Issued
13-Aug-02
Revised
27-Jan-09
CONSTRUCTION STANDARD
ELECTRIC OPERATIONS ORGANIZATION
C3801
Revision #2
Page 1 of 7
C3801
****This standard supercedes BECo CS2.12-2.18, COM/E 4-0835 NE-2 and COM/E
1-0796 NE-13, NE-13A, NE-14
INSTALLATION OF PRECAST PAD FOUNDATIONS FOR 4KV, 15KV, & 25KV PAD
MOUNTED, THREE-PHASE DISTRIBUTION TRANSFORMERS
Table of Contents
Page
1) Scope……………………………………………………………… 2
2) Bill of Materials…………………………………………………… 2
3) General Notes……………………………………………………. 2-4
4) Construction Drawings…………………………………………... 5-7
5) Related Standards……………………………………………….. 7
6) Approvals…………………………………………………………. 7
Issued
13-Aug-02
Revised
27-Jan-09
1.0
CONSTRUCTION STANDARD
ELECTRIC OPERATIONS ORGANIZATION
C3801
Revision #2
Page 2 of 7
PURPOSE
This standard illustrates the approved construction practices for the installation of
precast pad foundations for three-phase distribution transformers up to 2500 kVA
installed outdoors.
2.0
BILL OF MATERIALS
Material Description
Precast Concrete Pad Foundation
Pad #1 (Transformers up to 300kVA)
Pad #2 (Transformers 500 to 2500kVA)
1/2” Thick Plywood - 60” x 24” (minimum size)
42” x 15” (cutout opening for Pad #1)
54” x 15” (cutout opening for Pad #2)
1-1 ½” Diameter Crushed Stone
5/8” x 8’ Ground Rod
4/0 AWG Bare Copper Wire
1/0 AWG Bare Copper wire
4/0 to Ground Rod Connector, 4/0 Wire – Rod
Sand/Toping Concrete Mix, Quickrete
Product No. 1103 or approved equal
Sikaflex – 1a Concrete Sealant or equal
4” Diameter Insulated Grounding Bushing
5” Diameter Insulated Grounding Bushing
#4 AWG Bare Copper Wire
Ground Wire Connector – 4/0-1/0, 1/0-#4 Cu
Catalog ID
14706
14715
N/A,
See M3801
N/A
9229
104
100
597
N/A
N/A
1188
1189
114
608
3.0 GENERAL NOTES
3.1
Prior to backfill NSTAR inspection and approval of grounding and pad
installation is required. The contractor shall be responsible for ensuring that
NSTAR inspection is scheduled and completed.
3.2
The installation (including grounding), using the appropriate precast pad,
(based on transformer size, see “Bill of Materials” above) is detailed in
Figures 1 & 2. Grounding connection between the ground rods and 4/0
ground wire shall be direct burial rated Hy-Ground compression, Cadweld, or
mechanical grounding connectors as approved by NSTAR.
Issued
13-Aug-02
Revised
27-Jan-09
CONSTRUCTION STANDARD
ELECTRIC OPERATIONS ORGANIZATION
C3801
Revision #2
Page 3 of 7
3.3
All Catalog IDs that are “N/A” shall be furnished by contractor.
3.4
Ground surface shall be level for at least two feet away from the edge of the
pad on all sides. Top of pad shall be 4” min., 5” max. above finished grade.
3.5
If possible, avoid locating pad at low points in final grade or on abrupt slopes.
A pre-cast retaining wall (Catalog ID 14672) may be required if there is a
possibility of equipment being undermined or covered over. Avoid locations
likely to be covered by plowed snow or made inaccessible.
3.6
Pad shall be set on mechanically compacted 1 - 1 ½” diameter crushed
stone (refer to appropriate figure for depth of stone) over virgin or
mechanically compacted soil. All deleterious soils including organics, ashes,
cinders, shells or frozen materials shall be removed and replaced with
suitable backfill before installing the foundation.
3.7
For recommended minimum clearances from foundation (pad) to buildings,
building openings, landscaping, or traveled ways refer to NSTAR
Construction Standard C3802 for Clearance Requirements around
Padmounted Electrical Equipment.
3.8
A maximum number of eight (8) secondary conduits shall be permitted with
conductor sizes up to 750 kcmil copper or aluminum. For installations with
more than eight sets of secondary conductors (in parallel), a secondary bus
transclosure cabinet is required. The secondary bus transclosure cabinet
shall be sized and installed in accordance with the National Electrical Code
and state/local requirements. Refer to Figure 3 for location.
