Grounding Direct Current Systems, Part 2

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:

1. At the source.
2. At the first system disconnecting means or overcurrent device.
3. 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)].