Transformer Secondary Conductor Sizing

[Gategator37]

A 7.2KV - 120/240volt 75KVA transformer is being installed by the utility. The engineer is showing 3-#3/0 conductors going from this Transformer to a 250 amp OCPD (Service Disconnect). I believe the engineer meant to say 2 parallel sets of #3/0 because #3/0 is only good to 200 amps. Now, to change this can one size the conductors from the transformer to the OCPD off of the OCPD size (250 amps) or do I have to size it based on the secondary amps of the transformer?

Thanks.

 

[pete m.]

The conductors would be sized to the OCPD (since you have a single main). It appears that you would need 250 kcmil copper for a single run or you could parallel 1/0 copper.

Pete

 

[cripple]

Transformer Secondary Conductor Sizing

The sizing of these conductor would depend on how is installing the conductor. If installed by NEC electricians they should be sized per Article 220 and installed per 240.21(C), is installed by the serving utility the sizing requirement would be per the NESC.

 

[david luchini]

Assuming that the non-continuous load plus 125% of the continuous load does not exceed 230 amps, you could install #4/0 conductors, per 230.42 and 230.90(A)Ex.No.2.

 

[pete m.]

Assuming that the non-continuous load plus 125% of the continuous load does not exceed 230 amps, you could install #4/0 conductors, per 230.42 and 230.90(A)Ex.No.2.

Agreed as long as the transformer is utility owned which it appears to be from the OP.

Pete

 

[augie47]

agree with others
If the conductors are installed/maintained by the utility they can size them per NESC.
If the are customer owned then NEC applies and if the calculated load is 230 or less, a 4/0 is sufficient, over 230, then a 250 kcmil is required. (barring any applicable de-rating factors)