Size of Equipment Grounding Conductor on a 4,160 Volt Circuit

[frohara]

I understand the equipment grounding conductor is sized based on NEC 250.122(A): "General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122,…”. Table 250.122 lists the minimum size equipment grounding conductor per the "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment,...".

On a 4,160 volt circuit, is this the setting of the 50/51 relay (2000 Amp setting which requires a 250 kcmil copper equipment grounding conductor in my case), or the setting of the 51G relay (400 Amp setting which would require a #3 copper equipment grounding conductor in my case)?

 

[davidbeach]

Based on extending the low voltage device logic to this application, I'd say it is your phase relay setting. When a low voltage breakers has a built in ground trip unit, the setting of the ground unit isn't considered when sizing the EGC.

 

[jghrist]

Note that the EGC doesn't have to be larger than the phase conductor. For feeders 600V and below, the overcurrent protection is based mostly on the ampacity of the conductor. Overcurrent protection for 600V and below normally results in phase conductors larger than that required by Table 250.211 for EGCs.

Other criteria govern for feeders over 600V, based on Article 240 Part IX.

 

[dpc]

I would use the phase pickup. The reference in the table is to "overcurrent", not ground fault. The NEC defines "overcurrent" in relation to the ampacity of the conductors.

The underlying logic of the EGC sizing is to provide sufficient ground fault current back overcurrent device to allow it to operate before the EGC is damaged.

As a practical matter, I would be concerned about the ground pickup being changed in the future by someone who doesn't know what the ground wire size is.

This what I do even for resistance-grounded systems.

David Castor

http://www.cvoes.com