: 0.8–0.9 for industrial loads.
Load / Motor Efficiency (%)
Equipment efficiency (1–100%). Typical motors: 90–96%. Use 100% for resistive loads.
Demand Factor
Ratio of maximum actual demand to installed capacity (0.1–1.0). Use 1.0 for full-load design.
Load Type
It affects the Short-Circuit Pickup (STPU) calculation. Select Motor Load to reveal the starting current factor field.
Motor Starting Current Factor
It is visible only for motor loads. Typical values: 6x for DOL starting and 3x for Star-Delta starting.
Protection & Derating
Safety Multiplier
It is applied to derated FLC to determine design current. 1.25x is the NEC/IEC standard. Use 1.5x for essential services.
Ambient Temperature (°C)
Make installation as per environment temperature. Derating applies above 40°C: 0.95 at 50°C, 0.90 at 60°C, 0.85 above 60°C.
Installation Altitude (m)
Altitude derating: 0.97 at 1001–2000 m, 0.94 at 2001–3000 m, 0.91 above 3000 m.
Harmonic Derating Factor
It accounts for additional heating due to harmonic currents (0.5–1.0). Use 1.0 if no harmonic analysis is available.
Prospective Short-Circuit Current (kA)
The maximum fault current at the installation point. The ACB should have a breaking capacity equal to or greater than this value.
Reference table of all input parameters:
| Input Field | Values / Range | Notes |
|---|---|---|
| System Type | 3-Phase / 1-Phase | Required |
| System Voltage (V) | e.g. 415 | Required |
| System Frequency (Hz) | 50 Hz / 60 Hz | Default: 50 Hz |
| Design Standard | IEC 60947 / NEC / IS 13947 | Default: IEC 60947 |
| Connected Load / Power (kW) | e.g. 100 | Required |
| Power Factor | 0.1 to 1.0 (e.g. 0.85) | Required |
| Load / Motor Efficiency (%) | 1 to 100 (e.g. 95) | Required |
| Demand Factor | 0.1 to 1.0 (e.g. 1.0) | Default: 1.0 |
| Load Type | General / Motor / Transformer / Lighting / UPS | Default: General |
| Motor Starting Current Factor | e.g. 6 (DOL), 3 (Star-Delta) | Motor load only |
| Safety Multiplier | 1.10 / 1.25 / 1.30 / 1.50 | Default: 1.25 (NEC/IEC) |
| Ambient Temperature (°C) | e.g. 40 | Default: 40°C |
| Installation Altitude (m) | e.g. 500 | Default: 0 m |
| Harmonic Derating Factor | 0.5 to 1.0 (e.g. 1.0) | Default: 1.0 |
| Short-Circuit Current (kA) | 10 / 16 / 25 / 36 / 50 / 65 / 85 / 100 | Required |
| Installation / Duty Type | Normal / Heavy / Outdoor / Critical | Default: Normal Duty |
Standard ACB Rating Table
The calculator selects the next standard rating above the computed design current from the following table:
| ACB Rating (A) | Frame Size | Typical Application |
|---|---|---|
| 100 A | 630A Frame | Small distribution panels |
| 125 A | 630A Frame | Small distribution panels |
| 160 A | 630A Frame | LV motor control centers |
| 200 A | 630A Frame | LV motor control centers |
| 250 A | 630A Frame | Commercial distribution |
| 315 A | 630A Frame | Commercial distribution |
| 400 A | 630A Frame | Industrial feeders |
| 500 A | 630A Frame | Industrial feeders |
| 630 A | 630A Frame | Main LV switchboards |
| 800 A | 1600A Frame | Main LV incomer |
| 1000 A | 1600A Frame | Large industrial plants |
| 1250 A | 1600A Frame | Large LV switchgear |
| 1600 A | 1600A Frame | MV/LV transformer secondary |
| 2000 A | 2500A Frame | Main HV/LV switchboards |
| 2500 A | 2500A Frame | Grid substation LV side |
| 3200 A | 4000A Frame | Heavy industry / data centres |
| 4000 A | 4000A Frame | Utility / generation |
