deepupaulji
Member
- Location
- Lenexa
- Occupation
- Electrical Engineer
How to size a cables/wires on the secondary side of the transformer has always been a discussion everyone does it differently.
how i size the secondary cables from the transformer to the SWGR as a ratio of the primary to secondary voltages multiplied by the primary over current protection Amps ( breaker or Fuse size) and others do it as 1.25 x FLA of the transformer to size the cable.
For Example a 750 KVA Transformer @ 4.16KV /0.480KV Delta- Star , Primary side Switchgear is 160Ft away and the secondary side SWGR is 20Ft Away from the transformer. So transformer has a primary protection then the SWGR is downstream has a Main Breaker.
As per My Calculation
the Primary Side Over current Protection is 150A (Standard fuse size as the FLA for a 750KVA @ 4.16KV = 104.1A) and i size the Wire to 150A rated wire
The secondary side wire i size it based on the 150A x 4.16KV /0.48 kV = 1300A
the difference i find how others do it is they size the secondary wire as 1.25 x Secondary side FLA of the Transformer
Secondary FLA = 750KVA /(1.732 x 0.480KV) = 902.13A
Secondary Wire Size to be 1.25 x 902.13 = 1127.67A.
i tried Looking at NEC code and its not that clear to me
the section i referred were NEC 240.4 and 240.21(C).
Please shed some light on to this calculation and let me know about the NEC Section that needs to be referred to size the wire.
how i size the secondary cables from the transformer to the SWGR as a ratio of the primary to secondary voltages multiplied by the primary over current protection Amps ( breaker or Fuse size) and others do it as 1.25 x FLA of the transformer to size the cable.
For Example a 750 KVA Transformer @ 4.16KV /0.480KV Delta- Star , Primary side Switchgear is 160Ft away and the secondary side SWGR is 20Ft Away from the transformer. So transformer has a primary protection then the SWGR is downstream has a Main Breaker.
As per My Calculation
the Primary Side Over current Protection is 150A (Standard fuse size as the FLA for a 750KVA @ 4.16KV = 104.1A) and i size the Wire to 150A rated wire
The secondary side wire i size it based on the 150A x 4.16KV /0.48 kV = 1300A
the difference i find how others do it is they size the secondary wire as 1.25 x Secondary side FLA of the Transformer
Secondary FLA = 750KVA /(1.732 x 0.480KV) = 902.13A
Secondary Wire Size to be 1.25 x 902.13 = 1127.67A.
i tried Looking at NEC code and its not that clear to me
the section i referred were NEC 240.4 and 240.21(C).
Please shed some light on to this calculation and let me know about the NEC Section that needs to be referred to size the wire.
