Major Underground Design
John Perez
Underground
Engineering Design
Practices
Three Phase Circuit
– Infrastructure: Ductbanks & Manholes
• All three phase ductbanks are 4” & 2” PVC conduits encased in 3 inches of concrete
– Conduits are separated by 3 inches held in place with spacers (every 4’)
– One 4/0 Copper bare Neutral per circuit feeder is placed above the spacers within concrete
• Ductbanks have 30” of cover in private property, 36” in public ROW and 48” of cover in substations
• Ductbanks are sized in the planning stage between Developers, Distribution Planning and
Underground Engineering
Underground
Engineering Design
Practices
– Ductbanks & Manholes (continued)
• Manholes are spaced approximately 350’-400’ and encased in flowable fill
– Distances may vary depending on ductbank routing/ design
– Typically manhole alignment allows the duct to entire on center of the corners (diamond)
• Manholes range from 5’x5’x6’ to12’x12’x12’ according to the ductbank size
– Manholes are H20 rated and contain thin wall knockouts in the corners
– 12” diameter sump in the middle to allow for pumping
Underground
Engineering Design
Practices
Neutrals
Hooks/ Racks
Circuit Tag
Permanent Ladder
Color Coded
Corner window
– Design Philosophy
Underground
Engineering Design
Practices
Substations
• Ductbanks are typically 33-4” & 3-2” PVC conduits inside the station with 6-4” conduits between
substation switchgear tie cubicles
– Provides Cooling conduits, Circuit reroutes and new feeder circuits
– 750 kCM, Copper, EPR conductors are used for circuit exits (both 15 kV & 35 kV circuits)
Dips
• Installed to avoid overhead conflicts, customer requested jobs, underground conversion areas
• Circuit Dips when feasible contain an “Express Feeder”
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Express feeders are fed directly between overhead “P-Switches” and do not feed any padmount equipment
Circuits dips are fed with 750 kCM, Copper, EPR conductors (both 15 kV & 35 kV circuits)
Separate Risers feed customer loads
Avoids prolonged outages (SADI, SAFI) due to Padmount Switchgear replacements/ failures
Loop Feeds
• CPS Energy padmount transformers are fed between: Risers; Riser to Padmount Switchgears;
and Padmount Switchgear
• Padmount Switchgears are fed from a solid blade “P Switch” with 4/0 AWG Copper EPR
conductors or greater
• Padmount transformers are typically fed with bundled #2 AWG Aluminum XLPE 15 kV or 1/0 AWG
EPR Aluminum 35 kV conductors
• Padmount transformer usually have three 4” PVC conduits in the slab
• Loop connected kVA are a maximum of 2,500 kVA and 6,000 kVA for 15 kV primary and 35 kV
primary respectively, with open points at the load midpoint
Underground
Engineering Design
Practices
Loop Feeds (continued)
• When designs require manholes and/or padmount switchgears, the transformers are fed from
manholes only
– To allow future expansion with minimal disruption transformers are looped through manholes
Underground
Engineering Design
Practices
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Job Sketches
• 1-lines are created for 3 or more transformers and kept on the server
– Dashed lines indicate planned infrastructure/ cable installation
– Solid lines indicate in the field conditions
– Open points, circuit numbers, footages and size/number of conduits on the drawing
• MicroStation sketches are required for detailed drawings
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Padmount details showing rebar layouts, conduit locations
Manhole/ ring & cover apron specifications
Ductbank requirements and routing
Cable pulling schedule
Manhole cable routing
• GIS also logs detailed information
– Circuit conductivity & general placement of facilities
– Circuit number, facilities information, open points, etc
Underground
Engineering Design
Practices
1-line example
Underground
Engineering Design
Practices
Underground
Engineering Design
Practices
Underground
Engineering Design
Practices