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Recent content by cherryg

  1. cherryg

    Disconnect sizing question

    The value of 65kA appears to be grossly overestimated. Typically to derive 208/120V, you will need a small distribution Transformer. Lets say the Transformer is 500kVA (we need to know this rating) and the % impedance of this Transformer is 5. The calculated nominal short ckt value is 27kA and...
  2. cherryg

    Step-up transformer = Star-grounded LV / Star-grounded HV. How to calculate EPR?

    In TN-C-S system, the earth fault path is through the least impedance path i.e the PEN conductor back to the source. So, for the solidly earthed Generators (each contributing 6.67kA; calculated % reactance = 10.43%), the earth fault current will not enter earth and contribution to GPR is...
  3. cherryg

    VFD increasing the motor winding temperature

    You can specify more expensive VFD with restrictions on the THD on the load side. You need to discuss with the VFD vendor and read the manual. Load side filters are available for a price.
  4. cherryg

    Air India 787 crashes on take off

    One grim fact with the take-off is even after using the 3km approx. runway, the plane just managed to take off. The flying dust in the video, shows how close this plane was in reaching dirt.
  5. cherryg

    NEC Temperature Class

    There is an assumption that when liquid escapes, it passes on to vapor phase. What is the flash point of this liquid? Will it remain in liquid phase or transition to vapor phase?
  6. cherryg

    LV MCCB at Incomer / Short circuit withstand time

    I have two questions for che12345: a. If the Incomer breaker fails to trip and the upstream Swgr breaker trips, the let through energy still far exceeds the Incomer MCCB short time withstand rating. So how is this fine? b. For Incomer MCCB to cooridnate with the feeder breakers, the feeder...
  7. cherryg

    LV MCCB at Incomer / Short circuit withstand time

    This question is specific to a MCC Incomer where ABB MCCB is being used rather than an ACB. The MCCB short ckt interruption capcity is 70kA at 380V. But its short time withstand rating is only 7.3kA at 1 sec. Now the prospective short circuit current at the MCC bus is 62kA. If the Manin Incomer...
  8. cherryg

    Voltage drop for Motor Feeder

    Hello Waross I have the 2021 version of CEC which covers in Rule 8-102, the requirements of voltage drop. In Rule 8-102 (2) the Code recognizes the need to size the feeder or branch circuit as per the actual load seen by the circuit (and not the nameplate rating): "Notwithstanding Subrule 1)...
  9. cherryg

    Voltage drop for Motor Feeder

    Hello Bill... Yes you are correct. But CEC unlike NEC also allows variances and a qualified engineer can use the shaft kW value to reduce the load amps to actual value, and show that the permissible voltage drop is maintained. This as I said above is one of a case, not a general practice.
  10. cherryg

    Voltage drop for Motor Feeder

    I have a simple question. Does any Code or Standard or any guideline ever specify that voltage drop of motors needs to take into consideration: a. Full load Amps (Nameplate rating) and b. Resistance of the conductor needs to be at the max temp of its insulation i.e 90degC? . The reason I ask...
  11. cherryg

    Transformer Secondary Side - Bus sizing & Breaker sizing (NEC)

    Thanks xnuke & dpc. The main reason for this query is: At a later point, say 10 years down the line, when we add loads, an adequacy check is always done. This check is only on the Transformer capacity and not on the downstream bus / breaker. If we choose lower than the 125% rated value of the...
  12. cherryg

    Hazardous Area Classification (HAC) for Pressure Relief Valve

    Grades of release as defined in API RP 505 is: Continuous : 1000 or more hours/year Primary: 10 hours/year to 1000 hours/year Secondary: less than 10 hours/year In your case it appears to be an engineering judgement between Primary & Secondary. This relates directly to Zone 1 or 2. Even if you...
  13. cherryg

    Transformer Secondary Side - Bus sizing & Breaker sizing (NEC)

    I believe that NEC Article 450.3 Overcurrent Protection (and Table 450.3) directly determines the secondary side breaker sizing (and bus sizing). For example, if I have a 2500kVA, 13.8-0.48kV Transformer, the rated secondary side current is 3007A. As per Table 450.3, for 480V secondary (<1000V)...
  14. cherryg

    Need for EGC in High Impedance Grounded System - NEC design

    Once again posting this question for NEC based power distributiin design: Is it required for NEC based designs to use Equipment Grounding Conductor? Article 250.36 High Impedance Grounded Neutral systems does not mention it (see A to G). Similarly, later in Part VII Methods of EGC Connection...
  15. cherryg

    Help in Calculation

    Considering the Ampacities as given in IEC 60364-5-52, and the derating factors given in this std, you are still good with a 4 sq.mm cable, based on ampacity. Method D - laid in ducts, Method D2 - laid in ground and Method E - laid in tray give ampacities 35A,39A and 42A reply. You may take 6...
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