Dry-type transformer grounding

[2450]

What is the correct size of the grounding electrode conductor for a 30KVA, 480v-120/208v, 3ph, 4wire dry type transformer that supplies 120/208v power to a building distribution panelboard with a 100A, MCB? Is the grounding electrode conductor be sized per Table 250-66 or Table 250-122? The 1999 NEC code no longer classifies step down transformers as separately derived system. Is this true?

 

[dereckbc]

Re: Dry-type transformer grounding

Step downs are still considered SDS.

The GEC is based on the size of the secondary conductors size from table 250.66

 

[bob]

Re: Dry-type transformer grounding

You need both. The grounding electrode conductor for the secondary is based on 250.66 and a equipment grounding conductor base on 250.122
based on the primary OC device.