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Feeder Tap Grounds in Parallel

Question asked by grady_g on Aug 28, 2019
Latest reply on Sep 9, 2019 by chiefns830

#250.122(G) Equipment Grounding Conductors for Feeder Taps


250.122(G) says  Equipment grounding conductors run with feeder taps shall not be smaller than shown in Table 250.122 based on the rating of the overcurrent device ahead of the feeder but shall not be required to be larger than the tap conductors. 

This is easy to comply with when using a single raceway and a single set of wires for the tap. However I run into conflicting opinions on how this should be applied when the tap conductors are run in a parallel installation.


Example: Taping from a 4000A busway protected by a 4000A breaker to a 400A panel with a 400A main breaker.Table 250.122 says the EGC should be a 750 KCMIL ground when using AL wire. My tap conductor will be 10' in length. This will comply 240.21(B)(1)(4) My tap will be meeting the 1/10th the rating of the overcurrent device protecting the feeder conductors. I want to parallel (2) sets of 250 KCMIL AL for my tap conductors. 

Question: Do I need a full sized 750 KCMIL EGC in each of my tap raceways or can I use the language (but shall not be required to be larger than the tap conductors) to reduce Table 250.122's 750 KCMIL requirement to a 250 KCMIL EGC because no single ungrounded conductor is larger than a 250 KCMIL even though they are paralleled? 

Please provide solid reasoning in any answer you provide along with any engineering related information to support the answer.