GEC sizing, bussbar and wire how precise do you get?

[tortuga]

Greetings all
I am curious how precise you get with GEC sizing when a service is part or all bussbar ?
Say you have two service laterals from transformer bank to one service location on a commercial building.

• First lateral lands on a 400A bus bar (meter mod with three 200A tenant meters).
• Second goes to a 400A CT can that will have two 200A disconnects tapped off it fed with 3/0 CU.

Utility owns /provides the service laterals, so they are outside the NEC

Per 250.66 Note 1 we need to add up all the kCMIL of all the service conductors and size the GEC off the equivalent size:

1. If multiple sets of service-entrance conductors connect directly to a
service drop, set of overhead service conductors, set of underground
service conductors, or service lateral, the equivalent size of the largest
service-entrance conductor shall be determined by the largest sum of
the areas of the corresponding conductors of each set.

There may be other situations where service conductors are busbar or combinations of wire and busbar.
Do you just look up the copper wire @75C that matches the busbar amps?
Or do you actually measure the busbar, convert square mills to circuilar mills?
Granted it probably does not often make a difference given the range of 250.66, but it might if you have a very long run of copper to do...
Or would some of you go off the utility demand sized laterals 350 Aluminum?
Or perhaps this is just too rare a situation.

 

[don_resqcapt19]

I would just use Note 2 to that table, and select a wire size like you would if you were installing single conductors for that size service, and use that wire size with Table 250.66 to select the size of the GEC.

 

[tortuga]

I would just use Note 2 to that table, and select a wire size like you would if you were installing single conductors for that size service, and use that wire size with Table 250.66 to select the size of the GEC.

Thanks Don
I've been working with allot of busbar lately so
I converted some 400A bar thats 1/4" X 1-1/2" to circular mills and came up with 477465
The equivalent in copper for a 400A service would be 500 kcmil so yeah not much difference.