Where service-entrance conductors are installed in parallel in two or more raceways or cables, the size of the grounded conductor in each raceway or cable shall not be smaller than ______.
And the answer is...D, 1/0 AWG
In general, the smallest conductor allowed to be installed in parallel is a 1/0 AWG. When the ungrounded conductors are installed in parallel in more than one raceway or cable, the grounded conductor is required to be installed in each raceway or cable and sized based on the total circular mil area of the parallel ungrounded conductors in the raceway or cable. Using Table 250.102(C)(1), this method can lead to specification of a minimum size grounded conductor that is smaller than what is allowed to be installed in parallel. Therefore, the smallest grounded conductor allowed to be installed in two or more raceways is going to be limited at a 1/0 AWG.
I have a 1000 amp feed to a MCC for a process within a building and the feed is in 3 parallel sets of 500 kcmil cables coming from switchgear supplied by the service entrance. Since the MCC is not technically the "service" to the building does this requirement still apply?
Looks like 250.30, 3, (b) is clear that it does apply and the exceptions in 310.10, H do not apply.
No, the requirements of section 250.24 only apply to services. The difference is that now your conductors between the service equipment and the MCC are feeder conductors. There is not a requirement to install a grounded conductor with the feeder unless you have a load that utilizes the grounded conductor, which an MCC doesn't typically have outside of a corner grounded delta. The intent of the grounded conductor being required to be installed with every service is that it is the fault current path back to the utility transformer, hence why the requirement is found in Article 250. So a feeder is going to follow the requirements for feeder conductors. For example, if the system is 480/277 Volt, WYE connected system and is feeding a 480V MCC that is feeding all 3-phase motors, a grounded conductor is not needed. However, if the system was 240/120 Volt, Delta, 3 phase and the MCC fed a mixture of 3-phase and single phase motors, a grounded conductor would be needed to serve the load. This conductor would need to large enough to serve the load but would also be required to be installed in parallel just as the ungrounded conductors are. Section 310.10(H) would then kick in and require a minimum of 1/0 AWG. Section 250.30 doesn't apply in this situation unless there is a separately derived system in between the service and the MCC. I hope this helps to answer your question.
The MCC is 575 VAC 3 phase 3 wire with a ground conductor going into a facility in Canada but the installations in the US would be similar in that they are 480 VAC 3 phase 3 wire with a ground. All of the motors require an equipment ground provided by the MCC ground bus, this is typically connected to the service ground for the entire building. We do not typically utilize the service neutral in our systems. We do utilize the ground and require it to protect personnel and equipment. Our MCCs typically fall in the 800 – 2000 amp range and are feed as I described from service entrance switchgear. For ease of installation parallel conductor sets are the norm. in the past I have always sized ground conductors in these sets based on table 250.122, is this the correct approach? For example the 1000 amp feed would be 3 500 kcmil per phase with 3 #3 ground conductors in 3 conduits.
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