gk351 said:
ok....So if you have a 480/277 XFMR from the utility company that feeds a 600A MDP then its fine to bond in both places....whats the story if you had fed a 120/208 Step-down XFMR that feeds 100 Amp Lighting Panel from the 600A MDP. Would you bond the 120/208 XFMR? The lighting panel? This subject has always thrown me off, enlighten me please!
I focus in on the word in your above post, "
utility"
Utility distribution uses a different system, different philosophy, different code requirements/rules.
Utility uses
multipoint grounding. . They ground their neutral at multiple points thruout their distribution and don't use a separate equipment ground. . Both unbalanced operating current and fault current use the neutral. . That concept continues on thru your service drop/lateral up until your main/service disconnect. . That's why your service entrance conductors don't include an equipment ground. . And that's why you're allowed by NEC to bond one electrode conductor [such as from the ground rod] to a lineside meterbase outside and then bond a different electrode conductor [such as from the watermain] to an inside service disconnect [250.24(A)(1)]. . Sometimes on the service, multipoint is not only allowed, but it's required [250.24(A)2)]. . It's a continuation of utility multipoint grounding.
Usage [service loadside] uses
single point grounding and use a separate equipment ground for fault current. . The concept is to keep operating current flow off of the conductor that will be used to clear the fault. . That keeps current flow off of the conductor that is attached to metal parts that are accessible to John Q public [250.24(A)(5)].
For a separately derived system, such as a transformer, you can choose which spot you want to pick for your single point ground. . If you choose the
transformer, you leave the XO to case jumper installed and isolate your neutrals from your equipment grounds and enclosure inside your panel. . If you choose the
first disconnect, you remove the XO to case jumper and treat your first disconnect like a service panel. . No separation/isolation of neutrals from equipment grounds or enclosure.
