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  • Users: eeprom
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  1. eeprom

    Inexplicable motor failures

    Hi, I have two pumps, both 500 Hp, running on VFDs or across the line in bypass mode. Both motors have a history of failing windings. Both motors are inverter rated. There are no RTDs on either. Both motors are within 150ft of their VFDs, and neither motor is using VFD cabling. Both motors...
  2. eeprom

    Synchronizing solar,wind, and grid

    Hello, I'm trying to figure out how to tie solar panels and a wind turbine to a building with batteries, but also have these wired so that they are providing power whenever possible. They will provide power if the utility grid is active or not. My concern is synchronization. If the grid is...
  3. eeprom

    Feeder sizing conflict in code

    Hello, In article 215.2(A) the code says a feeder has to be size by either (1) 125% of the continuous load plus the non-continuous load OR (2) ampacity adjustment after load correction factors. This implies that if I have a commercial installation and after adjustments I end up with 310A, that...
  4. eeprom

    Seeking a sane explanation for NEC 110.14(C)

    All, I want to first point out that I understand the rules about not using conductors that are higher rated than the terminal lugs or other circuit devices. My question is not about applying the rule. It is in regards to why this rule exists as it does. It would be logical to say that if I...
  5. eeprom

    Calculation of air flow through furnace grate

    All, I'm trying to calculate the flow of air into a furnace. The flow of air goes through a grate that comes from a large duct and discharges into the furnace through very small holes. I know the size of the grate holes, and the air pressures on the upstream and downstream sides of the hole...
  6. eeprom

    Effect of imbalanced currents

    Hello, and Merry Christmas, I am working with another engineer on a small project on which we have come to a disagreement about imbalanced currents. Specifically, what we have is a 500 kVA three phase transformer with wye secondary, 480/277V. There is one three-phase 250 Hp motor, and one...
  7. eeprom

    Looking for high temperature psychometric chart

    Hello, I'm looking for a psychometric chart with a dry bulb temperature that goes to 500F, in imperial units. I've done numerous online searches. Most go to 120F. So far the highest temperature range I found was 300F. Any help would be appreciated.
  8. eeprom

    86 relay current draw?

    Hello, I'm troubleshooting a failed trip circuit for an 86 lockout relay. This is a GEI-28712E, similar to an Electroswitch. The 86 coil is tripped by one of several relays. The contacts for one of the relays has been welded closed, and I'm trying to figure out how that could have happened...
  9. eeprom

    Easy question

    Hello all, I'm an electrical engineer, not an HVAC person. And this question might be too easy for this forum. I wouldn't waste your time here, but it's about impossible to get a manual for a home furnace. And if you call the manufacturer, they won't give you tech support unless you are a...
  10. eeprom

    Conduit stub ups in concrete?

    I have been asked why I don't call out on my drawings details of concrete encasement for conduit stub ups. This particular situation is for three 2" conduits, rigid steel, coming out of the ground and going into a junction box mounted to the side of the building. The cables in the conduits are...
  11. eeprom

    Input cables for VFDs - sizing the EGC

    Hello, I am working with an Eaton SPX 300 Hp drive. The drive manual recommends that for incoming power cables, phase conductors should be 2 @ 300 kcmil, and the ground conductor should be 1 @ 300 kcmil. The question I have is about the ground. The upstream breaker is 500A. According to NEC...
  12. eeprom

    Cable sizing for VFD application

    Hello, I am working on a 300 HP VFD application, running a pump. The breaker upstream of the VFD is 500A. The drive is rated at 385A. The question I have is about cable sizing. My motor nameplate is 330A, and following NEC I can use 2 runs of 4/0, with #2 for a ground wire. If I use 90C...
  13. eeprom

    Single Phase Arc Flash

    Hello, I have been asked to do an arc flash study on a small single phase system, a 75 kVA transformer, 1 phase, 240VAC. ETAP uses three phase for arc flash. If I do the study using 3 phase, the fault currents will be smaller, because a single phase 75kVA will produce greater fault currents...
  14. eeprom

    Resistance of a sphere

    This is not a very practical question, but one I have thought about quite a bit. Just curious. If I took a copper sphere and measured the resistance across the sphere, I would get some value of resistance. If I tripled the size of the sphere, how would the resistance change? EE
  15. eeprom

    Frequent failing of RTDs in stator windings

    Hello, I am investigating a series of RTD failures in stator windings. All the RTDs are 100 ohm PT, and the motors are mostly 250 to 300 Hp. All RTDs are wired to multilin 469s. From what I have checked, the wiring is correct. All RTD failures were checked by techs. They are open circuits...
  16. eeprom

    Underground conduit requirements

    Hello, I have been debating with a fellow engineer the need to use a concrete encased duct bank, as opposed to just running PCV, packed in dirt, and covered with a 2" layer of concrete. A duct bank reduces conductor ampacity and costs a lot of money to build. I don't think it's necessary...
  17. eeprom

    VFD in Class II Div 1 Area

    Hello, I have a design for which it has been requested that the VFD be placed near the equipment. The equipment area is class II, division 1, grain dust. NEC only says that the enclosures have to be dust tight, which I can achieve with a Nema 12 panel and filtering fans (also rated for 12)...
  18. eeprom

    Book on centrifugal pump control

    Hello, I am looking for a book(s) which may explain the different methods of centrifugal pump control, especially with respect to using a variable frequency drive. I have The Pump Handbook, and it makes no significant mention of this topic. I work with lots of systems which have a pump on a...
  19. eeprom

    Darcy Weisbach equation - units?

    All, If I use the Darcy Weisbach equation for head loss in a pipe, the resulting units are feet. I have always assumed this to be feet of water. But if I am calculating head loss on a fluid with an SG of 0.8, is the head loss still in feet of water? Or is in feet of water/0.8? Thanks EE
  20. eeprom

    Arc Flash Hazard Assessment on SWGR CB

    Hello. I am trying to assess whether or not it is safe to operate switchgear with possibly very high incident energies. NFPA 70e table 130.7(c)(15)(A)(a) says that normal operation of a circuit breaker or switch does not require arc flash PPE. My question is, what if the calculated incident...

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