You must ground direct-current systems, but where? The answer depends upon whether the source is off-premises or on-premises. Note that the connection in question is to the earth [see the definition in NEC Art. 100], not a metallic path (bonding).
If the source is off-premises, then make the grounding connection at one or more supply stations [NEC Sec. 250.164(A)]. Note that you cannot make a grounding connection at individual services or at any point on the premises wiring.
If the source is on-premises, then you have three options [250.164(B)]. You can make this connection:
- At the source.
- At the first system disconnecting means or overcurrent device.
- By other means that accomplish equivalent system protection and that use equipment listed and identified for the use.
Size the grounding electrode conductor as follows:
- Not smaller than the neutral and not smaller than 8 AWG copper, 6 AWG aluminum, if on a 3-wire balancer set with overcurrent protection per 445.12(D).
- Not smaller than the largest conductor, if not like the preceding system.
Depending on what it’s connected to (e.g., ground ring), the minimum size is capped [250.166(C), (D), and (E)].
