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Thread: Transformer

  1. #1
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    Transformer

    I have 75kva three phase 480/277v to 120/208v transformer. The secondary transformer feeder is 145 feet long and terminates at panel with 200 amps main breaker. The primary has 100 amp fused disco in same room. Does the secondary require breaker due to feeder being 145 feet per code 245.21 or other article?

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    Last edited by codequestion; 06-08-18 at 07:54 PM.

  2. #2
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    Yes, unless the secondary conductors are outside, they need overcurrent protection per one of the rules in 240.21(C).
    Don, Illinois
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    (All code citations are 2017 unless otherwise noted)

  3. #3
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    Quote Originally Posted by don_resqcapt19 View Post
    Yes, unless the secondary conductors are outside, they need overcurrent protection per one of the rules in 240.21(C).
    Even though their is main breaker in the panel secondary? Also their is no over 25 feet 240.21 (C). My length is 145 feet.

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    Quote Originally Posted by codequestion View Post
    Even though their is main breaker in the panel secondary? Also their is no over 25 feet 240.21 (C). My length is 145 feet.

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    Correct, maximum secondary length before first overcurrent device is 25 feet, unless it is outdoors.
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    Quote Originally Posted by kwired View Post
    Correct, maximum secondary length before first overcurrent device is 25 feet, unless it is outdoors.
    So I would need total of 5 overcurrent devices one very 25 feet or just one 25 feet and I am good? Also their is 10 feet as well 240.21 (C). So how does that work?

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    Quote Originally Posted by codequestion View Post
    So I would need total of 5 overcurrent devices one very 25 feet or just one 25 feet and I am good? Also their is 10 feet as well 240.21 (C). So how does that work?

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    Actually unless this is an industrial installation where conditions of maintenance and supervision ensure that only qualified persons service the systems - the 10 foot rule is what applies, and no you don't need an overcurrent device every 25 feet, once you hit the first device you no longer have feeder tap or secondary conductors you have a "feeder" beyond that point.
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    Quote Originally Posted by kwired View Post
    Actually unless this is an industrial installation where conditions of maintenance and supervision ensure that only qualified persons service the systems - the 10 foot rule is what applies, and no you don't need an overcurrent device every 25 feet, once you hit the first device you no longer have feeder tap or secondary conductors you have a "feeder" beyond that point.
    What about 240.21 (c)(6)? Is it for 25 feet rule for non industrial location? I am still confuse so I can either apply 10 feet or 25 feet rule?

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    Quote Originally Posted by codequestion View Post
    What about 240.21 (c)(6)? Is it for 25 feet rule for non industrial location? I am still confuse so I can either apply 10 feet or 25 feet rule?

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    See 240.21(C)(2), 10 foot transformer secondary tap rule -
    240.21(C)(2)(4):
    • Find the ratio between primary and secondary voltages, i.e. 480/208 = 2.31
    • Find the rating of the primary side overcurrent device...Primary Rating = 75kVA/480 x 1.732 = 90 x 1.25%=112.7 - round up to next std size = 125A - ref Art 240.6.
    • Divide rating of 125A by 10 = 12.5.

    Minimum allowable conductor would be, 2.31 x 12.5 = 28.875 =29A, which equals min #10 Awg for secondary side. The caveat is the conductors cannot be less than the loads supplied by the secondary, see 240.21(C)(2)(1). This is why the 10ft tap rule is seldom used.
    Last edited by Dale001289; 06-09-18 at 07:18 AM.

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    Quote Originally Posted by codequestion View Post
    What about 240.21 (c)(6)? Is it for 25 feet rule for non industrial location? I am still confuse so I can either apply 10 feet or 25 feet rule?

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    Yes, thought something wasn't quite right when I posted before because I was pretty sure you could go up to 25 feet in any location - must meet all the conditions mentioned though.
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    Quote Originally Posted by Dale001289 View Post
    See 240.21(C)(2), 10 foot transformer secondary tap rule -
    240.21(C)(2)(4):
    • Find the ratio between primary and secondary voltages, i.e. 480/208 = 2.31
    • Find the rating of the primary side overcurrent device...Primary Rating = 75kVA/480 x 1.732 = 90 x 1.25%=112.7 - round up to next std size = 125A - ref Art 240.6.
    • Divide rating of 125A by 10 = 12.5.

    Minimum allowable conductor would be, 2.31 x 12.5 = 28.875 =29A, which equals min #10 Awg for secondary side. The caveat is the conductors cannot be less than the loads supplied by the secondary, see 240.21(C)(2)(1). This is why the 10ft tap rule is seldom used.
    10 foot tap rule is used a lot. In your example above you have a transformer secondary rating of 208 amps. If you are supplying a 200 amp overcurrent device you will have 200 amp conductor - which is well above the minimum needed 29 amps. The limitation of that section mostly only effects you if you were supplying say an individual 15 amp breaker from said transformer - it would need to be supplied with 10AWG conductor even though the load is 15 or less. Load side of the breaker could then be 14 AWG.
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