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  1. with my local utilities, I have always used a...

    with my local utilities, I have always used a ball park of 1 ring =5kV to ground on systems 24.9kV L-L and higher. But there are usually too many variable for this to be used universally.
  2. Actually, the resultant autotransformer (i.e....

    Actually, the resultant autotransformer (i.e. buck - boost) needs to be able to carry the entire load. However, as has been pointed out, the isolation transformer which is connected into the proper...
  3. Nothing really. An isolation transformer creates...

    Nothing really. An isolation transformer creates a separately derived system, so what happens on one transformer secondary has little to no impact on the grounding conductor of another.
    However, I...
  4. What type of load was the breaker feeding? ...

    What type of load was the breaker feeding?

    During a primary side fault, you would not expect to see an fault current on the transformer secondary. However, your loads will experience a loss of...
  5. The conversion between HP and kW will always...

    The conversion between HP and kW will always result in a lower value than what you calculate for the input power.
    Remember motors are rated in output power, you must account for the losses (e.g.....
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    Transformers smaller than 500KVA are usually...

    Transformers smaller than 500KVA are usually supplied with impedances less than 5.75%. I would have used 3.5% for a maximum fault current of 6kA, which is still less than the standard breaker AIC of...
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    What is the restricted boundary distance when the...

    What is the restricted boundary distance when the cover is on and when it is removed?

    If removing the covers, solely for data collection, is considered as energized work how is that task even...
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    As an option or as standard?

    As an option or as standard?
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    Yes, The NEC rules for

    Yes,

    The NEC rules for <=600V transformers are about the maximum protective device sizing, not the minimum.
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    It is not specifically addressed in NFPA70E. Are...

    It is not specifically addressed in NFPA70E. Are using using a commercial software package or the formulas in the appendixes? Most software is based on IEEE 1584 equations, so you might look there...
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    It depends on the size of the primary side...

    It depends on the size of the primary side protective device.
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    No. It takes a somewhat special Scott-Tee...

    No.
    It takes a somewhat special Scott-Tee connection to go between 3ph and 2ph. A standard run of the mill T connection is strictly 3ph to 3ph.
    You can tell the difference by looking at the...
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    Symmetrical means there are equal parts above and...

    Symmetrical means there are equal parts above and below the zero axis line of the waveform. Asymmetrical means there are unequal parts above and below, as would be found with a DC offset.
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    The NEC treats a transformer with T windings the...

    The NEC treats a transformer with T windings the same as any other three phase transformer bank. Just like it does open-delta and open-wye configurations.
    Most(?) three -phase transformers smaller...
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    One of the reason for CTI is to accommodate...

    One of the reason for CTI is to accommodate tolerances in the protective device operation. It is pretty important when you are dealing with different types of protective devices.
    LV circuit breaker...
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    I know what used to be a common practice, but the...

    I know what used to be a common practice, but the panelboard cannot be a Main Lug Only design, since at least 2014. 408.36 requires the panelboard to have an overcurrent protective device. 408.36(B)...
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    240.21(C)(1)(2) is about protecting only the...

    240.21(C)(1)(2) is about protecting only the conductors on the secondary of a transformer.
    However, the panelboard also needs protection per 408.36, as others have mentioned.
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    I don't see a problem. NEC 2017 ...

    I don't see a problem.

    NEC 2017
    670.3(A)(2) - nameplate must list the maximum possible overcurrent protective device.
    670.4(A) - The conductor sizing shall take into account the loads that...
  19. Thread: 230.71(A)

    by jim dungar
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    NFPA70E does not exempt single phase systems from...

    NFPA70E does not exempt single phase systems from Arc Flash analysis. You just cannot simply use IEEE-1584 as you can with three phase systems.

    The 125kVA and smaller 'exception' was removed from...
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    The primary fuse is often intended to remove the...

    The primary fuse is often intended to remove the transformer from the system after it has failed (i.e. short circuit protection). Overload protection is usually provided the secondary protective...
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    Schneider Electric also lists multi breaker...

    Schneider Electric also lists multi breaker combinations, but they are for use only with multi-metering centers.
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    It depends on how the buck-boost was wired. A...

    It depends on how the buck-boost was wired. A single transformer wired 208V to 240V would have the common 'hot' leg which would be 120V to neutral.
    However if, as in a previous thread, someone used...
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    That was the right way to go.

    That was the right way to go.
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    Please do the math. Remember, you are starting...

    Please do the math. Remember, you are starting with two legs of a 'star' connected transformer.
    If you have 208V L-L and 120VL-G then you increase the L-L voltage by 10% to 240V, the L-G voltage...
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    If you use a buck-boost to raise the L-L voltage...

    If you use a buck-boost to raise the L-L voltage from 208 to 240V then you will also raise the L-G voltage to roughly 136V.
    You most likely need a 208 - 120/240 isolation transformer.
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