sparkyguy545
Member
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.
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.
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