In 250.24(C) (1) and (2), the minimum sizing of the grounded conductor brought to service equipment is spelled out. 250.24(C) (1) states that this conductor shall be smaller than the required grounding electrode conductor specified in 250.66.
As an example, take a 480V, 3-phase, 4-wire, 2000 amp service that is fed from a utility transformer with aluminum conductors (6 parallel underground conduit runs of 3-750 kcmil per phase, and a #3/0 AL grounded conductor per phase), which terminates in a 2000A main switchgear. This service is grounded at the main switchgear with a #4/0 copper grounding electrode conductor that is connected to building steel and a concrete encased electrode. The service is in an industrial plant with mainly balanced 3-phase motor loads.
The neutral load for the service was calculated to require no more than a #3/0 AL conductor in parallel, which exceeds the minimum of 12.5% of the phase conductor in circular mils, and also exceeds the minimum conductor size of #1/0 AWG required for parallel runs.
The question is does this installation meet code?
If you follow 250.24(C) (1), the size for the grounded conductor shall not be smaller than the required grounding electrode conductor in table 250.66, which is a #3/0 Copper or 250 kcmil AL, and the code is not met.
If you apply 250.24(C) (2) for parallel conductors, it only states the grounding conductor needs to be sized based on the size of the ungrounded service conductor and not smaller than #1/0 AWG based on 310.4.
Does 250.24(C) (1) still apply for the minimum size of the grounded conductor in a parallel run, making the minimum size of the parallel conductors 250 kcmil AL, or should 250.24(C) (2) take precedence for the minimum size?
As an example, take a 480V, 3-phase, 4-wire, 2000 amp service that is fed from a utility transformer with aluminum conductors (6 parallel underground conduit runs of 3-750 kcmil per phase, and a #3/0 AL grounded conductor per phase), which terminates in a 2000A main switchgear. This service is grounded at the main switchgear with a #4/0 copper grounding electrode conductor that is connected to building steel and a concrete encased electrode. The service is in an industrial plant with mainly balanced 3-phase motor loads.
The neutral load for the service was calculated to require no more than a #3/0 AL conductor in parallel, which exceeds the minimum of 12.5% of the phase conductor in circular mils, and also exceeds the minimum conductor size of #1/0 AWG required for parallel runs.
The question is does this installation meet code?
If you follow 250.24(C) (1), the size for the grounded conductor shall not be smaller than the required grounding electrode conductor in table 250.66, which is a #3/0 Copper or 250 kcmil AL, and the code is not met.
If you apply 250.24(C) (2) for parallel conductors, it only states the grounding conductor needs to be sized based on the size of the ungrounded service conductor and not smaller than #1/0 AWG based on 310.4.
Does 250.24(C) (1) still apply for the minimum size of the grounded conductor in a parallel run, making the minimum size of the parallel conductors 250 kcmil AL, or should 250.24(C) (2) take precedence for the minimum size?
