1 © 2015 Eaton. All Rights Reserved.. Suggestion on How to Use Industry Trainers are encouraged to use this material in their sessions Download the presentation file Print the Notes pages and read them as you view the presentation in the “Slide Show” view. In this way you see the slides in large format and have animation (when available)

© 2015 Eaton. All Rights Reserved.. Types of Overcurrent & Calculations

3 © 2015 Eaton. All Rights Reserved.. 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.

4 © 2015 Eaton. All Rights Reserved.. 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.

5 © 2015 Eaton. All Rights Reserved.. Normal Load Source RR RR R R Load R I L = V S / R T VSVS ILIL ILIL

6 © 2015 Eaton. All Rights Reserved.. Overload Source Within the normal path RR RR R R Load R VSVS I OL = V S / R T

7 © 2015 Eaton. All Rights Reserved.. Short Circuit Source Outside the normal path RR RR R R I SC = V S / R T VSVS Load R Out of the circuit

8 © 2015 Eaton. All Rights Reserved.. Types of Faults Bolted Faults Arcing Faults

9 © 2015 Eaton. All Rights Reserved.. Bolted Short Circuit AB Arcing Fault AB Current Thru Air

10 © 2015 Eaton. All Rights Reserved.. Bolted Faults –Various Types A B C Ground N 480Y/277 V 3  / 4W Solid Grd

11 © 2015 Eaton. All Rights Reserved.. Bolted Faults – Three Phase A B C Ground N 480Y/277 V 3  / 4W Solid Grd Typically considered the “worst case” or highest magnitude

12 © 2015 Eaton. All Rights Reserved.. Bolted Faults – Line to Line A B C Ground N 480Y/277 V 3  / 4W Solid Grd 87 % of the three phase bolted fault

13 © 2015 Eaton. All Rights Reserved.. Bolted Faults – Line to Ground A B C Ground N 480Y/277 V 3  / 4W Solid Grd Typically much lower than 3  fault, but can be > 3  fault near Xfmr terminals

14 © 2015 Eaton. All Rights Reserved.. AB 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

15 © 2015 Eaton. All Rights Reserved.. Arcing Faults – Progression A B C Ground N 480Y/277 V 3  / 4W Solid Grd

16 © 2015 Eaton. All Rights Reserved.. Arcing Faults – Three Phase A B C Ground N 480Y/277 V 3  / 4W Solid Grd Can vary widely possibly up to 89% of 3  bolted fault

17 © 2015 Eaton. All Rights Reserved.. Arcing Faults – Line to Line A B C Ground N 480Y/277 V 3  / 4W Solid Grd Can vary widely possibly up to 74% of 3  bolted fault

18 © 2015 Eaton. All Rights Reserved.. Arcing Faults – Line to Ground A B C Ground N 480Y/277 V 3  / 4W Solid Grd Can vary widely

19 © 2015 Eaton. All Rights Reserved.. Arcing Faults – Sustainability A B C Ground N 480Y/277 V 3  / 4W Solid Grd Rule of thumb: Arcing faults will typically not sustain at less than 38% of 3  bolted fault

20 © 2015 Eaton. All Rights Reserved.. How do you know what the short-circuit current is throughout a system? and What are some typical values

21 © 2015 Eaton. All Rights Reserved.. MM MSB Short Circuit Currents Vary Depending on Many Factors Transformer Size & % Z Voltage Conductor Size & Length

22 © 2015 Eaton. All Rights Reserved.. MM MSB Short Circuit Currents Vary Depending on Many Factors 60,000 A 27,000 A40,000 A 60,000 A 18,000 A 9,000 A

23 © 2015 Eaton. All Rights Reserved KVA 480/277V 1 5 % Z 500 KVA 480/277V 1A 2 % Z 1500 KVA 480/277V 2 5% Z 1500 KVA 480/277V 2A 2 % Z Short Circuit Current Examples # KVA 208/120V 3 5% Z 1500 KVA 208/120V 3A 2 % Z

24 © 2015 Eaton. All Rights Reserved.. How to Calculate Transformer Secondary (assuming infinite primary) I sca = (Xfmr FLA) x 100 / %Z (increase result by 10% due to UL tolerance for transformer impedances)

25 © 2015 Eaton. All Rights Reserved KVA 480/277V 1 5 % Z 500 KVA 480/277V 1A 2 % Z 13,222 A 33,055 A 1500 KVA 480/277V 2 5% Z 1500 KVA 480/277V 2A 2 % Z 39,666 A 99,165 A Short Circuit Current Example #1 Answers 1500 KVA 208/120V 3 5% Z 1500 KVA 208/120V 3A 2 % Z 91,608 A 229,020 A

26 © 2015 Eaton. All Rights Reserved.. 208/120V 4 4A 6 6A 480/277V 5 5A 480/277V 40,000 A. 50 ft # 150 ft 250 kcm Short Circuit Current Example #2

27 © 2015 Eaton. All Rights Reserved.. Calculating Short Circuit Currents Utilize Point-to-Point Method Steps 4, 5 & 6 for 3  Faults

28 © 2015 Eaton. All Rights Reserved.. Short Circuit Current Example #2 Answers 208/120V 4 4A 40,000 A. 50 ft # 1 12,367 A. 480/277V 5 5A 40,000 A. 20,322 A. 6 6A 480/277V 40,000 A. 50 ft 250 kcm 28,818 A.

29 © 2015 Eaton. All Rights Reserved.. 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

30 © 2015 Eaton. All Rights Reserved..