ELECTRICAL SERVICE RULE BOOK BULLETIN
NUMBER:
DATE:
EFFECTIVE:
ORIGINATED:
INDIVIDUALS AFFECTED:
SUBJECT:
02-Rev1
August 18, 2011
Immediately
Electrical Standards
Internal and External Customers
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
ELECTRIC SERVICE RULES
GROUNDING
Chapter 5
Issued JAN 2011
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)
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.
A.
Page 2 of 6
Page 57
IP&L / WP&L
ELECTRIC SERVICE RULES
GROUNDING
Chapter 5
Issued JAN 2011
FIG 501C
FIG 501D
Grounding Instrument Transformer Cabinets
Outside Wall
C.T. Cabinet
Meter Socket Equipment
Bonding Conductor
Neutral Bus
Service Disconnect
min. #8 CU
Neutral Bus
Equipment
Bonding
Conductors
Raceway for Service
Conductors
(See Section 503)
Cabinet Bonding
Point
Ground Line
Page 3 of 6
Grounding Electrode
Conductor
Page 59
IP&L / WP&L
ELECTRIC SERVICE RULES
GROUNDING
Chapter 5
Issued JAN 2011
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 4wire 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
ELECTRIC SERVICE RULES
GROUNDING
Chapter 5
Issued JAN 2011
TABLE 503
Minimum Equipment
Metering Service Entrance
Bonding Conductor Size
Cabinet
Conductor Size
or Equivalent Area for
Rating,
(75°C Copper,
Number Parallel Conductors
Amps
AWG/kcmil)
of Runs (Copper, AWG/kcmil)
3/0
2
#2
400
4/0
2
1/0
600
1
1/0
3/0
3
1/0
600
4/0
3
2/0
350
2
2/0
3/0
4
2/0
800
350
3
2/0
350
4
4/0
1200
500
4
250
600
3
250
350
6
300
1600
500
5
350
600
4
300
350
7
350
2000
500
6
400
600
5
400
350
9
400
2500
500
7
500
600
6
500
350
10
500
500
8
500
3000
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)
Main Disconnect
CT Cab
Transformer
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