| 5000 A | 6300A Frame | Extra-large industrial |
Derating Quick Reference
Temperature Derating
| Ambient Temp | Derating Factor | Remark |
|---|---|---|
| ≤ 40°C | 1.00 | No derating |
| 41 – 50°C | 0.95 | 5% reduction |
| 51 – 60°C | 0.90 | 10% reduction |
| > 60°C | 0.85 | 15% reduction |
Altitude Derating
| Altitude | Derating Factor | Remark |
|---|---|---|
| ≤ 1000 m | 1.00 | No derating |
| 1001 – 2000 m | 0.97 | 3% reduction |
| 2001 – 3000 m | 0.94 | 6% reduction |
| > 3000 m | 0.91 | 9% reduction |
Formulas
| Parameter | Formula |
|---|---|
| FLC (3-Phase) | P × 1000 ÷ (√3 × V × PF × η × DF) |
| FLC (1-Phase) | P × 1000 ÷ (V × PF × η × DF) |
| Combined Derating Factor (CDF) | Temperature Factor × Altitude Factor × Harmonic Factor |
| Derated FLC | FLC ÷ CDF |
| Design Current | Derated FLC × Safety Multiplier |
| ACB Rating | Next standard rating ≥ Design Current |
| LTPU Setting | FLC × 1.05 |
| STPU (General Load) | FLC × 10 |
| STPU (Motor Load) | FLC × Starting Factor × 1.1 |
| GFP Setting | FLC × 0.20 |
| Utilization Level (%) | (FLC ÷ ACB Rating) × 100 |
Note: P is the connected load in kW, V is the line voltage in Volts, PF is the power factor, η (eta) is the efficiency fraction and DF is the demand factor.
Calculation
Step-by-Step Calculation
Step-1: Full Load Current (FLC) is calculated from the load power, voltage, power factor, efficiency and demand factor.
Step-2: Temperature Derating Factor is determined from the ambient temperature input.
Step-3: Altitude Derating Factor is determined from the installation altitude input.
Step-4: Combined Derating Factor (CDF) is the product of the three individual derating factors.
Step-5: Derated FLC is the FLC divided by the CDF representing the effective current under derated conditions.
Step-6: Design Current is the Derated FLC multiplied by the chosen Safety Multiplier.
Step-7: Recommended ACB Rating is the next standard catalogue rating at or above the design current.
Step-8: Protection settings (LTPU, STPU, GFP) are calculated as multiples of FLC per standard practice.
Output Results
After clicking Calculate ACB Size the results panel displays all calculated values with a summary recommendation.
| Output Parameter | Description |
|---|---|
| Full Load Current (FLC) | Calculated full load current (FLC) in Amperes |
| Temperature Derating Factor | Derating multiplier based on the ambient temperature |
| Altitude Derating Factor | Derating multiplier based on installation altitude |
| Harmonic Derating Factor | User entered harmonic derating value |
| Combined Derating Factor | Product of all 3 derating factors |
| Derated FLC | FLC adjusted for all derating conditions |
| Design Current | Derated FLC x safety multiplier |
| Recommended ACB Rating (A) | Next standard rating above design current |
| ACB Frame Size | Standard ACB frame classification |
| Required Breaking Capacity | Minimum short circuit breaking capacity in kA |
| Overload Relay / LTPU Setting | Long-Time Pickup setting (~105% of FLC) |
| Short-Circuit Pickup (STPU) | Short-Time Pickup setting (10x FLC or motor factor) |
| Ground Fault Protection (GFP) | Ground fault pickup (~20% of FLC) |
| ACB Utilization Level | FLC as percentage of ACB rated current |
Utilization Level is flagged with a warning badge if it exceeds 80% indicating the ACB may be undersized for adequate operational margin.
