Mike Dolt Enterprises
Mike Dolt Enterprises
User: sparkyguy545, Location: Fl
Asked on: Tuesday, April 26, 2011 at 12:38 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:38 am EDT
Question: Conditions:I have an installation with an INDOOR dry-type 500kVA transformer with 3-wire 480 Volt Delta with ungrounded secondary. The transformer is scheduled for replacement. If possible, I would prefer to replace it with a transformer with a 480Y/277 solidly-grounded secondary. My proposed arrangent would have virtually no neutral current and would be installed for the sole purpose of obtaining a solidly-grounded system without going to a corner-grounded delta (which I hate) or adding a zig-zag grounding xfmr (which I also don't care for and it complicates the system). My concern is that the existing conduits which interconnect the transformer to the downstream switchboard consists of 2 x 4" PVC (non-metallic) each containing 3 x 600 kcmil THWN and 1 x 1/0 Insulated Ground (note: NO neutral). Due to physical constraints, we can't replace the ductbank or the number of installed conductors.My question: "Does Exception #1 in 250.30A1 permit me to bond X0-G within the xfmr, tape my 1/O White and re-bond N-G in the new switchboard? I realize I still need to furnish the necessary grounding external to the swbd, but I don't want to make this question any longer than it already is. The white-taped 1/0 would appear to meet the sizing requirements as reduced capacity neutrals and therefore seem to meet the NEC requirements. I've always assumed this exception was "intended" to apply to outdoor pad-mount xfmrs with an indoor swbd. However, my transformer, conduit & wire and swbd would all be within the same building.
Answer
Yes, Exception #1 in 250.30(A)(1) permits the proposed arrangement with the following conditions: The transformer and the switchboard must be located within the same building. The conduit system must be non-metallic (PVC) and must contain only the transformer secondary conductors and the ground wire. The 1/0 insulated ground wire must be white-taped to indicate that it is being used as a neutral. The neutral (N) and ground (G) conductors must be bonded together within the switchboard. Proper grounding must be provided external to the switchboard, as required by NEC Article 250.
Explanation:
Exception #1 in 250.30(A)(1) states that the conductor size of a neutral conductor tapped from a solidly grounded secondary transformer winding shall not be required to be larger than the size of the ungrounded conductors tapped from the same winding if: The grounded neutral conductor is connected to the grounded neutral conductor of the system to be supplied.* The grounded neutral conductor is not used for fault-current return.In this case, the 480Y/277 transformer will have a solidly grounded secondary, and the 1/0 white-taped conductor will be used as the neutral. The neutral will be bonded to the ground in the switchboard, and no fault current will be returned through the neutral conductor. Therefore, the 1/0 white-taped conductor meets the requirements of Exception #1 and can be used as the neutral.It is important to note that this exception is only applicable to transformers and switchboards located within the same building and where the conduit system contains only the transformer secondary conductors and the ground wire. If any other conductors are present in the conduit system, such as primary conductors, the neutral conductor must be sized according to Table 310.16. Flag for review