Services Ground Wire Sizing......

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jsinclair

Member
Location
Pennsylvania
Nagging and troublesome question here that I am looking for some feedback on. Let me use the following scenario:

A new 200A, single phase, 120/240v service. The service passes through the meterbase, then to a 200A, single phase, 120/240v fused disconnect. At which point the neutral is bonded to earth ground. A conduit connects the disconnect with a 42 space electrical space located elsewhere in the building. The electrical panel is a main lug type rated for 200A.

Assuming the feeds are 4/0's for the "hots", a 4/0 for the neutral, and a #2 on the ground to the disconnect, what size ground wire should be carried from the disconnect to the panel? Why? References?

I would like to give my thoughts on the subject now, but I am trying to persuade others with my reasoning and I don't want to "stack the deck".

Thnx,

John

[ July 08, 2004, 06:30 PM: Message edited by: jsinclair ]
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: Services Ground Wire Sizing......

Oh I missed a couple of questions. :)

Why?

The 6 cu or 4 al is sufficient to operate the 200 amp overcurrent protection.

References?

Table 250.122
 
ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
Re: Services Ground Wire Sizing......

This is assuming copper, so correct me if I'm wrong.

Since you increased the size of ungrounded conductors, you will also need to increase the size of the equipment grounding conductor.

For a 200A overcurrent device, you would normally install 3/0, which is 167,800 circular mils. You elected to install 4/0, which is 211,600 circular mils...a difference of 126%. You must also increase the EGC by this value of 126%. So, normally a #6 would work, but you must take its size (26240) and multiply it by 1.26, and you will come up with a value of 33089, which is a #4.

References:
310.16
250.122(B)
Table 8, chapter 9.

Edit: If you are using aluminum, just do what Bob said.
:)

[ July 08, 2004, 06:53 PM: Message edited by: ryan_618 ]
 
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