1. Ampacity Calculations and Overcurrent Protection 1
© Ted “Smitty” Smith
E Light Electric Services, Inc.

2. The first several slides of this presentation are a review of calculations. Read them over and take some notes as they will help you on your journeyman’s exam as well as while doing this drill.
Click the slide to advance to the next slide as you are ready.
You will reach a slide which says “START”. This is the beginning of the timed portion of the drill. Click that slide and the questions slides will start running. Each slide will remain in place for 3-5 minutes depending on the difficulty of the calculation. A sound will go off letting you that it has moved to the next slide. Attempt to answer the question. If you finish before the timer goes off, click the slide and it will go to the next question. If you do not finish, simply stop working that problem and start on the next. When you reach the end of the timed portion you will see a slide that says “STOP”. Click on that slide and each of the questions will reappear and this time you can click them to move to the next slide, which will have the answer.
Please call me if you have any issues or questions and I will be happy to arrange a time for us to review.
Directions

3. TAP RULES: Transformer Secondary
Conductors not longer than 10’
Ampacity not less than the combined calculated loads supplied
Ampacity not less than the rating of the equipment of OCPD supplied
Ampacity must be at least 1/10th the current rating of the OCPD protecting the feeder conductors multiplied by the Primary to Secondary Voltage Ratio.
Tap conductors do not extend beyond the equipment they supply
Enclosed in a raceway
25’ Tap Rule
Ampacity of the tap conductors must be at least 1/3 the rating of the OCPD protecting the feeder conductors multiplied by the primary to secondary voltage ratio
Conductors must terminate in a single OCPD
Conductors enclosed in a raceway or other approved manner

4. Primary to Secondary Voltage Ratio
This is not the turns ratio
This is the ratio of the Line voltage of the primary to the line voltage of the secondary
For example
A typical 408V to 208V three phase transformer is actually a 4:1 turns ratio transformer.
The primary to secondary voltage ratio is 480/208= 2.308
The voltage ratio therefore is 2.308:1

5. 10 Foot Tap Rule Example Situation
A three phase transformer feeds a 200A panel rated at 208V.
The transformer is fed from a 60A, 480V inverse time circuit breaker.
What is the minimum sized secondary conductor allowed for the secondary conductors using THHN if they are no longer 10 feet in length?
All equipment is rated at 90 degrees C.
A 3/0 THHN has an ampacity of 225 A and meets the first rule of having an ampacity of at least the calculated load and the OCPD rating the conductors are feeding. (200A)
The voltage ratio here is 480/208= 2.308:1
The OCPD protecting the feeder conductors is 60 A
That rating times the voltage ratio is 60 X 2.308=
1/10 of is 13.8 amperes and that is far less than the 225 A ampacity of the THHN.
Another way to look at it is that 225A is not greater than three times the

6. 25 Foot Tap Rule Example Situation
A three phase transformer feeds a 200A panel rated at 208V.
The transformer is fed from a 60A, 480V inverse time circuit breaker.
What is the minimum sized secondary conductor allowed for the secondary conductors using THHN if they are no longer 25 feet in length?
All equipment is rated at 75 degrees C.
We will have to use the 75 C column of T310.15(B)(16) here due to the requirement of (C) and the rating of the equipment being 75 C. So a 4/0 THHN will be required.
The actual ampacity of a 4/0 THHN is 260A
The voltage ratio here is 480/208= 2.308:1
The OCPD protecting the feeder conductors is 60 A
That rating times the voltage ratio is 60 X 2.308=
1/3 of is amperes and that is far less than the 260 A ampacity of the THHN.
Another way to look at it is that 260A is not greater than 3 times the

7. It is always marked on the OCPD unless the OCPD has a “default” AIC.
AIC Ratings
This is the amount of current an OCPD can take without physically exploding
This is not the current rating. The current rating is the amount of current that will cause the OCPD to trip.
It is always marked on the OCPD unless the OCPD has a “default” AIC.
FUSES: If an AIC is not marked on a fuse it has an AIC of 10,000 Amperes [240.60(C)(3)]
Circuit Breakers: If an AIC is not marked on a circuit breaker it has an AIC of 5,000A. [240.83(C)]
Default Ratings

8. Click here to start the timed portion of the Drill

9. What is the ampacity of a THHN aluminum conductor, size 250 kCMIL, installed in an ambient temperature of 104 degrees F?

