What's wrong (if anything) with these calcs?

[mr_rhino]

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

 

[peter]

Jeff,
You seem to be able to wade through this NEC swamp very well.
However I wonder about "because the wye-delta starter will require 6 current-carrying conductors in one conduit to the motor". Why would this require 6 conductors? Isn't there a set of relays which would transfer from delta to Y automatically?
~Peter

 

[Smart $]

Why would this require 6 conductors? Isn't there a set of relays which would transfer from delta to Y automatically?
~Peter

Three windings, 2 leads for each. The relays make the winding connections at the controller, therefore the leads must remain separated back to the controller.

 

[peter]

I'll buy that. But then only three of the conductors would be carryung current at one time. Nicht wahr?
~Peter

 

[don_resqcapt19]

Peter,
All 6 carry current. In the wye start mode one end of each winding is connected to the phase conductors and the other ends are shorted to make the wye. In the delta run mode both ends of each winding are connected to the phase conductors to make the delta.
Don

 

[bcorbin]

This is what is called a "consequent pole" motor, right?

 

[sceepe]

How would you do if this was a question on an exam? Or better yet, a real-life situation?

-Jeff

In all honesty, I would use some rules of thumb that would give me an answer in a lot less time but would end up with slightly larger conductors.

But in real life, you would never have an ambient temp of 40 deg c (104 deg f) in an elevator equipment room. They have to be conditioned.

And as long as we are talking about real life, the wiring from the starter (i.e. elevator controller) to the elevator pump is installed by the elevator equipment manufacturer. Elec contractor and the PE only have to provide power to the elevator controller. So the design is a lot more simple.

But you did a fine job flushing out all the details. I don't think many folks would pick up on the 6 wires and the conduit jam ratio. If you are trying to make a test question, I think you will stump a lot of folks. However, this would be a great question for the engineers at the elevator company.

 

[kingpb]

#1. I don't think the FLA in 430.150 applies because it is a special application motor, not general purpose.

#2. If FLA=143A, then derating for Y/D starter, and non-continuous motor (up to 60 min) is 143 x .58 x 0.9 = 74.7A, OK use 75A.

#3. Select #4 AWG from 75 deg C column good for 85A, derate for 40 deg C temp, 85 X 0.88 = 74.8A, OK 75A.

#4. Annex C allows for 7-#4AWG THHN in 1-1/4" EMT CND.

#5. Use an MCP in the combination starter, lookc at Time Coordination Curve and set it to trip for just over locked rotor current, (as stated in problem 3X, so set at 450A) to protect on start-up and in case of short circuit. Adjust overloads to match the starting curve of motor.

#6. Use a #4 AWG ground conductor.

Just one opinion. What an effort, I need a beer!

 

[sceepe]

I started to fall into this trap too. You multiply the allowable ampacity by the derating factors. Or you divide the FLA by the derating factors. Either way, you should end up with BIGGER wire after you derate. Your method gives you smaller wire; so something must be wrong.

I with you on the cold beer idea. Cheers....

 

[noxx]

Two quick questions, cause I don't have my book on my desk and I'm not going to go out to the car

1) Did you de-rate from native? i.e. when derating start with the 90c table if you're using THHN/THHWN.

2) where the heck did you get 1.15x on an inverse time breaker?

 

[kingpb]

The smaller wire is due to two considerations. First, the fact that due to the non-continuous load, the wire will disipate some of the heat developed in between usages. This causes the conductor to operate at a lower average temperature then the maximum allowed, which is the 75 deg C rating. A lower temperature, means a lower resistance, which in turn translates to higher current for a shorter time, before cable would be damaged. The values listed in the various tables throughout the NEC are based on continuous current, and thus, if you plot the cable on a curve of current vs. time you will see that cables can handle much larger amounts of current for smaller amounts of time. For example, a #4AWG rated for 85A at 75 Deg C, can actually handle approximately 800A for 10 secs, before damage would occur.

Second reason for smaller size conductors is due to the wye/delta starter. The current in each conductor during both operations is 1/sqrt3 or 0.58 x FLA (Line side). The conductors are carrying winding current only, remember there are 6 conductors.

You do have to derate for 6 current carrying conductos in a single conduit because all 6 are carrying current at the same time, which, I think I forgot to take into consideration in my earlier response, so using the 90 deg C column, derating for temp, and 6 conductors, I would have to use a #3 AWG not #4 AWG. (oops) Conduit size remains unchanged.

Now I need two beers!