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Short-Circuit Current

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Starting point *

System voltage *

Line-to-line voltage at fault location

Transformer rating

kVA

Primary voltage

Transformer impedance

Leave blank to use standard table values

Primary fault current

Optional - for Method 2 with known primary fault

Source fault current

Known fault current at source

Add cables and busbars in physical order from source to fault point.

[[ index + 1 ]]. [[ comp.component_type === 'cable' ? 'Cable' : 'Busbar' ]]

Motors near the fault contribute 4 × full-load current (NEC Table 430.250).

Motor [[ index + 1 ]]

Motor HP

Quantity

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Title

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Company name

Project number

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Project name

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Calculation No.: Calc-001
Revision: A
Description:
Created By: Engineer Name

Project: Project Name
Project No.: P001
Client: Client Name

Short-Circuit Current Calculation

Three-phase fault current analysis using Bussmann point-to-point method per Cooper Bussmann SPD tables.

1. Input Parameters

System configuration and parameters for fault current calculation:

Parameter	Symbol	Value	Notes
Calculation Method		Transformer	Starting point for analysis
System Voltage	V_LL	480 V	Line-to-line voltage
Transformer Rating	S	1 kkVA	Transformer capacity
Primary Voltage	V_p	4.16 kV	Primary side
Transformer Impedance	Z%	5.75 %	Percent impedance
System Components		0	0 cable(s), 0 busbar(s)
Motors		0	4 x FLA (NEC Table 430.250)

2. Base Fault Current (Transformer)

Calculate fault current at transformer secondary terminals using transformer rating and impedance.

2.1 Transformer Full Load Current

Calculate transformer secondary full load current:

$$ I_{FL} = \frac{S \times 1000}{\sqrt{3} \times V_{LL}}
                        = \frac{1000 \times 1000}{1.732 \times 480}
                        = 1202.8 \; \text{A} $$

Where S = 1000 kVA (transformer rating), V = 480 V (secondary voltage)

2.2 Transformer Fault Current

Calculate maximum fault current based on transformer impedance (infinite primary):

$$ I_{sc} = \frac{I_{FL} \times 100}{Z\%}
                             = \frac{1202.8 \times 100}{5.75}
                             = 20,918 \; \text{A} $$

Where Z% = 5.75% (transformer impedance). This is the bolted fault current at the transformer secondary terminals.

3. Final Results

Total available fault current at the load point.

3.1 Total Fault Current

**Total Fault Current: 20,918 A (20.92 kA)**

(No motor contribution)

Parameter	Symbol	Value	Notes
Base Fault Current	I_sc,base	20.9 kA	Without motor contribution
Motor Contribution	I_motor	0 A	Locked rotor current
Total Fault Current	I_sc,total	20.9 kA	20.92 kA

4. Equipment Rating Requirements

All electrical equipment at the fault location must be rated to safely withstand and/or interrupt the available fault current.

Parameter	Symbol	Value	Notes
Interrupting Capacity (AIC)	I_interrupt	20.9 kA	Breakers/fuses must be >= 22 kAIC
Recommended AIC Rating		22 kAIC	Next standard rating above fault current
Short-Circuit Current Rating (SCCR)	I_SCCR	20.9 kA	Panelboards, switchboards, MCCs
Withstand Rating (RMS)	I_withstand	20.9 kA	Bus bracing, equipment withstand
Peak Asymmetrical (x1.6)	I_peak	33.5 kA	Maximum instantaneous current

4.1 Important Rating Notes

Interrupting Capacity (AIC):
Use fault current WITHOUT motors for AIC ratings
Motor contribution decays before breaker interruption (~0.1 seconds)
Standard AIC ratings: 10, 14, 22, 42, 65, 100, 200 kAIC


Short-Circuit Current Rating (SCCR):
Use TOTAL fault current including motors for SCCR
Applies to panelboards, switchboards, motor control centers
All components in series must meet or exceed SCCR requirement


Withstand Rating:
Use TOTAL fault current including motors
Bus bracing must withstand mechanical forces
Peak asymmetrical current used for momentary ratings

5. Standards & References

This calculation follows the Cooper Bussmann point-to-point method for short-circuit current analysis.

5.1 Applicable Standards

Cooper Bussmann SPD (Selecting Protective Devices), 2005 edition
Point-to-point method for fault current calculation
Table 2: conductor C values, three single conductors in steel or nonmagnetic conduit, 600 V / 5 kV / 15 kV
Table 1: busway C values (plug-in, feeder, high-impedance; copper and aluminium)
Note 1: motor contribution = 4 x total motor full-load current
M = 1 / (1 + f); the report shows the divisor 1 + f


NEC (NFPA 70) Table 430.250
Full-load current, three-phase alternating-current motors


NEC (NFPA 70) - National Electrical Code
Article 110.9: Interrupting Rating
Article 110.10: Circuit Impedance and Short-Circuit Ratings


IEEE Std 141 (Red Book) - Electric Power Distribution for Industrial Plants
Chapter 4: Short-Circuit Current Calculations
Point-to-Point Method for fault current analysis


IEEE Std 242 (Buff Book) - Protection and Coordination
Motor contribution to fault current
Protective device coordination principles


IEEE Std 551 - Calculating Short-Circuit Currents
Detailed calculation methodologies
System modeling and impedance calculations

5.2 Calculation Methodology

Bussmann Point-to-Point Method:This method progressively calculates fault current through each component of the electrical distribution system:
Base Fault Current: Start at transformer secondary or known source fault level
Component Impedance: Process each cable and busway in physical order using Bussmann C values
f-factor: f = (1.732 x L x I_upstream) / (C x n x V) for cables, f = (1.732 x L x I_upstream) / (C x V) for busway
M multiplier: M = 1 + f (downstream current = upstream / M)
Motor Contribution: Add 4 x the total motor full-load current (NEC Table 430.250)
Total Fault Current: Sum impedance-limited fault plus motor contribution
Key Parameters:
Transformer %Z: Lower impedance results in higher fault current
Bussmann C Values: Reciprocal impedance figures from Cooper Bussmann SPD tables
M Multiplier: M = 1 + f; reduces downstream fault current proportional to impedance
Motor contribution: 4 x FLA, added at the fault location (Bussmann Note 1)

5.3 Design Verification Requirements

Engineer Responsibilities:

Verify all input parameters (transformer data, cable sizes, lengths)
Confirm conductor types and installation methods
Obtain utility fault level data if using source fault method
Verify motor data and operating conditions
Select equipment with adequate ratings (AIC, SCCR, withstand)
Perform protective device coordination study
Verify conductor ampacity and short-circuit withstand capability
Update calculation when system changes occur
Conduct arc flash hazard analysis per IEEE 1584-2018
Ensure proper equipment labeling per NEC 110.16


Important Notes:
Fault current calculated is three-phase bolted fault (maximum possible)
Results represent worst-case available fault current
All equipment must be rated for the calculated values
Calculation must be reviewed by licensed professional engineer

Created By
Engineer Name
Date: ..........................

Checked
By: .............................
Date: ..........................

Approved
By: .............................
Date: ..........................

Client Approved
By: .............................
Date: ..........................

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