10. 209.3 A
T310.15(B)(16)
T310.15(B)(2)(a)

11. What is the ampacity of a 10 AWG THWN conductor installed with a total of 9 current carrying conductors in an ambient temperature of 73 degrees F?

12. 25.73 Amperes
T310.15(B)(16)
T310.15(B)(2)(a)
T310.15(B)(3)(a)

13. What is the minimum sized THWN conductor which can be used to feed a 100 Ampere 208V three phase panel with a 100A main breaker if the panel is fed from a three phase 480V:208V transformer. The secondary conductors will not have overcurrent protection. The secondary conductors will be 8 feet in length and they will leave the transformer enclosure. The feeder conductors for the transformer are protected by a 60A inverse time circuit breaker? Assume everything has a temperature rating of 75 degrees C.

14. 3 AWG
240.21(C)
T310.15(B)(16)

15. Copyright 2005 Ted "Smitty" Smith
What is the maximum sized inverse time circuit breaker that can be used to protect the 10 AWG THHN conductors which are feeding a load rated at 22 Amperes which is considered to be continuous duty?
Copyright 2005 Ted "Smitty" Smith

16. 30 A
240.4

17. Copyright 2005 Ted "Smitty" Smith
What is the minimum sized circuit breaker(s) required for a range cooking unit in a single family dwelling that has a nameplate rating of 29.2 kW?
Copyright 2005 Ted "Smitty" Smith

18. Power supply would need to be divided into two circuits of 240V each
Power supply would need to be divided into two circuits of 240V each. It would require (2) 35A, two pole circuit breakers, one for each circuit.
T W x 1.85=14,800 W of demand
14,800/240=61.66 Amps
422.11(B) requires the ranges with over 60A of demand be fed with two circuits and each circuit have an OCPD of not greater than 50A.

19. What is the minimum sized THW Aluminum conductor which can be used to feed a 23 ampere continuous load that is located in an ambient temperature of 33 degrees C. The branch circuit will be installed in a raceway with a total of 11 current carrying conductors?

20. 4 AWG
23 amperes continuous= 23 x 1.25= 28.75
33 degrees Celsius for a 75C conductor is .94= 28.75/.94=30.585
11 current carrying conductors is .50= /.50=61.17 A

21. What is the minimum TBS aluminum conductor which can be used to feed a 73 ampere load if the conductors are installed in an ambient temperature of 63 degrees F and they are installed in a raceway with a total of 13 current carrying conductors?

22. 2/0
T310.15(B)(16)
T310.15(B)(2)(a)
T310.15(B)(3)(a)

23. What is the ampacity of each 12 AWG THHN conductors in a set of 12/3 MC cables if there are a total of eight MC cables and all of the cables are installed in a bundle on bridle rings above a drop ceiling? The conductors are installed in ambient temperature of 86 degrees F.

24. 18 amperes
T310.15(B)(16)
310.15(B)(3)(4)Exception to (4)
30 x .60= 18

25. Copyright 2008 Ted "Smitty" Smith
What is the ampacity of a 3/0 THWN-2 copper conductor that is installed in a ladder type cable tray that is 14” wide? The conductor is installed with a total of 8 current carrying conductors. The conductors are installed in a single layer and are separated from each other so that at least one cable diameter of space is between each cable. The cable tray does not have a cover.
Copyright 2008 Ted "Smitty" Smith

26. 350 Amperes
T310.15(B)(17)
392.80(A)(2)(c)

27. An industrial X-ray machine has a momentary rating of 55 amperes amperes and long time rating of 38 amperes. What minimum sized inverse time circuit breaker can be used to protect the x-ray machine?

28. 40 Amperes
660.6(A)
50% of the momentary rating= 27.5 A
100% of the long time rating= 38 A
Can not be less the larger of the above. The next size above 38 is 40 A.

29. A PV DC circuit has a maxuimum calculated load of 32 amperes
A PV DC circuit has a maxuimum calculated load of 32 amperes. What is the minimum sized overcurrent protection required for this circuit?

