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Friday, July 28, 2017
Question:
Rule 250.122 (B) says "Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately...". What defines "the minimum size"? For example, (using values for copper from 75° C column of T310.15 (B) (16) and assuming no factoring is required), 600 kcmil is frequently used. But parallel # 3/0 (167.8 kcmil) could also be used (obviously with paralleled EGCs). Using this option, is the minimum size to be considered 335.6 kcmil?
John Duggan
A
Answer:
The minimum conductor size is determined by what is necessary for the load such as required by 210.19 or 215.2. Whether you choose to use 600 kcmil conductors or parallel two- 3/0 conductors (such as for a 400 amp load) the equipment grounding conductor is sized using Table 250.122 based on the overcurrent device. If you used larger conductors, whether in a parallel arrangement or not - for voltage drop, future use, etc., the equipment grounding conductors would need to be increased in size proportional to the increase of the ungrounded conductors.