More misconception, I think, based purely on 'we've always done it this way'.
We are working on a high-rise concrete building and bringing power from the main service boards in the basement to an electrical room up in the building. Someone came up with the idea that we should run a single GEC size 500mcm for a handful of new transformers we are installing in said room. The problem now is bonding the raceway in which the 500mcm is running. Since there is a paragraph stating that the bonding jumper should be the same size as the GEC, we're bonding a 2'' emt with a 500 mcm bonding jumper. That does not make sense to me. I questioned it and brought the section below. The common GEC should have never been larger than 3/0, am I wrong?
250.30(A)(6) Grounding Electrode Conductor, Multiple Sepa-
rately Derived Systems. A common grounding electrode
conductor for multiple separately derived systems shall be
permitted. If installed, the common grounding electrode
conductor shall be used to connect the grounded conductor
of the separately derived systems to the grounding elec-
trode as specified in 250.30(A)(4). A grounding electrode
conductor tap shall then be installed from each separately
derived system to the common grounding electrode con-
ductor. Each tap conductor shall connect the grounded con-
ductor of the separately derived system to the common
grounding electrode conductor. This connection shall be
made at the same point on the separately derived system
where the system bonding jumper is connected.
Exception No. 1: If the system bonding jumper specified in
250.30(A)(1) is a wire or busbar, it shall be permitted to
connect the grounding electrode conductor tap to the equip-
ment grounding terminal, bar, or bus, provided the equipment
grounding terminal, bar, or bus is of suffıcient size for the
separately derived system.
Exception No. 2: A grounding electrode conductor shall not
be required for a system that supplies a Class 1, Class 2, or
Class 3 circuit and is derived from a transformer rated not
more than 1000 volt-amperes, provided the system grounded
conductor is bonded to the transformer frame or enclosure by
a jumper sized in accordance with 250.30(A)(1), Exception
No. 3, and the transformer frame or enclosure is grounded by
one of the means specified in 250.134.
(a) Common Grounding Electrode Conductor. The com-
mon grounding electrode conductor shall be permitted to be
one of the following:
(1) A conductor of the wire type not smaller than 3/0 AWG
copper or 250 kcmil aluminum
(2) The metal frame of the building or structure that com-
plies with 250.52(A)(2) or is connected to the ground-
ing electrode system by a conductor that shall not be
smaller than 3/0 AWG copper or 250 kcmil aluminum
(b) Tap Conductor Size. Each tap conductor shall be
sized in accordance with 250.66 based on the derived un-
grounded conductors of the separately derived system it
serves.
We are working on a high-rise concrete building and bringing power from the main service boards in the basement to an electrical room up in the building. Someone came up with the idea that we should run a single GEC size 500mcm for a handful of new transformers we are installing in said room. The problem now is bonding the raceway in which the 500mcm is running. Since there is a paragraph stating that the bonding jumper should be the same size as the GEC, we're bonding a 2'' emt with a 500 mcm bonding jumper. That does not make sense to me. I questioned it and brought the section below. The common GEC should have never been larger than 3/0, am I wrong?
250.30(A)(6) Grounding Electrode Conductor, Multiple Sepa-
rately Derived Systems. A common grounding electrode
conductor for multiple separately derived systems shall be
permitted. If installed, the common grounding electrode
conductor shall be used to connect the grounded conductor
of the separately derived systems to the grounding elec-
trode as specified in 250.30(A)(4). A grounding electrode
conductor tap shall then be installed from each separately
derived system to the common grounding electrode con-
ductor. Each tap conductor shall connect the grounded con-
ductor of the separately derived system to the common
grounding electrode conductor. This connection shall be
made at the same point on the separately derived system
where the system bonding jumper is connected.
Exception No. 1: If the system bonding jumper specified in
250.30(A)(1) is a wire or busbar, it shall be permitted to
connect the grounding electrode conductor tap to the equip-
ment grounding terminal, bar, or bus, provided the equipment
grounding terminal, bar, or bus is of suffıcient size for the
separately derived system.
Exception No. 2: A grounding electrode conductor shall not
be required for a system that supplies a Class 1, Class 2, or
Class 3 circuit and is derived from a transformer rated not
more than 1000 volt-amperes, provided the system grounded
conductor is bonded to the transformer frame or enclosure by
a jumper sized in accordance with 250.30(A)(1), Exception
No. 3, and the transformer frame or enclosure is grounded by
one of the means specified in 250.134.
(a) Common Grounding Electrode Conductor. The com-
mon grounding electrode conductor shall be permitted to be
one of the following:
(1) A conductor of the wire type not smaller than 3/0 AWG
copper or 250 kcmil aluminum
(2) The metal frame of the building or structure that com-
plies with 250.52(A)(2) or is connected to the ground-
ing electrode system by a conductor that shall not be
smaller than 3/0 AWG copper or 250 kcmil aluminum
(b) Tap Conductor Size. Each tap conductor shall be
sized in accordance with 250.66 based on the derived un-
grounded conductors of the separately derived system it
serves.
