I am installing a subpanel in the basement. The main service is in the garage. The main service has its ground wire runs to the basement and it connects to the cold water pipe as it enters the basement from the outside. This location is about 5 feet from the subpanel location.
I know the subpanel needs to be grounded to the main service panel using a correctly sized wire.
My question is whether I can ground the subpanel to the same cold water pipe at about the same location that the main service connects to the pipe. Since they both ground to the same grounding item, would this be allowed in the NEC?
(My appologies to any pros that may be cringing at my terminology).
As long as you run a properly sized Equipment Grounding Conductor in the wiring method you use to supply the sub panel then establishing an auxiliary grounding connection from the sub panel's Equipment Grounding Conductor buss bar, but not from it's neutral buss bar, to the underground metal water piping will do no harm. In that case you will be using the metal water piping as a Supplementary Grounding Electrode.
It is important to note that you must keep the grounded current carrying conductor (neutral) aloof from ground at all points beyond the service disconnecting means and that definitely includes in your sub panel. You must not bond the neutral to ground in that sub panel. Any bonding screw or strap that is supplied with the panel to connect the neutral buss to the panels cabinet must be removed and discarded. The Grounding Electrode Conductor (GEC) must not be terminated to the same buss bar as the neutral or otherwise connected to the neutral within the sub panel's cabinet. If you fail to keep the neutral and EGCs isolated from each other in the sub panel then neutral current will be flowing on the Supplementary electrode GEC and on the Service Equipment's GEC under normal operating conditions. Please take my word for it that that is not a good thing.
-- Tom H
300.3 Conductors. (B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (4).
250.54 Supplementary Grounding Electrodes. Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or
250.53(C) or the resistance requirements of 250.56, but the earth shall not be used as the sole equipment grounding conductor.
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