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Wednesday, October 4, 2017
Question:
Re: CQD answer published Thursday, September 28, 2017 -Number of Ground Rods
Hey Charlie.
Regarding the answer to today's CQD, here's the "Rest of the Story'". The second sentence of 250.50 states, "Where none of these grounding electrodes exist, one or more of the grounding electrodes specifed in 250.52(A)(4) through (A)(8) shall be installed and used."
Rod-type grounding electrodes are included in 250.52(A)(5). Two ground rods are required according to 250.53(A)(3) unless one ground rod has a resistance to earth of 25 ohms or less as provided in the Exception. Then, one ground rod is permitted.
This writer is aware that some AHJs permit only one ground rod to be installed for temporary services so the installer should verify what the local enforcement practices are.
Phil Simmons
A
Answer:
Hey Phil thanks for your comment, that should have been included in the original answer and it is no surprise that you would notice given all your experience. As you point out if a ground rod is the only type of grounding electrode used then it must have a resistance to ground of 25 ohms or less. If not then an additional electrode is required and it is common to use a second ground rod. This applies even for temporary wiring because nothing in Article 590 changes the rule as stated in 590.2(A).