How to use the Calculator?
1). Open the ACB Sizing Calculator.
2). Fill in the System Configuration section: select System Type & enter System Voltage.
3). Fill in the Load Parameters section: enter kW, Power Factor, Efficiency & select Load Type.
4). For Motor loads enter the Starting Current Factor (6 for DOL, 3 for Star Delta).
5). Fill in the Protection & Derating section: select the Safety Multiplier, enter Ambient Temperature & Altitude.
6). Select the prospective short circuit current (kA) from the dropdown.
7). Click the "Calculate ACB Size" button.
8). Review the results panel. Note the Recommended ACB Rating, Frame Size and protection settings.
9). If the Utilization Level exceeds 80% consider selecting the next higher ACB rating.
10). Use the "Reset All Fields" button to clear all inputs and start a new calculation.
Solved Example
Size an ACB for a 3 phase, 415 V, 75 kW induction motor (DOL starting), PF = 0.85 and efficiency = 94%, installed in an industrial plant at 45°C ambient, 500 m altitude with no harmonics, SCC = 36 kA.
| Step | Calculation | Result |
|---|---|---|
| Full Load Current (FLC) | 75000 ÷ (√3 × 415 × 0.85 × 0.94 × 1.0) | 137.6 A |
| Temperature Derating (45°C) | Between 40–50°C → 0.95 | 0.95 |
| Altitude Derating (500 m) | Below 1000 m → 1.00 | 1.00 |
| Harmonic Derating | No harmonics → 1.00 | 1.00 |
| Combined Derating Factor | 0.95 × 1.00 × 1.00 | 0.950 |
| Derated FLC | 137.6 ÷ 0.95 | 144.8 A |
| Design Current (×1.25) | 144.8 × 1.25 | 181.0 A |
| Recommended ACB Rating | Next standard rating ≥ 181 A | 200 A |
| ACB Frame Size | ≤ 630 A | 630A Frame |
| LTPU Setting | 137.6 × 1.05 | 144.5 A |
| STPU Setting (DOL, 6×) | 137.6 × 6 × 1.1 | 907.6 A |
| GFP Setting | 137.6 × 0.20 | 27.5 A |
| Utilization Level | (137.6 ÷ 200) × 100 | 68.8% ✓ OK |
Result: Install a 200 A / 36 kA ACB in a 630A Frame. Set LTPU at 145 A, STPU at 908 A, GFP at 28 A.
Limitations
This calculator is intended for preliminary guidance only. It does not replace a full engineering study. The following limitations apply:
- The tool does not account for cable impedance, upstream protection coordination, or selectivity (discrimination) studies.
- Motor starting methods beyond DOL and Star-Delta (e.g. soft starters, VFDs) require separate analysis.
- The harmonic derating factor must be determined from a separate power quality study; the tool accepts a user-supplied value only.
- Ground fault protection settings shown are indicative; TN, TT and IT system earthing arrangements may require different approaches.
- Breaking capacity requirements may be higher when upstream sources can feed fault current independently.
- The results are for single-feeder ACB sizing; bus-tie and coupler ACBs require additional selectivity verification.
Points to Remember
All final designs should be verified and approved by a licensed electrical engineer in accordance with applicable local standards and regulations (IEC 60947, NEC, IS 13947 or equivalent).
Standards Reference
| Standard | Title | Scope |
|---|---|---|
| IEC 60947-2 | Low-Voltage Switchgear & Control gear - Part 2: Circuit Breakers | International standard for LV circuit breakers |
| NEC (NFPA 70) | National Electrical Code | USA electrical installations |
| ANSI / IEEE C37 | Switchgear Standards | USA switchgear ratings |
| IS 13947-2 | Low Voltage Switchgear & Control gear | Indian equivalent of IEC 60947 |
| IEC 60364 | Low Voltage Electrical Installations | Installation wiring & protection |