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Thread: Table 450-3(b)

  1. #1
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    Table 450-3(b)

    Table 450-3(b) note 2 ...

    How do I determine when secondary protection is required. I use the 4 to 1 ratio now but am unable to hang my hat on anything.

    Marty

  2. #2
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    Re: Table 450-3(b)

    Never.

    Overcurrent devices on the secondary of a transformer (600V max primary) only provide protection for the secondary conductors and so are only required based on the conductor application. Application considerations include transformer winding configuration and "tap rules".

    If you happen to have a secondary overcurrent protective device, which is sized correctly, you then have the option of increasing the rating of the primary protection
    Just because you can, doesn't mean you should.

  3. #3
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    Re: Table 450-3(b)

    Marty, Re: "How do I determine WHEN secondary protection is required."
    Refer to 2002 NEC art 240.4 (F) Transformer secondary conductors.

  4. #4
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    Re: Table 450-3(b)

    Jim
    I am confused by your statement. It seems that secondary OCPD's will provide overload protection for the transformer as well as conductors. The fact that note 2 requires you to add the values of OCPD's and not exceed the value of a single OCPD (vs service protection for example,) would seem to indicate that overload protection is being provided as well.

    To answer molsen's question, you need secondary protection almost every time, because even when you have the transformer protected from overload, you won't have the secondary conductors protected.
    Two exceptions could be delta-delta transformers with minimum primary protection, and two wire primary-two wire secondary transformers.

  5. #5
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    Re: Table 450-3(b)

    jtester,

    The NEC allows this transformer installation. The secondary breaker is larger than 125%, so it does not provide any transformer protection.

    Primary protection - 20A
    Primary conductor - #12AWG (any length)
    Xformer 15KVA 3phase 480V(18A)->208Y/120V(41.6A)
    Secondary conductor - #6 AWG THHN (10 ft)
    Panelboard with main breaker - 60A
    Just because you can, doesn't mean you should.

  6. #6
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    Re: Table 450-3(b)

    I agree the installation is legal, but in this case, technically, no secondary protection is required, just secondary conductor protection as provided by the 60 amp breaker.

    Your example could just as easily have had a 40 amp primary breaker and then I would have needed secondary protection.
    Section 450.3(B) tells you that secondary protection is achieved when you protect to 125% and round up to the next sized breaker per Note 1 of the Table. If I follow Table 450.3(B) 41.6x1.25=52 applying note 1, gives me a 60 amp breaker for secondary protection.
    I'm not sure that you can assume that since flc is 41.6 amps, anything higher will damage the unit. I wonder what the threshold of damage curve for a dry type transformer looks like. I have some for oil filled, but not for dry type.

    Jim T

  7. #7
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    Re: Table 450-3(b)

    Although I don't have a copy of the damage curve my reference refers to IEEE C57.12.59 which is for transformers 501KVA and larger. However there sould be a foot note at the end of the standard which indicates that for 3ph units from 15-500KVA and 1ph units from 5-500kva refer to the same IEEE standard and the curve in fig 1 for catagory I transformers.

  8. #8
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    Re: Table 450-3(b)

    As an after thought I would like to add that the notes that I have from a retired dry type distribution transformer design engineer of a major manufacturer indicates that transformer life should be about 30 years based upon 35% avg. loading per accepted industry standards. With 100% loading 3-5 years may be expected.
    This is based upon an avg. of 30degC amb. w/40degC max.
    My notes also show a 50% reduction of life expectance for each 10degC above 30degC.
    So it certainly appears as though those who are reducing cost by down sizing transformers may be saving money up front but doesn't consider the long range consequences.

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