Duct Bank

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Caligula

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I've encountered a detail for a duct bank that I find a little strange. The duct bank has 6 conduits, two high three across, and the detail shows a note "#2 bare copper ground wire installed in alternating positions" and points to two wires shown in every other conduit spacer. To clarify, that means with six conduits you have two spacers on the top, and one on the bottom, which each have two #2 bare copper ground wires.

Does anyone know why these ground wires are shown? They are not indicated to be bonded to rebar, they are not the ground from the circuit, and it is not indicated as to what to do with them on the ends of the duct bank.

The circuits in the bank are 277/480V and are not service conductors.
 
Caligula said:
I've encountered a detail for a duct bank that I find a little strange. The duct bank has 6 conduits, two high three across, and the detail shows a note "#2 bare copper ground wire installed in alternating positions" and points to two wires shown in every other conduit spacer. To clarify, that means with six conduits you have two spacers on the top, and one on the bottom, which each have two #2 bare copper ground wires.

Does anyone know why these ground wires are shown? They are not indicated to be bonded to rebar, they are not the ground from the circuit, and it is not indicated as to what to do with them on the ends of the duct bank.

The circuits in the bank are 277/480V and are not service conductors.
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If they are not service conductors, and the 2008 NEC is enforced, EGCs are required to be installed. Why/how they have specified them this way is a mystery.
 
jrannis said:
I have seen a bare copper conductor run with duct banks. Is your concern as to why two are specified?
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First, why are they there at all? What code requires them, or if not code, why is it good practice. What do they DO?

Second, why are there two?

I just don't understand the reason for having them period.

And this detail is pre-2008 code.
 
250.4(A)
250.24(A)(5)
250.32(A)(&(B) - ['05]
250.122

Once electrically past the first point of disconnect (which you mentioned these were not service conductors), one is required to install an EGC. The grounded conductor is not permitted to be bonded to the EGC past the first disconnecting means (service disconnect). Once past the first disconnect, the grounded conductor is isolated from the EGC, so the grounded conductor will not carry ground fault current.
The bare equipment ground conductors are designed for that purpose.
I am guessing that the conductors we are discussing are the EGCs.
 
Caligula said:
I've encountered a detail for a duct bank that I find a little strange. The duct bank has 6 conduits, two high three across, and the detail shows a note "#2 bare copper ground wire installed in alternating positions" and points to two wires shown in every other conduit spacer. To clarify, that means with six conduits you have two spacers on the top, and one on the bottom, which each have two #2 bare copper ground wires.

Does anyone know why these ground wires are shown? They are not indicated to be bonded to rebar, they are not the ground from the circuit, and it is not indicated as to what to do with them on the ends of the duct bank.

The circuits in the bank are 277/480V and are not service conductors.
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These bare conductors I have seen used for tying the ground grid together throughout the plant. They are used to tie various points of the grid together, as well as attach to manholes and other structures along the way.
 
mull982 said:
These bare conductors I have seen used for tying the ground grid together throughout the plant. They are used to tie various points of the grid together, as well as attach to manholes and other structures along the way.
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Such as in equal potential grounding.
 
I have actually seen it used as the Grounding Electrode.
Never seen two, only one #2 bare stranded.
But somewhere in the specs it would mention 'grounding electrode'.
db
 
barclayd said:
I have actually seen it used as the Grounding Electrode.
Never seen two, only one #2 bare stranded.
But somewhere in the specs it would mention 'grounding electrode'.
db
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Double it up for redundancy. It is encased in concrete and run all over so if portions are lost of one the other is still providing the service.

These should also be bonded together at every manhole at every 50' feet in the embedded run.
 
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