xfmr calcs.

Status
Not open for further replies.
paul renshaw

paul renshaw

Senior Member
Location
Cordova, Maryland
When calculating the conductor size for the secondary conductors of a xfmr, do you have to add anything to the max output amps of the xfmr? Say it is a dry type step down and the output amps are 125amps just for an example. Can you size the wire for 125 amps, because it can be actually fused higher if the xfmr has primary and secondary OCPD.
 
paul renshaw said:
When calculating the conductor size for the secondary conductors of a xfmr, do you have to add anything to the max output amps of the xfmr? Say it is a dry type step down and the output amps are 125amps just for an example. Can you size the wire for 125 amps, because it can be actually fused higher if the xfmr has primary and secondary OCPD.
Click to expand...

Blindly following the NEC guidelines for protection of transformers, can actually lead to transformers not properly being protected. Properly selecting the OCPD for the transformer requires the OCPD to trip before the current level reaches the transformer damage curve. Consequently, the cable damage curve, if sized for current of the transformer will be to the right and above the transformer damage curve. Therefore there is no need to oversize the cable, since the transformer protection will trip long before the cable is damaged.

Oversizing cable is something that is done far to often. Especially, when there is a lack of understanding in the principles of correctly applying the protective devices and selecting trip settings.
 
paul renshaw said:
kingpb,
If the xfmr primary amps is 54 amps, and the secondary is 125 amps (3ph, 480 to 208/120, 45kva) what size wire would you suggest?
Click to expand...

Assuming 75deg C terminations, using 310.16, with no temperature adjustments, #6AWG on the 480V side, and #1AWG on the 208V side.

Certainly, this is based on not knowing anything about actual installed location or conditions, client requirements, etc, etc, etc.
 
The secondary feeds into a 150 amp main breaker panel that serves lighting and outlet loads, it is currently fed with #2, which I have brought to the installers attention, is too small. There is also no OCPD at the secondary, and the conductors run for 65' in the building before going into the main breaker panel.
 
Status
Not open for further replies.
Top