Generator Fault Current Calculator
Calculate a generator's symmetrical short-circuit current to ensure circuit breakers and transfer switches have the correct AIC rating.
Generator Specifications
Available Fault Current
Full Load Amps (FLA): -- Amps
What does this mean?
Your breakers and transfer switch must have an AIC (Ampere Interrupting Capacity) rating higher than this value to safely clear a short circuit without exploding.
Part Finder: Protection & Safety
Circuit Breakers
High Interrupting Rating (AIC)
Current Limiting Fuses
Fast acting protection
Arc Flash PPE
Safety gear for live work
Breaker Rating Suitability Guide
This chart compares your calculated fault current against standard
breaker interrupting ratings (kAIC).
Green = Safe to use.
Red = Unsafe/Explosion
Hazard.
Typical Fault Current Values (Estimates)
| Generator Size | Fault Current @ 208V | Fault Current @ 480V |
|---|---|---|
| 50 kW | ~1,450 A | ~625 A |
| 100 kW | ~2,900 A | ~1,250 A |
| 250 kW | ~7,200 A | ~3,100 A |
| 500 kW | ~14,500 A | ~6,250 A |
| 1000 kW | ~29,000 A | ~12,500 A |
* Assumes 12% Subtransient Reactance (X"d). Actual values vary by manufacturer.
Understanding Generator Fault Current
Fault current (or short-circuit current) is the maximum amount of amperage that a generator can supply under a bolted three-phase fault condition. This value is critical for a process called a short-circuit coordination study.
- Why It's Important: Every circuit breaker and fuse has an Ampere Interrupting Capacity (AIC) rating. This is the maximum fault current that the device can safely interrupt without failing, exploding, or causing a fire. The calculated fault current must be less than the AIC rating of all downstream protective devices.
- Subtransient Reactance (X"d): This value, provided by the generator manufacturer, represents the generator's internal opposition to the flow of current immediately following a fault (in the first few cycles). A lower X"d percentage means the generator has less internal impedance and will produce a higher fault current.
Critical Safety Warning
Fault current calculations are a critical component of a safe electrical system design. Incorrectly rated protective devices can lead to catastrophic equipment failure, arc flash incidents, and serious injury or death.
This calculator is for estimation purposes only. All electrical system design and component selection must be performed by a qualified and licensed electrical engineer in accordance with the National Electrical Code (NEC) and all applicable local regulations.
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