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
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.