DustinPhillips
New member
- Location
- Richland, WA
The code requirement I'm looking at is NEC 450.3 (b) and 240.4(F) out of the 2017 NEC.
I am working on a furnace design that is being powered by a 3-phase 480 V supply and controlled by an SCR.
We are stepping down the voltage after the SCR to 45 V with a 480/45 delta-wye transformer.
SCR manufacturers don't recommend fusing downstream of the SCR because of potential equipment damage. Some manufacturers have examples with our same delta-wye setup that don't include secondary fusing.
240.4 (F) allows you to fuse the primary for secondary conductor protection in the case of a single-phase transformer having a 2 wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary.
I want to understand the reason why the code does not allow the same provision for a delta-wye.
This is also a system that would only be serviced by a "Qualified Person" and the transformer is only used for the furnace heater coils so I am also wanting to understand if this code requirement applies to systems like the one described above.
I am working on a furnace design that is being powered by a 3-phase 480 V supply and controlled by an SCR.
We are stepping down the voltage after the SCR to 45 V with a 480/45 delta-wye transformer.
SCR manufacturers don't recommend fusing downstream of the SCR because of potential equipment damage. Some manufacturers have examples with our same delta-wye setup that don't include secondary fusing.
240.4 (F) allows you to fuse the primary for secondary conductor protection in the case of a single-phase transformer having a 2 wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary.
I want to understand the reason why the code does not allow the same provision for a delta-wye.
This is also a system that would only be serviced by a "Qualified Person" and the transformer is only used for the furnace heater coils so I am also wanting to understand if this code requirement applies to systems like the one described above.
