Sizing conductors for transformers

[gman2003]

Do the primary and secondary conductors of a transformer need to be sized for continuous load? Example: for a 225kVA transformer feeding a 500 amp, 208Y/120 volt, 3 phase panel from a 480 volt three phase panel, the primary of the transformer is rated at 271 amps. Can this be fed from a 300 amp breaker or do you need to rate for continuous load? Thanks.

 

[websparky]

Re: Sizing conductors for transformers

Kind of depends......

Is there primary AND secondary OCD?

See 450.3(B)

 

[gman2003]

Re: Sizing conductors for transformers

Yes, there is OCPD on both sides. The primary has a 300 amp breaker, and the panel fed from the secondary has a 500 amp main. I see that the section you referred to states that 9 amp or more have maximum protection of 125%. Since this states "maximum" protection, it's not a requirement to rate it at 125%, is it?

Also, I don't see any references to the secondary being sized as such. Am I missing it somewhere, or do I size it at 100%?

[ January 13, 2004, 08:45 PM: Message edited by: gman2003 ]

 

[jerryb]

Re: Sizing conductors for transformers

A tranformer is capable of delivering it's rated KVA and could be continuous. Therefore I would think that the primary and secondary overcurrent devices (and associated wiring) need to be rated for 125% of unit nameplate. Anything less would be cheating the user from its full capability.

 

[pierre]

Re: Sizing conductors for transformers

450.3(B), if that is where you are referencing from to get the 125%, is for protection of the transformer windings (primary and secondary), not the conductors. Conductor overcurrent protection referencing is found in Article 240.

Pierre