winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Given a 75kVA 240V delta to 480/277V wye transformer.
Primary full load current is 180A. Secondary is 90A.
Article 450 (specifically 450.3(B) ) tells us that the maximum OCPD on the primary side can be 450A (180*2.5), as long as the secondary protection is less than 112.5A.
If the transformer primary is protected by a 450A breaker, what is the required ampacity for the primary conductors? Analogously to the feeder for a motor, I would have guessed 225A, the transformer rated primary current * 1.25.
However I looked in 240.4(G), the normal location where _conductor protection_ is permitted for specific applications as described in other articles, and transformer primary conductors are not mentioned there. Did I miss a reference here? This suggests that if I want to use a 450A breaker for protecting both the transformer primary _and_ the primary conductors, then the conductors must have an ampacity of 450A (perhaps somewhat less, given 240.6(A), but still lots more than the load served).
240.21(B)(3) (tap rules for transformers) appears to apply, although it is not clear if paragraph 5 applies to both the primary and secondary conductors, or just the secondary conductors. Are the primary conductors considered protected if their ampacity, as scaled by the transformer ratio, matches the secondary OCPD?
For a 'normal' 75kVA transformer, is it general practise to size the primary OCPD at maximum allowed 250% of rated primary current, or at a lower value?
Thanks
Primary full load current is 180A. Secondary is 90A.
Article 450 (specifically 450.3(B) ) tells us that the maximum OCPD on the primary side can be 450A (180*2.5), as long as the secondary protection is less than 112.5A.
If the transformer primary is protected by a 450A breaker, what is the required ampacity for the primary conductors? Analogously to the feeder for a motor, I would have guessed 225A, the transformer rated primary current * 1.25.
However I looked in 240.4(G), the normal location where _conductor protection_ is permitted for specific applications as described in other articles, and transformer primary conductors are not mentioned there. Did I miss a reference here? This suggests that if I want to use a 450A breaker for protecting both the transformer primary _and_ the primary conductors, then the conductors must have an ampacity of 450A (perhaps somewhat less, given 240.6(A), but still lots more than the load served).
240.21(B)(3) (tap rules for transformers) appears to apply, although it is not clear if paragraph 5 applies to both the primary and secondary conductors, or just the secondary conductors. Are the primary conductors considered protected if their ampacity, as scaled by the transformer ratio, matches the secondary OCPD?
For a 'normal' 75kVA transformer, is it general practise to size the primary OCPD at maximum allowed 250% of rated primary current, or at a lower value?
Thanks