3.9
When installed the front (door side) of the distribution transformer will cover
the pad cutout. Primary cable sweeps shall terminate within the pad cutout
below the primary compartment of the transformer (the left side of the pad
cutout when looking at the front of the transformer). The secondary conduit
risers shall terminate within the pad cutout below the secondary
compartment of the transformer (the right side). Refer to Figures No. 1 & 2
for typical positioning of installed conduits within the precast pad cutout.
3.10 Plug all conduit ends to prevent moisture and debris from entering conduits
prior to transformer installation. Do not install mule tape in new conduits
until after the bottom of the cutout is sealed with sand/topping concrete mix
per 3.11.
Issued
13-Aug-02
Revised
27-Jan-09
CONSTRUCTION STANDARD
ELECTRIC OPERATIONS ORGANIZATION
C3801
Revision #2
Page 4 of 7
3.11 After pad is installed and set level on the crushed stone with the conduits in
place, one inch depth of the crushed stone shall be removed (and properly
disposed of) from the entire area within the bottom of the cutout. A
sand/topping concrete mix (refer to Bill of Materials) shall be placed within
the cutout to seal the bottom as shown in Figures 1 & 2. All conduits shall be
sealed with PVC fitted caps before the mix of concrete is placed within the
cutout. The sand/topping mix of concrete shall be hand troweled and pitched
so that the middle of the cutout bottom is lower than the surrounding edges.
The purpose of sealing the bottom of the cutout is to prevent small amounts
of leaking and dripping transformer fluids from seeping through the crushed
stone beneath the precast pad. A cleanup of small amounts of transformer
fluids can be cleaned up more efficiently and less costly within the cutout.
3.12 Grounding bushings shall be installed on all primary and secondary conduits.
A #4 AWG Bare Copper Wire shall be shall be mechanically connected to all
conduit bushings and to the 1/0 AWG Bare Copper Grounding Wires within
the cutout as shown in Figures No. 1 & 2.
3.13 Around the perimeter of the cutout opening apply a ½” wide x ½” high bead
of “Sikaflex-1a” polyurethane elastomeric sealant/adhesive (or approved
equal). This caulking material is to be applied ¼” from the edge of the
opening. The purpose of this “dam” is to divert small oil leaks away from the
cutout out and to the edge of the pad where the leak will be noticed sooner.
Allow material to cure before installing diamond plate cover.
3.14 Cable pulling mule tape may be installed after all prior steps are complete.
Temporary caps may be used until all cable is pulled in, but known spare
ducts must be plugged with expanding type plugs that seal against the inner
duct wall. Coil all ground wires and mule tape ends inside cutout.
3.15 Opening in top section of pre-cast pad shall be completely covered with ½
thick plywood and secured with ½ diameter bolts aligning with the predrilled
inserts in the pre-cast pad. Plywood cover shall remain in place until the
transformer is installed. Refer to M3801 “Material Standard for Precast
Pads” for plate details.
Issued
13-Aug-02
Revised
27-Jan-09
CONSTRUCTION STANDARD
ELECTRIC OPERATIONS ORGANIZATION
C3801
Revision #2
Page 5 of 7
4.0 CONSTRUCTION DRAWINGS
4" D IA . G A L V . S C H . 40 S T EEL C O N D U IT
S EC O N D A R Y S E R VIC E
(36 " R A D IU S Q U A R T E R -B E N D )
5
X 8 '-0" G R O U N D
R O D - 2 (T YP) W /
C O M P R E SS IO N
G R O U N D IN G
C O N N E C T IO N O R
C AD W E LD
4 /0 A W G ST R C U
8"
6 '-6"
1 " T H IC K SE ALA N T
2
B E A D A P P LIED
A LL A R O U N D
1 '-6"
1 '-6"
3'-6"
6 '-0"
A
1'-3"
6"
6"
4"
4"
A
9"
G R O U N D W IR E
C O N N E C T O R (T YP)
C PAD
3"
4" O R 5 " D IA . G A L V . S C H . 40
S T E EL C O N D U IT
P R IM A R Y SER VIC E
6"
11"
11"
6"
4"
# 4 A W G C U (T YP)
PLAN
1
A W G S T R C U 48" LEN G T H
A V A IL A B L E (M IN )
A B O V E T O P O F PAD (T Y P)
0
7 '-6"
T O P O F C O N D U IT G R O U N D IN G
B U S H IN G S S H A L L B E 8" B ELO W
T H E T O P O F T H E PA D
P R E C A ST P AD
S A N D / T O P PIN G
M IX C O N C R E T E
3 /4 " C H A M F ER
A LL A R O U N D
S E A L A N T B E A D (T Y P)
F IN IS H
G R AD E
4"
1 " (T Y P ) @ L O W P O IN T
8"
2 " (T YP)
4"
S LO P E (TY P )
8"
1 8 " M IN .
