I must refer you to the NEC Handbook for elaboration.
2005 NEC Handbook page 162 explaining 240.4(F):
The fundamental requirement of 240.4 specifics that conductors are to be protected against over-current in accordance with their ampacity, and 240.21 requires that the protection be provided at the point the conductors receive their supply. Section 240.4(F) permits the secondary circuit conductors from a transformer to be protected by over-current devices in the primary circuit conductors of the transformer only in the following two special cases:
1. A transformer with a 2-wire primary and 2-wire secondary, provided the transformer primary is protected in accordance with 450.3
2. A 3-phase, delta-delta-connected transformer having a 3-wire, single-voltage secondary, provided its primary is protected in accordance with 450.3
Except for these two special cases, transformer secondary conductors must be protected by the use of over-current devices, because the primary over-current devices do not provide such protection....
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The basic math of 4:1 current multiplication of primary-to-primary vs line-to-neutral is a cooker.
Again and again I must proclaim: AHJ -- handbook in hand -- will be shooting down delta-wye transformers without OCP at the secondary.
Last edited by Tesla; 02/20/10 10:09 PM.
