I'm getting differing opinions on the interpretation of Note 2 to Table 450.3(B). I have a 150 kVA, 480-208Y/120 V indoor dry-type transformer feeding three main breaker panelboards. The question comes from the phrase in Note 2 "shall not exceed the allowed value of a single overcurrent device". Is that value for multiple overcurrent devices (a) strictly a maximum of 125% of transformer rated current, or (b) permitted to be as high as "the next higher standard rating" of Note 1 of Table 450.3(B). If "value" means (a), the sum of the three main breakers is limited to 520 A, but if "value" means (b), I can go to 600 A.
Mike, The value is per note 1, in my opinion. This permits the rounding up to a total of 600 amps for the three overcurrent protective devices. Don't forget that the conductors between the transformer and the panles must be protected per 240.21(C). Don
Re: Transformer Overcurrent Protection#89459 09/16/0411:41 AM09/16/0411:41 AM
If I'm reading note two correctly it appears to only apply if the primary overcurrent protection is sized larger than 125% of the pri FLA?? In that are compelled to additionally protect the secondary winding.
If the pri OCPD is sized to protect the transformers pri. per 450.3 (B) [pri protection only} , then OCP for the secondary winding is not required.
You may have multiple taps from the secondary of the transformer (as mentioned 240.21 (C) ) to several loads the secondary conductors must be protected per that section and article 240. If your pri overcurrent protection exceeds 225 A (480/.83)I think Note two is applicable.??
RYAN could you indicate the paragraph in the 05 that references 240.4 I'm looking at the pre-print and appear to be missing it!!
[This message has been edited by cpal (edited 09-17-2004).]
240.21(C) Transfomrer secondary conductors. Each set of conductors feeding seperate loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
*(C)(1)through(C)(6) are the 10', 25', etc., rules, similiar to the 2002.