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© 2015 Eaton. All Rights Reserved..
1
Types of Overcurrent
&
Calculations
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NEC® Article 100 – Definitions
Overcurrent
Any current in excess of the rated current of
equipment or the ampacity of a conductor. It may
result from overload, short circuit, or ground fault.
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3
NEC® Article 100 – Definitions
Overload
Operation of equipment in excess of normal, full-load rating,
or of a conductor in excess of rated ampacity that, when it
persists for a sufficient length of time, would cause damage
or dangerous overheating. A fault, such as a short circuit
or ground fault, is not an overload.
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4
Normal Load
IL= VS / RT
IL
VS
R
Source
R
R
Load
IL
R
R
R
R
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5
Overload
IOL= VS / RT
VS
R
R
Source
R
Load
R
R
R
R
Within the normal path
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6
Short Circuit
ISC= VS / RT
VS
R
R
Source
R
Load
R
R
R
R
Out of the circuit
Outside the normal path
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7
Types of Faults
• Bolted Faults
• Arcing Faults
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8
Bolted
Short Circuit
Arcing
Fault
Current Thru
Air
A
B
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A
B
9
Bolted Faults –Various Types
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
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10
Bolted Faults – Three Phase
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Typically considered the “worst case” or highest
magnitude
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11
Bolted Faults – Line to Line
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
87 % of the three phase bolted fault
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12
Bolted Faults – Line to Ground
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Typically much lower than 3
 fault, but can be > 3
fault near Xfmr terminals
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13
Arcing Faults – Many Variables Affect Current
& Whether It Is Sustainable
System Voltage
Gap spacing
Available 3
 Short Circuit Amps
Amount of Copper Vaporized
Degree of Containment
Configuration of Equipment
Typically does not sustain on 208Y/120V
A
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B
14
Arcing Faults – Progression
A
480Y/277 V
B
3 / 4W
Solid Grd
C
N
Ground
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15
Arcing Faults – Three Phase
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Can vary widely possibly up to 89% of 3

bolted fault
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16
Arcing Faults – Line to Line
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Can vary widely possibly up to 74% of 3

bolted fault
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17
Arcing Faults – Line to Ground
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Can vary widely
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18
Arcing Faults – Sustainability
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Rule of thumb:
Arcing faults will typically not sustain at less than
38% of 3
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 bolted fault
19
How do you know what the short-circuit current
is throughout a system?
and
What are some typical values
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20
Short Circuit Currents Vary
Depending on Many Factors
Transformer Size & % Z
Voltage
Conductor Size & Length
MSB
M
M
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21
Short Circuit Currents Vary
Depending on Many Factors
60,000 A
60,000 A
MSB
40,000 A
18,000 A
27,000 A
9,000 A
M
M
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22
Short Circuit Current Examples #1
500 KVA
1500 KVA
5%Z
5% Z
480/277V
1
1500 KVA
5% Z
208/120V
480/277V
2
3
500 KVA
1500 KVA
1500 KVA
2%Z
2%Z
2%Z
480/277V
1A
480/277V
2A
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208/120V
3A
23
How to Calculate Transformer Secondary
(assuming infinite primary)
Isca = (Xfmr FLA) x 100 / %Z
(increase result by 10% due to UL
tolerance for transformer impedances)
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24
Short Circuit Current Example #1 Answers
500 KVA
1500 KVA
5%Z
5% Z
480/277V
5% Z
208/120V
480/277V
13,222 A
1
1500 KVA
39,666 A
2
91,608 A
3
500 KVA
1500 KVA
1500 KVA
2%Z
2%Z
2%Z
480/277V
480/277V
33,055 A
1A
208/120V
99,165 A
2A
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3A
229,020 A
25
Short Circuit Current Example #2
208/120V
4
480/277V
40,000 A.
5
480/277V
40,000 A.
6
50 ft # 1
4A
40,000 A.
50 ft 250 kcm
5A
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6A
26
Calculating Short Circuit Currents
Utilize Point-to-Point Method
Steps 4, 5 & 6 for 3 Faults
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27
Short Circuit Current Example #2 Answers
208/120V
4
480/277V
40,000 A.
5
480/277V
40,000 A.
6
50 ft # 1
4A
50 ft 250 kcm
5A
12,367 A.
40,000 A.
6A
20,322 A.
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28,818 A.
28
Summary / Questions / To come
• Overcurrents – overloads, bolted faults, & arcing
faults
• Fault currents can be determined throughout
distribution system
• Available short circuit currents are needed to
assess I.R., short-circuit current ratings, selective
coordination, arc flash hazards & OCPD selection
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© 2015 Eaton. All Rights Reserved..
30