75 KVA Tansformer

[CCCI]

We are going to move a 75 KVA transformer it is now on a 90 Amp 3 pole breaker 277/480 with #3 THHN and has 225 amp fuse on the 120/208 load side and 3/0 THHN. The ground is a #6.
My math has a 90.2 amp line and a 208 amp load. Should the wire have been:LINE 3 #1 & 1#6 Ground
LOAD 4 #4/0 & 1#4 Ground
Thanks
MIKE

 

[rr]

The LOAD looks correct. Look at your LINE again. 2005 NEC Table 310.16 and Table 250.122

 

[petersonra]

We are going to move a 75 KVA transformer it is now on a 90 Amp 3 pole breaker 277/480 with #3 THHN and has 225 amp fuse on the 120/208 load side and 3/0 THHN. The ground is a #6.
My math has a 90.2 amp line and a 208 amp load. Should the wire have been:LINE 3 #1 & 1#6 Ground
LOAD 4 #4/0 & 1#4 Ground
Thanks
MIKE

A 90A breaker can certainly protect a #3 wire. The 225A fuse on the secondary side is iffy.

The EGC are sized based on the size of the OCPD. A 90A CB requires a #8 ECG, and a 225A OCPD requires a #4.

It seems like for whatever reason they chose to reduce the size of the primary conductors and OCPD from what it could have been, but it is not a code violation to make the conductors smaller than the maximum size allowed as long as they are large enough to supply the current required and are properly protected.

It appears this would qualify as an SDS, meaning the grounded conductor of the secondary side needs to be bonded to the GES.

 

[infinity]

The EGC are sized based on the size of the OCPD. A 90A CB requires a #8 ECG, and a 225A OCPD requires a #4.

The equipment bonding jumper between the transformer and the 225 amp OCPD is required to be sized according to the secondary conductor size and table 250.66. This would require a #2 CU for #4/0 secondary conductors. Look at 250.30(A)(2).

 

[petersonra]

The equipment bonding jumper between the transformer and the 225 amp OCPD is required to be sized according to the secondary conductor size and table 250.66. This would require a #2 CU for #4/0 secondary conductors. Look at 250.30(A)(2).

You are right. This is the connection to the GES. The EGC going along with the wires from the fuses are sized according to 250.122.

 

[kingpb]

...277/480 with #3 THHN and has 225 amp fuse on the 120/208 load side...

Correct usage of voltage descriptions would dictate that you have a 408Y/277V and 208Y/120V system.