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  1. JensenDrive

    MCC Bus Temperature based on 105C

    I noticed recently that Eaton sent in some submittals for 480V MCCs with bus where they say ampacity is based on 105C max operating temperature in a 40C ambient. Then they connect circuit breakers directly to the bus, and the circuit breakers have terminal temperatures limited to 75C. The also...
  2. JensenDrive

    90C Lugs for MCCBs - Why?

    Why does Eaton offer to sell me 90C lugs on standard 75C MCCBs? From what I can tell, the higher lug temperature rating does not allow me to use 90C ratings of cable since the lug passes cable temperature directly to the 75C rated breaker; the lug has no insulation effect and I still have to use...
  3. JensenDrive

    DTT for small distribution generator

    I have a case of a small distribution generator that is sized larger then the minimum load on the line, so the generator has the ability to carry line load if the utility trips. The utility is saying a direct transfer trip (DTT) scheme will be needed to ensure the gen trips if the utility trips...
  4. JensenDrive

    Generator and Motors in TCC

    If you have a TCC, first run with motor contribution and generator impedance set at Xd", and the TCC says the trip will take over maybe 0.5 seconds to clear, the TCC seems a bit misleading. Fault current is not as high as the TCC implies by the time the elements operate. Do you rerun the study...
  5. JensenDrive

    Wikipedia, Electric Power System

    I have been browsing Wikipedia. I am impressed at how much good stuff it has on electric power systems. The site might be worth referring people to that have basic questions on how power systems work. http://en.wikipedia.org/wiki/Three-phase http://en.wikipedia.org/wiki/Three_phase_power...
  6. JensenDrive

    Why is circuit breaker ground fault max delay = 0.5s?

    Why do most low voltage breaker ground overcurrent units (at least the ones I have dealt with) seem to have a maximimum delay setting of 0.5 seconds? The NEC 230.95.A says delay can be as long as 1 second @3000A. What piece of the story am I missing?
  7. JensenDrive

    NEC Ungrounded Systems

    NEC 250.36 covers high impedance grounding, and 250.36.a states that the ground reference has to be to a neutral. It says: "The grounding impedance shall be installed between the grounding electrode conductor and the system neutral. When a neutral is not available, the grounding impedance...
  8. JensenDrive

    Arc Flash at end loads

    NEC 110.16 says arc flash labeling is done at "switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers." I am not sure how to interpret "industrial control panels." Note 110.16 does not metion arc flash labeling the end load, but broadly...
  9. JensenDrive

    IEEE Red Book Cable Impedances, 600V vs 5kV nonshielded

    At the end of chapter 4 of the 1993 IEEE Red Book (Std 141) there are some typical cable impedances (the 1986 version does not seem to have the tables). For a given AWG or kcmil, the tables give the same reactance for 600V and 5kV nonshielded cablereactance, XL. I know the tables is a...
  10. JensenDrive

    Arc Flash - Where do you stop?

    How far down in the electrical system do you carry arc flash analysis? It seems one has to go to every load hanging off of a panel that an electrician might work on live. Carried to an extreme, that means that every 277V light fixture and every load downstream of a 480V distribution panel needs...

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