adalewhite
Master Electrician, Electrical Design, Veteran
- Location
- Georgia
- Occupation
- Licensed Electrician, Electrical Designer, A&E PM
Would love to hear some thoughts on this.
The Condition:
This is a recently completed existing installation with a 4000A Outdoor NEMA 3R switchboard fed by a pad-mount transformer, we’ll call this External-MSB. This switchboard is a manual transfer switch and has a CT Section, a 4000A Main Circuit Breaker connected to the Utility pad-mounted transformer, a 4000A Breaker connected to a cam-lock section for the connection of a temporary generator. There are (12)sets of (4)600kCMIL (AL) conductors from the transformer to the main.
There is a set of three 10’ ground rods driven 20’ apart and connected to the ground bus in the External-MSB with 1/0(CU) cadwelded to each ground rod. The ground bus is factory bonded to the neutral bus.
From External-MSB bus downstream of the Main and the Generator breaker there are (12) sets of (4)600kCMIL(AL),500kCMIL(AL);G each set feeding 4000A MLO Internal-MSB. NEC 2017 is the current code.
Questions:
Based on 250.66(A) are the #1/0(CU) to the three ground rods sufficient to meet code and not required to be sized by Table 250.66?
250.66(A) Connections to a Rod, Pipe, or Plate Electrode(s).
If the grounding electrode conductor or bonding jumper connected to a single or multiple rod, pipe, or plate electrode(s), or any combination thereof, as described in 250.52(A)(5) or (A)(7), does not extend on to other types of electrodes that require a larger size conductor, the grounding electrode conductor shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.
Discussion:
Since the External-MSB Main is the service disconnect and is outdoors, the other electrodes described in 250.52(A)(5) or (A)(7) are not available.
The connection between External-MSB and Internal-MSB is a feeder by definition and has the 500kCMIL(AL) equipment grounding conductors. I do believe these will need to be re-pulled to comply with 250.122.
The Condition:
This is a recently completed existing installation with a 4000A Outdoor NEMA 3R switchboard fed by a pad-mount transformer, we’ll call this External-MSB. This switchboard is a manual transfer switch and has a CT Section, a 4000A Main Circuit Breaker connected to the Utility pad-mounted transformer, a 4000A Breaker connected to a cam-lock section for the connection of a temporary generator. There are (12)sets of (4)600kCMIL (AL) conductors from the transformer to the main.
There is a set of three 10’ ground rods driven 20’ apart and connected to the ground bus in the External-MSB with 1/0(CU) cadwelded to each ground rod. The ground bus is factory bonded to the neutral bus.
From External-MSB bus downstream of the Main and the Generator breaker there are (12) sets of (4)600kCMIL(AL),500kCMIL(AL);G each set feeding 4000A MLO Internal-MSB. NEC 2017 is the current code.
Questions:
Based on 250.66(A) are the #1/0(CU) to the three ground rods sufficient to meet code and not required to be sized by Table 250.66?
250.66(A) Connections to a Rod, Pipe, or Plate Electrode(s).
If the grounding electrode conductor or bonding jumper connected to a single or multiple rod, pipe, or plate electrode(s), or any combination thereof, as described in 250.52(A)(5) or (A)(7), does not extend on to other types of electrodes that require a larger size conductor, the grounding electrode conductor shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.
Discussion:
Since the External-MSB Main is the service disconnect and is outdoors, the other electrodes described in 250.52(A)(5) or (A)(7) are not available.
The connection between External-MSB and Internal-MSB is a feeder by definition and has the 500kCMIL(AL) equipment grounding conductors. I do believe these will need to be re-pulled to comply with 250.122.
