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Thread: NEC 450.4(A)

  1. #1
    Join Date
    Jun 2018
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    NEC 450.4(A)

    I have auto transformer going from supply side 208V single phase to load side 240V single phase, 5kVA. Load drawn is 16 amps by kitchen cooking appliance. On the 240V side outside of the auto transformer I have 30 amps disconnect fused at 20 amps then going to load.

    I am confused by what NEC 450.5(A) says at the end "an overcurrent device shall not be installed in series with the shunt winding of the autotransformer between Points A and B as shown in Figure 450.4(A)". So does that mean the secondary side 20 amps fused disco is not acceptable?

  2. #2
    Join Date
    Jan 2007
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    3,773
    Quote Originally Posted by hhsting View Post
    I have auto transformer going from supply side 208V single phase to load side 240V single phase, 5kVA. Load drawn is 16 amps by kitchen cooking appliance. On the 240V side outside of the auto transformer I have 30 amps disconnect fused at 20 amps then going to load.

    I am confused by what NEC 450.5(A) says at the end "an overcurrent device shall not be installed in series with the shunt winding of the autotransformer between Points A and B as shown in Figure 450.4(A)". So does that mean the secondary side 20 amps fused disco is not acceptable?
    The 20a fused disconnect on the secondary it fine.

    It sounds to me like you may have a Buck/Boost Transformer.

    Regardless, it's just telling you where "Not" to place an overcurrent device.

    Your overcurrent devices need to be installed on the input and output leads.

    JAP>

  3. #3
    Join Date
    Jan 2007
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    Or possible overcurrent devices on the input only.

    JAP>

  4. #4
    Join Date
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    You may find this ECN thread on the topic useful.

    https://www.electrical-contractor.ne...t&Number=98810
    "Electricity is really just organized lightning." George Carlin


    Derek

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