wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
240.4(F) allows us to treat conductors on the secondary side of certain transformers as being protected by a primary-side OCPD after properly adjusting the OCPD rating. The transformers for which this is allowed are the ones for which there is a strict ratio between primary currents and secondary currents. I.e. there is a ratio C such that Isecondary = C * Iprimary. And then 240.4(F) requires the secondary conductor ampacity to be at least C * the primary OCPD rating.
It seems to me that this could be expanded in two related ways. First, rather than restrict 240.4(F) to conductor protection, it could simply say (to paraphrase) "for those transformer types, any OCPD on the primary side may be considered to be an OCPD on the secondary side of C times the actual rating." So for example this would let us stack 240.4(B) with 240.4(F). Or use 240.4(F) to satisfy the 408.36 requirement for protection of a panelboard.
Second, even for the transformer types for which there is no such strict ratio C, there is always a worst case ratio D such that Isecondary <= D * Iprimary". E.g. on a 480D : 208Y/120 transformer, D = 4. You can't have 100A flowing through any secondary conductor unless there is some primary conductor carrying at least 25A (perhaps barring some perverse combination of differing loads with very different power factor angles). So 240.4(F) could be expanded to allow the use of the ratio D for all transformer types.
Is there some potential problematic side effect of either of these two changes that I'm not seeing?
Thanks,
Wayne
It seems to me that this could be expanded in two related ways. First, rather than restrict 240.4(F) to conductor protection, it could simply say (to paraphrase) "for those transformer types, any OCPD on the primary side may be considered to be an OCPD on the secondary side of C times the actual rating." So for example this would let us stack 240.4(B) with 240.4(F). Or use 240.4(F) to satisfy the 408.36 requirement for protection of a panelboard.
Second, even for the transformer types for which there is no such strict ratio C, there is always a worst case ratio D such that Isecondary <= D * Iprimary". E.g. on a 480D : 208Y/120 transformer, D = 4. You can't have 100A flowing through any secondary conductor unless there is some primary conductor carrying at least 25A (perhaps barring some perverse combination of differing loads with very different power factor angles). So 240.4(F) could be expanded to allow the use of the ratio D for all transformer types.
Is there some potential problematic side effect of either of these two changes that I'm not seeing?
Thanks,
Wayne
