This article was posted before 01/01/2011 and is most likely outdated.

Grounding Electrode Conductor at Transformer

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Mike, I have a transformer installed on a fishing pier (depth of water is 20-30 feet deep). The feeder to the transformer is supplied from service equipment located on the shoreline (200 feet away). This feeder is constructed of rigid nonmetallic conduit (PVC), and it contains three ungrounded (hot) conductors with an equipment grounding conductor (sized in accordance with Table 250-122). There is no effectively grounded structural metal member or effectively grounded metal water pipe to be used as the grounding electrode [250-32(a)(3)]. My questions are as follows:

1.     What safety purpose does grounding the secondary to the earth (electrode) serve?

2.     Can I use the primary feeder equipment grounding conductor (if properly sized in accordance with Table 250-66 and 250-122) as the grounding electrode conductor for the transformer? If not why?

It seems to me that the feeder equipment grounding conductor will bonded the secondary Xo to the earth (electrode), because:

a.     the secondary grounded (neutral) terminal Xo must be bonded to the metal enclosure of the transformer [250-30(a)(1)],

     the metal enclosure of the transformer must be bonded (equipment grounding conductor) to the metal parts of the service equipment [250-24(b)], by a main bonding jumper [250-28]

c.     the service equipment is grounded to the earth [250-24(a)]

3.     If the answer to the above question is No, can I install the grounding electrode conductor (sized to Table 250-66) for the transformer (separately derived system) within the primary feeder raceway and terminate this conductor to the service grounding electrode?

Mike Holt's Comment:

1. The purpose of grounding the secondary Xo terminal of the transformer (separately derived system) to the earth (system grounding*) is to quickly remove any high-voltage surges imposed by lightning, line surges, or unintentional contact with higher voltage lines on the premise wiring by providing a low impedance (resistive) path to the earth. In addition, system grounding stabilizes phase-to-ground voltage during normal operation and during line-to-ground faults. System grounding for transformers is accomplished by bonding the secondary grounded (neutral) conductor (Xo) to the earth [250-30(a)].

*System grounding is the intentional bonding of the secondary grounded (neutral) conductor to earth at the power supply.

2. No, the primary feeder equipment grounding conductor cannot be used for the grounding electrode. Why, well ... um...YOU JUST CAN'T BECAUSE THAT'S THE CODE AND DON'T GIVE ME A HARD TIME. Another way to say this is I have no clue. I do know the electrons won't know the difference.

3. YES, the grounding electrode conductor can be installed with the feeder circuit conductor, because there is no rule in the NEC prohibiting the installation of grounding electrode conductors within the same raceway with service, feeder, or branch circuits [230-7 Exception No. 1].