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  1. cherryg222

    Code Query - Underground ducts (RGS-metallic)

    We know that for underground ducts (in concrete), with arrangement as per Fig.310.60(C)(3), with MV cables, we use the Tables 310.60(C)(77)till (80).However, as detailed in Handbook, the term "electrical ducts" refers to nonmetallic conduits. Does this mean these tables pertain to non-metallic...
  2. cherryg222

    NEC Ampacity - Code Interpretation

    After listening to a presentation on ampacities and reading the details of Art. 310.15(B)(3), I am back to square 1. The Article says.........."where number of ......., or where single conductors or multicoductor cables are installed without maintaining spacing for a continuous length longer...
  3. cherryg222

    NEC Ampacity - Code Interpretation

    In Art.392.80(1)(a),it has been indicated that...."....Adjustment factors shall be limited to the number of current carrying conductors in the cable and not to number of conductors in the cable tray". How is this statement to be interpreted? If I have 5 runs of 3C - #4 cables in a tray in random...
  4. cherryg222

    NEC Ampacity - Code Interpretation

    So the statement, 2.1) the lowest temperature rating of the termination points is 75 degrees C (or 60/75 degrees C), the conductor's ampacity can be selected from the 75 degrees C column and 90deg C ampacity to be considered for derating by David; are these self contradicting?
  5. cherryg222

    NEC Ampacity - Code Interpretation

    7anoter4..Thank you for the elaboration. This is the application which I was expecting, where trefoil arrangement is followed with the required separation. I wish they had a cross reference to Sec 392.80 on this particular table. Now, if we do not maintain this separation but go in...
  6. cherryg222

    NEC Ampacity - Code Interpretation

    thx David. I also wanted to know under what circumstances Table 310.15 (B)(20) can be used. For example, can it be used for tray laying? Again, three Single Insulated Conductors refers to triplexed cables? Does a trefoil on tray qualify for these ampacities?
  7. cherryg222

    NEC Ampacity - Code Interpretation

    David...That's interesting. For.eg for 250kcmil cable, the listed ampacities at 75 and 90degC are 255 & 290A. What you are saying is the ampacity will be 290A x d OR 250A whichever is lower. Let us say the derating factor is 0.85. Then 290 x 0.85 = 246.5A. Since 246.5A < 250A, hence 246.5 will...
  8. cherryg222

    NEC Ampacity - Code Interpretation

    Recently read an article in EC&M from 2007 which said: When correcting conductor ampacity for elevated ambient temperature, the correction factor used for THHN conductors is based on the 90°C rating of the conductor, based on the conductor ampacity listed in the 90°C column of Table 310.16...
  9. cherryg222

    LV MCC Incomer sizing - 100% rated breaker

    Jraef...My concern is upto 2129A only. Any current carrying part with any deration should be able to meet that amps. If the breaker is 100% rated in it's enclosure, and the bus bars are rated at 2500A and certified for it, everything will fall into place. We know that the design is 2x100% which...
  10. cherryg222

    LV MCC Incomer sizing - 100% rated breaker

    Thanks all for your feedback. We are following the 75degC ampacities. Along with a 100% rated breaker and enclosure, we will be able to meet the requirement. I still had the question lurking for the bus of 2500A. I am guessing that the bus requires no derating and is good for continuous 2500A...
  11. cherryg222

    LV MCC Incomer sizing - 100% rated breaker

    We have a situation where the transformer (dry type) is rated at 1500/1770kVA (85/115degC). The secondary side current @ 480V for 1770kVA is 2129A. As per NEC Article 450.3(A), the secondary side OCPD needs to be sized at 125% of FLC i.e 2661A. If we choose to use 100% rated breaker (UL 1066 for...
  12. cherryg222

    Savitzky-Golay filter

    Doug. Thank you for this. I hope it helps other users.
  13. cherryg222

    Savitzky-Golay filter

    Recently I came across the versatile Savitzky-Golay filter on a different forum. The creator for the code Conrad Hoffman has kindly shared the code, as below. This filter is very useful for smoothing data which is voluminous and has too many spikes. Try it. Sub SG_five() '5 Point...
  14. cherryg222

    LV ungrounded system - HVAC power supply

    Thank you all for your valuable comments. Indeed, some old practices are in play here. I have used 5A HRG for 480V system, with gf detection both on Transformer neutral as well as on individual feeders. Usually, the neutral gf detector provides an annunciation for action. Once technician arrives...
  15. cherryg222

    LV ungrounded system - HVAC power supply

    We have a project in Mexico, where the client has insisted on ungrounded (delta-delta connection of Transformer) system for Low voltage (480V). From the design basis, it is clear that the purpose is to maintain power to process loads, which cannot be shut down without proper supervision and...
  16. cherryg222

    Derating of IEC 62271-100 HV Breakers

    Just to add to this post. The exercise of determining a derating factor actually got changed to determining if a certain breaker with a test certificate (T100a and three different time constants; 45ms, 90ms and 135ms) is suitable to the system condition. Was successful in generating the actual...
  17. cherryg222

    Derating of IEC 62271-100 HV Breakers

    cuky2000..Thanks for providing the graphical derating factor chart. I had a question. The derating factor is definitely taking care of the asymmetrical current. Does it take care of the energy carried by the parting contacts also? CIGRE Tech Brochure 304 says: The arc energy represented by the...
  18. cherryg222

    Derating of IEC 62271-100 HV Breakers

    How may of you have actively derated breakers from their test certificate (IEC 62271-100 HV breakers)which are tested at standard time constant of 45 ms? What method did you employ? Did any of you use the CIGRE Technical Brochure No.304 OR the ENA Engineering Recommendation G89 (2011)? If you...
  19. cherryg222

    Oil filled Transformer Ambient Derating- ANSI C57.91

    magoo2 I understand what you are saying. The approximation can be followed but later validated by Transformer vendor. If the derating is within a reasonable margin of 10%. it can easily be accommodated. If the derating factor is more than 15% +10% of 15% = 16.5% then it means that we may have...
  20. cherryg222

    Oil filled Transformer Ambient Derating- ANSI C57.91

    magoo2...Isn't that precisely the reason that ANSI/IEEE is derating the transformer at the rate of 1% per 1degC rise above 30degC reference (for OA/FA transformers)? Here for 15degC rise, the derating will be 15%. 60/80MVA @ 30degC ONAN/ONAF will be derated to 51/68MVA @ 45degC.

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