Transformer secondary overcurrent protection

[dpb]

Hello, I haven't done a whole lot of transformer hook ups so i wanted to make sure i have this correct. I'm installing a 14kva step up 208D to 480Y transformer and wanted to make sure that i am interpreting 240.21 C 1 correctly. The primary is protected at the box with a 40a, and will have a disconnect immediately before the transformer on the primary side. the load side will have minimum 12awg conductors and will only feed one device mounted right next to the transformer (less than 5 feet away). So with that information i would not need a disconnect or overcurrent protection on the secondary, correct? device on the 480 side is a 13.6 max amperage forklift charger

208x40=8320w
8320/480=17.33a

13.6x1.25=17a

 

[wwhitney]

240.21(C)(1) doesn't help you with a delta-wye transformer. The last paragraph says "Single-phase (other than 2-wire) and multiphase (other than
delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device."

You don't have a delta-delta, so you will need to use one of the other subsections of 240.21(C), all of which require secondary OCPD.

Cheers, Wayne

P.S. As a technical aside, 240.21(C)(1) could be written more broadly to use the actual turns ratio of the transformer coils for any transformer topology. For a 208D : 480Y transformer, each 208V primary coil is cowound with a 277V (L-N) secondary coil, so that turns ratio is 208:277. That means your 40A primary OCPD would be equivalent to 40A * 208:277 = 30A secondary OCPD. I.e. if a secondary load drew 30A L-N, the primary side would see 40A in two of the primary conductors. However all of this is moot for you, as 240.21(C)(1) is not written that way.

 

[dpb]

ah did not notice that, didn't have my book on me in the field and was using my mike holt vol 1, the section seems to be missing that bit on info. gotta make sure to put my NEC back in the van every day. I'll install a fusible disconnect on the secondary then.
Thank you!