6"
(T Y P)
1"
4 " (T YP )
2 '-2"
1 '-6"
3 " M IN .
1 - 11 2 " D IA . C R U S H E D S T O N E (W E LL T A M P ED )
7 ' - 6 " x 7' - 0 " x 18 " D EPTH BELO W PAD
S E C T IO N A -A
Figure 1 – Pad #1: Three-Phase 75 - 300kVA Transformer Precast Pad Installation
& Grounding Detail
Note: Contractor is responsible to obtain NSTAR approval before backfilling conduit and precast pad.
Issued
13-Aug-02
Revised
27-Jan-09
CONSTRUCTION STANDARD
ELECTRIC OPERATIONS ORGANIZATION
C3801
Revision #2
Page 6 of 7
4" DIA. GALV. SCH. 40
STEEL CONDUIT
SECONDARY SERVICE
(36" RADIUS QUARTER-BEND)
4/0 AWG STR CU
5
8'-0"
8"
X 8'-0" GROUND ROD2 (TYP) W/
COMPRESSION
GROUNDING
CONNECTION OR
CADWELD
1 " THICK SEALANT
2
BEAD APPLIED
ALL AROUND
4'-6"
1'-9"
8'-0"
1'-9"
1' -3"
A
A
6"
10"
4"
9"
GROUND WIRE
CONNECTOR (TYP)
4"
6"
18"
10"
6"
6"
4"
2'-3"
4" OR 5" DIA. GALV.
SCH. 40 STEEL CONDUIT
PRIMARY SERVICE
#4 AWG CU (TYP)
C PAD
PLAN
1
0 AWG STR CU 48" LENGTH AVAILABLE (MIN)
ABOVE TOP OF PAD (TYP)
9' - 0"
PRECAST PAD
TOP OF CONDUIT GROUNDING
BUSHINGS SHALL BE 8" BELOW
THE TOP OF THE PAD
2" (TYP)
3/4" CHAMFER
ALL AROUND
SAND / TOPPING
MIX CONCRETE
SEALANT BEAD (TYP)
8"
FINISH GRADE
1" (TYP) @ LOW
POINT
4"
SLO PE (TY P)
4"
8"
18" MIN.
4" (TYP)
1"
6" MIN.
2'-2"
1'-6"
3" MIN.
1 - 11 2" DIA. CRUSHED STONE (WELL TAMPED)
9' x 9' x 18" DEPTH BELOW PAD
SECTION A-A
Figure 2 – Pad #2: Three-Phase 500 - 2,500 kVA Transformer Precast Pad
Installation & Grounding Detail
Note: Contractor is responsible to obtain NSTAR approval before backfilling conduit and precast pad.
Issued
13-Aug-02
Revised
27-Jan-09
CONSTRUCTION STANDARD
ELECTRIC OPERATIONS ORGANIZATION
C3801
Revision #2
Page 7 of 7
INCOMING 4, 13.8, 25KV PRIMARY
DISTRIBUTION
TO CUSTOMER
SWITCHGEAR
10' MINIMUM
NSTAR THREE-PHASE
PADMOUNTED TRANSFORMER
SECONDARY BUS
TRANSCLOSURE CABINET
(WHEN REQ'D)
Figure 3 - Transformer Pad and Transclosure Cabinet Detail
5.0 RELATED STANDARDS
5.1
5.2
5.3
5.4
5.5
M3801 Material Standard for Precast Pads and Covers
M3907 Three Phase Distribution Padmount Transformers 300-2500kVA
C3802 Clearance Requirements Around Padmounted Electrical Equipment
C1100 Distribution Duct Bank Construction and Materials
D3820 200 Amp Underground Distribution Design Standard
6.0 APPROVALS
Approved by: Amin Jessa
Director, Distribution Engineering