i'm not good at math. i saw some formulas that people had posted but still i am harving hard time figuring this out. i need to know what size wire i should use. primary is 4160, secondary is 120/208 wye. we are using a 150 kva transformer which is supplying a 400 amp panel. what should the conductor size be for the 5kv cable?

 

Why won't you leave this issue to a trained Engineer?
Meir

 

from the data provided it seems that the transformer is 4.16 kv/120v, 208v 150 kva rating. Accordingly primery current wil be around 36 amp. If it is an indoor type transformer, an XLPE 6.6 kv, armoured Aluminium cable of 10 squ mm size (if available) can be used.

If it is an outdoor type installation, ACSR conductor size 20 sq mm will be sufficient.

 

Responding to pbw's Fri, Apr 16, 1:19 pm comment:

Caution: The transformer is a three phase unit. Thus load current is not 36 A, but instead, 21 A. In most 4.16 kV installations the wire size determination is dictated by short-circuit withstand duty, never load current alone. Also, ground or earth-fault duty must be known for confirming shield or metal sheath adequacy.

The system parameters that affect both thermal and dynamic integrity should be ascertained. Some of the most salient ones are: fault-current level and asymmetrical factors; fault duratioon, i.e., clearing time of protective fuses or tripping time of protective circuit breakers; and
material/type of termination hardware. Load current alone could lead to underestimating conductor size. It could result in conductor "fusing" with catastrophic consequences.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected]) {[email protected]}