ELECTRICAL SERVICE RULE BOOK BULLETIN NUMBER: 02-Rev1 DATE: August 18, 2011 EFFECTIVE: Immediately ORIGINATED: Electrical Standards INDIVIDUALS AFFECTED: Internal and External Customers SUBJECT: Grounding CT Cabinets INTRODUCTION Figure 501D, Page 57 of the 2011 ESR Book shows an equipment bonding conductor, extending from the neutral bar, of the CT Cabinet, to the metal cabinet of the CT Cabinet. It references Section 503. Section 503.A. states: Equipment bonding conductors on the supply side of service shall be sized according to NEC 250.102C. The following table may be used as a guide in determining the minimum size for equip. bonding conductors for metering cabinet installations. Table 503, on page 60, lists the minimum equipment bonding conductor size for various CT Cab amp ratings and is based on the bonding jumper having an area not less than 12 1/2 percent of the area of the largest phase conductor. THE PROBLEM An Electrical Inspections Department found equipment bonding conductors, as small as #12, in some CT Cabinets. The #12 bonding conductor is multiple sizes too small and could have resulted in a catastrophic injury. To correct for this serious problem, the Electrical Inspections Department began requiring an equipment bonding conductor, sized according to NEC 250.122, extended from the main disconnect to the CT Cabinet. In a few cases, this required the extension of a large copper conductor, for more than 100 feet, from the main disconnect, to the CT Cabinet. THE SOLUTION The Electrical Inspections Department has determined that it will accept the grounding of the CT Cabinet, per Figure 501D, Page 57 of the 2011 ESR Book, provided the equipment bonding conductor, extending from the neutral bar of the CT Cabinet to the metal cabinet of the CT Cabinet and is properly sized according to NEC 250.102C, as shown in Table 503. These bonding conductors are about 1 to 2 sizes larger than required in NEC 250.122 and bonding the CT Cabinet to the neutral conductor is permitted by NEC 250.142.A. QUESTIONS Contact Tony Porch (319-786-4183) or Rob Weber (319-786-4743) in Electrical Standards with any questions concerning this bulletin. Page 1 of 6
Page 55 IP&L / WP&L Chapter 5 501. GROUNDING ELECTRODE SYSTEM Grounding Electrodes NEC 250.52: 1. Metal Underground Water Pipe. NEC250.52 (A)(1) 2. Metal Frame of a Building or Structure - NEC250.52 (A)(2) 3. Concrete Encased Electrode (Ufer) - NEC250.52 (A)(3) 4. Ground Ring - NEC250.52 (A) (4) 5. Rod and Pipe Electrodes - NEC250.52 (A)(5) 6. Plate Electrodes. - NEC250.52 (A)(6) A. If available on the premises at each building or structure served, each item in NEC 250.52 (A) 1-6, shall be bonded together to form the grounding electrode system. B. The grounding electrode conductor (GEC) 1) The grounding electrode conductor shall not be run through meter sockets, Instrument Transformer Cabinets (CT), or the utility portion of a metering pedestal. 2) The grounding electrode conductor may be terminated in one of the following. a) In the service disconnect(s). b) In the metering equipment containing a service disconnect c) In the termination compartment of multiple metering installations. C. Metal underground gas piping systems on the line side of the gas meter shall not be used as a grounding electrode or bonded to the grounding electrode system. D. If two ground rods are installed (NEC 250.56), both ground rods shall be installed to left or right of meter socket but not in front. The grounding electrode conductor shall not pass in front of meter socket or pedestal. Page 2 of 6
Page 57 IP&L / WP&L Chapter 5 FIG 501C FIG 501D Grounding Instrument Transformer Cabinets C.T. Cabinet Outside Wall Meter Socket Equipment Bonding Conductor Neutral Bus Service Disconnect min. #8 CU Neutral Bus Equipment Bonding Conductors (See Section 503) Cabinet Bonding Point Ground Line Raceway for Service Conductors Grounding Electrode Conductor Page 3 of 6
Page 59 IP&L / WP&L Chapter 5 502. BONDING SERVICE ENTRANCE AND METERING EQUIPMENT A. The electrical contractor is responsible for all bonding connections. B. The termination cabinet, the CT cabinet and the meter socket enclosure shall be bonded to: 1. The system neutral when they are located on the line side of or at the main disconnect. 2. The equipment grounding conductor, when all of the following apply: a. They are installed the load side of the main disconnect. b. No service ground-fault protection is installed. c. They are located immediately adjacent to the main disconnect. C. Bonding shall be provided where necessary to ensure electrical continuity and the capacity to safely conduct any fault current likely to be imposed. 503. BONDING CONDUCTOR SIZING A. Equipment bonding conductors on the supply side of service shall be sized according to NEC 250.102C. Table 503 may be used as a guide in determining the minimum size for equip. bonding conductors for metering cabinet installations. B. Table 503 assumes 75 C temperature ratings for service entrance conductors, with no adjustment factor for more than 3 current-carrying conductors. On a 3-phase 4- wire wye service where the major portion of the load consists of nonlinear (harmonic) loads, the neutral shall be considered as a current-carrying conductor (NEC 310.15B4c); in such cases a larger equipment bonding conductor shall be used. Page 4 of 6
Page 60 IP&L / WP&L Chapter 5 TABLE 503 Metering Cabinet Rating, Amps 400 600 800 1200 1600 2000 2500 3000 Service Entrance Conductor Size (75 C Copper, AWG/kcmil) Minimum Equipment Bonding Conductor Size or Equivalent Area for Parallel Conductors (Copper, AWG/kcmil) Number of Runs 3/0 2 #2 4/0 2 1/0 600 1 1/0 3/0 3 1/0 4/0 3 2/0 350 2 2/0 3/0 4 2/0 350 3 2/0 350 4 4/0 500 4 250 600 3 250 350 6 300 500 5 350 600 4 300 350 7 350 500 6 400 600 5 400 350 9 400 500 7 500 600 6 500 350 10 500 500 8 500 600 8 600 750 7 750 Page 5 of 6
ELECTRICAL SERVICE RULE BOOK BULLETIN 2011 NEC 250.142 Use of Grounded Circuit Conductor for Grounding Equipment (A) Supply-Side Equipment. A grounded circuit conductor shall be permitted to ground non current-carrying metal parts of equipment, raceways, and other enclosures at any of the following locations: (1) On the supply side or within the enclosure of the ac servicedisconnecting means (2) On the supply side or within the enclosure of the main disconnecting means for separate buildings as provided in 250.32(B) (3) On the supply side or within the enclosure of the main disconnecting means or overcurrent devices of a separately derived system where permitted by 250.30(A)(1) Transformer CT Cab Main Disconnect FAULT RED circuit, path of fault current back to transformer when BONDING in the CT Cabinet is per Alliant Energy (AE) Electric Service Rule (ESR) Book Section 502. BLUE circuit, path of fault current back to transformer when GROUNDING the CT Cabinet to the Main Disconnect. Page 6 of 6