National Electrical Installation Standards

Standards as High as Your Own

Thursday, December 20, 2018


When installing a 400A 240/120V service at a dwelling, service entrance conductors are 3/0 Cu paralleled from meter enclosure to service disconnecting means, 1 panel with 200A and 1 adjacent 200A fused service disconnect (connected by trough and bonded), the electrode grounding conductor to 2 ground rods (6' apart) and to the nearest building water lines using #2 Cu (Hot/cold/hot re-circulation and boiler zone lines all boded by same conductor) is it permissible to run the auxiliary ground from each service disconnecting means (2) with #4 cu from each if irreversibly connect to the #2 Cu equipment grounding conductor connected to the grounding electrode? {Note: the 2nd Service disconnecting means feeds a dislocated 200A sub-panel elsewhere in the dwelling with 4/0 SER Al.} Thank you.

Roger Miller



Hey Roger thanks for your question. If I understand your example correctly the 3/0 copper service entrance conductors are not in parallel (electrically joined at both ends) (310.10(H)(1) because each set supplies a separate service disconnecting means. One being the main circuit breaker in the panelboard and the other being the fusible switch. There are 3 different methods allowed for making grounding electrode conductor connections for buildings and structures with multiple service disconnecting means as stated in 250.64(D). It seems the simplest is to connect the grounding electrode conductor from the water pipe (2 AWG copper based on the parallel 3/0 ungrounded conductors) to the grounded conductor in the wireway before the panelboard and fusible switch as stated in 250.64(D). The grounding electrode conductor to the ground rods can also be connected at the same point using a 6 AWG copper conductor as stated in 250.66(A). Alternatively, a common grounding electrode conductor with taps as stated in 250.64(D)(1) or individual grounding electrode conductors as stated in 250.64(D)(2) can be used.

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