30. 40 A
690.9(B)(1)
240.6

31. CLICK HERE TO PROCEED WITHOUT THE TIMER AND REVIEW THE QUESTIONS AND ANSWERS

32. What is the ampacity of a THHN aluminum conductor, size 250 kCMIL, installed in an ambient temperature of 104 degrees F?

33. 209.3 A
T310.15(B)(16)
T310.15(B)(2)(a)

34. What is the ampacity of a 10 AWG THWN conductor installed with a total of 9 current carrying conductors in an ambient temperature of 73 degrees F?

35. 25.73 Amperes
T310.15(B)(16)
T310.15(B)(2)(a)
T310.15(B)(3)(a)

36. What is the minimum sized THWN conductor which can be used to feed a 100 Ampere 208V three phase panel with a 100A main breaker if the panel is fed from a three phase 480V:208V transformer. The secondary conductors will not have overcurrent protection. The secondary conductors will be 8 feet in length and they will leave the transformer enclosure. The feeder conductors for the transformer are protected by a 60A inverse time circuit breaker? Assume everything has a temperature rating of 75 degrees C.

37. 3 AWG
240.21(C)
T310.15(B)(16)

38. Copyright 2005 Ted "Smitty" Smith
What is the maximum sized inverse time circuit breaker that can be used to protect the 10 AWG THHN conductors which are feeding a load rated at 22 Amperes which is considered to be continuous duty?
Copyright 2005 Ted "Smitty" Smith

39. 30 A
240.4

40. Copyright 2005 Ted "Smitty" Smith
What is the minimum sized circuit breaker(s) required for a range cooking unit in a single family dwelling that has a nameplate rating of 29.2 kW?
Copyright 2005 Ted "Smitty" Smith

41. Power supply would need to be divided into two circuits of 240V each
Power supply would need to be divided into two circuits of 240V each. It would require (2) 35A, two pole circuit breakers, one for each circuit.
T W x 1.85=14,800 W of demand
14,800/240=61.66 Amps
422.11(B) requires the ranges with over 60A of demand be fed with two circuits and each circuit have an OCPD of not greater than 50A.

42. What is the minimum sized THW Aluminum conductor which can be used to feed a 23 ampere continuous load that is located in an ambient temperature of 33 degrees C. The branch circuit will be installed in a raceway with a total of 11 current carrying conductors?

43. 4 AWG
23 amperes continuous= 23 x 1.25= 28.75
33 degrees Celsius for a 75C conductor is .94= 28.75/.94=30.585
11 current carrying conductors is .50= /.50=61.17 A

44. What is the minimum TBS aluminum conductor which can be used to feed a 73 ampere load if the conductors are installed in an ambient temperature of 63 degrees F and they are installed in a raceway with a total of 13 current carrying conductors?

45. 2/0
T310.15(B)(16)
T310.15(B)(2)(a)
T310.15(B)(3)(a)

46. What is the ampacity of each 12 AWG THHN conductors in a set of 12/3 MC cables if there are a total of eight MC cables and all of the cables are installed in a bundle on bridle rings above a drop ceiling? The conductors are installed in ambient temperature of 86 degrees F.

47. 18 amperes
T310.15(B)(16)
310.15(B)(3)(4)Exception to (4)
30 x .60= 18

48. Copyright 2008 Ted "Smitty" Smith
What is the ampacity of a 3/0 THWN-2 copper conductor that is installed in a ladder type cable tray that is 14” wide? The conductor is installed with a total of 8 current carrying conductors. The conductors are installed in a single layer and are separated from each other so that at least one cable diameter of space is between each cable. The cable tray does not have a cover.
Copyright 2008 Ted "Smitty" Smith

49. 350 Amperes
T310.15(B)(17)
392.80(A)(2)(c)

50. An industrial X-ray machine has a momentary rating of 55 amperes amperes and long time rating of 38 amperes. What minimum sized inverse time circuit breaker can be used to protect the x-ray machine?

51. 40 Amperes
660.6(A)
50% of the momentary rating= 27.5 A
100% of the long time rating= 38 A
Can not be less the larger of the above. The next size above 38 is 40 A.

52. A PV DC circuit has a maxuimum calculated load of 32 amperes
A PV DC circuit has a maxuimum calculated load of 32 amperes. What is the minimum sized overcurrent protection required for this circuit?

53. 40 A
690.9(B)(1)
240.6