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Thread: Transformer

  1. #11
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    Quote Originally Posted by kwired View Post
    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.
    You could incorporate both the 10 and 25 foot tap rules from the same xfmr
    secondary - using my example of minimum 29A for a very lightly loaded panel along with your example of a 200A. The key factor is sizing the conductors to the load as I stated above - ref 240.21(C)(2)(1).
    In petro-chem we rarely use the 10 foot rule. In fact we don’t see the 25 ft tap rule often either since most of our transformers are located outdoors to begin with.


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  2. #12
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    Quote Originally Posted by Dale001289 View Post
    You could incorporate both the 10 and 25 foot tap rules from the same xfmr
    secondary - using my example of minimum 29A for a very lightly loaded panel along with your example of a 200A. The key factor is sizing the conductors to the load as I stated above - ref 240.21(C)(2)(1).
    In petro-chem we rarely use the 10 foot rule. In fact we don’t see the 25 ft tap rule often either since most of our transformers are located outdoors to begin with.


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    Yes, but for OP's transformer there is only an issue with a tapped load of under 29 amps when using the 10 foot rule, in such case the supply side of your overcurrent device needs to be at least 10 AWG instead of 14 AWG. Anything over 29 amps requires conductor sized at least to the load anyway.

    25 foot rule is where you would need larger conductor. With 125 amp primary device and a 2.31 voltage ratio you would need a 96 amp conductor minimum for 25 foot rule, even if only supplying a 15 amp breaker.
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  3. #13
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    Quote Originally Posted by kwired View Post
    Yes, but for OP's transformer there is only an issue with a tapped load of under 29 amps when using the 10 foot rule, in such case the supply side of your overcurrent device needs to be at least 10 AWG instead of 14 AWG. Anything over 29 amps requires conductor sized at least to the load anyway.

    25 foot rule is where you would need larger conductor. With 125 amp primary device and a 2.31 voltage ratio you would need a 96 amp conductor minimum for 25 foot rule, even if only supplying a 15 amp breaker.

    I agree. I was only giving a general example of the 10 foot tap rule since the OP had expressed some level of confusion. There are actually three transformer 25 foot tap rules: 240.21(C)(2), 240.21(C)(6) and 240.21(B)(3) - this last one gets overlooked quite a bit since it involves both primary and secondary conductors.
    Last edited by Dale001289; 06-09-18 at 12:01 PM.

  4. #14
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    Additional info to post #1: secondary side panelboard is going to be located in tenant space not relocatable and the transformer is in electrical room. Its 145 feet of secondary cables between the transformer and the secondary panelboard sized 3/0 awg.

    The secondary side panelboard calculated demand load is 191 amps, panelboard bus 200 amps, panelboard main circuit breaker 200 amps.

    1. If I use ten feet rule 240.21 (C)(2):

    -240.21 (2)(1)(a)(b): i am using 3/0 awg secondary conductors amapcity at 75C 200 amps.

    -240.21 (2)(2): they dont extend beyond secondary panelboard but thats more than 10 feet away. So not sure how this applies unless i provide 200 amp enclosed circuit breaker less than 10 feet transformer secondary.

    -240.21 (2)(3): that applies but i am guessing they are talking about less than 10 feet. Panelboard is 145 feet away. Not sure how this applies unless provide 200 amp enclosed circuit breaker less than 10 feet.

    -240 21 (2)(4): Primary overcurrent protrctive device I have is 100 amps. Ampacity of secondary conductor is 200 amps. ( 200 x10) which is less than 100x (480/208). I guess okay.

    2. If i use 25 foot rule 240.21(C)(6):

    - 240.21 (C)(6)(1): 3/0 awg secondary cable ampacity 200 amp is greater than (480/208)*(100/3).

    -240.21(C)(6)(2): Have to provide 200 amps enclosed circuit breaker less than 25 feet from transformer secondary. However from 15 feet from transformer secondary the cables would go to other rooms.

    -240.21 (C)(6)(3): That applies to secondary conductors less than 25 feet. I have secondary condcutors 145 feet. So the install up to 25 feet would have to comply and not the rest?

    3. 240.21(C) says condctors connected to transformer without overcurrent protection at the secondary as specified in 240.21 (C)(1) to (C)(6). C1 does not apply to post #1 i guess. C2 to C6 are either 10feet or 25 deet rules but I have 145 feet. So C1 to C6 does not apply in my case so I have to provide with overcurrent protection just where??


    I guess I could be interpretting #1 to #3 above wrong.

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    Last edited by codequestion; 06-10-18 at 01:00 PM.

  5. #15
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    Quote Originally Posted by codequestion View Post
    Additional info to post #1: secondary side panelboard is going to be located in tenant space not relocatable and the transformer is in electrical room. Its 145 feet of secondary cables between the transformer and the secondary panelboard sized 3/0 awg.

    The secondary side panelboard calculated demand load is 191 amps, panelboard bus 200 amps, panelboard main circuit breaker 200 amps.

    1. If I use ten feet rule 240.21 (C)(2):

    -240.21 (2)(1)(a)(b): i am using 3/0 awg secondary conductors amapcity at 75C 200 amps.

    -240.21 (2)(2): they dont extend beyond secondary panelboard but thats more than 10 feet away. Can't apply 10 foot rule then, simple as that. So not sure how this applies unless i provide 200 amp enclosed circuit breaker less than 10 feet transformer secondary.

    -240.21 (2)(3): that applies but i am guessing they are talking about less than 10 feet. Panelboard is 145 feet away. Not sure how this applies unless provide 200 amp enclosed circuit breaker less than 10 feet.

    -240 21 (2)(4): Primary overcurrent protrctive device I have is 100 amps. Ampacity of secondary conductor is 200 amps. ( 200 x10) which is less than 100x (480/208). I guess okay.

    2. If i use 25 foot rule 240.21(C)(6):

    - 240.21 (C)(6)(1): 3/0 awg secondary cable ampacity 200 amp is greater than (480/208)*(100/3).

    -240.21(C)(6)(2): Have to provide 200 amps enclosed circuit breaker less than 25 feet from transformer secondary. However from 15 feet from transformer secondary the cables would go to other rooms.

    -240.21 (C)(6)(3): That applies to secondary conductors less than 25 feet. I have secondary condcutors 145 feet. Again can't apply 25 foot rule because you are over 25 feet. So the install up to 25 feet would have to comply and not the rest?

    3. 240.21(C) says condctors connected to transformer without overcurrent protection at the secondary as specified in 240.21 (C)(1) to (C)(6). C1 does not apply to post #1 i guess. C2 to C6 are either 10feet or 25 deet rules but I have 145 feet. So C1 to C6 does not apply in my case so I have to provide with overcurrent protection just where??


    I guess I could be interpretting #1 to #3 above wrong.

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    You need to apply overcurrent protection within 10 feet or 25 feet and at same time comply with provisions that go with which ever rule you are applying. Once you have a place for the overcurrent protection - the rest of the downstream conductors are no longer feeder taps or transformer secondary conductors that must comply with 240.21, they are simply feeders or branch circuits.
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