Transformer primary overcurrent protection.

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don_resqcapt19 said:
It has to be specifically permitted. The code section says that the maximum permitted OCPD is 250%. If there is not a code section that says you are permitted to round up, then you have to round down or find a OCPD that exactly matches the maximum permitted OCPD.
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Why does it have to be specifically permitted. It's already permitted in 240.4(B) right. To me it seems like it has to be specifically restricted.
 
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In fact check out the FPN reminding you of 240.4

450.3 Overcurrent Protection. Overcurrent protection of
transformers shall comply with 450.3(A), (B), or (C). As
used in this section, the word transformer shall mean a
transformer or polyphase bank of two or more single-phase
transformers operating as a unit.
FPN No. 1: See 240.4, 240.21, 240.100, and 240.101 for
overcurrent protection of conductors.
 
don_resqcapt19 said:
It has to be specifically permitted. The code section says that the maximum permitted OCPD is 250%. If there is not a code section that says you are permitted to round up, then you have to round down or find a OCPD that exactly matches the maximum permitted OCPD.
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Why would you have to round down IF the Code does not specifically instructs to do so. Wouldn't principles of math prevail? I thought that the Code is written in the permissive mode, eg. if it is not prohibited, it is permitted.

Of course the argument can be made that in other areas they specifically permit to select the next largest size OCPD. If they specifically permitted it and it is not applicable here, then - for consistency - they should have required to select the next smaller standard OCPD.
 
weressl said:
Why would you have to round down IF the Code does not specifically instructs to do so. Wouldn't principles of math prevail? I thought that the Code is written in the permissive mode, eg. if it is not prohibited, it is permitted.
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The point is it is prohibited. The code rules says the maximum rating of the OCPD shall not exceed 250% of the primary full load current. How would any principles of math change this.
Of course the argument can be made that in other areas they specifically permit to select the next largest size OCPD. If they specifically permitted it and it is not applicable here, then - for consistency - they should have required to select the next smaller standard OCPD.
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There is no reason to specify the next standard smaller size...there is no reason that you can't have a custom made OCPD that is rated at exactly 250% of the rated primary current:D When the code rule states a maximum OCPD that is the maximum and in the cases where the CMP wanted to, they provided language to permit the rounding up to the next standard size. You are correct that many code users think that you can always do this, but that is often not the case.
 
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