Hi-
Check your knowledge, help others, and tell us if something has been miscalculated, misinterpreted, forgotten, or if unnecessary steps have been added:
Situation:
You?ve been requested to wire up a non-continuous duty 50HP motor used for a hydraulic elevator pump unit to a wye-delta (soft)starter. The starter, which includes a Class 10 Motor Overload, will be set to limit starting current to 3X FLA, and to trip at 1X FLA during run (Delta) mode.
- Line voltage is 208V 3phase (+15%, -10%).
- OCPD will be a standard (inverse-time) breaker.
- All conductors, shall be THHN &be installed in a combo of FMC (Greenfield) / EMT / LFNC conduit.
- Ambient = 40degC.
Proposed Solution: (All refs to NEC)
Per Table 430.150, Motor FLA = 143A.
Sizing the Motor Conductors:
1. Because the load >100A, per 110.14(C)(1) one can reasonably assume the use of terminals and devices rated for 75degC, allowing the wire ampacity to be also rated for 75degC.
2. Per Table 310.16, one must de-rate the conductor ampacity to 0.88 for 75degC-rated wire used at 40degC.
3. Per Table 310.15(B)(2)(a), one must further de-rate the conductor ampacity to 0.8 because the wye-delta starter will require 6 current-carrying conductors in one conduit to the motor (not 3).
4. Because of the use of a wye-delta starter, the individual conductor ampacity can be increased by 1/0.58 per 430.22(C).
5. Because of the use of a non-continuous duty cycle motor in an intermittent (elevator) application, per Table 430.22 (E), the ampacity can be further increased by 1/0.9. Therefore, the required motor conductor ampacity should be greater than (143 x 0.58 x 0.9 x 1.25) / (0.88 x 0.8) = 133A, requiring the use of a total of six 1/0 THHN copper conductors (or larger) per Table 310.16.
Sizing the OCPD & Equipment Grounding Conductor:
6. Per Table 430.52, we should be able to use an inverse-time circuit breaker rated for a maximum of 2.5 x 143A = 358A, and rounding to the next higher value, per 240.6 this would be 400A. (BTW- The minimum breaker size would be 1.15 x 143 = 165A, allowing the use a 175A breaker; however, it would likely become a nuisance-tripper; let?s size the breaker at maximum)
7. Per the opening paragraph of 250.122, the EGC is not required to be larger than the motor conductors.
8. Per Table 250.122, for a 400A OCPD, we should be able to use one 3AWG copper EGC (or larger).
Sizing the Motor Conductor Conduit:
10. There are six 1/0 (OD=0.486 in) and one 3AWG THHN wires in the EMT. Therefore, the total area required by these conductors per Chapter 9 Table 5 is: (6 x 0.1855) + (1 x 0.0973) = 1.2103 sq. In.
Per Chapter 9 Table 1, the conductors can utilize no more than 40% of the EMT internal cross-sectional area. Therefore, the EMT must have an internal cross-sectional area of greater than 1.2103/0.4 = 3.026 sq in. Per Chapter 9 Table 4, 2 inch EMT will meet this requirement at 3.356 sq. in. area (36% fill).
11. The conduit jam ratio = 3.356/0.486 = 6.9, and since it is not within the range of 2.8 to 3.2, no conductor jamming issues are anticipated.
How would you do if this was a question on an exam? Or better yet, a real-life situation?
-Jeff
Check your knowledge, help others, and tell us if something has been miscalculated, misinterpreted, forgotten, or if unnecessary steps have been added:
Situation:
You?ve been requested to wire up a non-continuous duty 50HP motor used for a hydraulic elevator pump unit to a wye-delta (soft)starter. The starter, which includes a Class 10 Motor Overload, will be set to limit starting current to 3X FLA, and to trip at 1X FLA during run (Delta) mode.
- Line voltage is 208V 3phase (+15%, -10%).
- OCPD will be a standard (inverse-time) breaker.
- All conductors, shall be THHN &be installed in a combo of FMC (Greenfield) / EMT / LFNC conduit.
- Ambient = 40degC.
Proposed Solution: (All refs to NEC)
Per Table 430.150, Motor FLA = 143A.
Sizing the Motor Conductors:
1. Because the load >100A, per 110.14(C)(1) one can reasonably assume the use of terminals and devices rated for 75degC, allowing the wire ampacity to be also rated for 75degC.
2. Per Table 310.16, one must de-rate the conductor ampacity to 0.88 for 75degC-rated wire used at 40degC.
3. Per Table 310.15(B)(2)(a), one must further de-rate the conductor ampacity to 0.8 because the wye-delta starter will require 6 current-carrying conductors in one conduit to the motor (not 3).
4. Because of the use of a wye-delta starter, the individual conductor ampacity can be increased by 1/0.58 per 430.22(C).
5. Because of the use of a non-continuous duty cycle motor in an intermittent (elevator) application, per Table 430.22 (E), the ampacity can be further increased by 1/0.9. Therefore, the required motor conductor ampacity should be greater than (143 x 0.58 x 0.9 x 1.25) / (0.88 x 0.8) = 133A, requiring the use of a total of six 1/0 THHN copper conductors (or larger) per Table 310.16.
Sizing the OCPD & Equipment Grounding Conductor:
6. Per Table 430.52, we should be able to use an inverse-time circuit breaker rated for a maximum of 2.5 x 143A = 358A, and rounding to the next higher value, per 240.6 this would be 400A. (BTW- The minimum breaker size would be 1.15 x 143 = 165A, allowing the use a 175A breaker; however, it would likely become a nuisance-tripper; let?s size the breaker at maximum)
7. Per the opening paragraph of 250.122, the EGC is not required to be larger than the motor conductors.
8. Per Table 250.122, for a 400A OCPD, we should be able to use one 3AWG copper EGC (or larger).
Sizing the Motor Conductor Conduit:
10. There are six 1/0 (OD=0.486 in) and one 3AWG THHN wires in the EMT. Therefore, the total area required by these conductors per Chapter 9 Table 5 is: (6 x 0.1855) + (1 x 0.0973) = 1.2103 sq. In.
Per Chapter 9 Table 1, the conductors can utilize no more than 40% of the EMT internal cross-sectional area. Therefore, the EMT must have an internal cross-sectional area of greater than 1.2103/0.4 = 3.026 sq in. Per Chapter 9 Table 4, 2 inch EMT will meet this requirement at 3.356 sq. in. area (36% fill).
11. The conduit jam ratio = 3.356/0.486 = 6.9, and since it is not within the range of 2.8 to 3.2, no conductor jamming issues are anticipated.
How would you do if this was a question on an exam? Or better yet, a real-life situation?
-Jeff