National Electrical Installation Standards

Standards as High as Your Own

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Wednesday, January 4, 2017

Question:

Re: CQD answer published Thursday, December 22, 2016 -Article 250 Tables

Something that may help David Cotter understand which table is appropriate to use is for him to understand that Tables 250.66 and 250.102(C)(1) are applicable where there isn't an overcurrent protective device immediately ahead of the ungrounded conductors, such as at services and separately derived systems.  Table 250.122 will always be used where there is an overcurrent protective device as part of the circuit, such as with feeders and branch circuits. This is why Tables 250.66 and 250.102(C)(1) are based on the ungrounded wire size and Table 250.122 is based on the fuse or circuit breaker used on the ungrounded conductors.

The only difference between conductor sizes between Tables 250.66 and Table 250.102(C)(1) is T250.66 has definitive maximum conductor sizes for grounding electrode conductors for corresponding ungrounded wire sizes >1100 kcmil CU and >1750 kcmil AL. This is because there is a limited amount of current that will flow on the grounding electrode conductors during a fault, and the wire sizes indicated have been proven to be of sufficient size to handle the current imposed on the grounding electrode system without damage to the grounding electrode conductor.  Table 250.102(C)(1), on the other hand, requires the grounded conductor, main bonding jumper, etc. [see list at top of T250.102(C)(1)] to be sized at 12.5% of the ungrounded conductors when the equivalent ungrounded conductor size exceeds 1100 kcmil CU and 1750 kcmil AL.

Before the 2014 NEC, the only tables we had to work with were T250.66 and T250.122.  The rules didn't change in the 2014 edition. T250.66 was broken out into two separate tables for clarification purposes.

Thanks, Eddie Guidry

A

Answer:

Hey Eddie thanks for your comment. Your description provided much more clarification.

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