ground ring for new switchgear

[philhanna@comcast.net]

According to NEC Table 250.122, I would use a 500kcmil equipment grounding conductor in the ground ring for a low voltage switchgear with a 4000A main breaker. I have seen designs that use a 4/0 with 4 ground rods for the ground ring when the main breaker is 4000A. Is there something in the code that allows a smaller quage (than 500kcmil) ground ring conductor when utilizing multiple ground rods.

 

[raider1]

Welcome to the forum.

As far as the NEC is concerned a ground ring only needs to be a #2 copper.

Here is what the NEC says about a ground ring.

250.52(A)(4) Ground Ring. A ground ring encircling the building or
structure, in direct contact with the earth, consisting of at
least 6.0 m (20 ft) of bare copper conductor not smaller
than 2 AWG.

There is also no requirement that any ground rods be attached to the ground ring.

Chris

 

[petersonra]

According to NEC Table 250.122, I would use a 500kcmil equipment grounding conductor in the ground ring for a low voltage switchgear with a 4000A main breaker.

The ground ring is a grounding electrode, not an EGC.

The GEC conductor connecting to it would need to be sized IAW 250.52(c).

I have seen designs that use a 4/0 with 4 ground rods for the ground ring when the main breaker is 4000A. Is there something in the code that allows a smaller quage (than 500kcmil) ground ring conductor when utilizing multiple ground rods.

People design all kinds of goofy things for no apparent reason.

 

[philhanna@comcast.net]

ground ring for new switchgear

Thank you, gentlemen.
I am still confused by what seems to be a conflict when I look at 250.53(c) and it says to size all bonding jumpers that connect grounding electrodes per 250.66.

 

[roger]

Thank you, gentlemen.
I am still confused by what seems to be a conflict when I look at 250.53(c) and it says to size all bonding jumpers that connect grounding electrodes per 250.66.

Yes it does but the Table is only a part of 250.66, read 250.66(A), (B), & (C)


Roger

 

[philhanna@comcast.net]

ground ring for new switchgear

Thanks, again. If there is a ground faullt, it must be assumed that the service conductor grounds will carry the fault until it is interrupted. Is that the way you see it?

 

[raider1]

Thanks, again. If there is a ground faullt, it must be assumed that the service conductor grounds will carry the fault until it is interrupted. Is that the way you see it?

The Earth can't be used as a fault current path for an NEC regulated system.

The grounding electrode system does nothing to help clear ground faults, that is the purpose of bonding, and the main bonding jumper.

Here is what the NEC says is the purpose of the grounding electrode system.

250.4(A)(1) Electrical System Grounding. Electrical systems that
are grounded shall be connected to earth in a manner that
will limit the voltage imposed by lightning, line surges, or
unintentional contact with higher-voltage lines and that will
stabilize the voltage to earth during normal operation.

Here is what the NEC says about bonding and the effective ground-fault current path.

250.4(A)(3) Bonding of Electrical Equipment. Normally non?
current-carrying conductive materials enclosing electrical
conductors or equipment, or forming part of such equipment,
shall be connected together and to the electrical supply
source in a manner that establishes an effective groundfault
current path.

250.4(A)(5) Effective Ground-Fault Current Path. Electrical equipment
and wiring and other electrically conductive material
likely to become energized shall be installed in a manner that
creates a low-impedance circuit facilitating the operation of
the overcurrent device or ground detector for high-impedance
grounded systems. It shall be capable of safely carrying the
maximum ground-fault current likely to be imposed on it from
any point on the wiring system where a ground fault may
occur to the electrical supply source. The earth shall not be
considered as an effective ground-fault current path.

Chris

 

[philhanna@comcast.net]

ground ring for new switchgear

Thank you, all, for the expeditious and thorough responses. I feel that I understand grounding and bonding much better.