Underground System Design
TADP 547
Thermal Considerations
Presentation 5.1
Instructor: George Matto
Key Factors that Affect Ampacity
Installation Factors - 1
• Conductor type and size (ohmic loss in conductor and neutral)
• Depth of burial
• Configuration - spacing to other loaded conductors 3-1/C Triplexed or Cradled, 3-1/C Flat or Equilateral Triangle, 3/C Cable
• Earth Ambient Temperature
• Shield Losses Size / Connection (OC or SC)
Installation Factors - 2
• Soil Resistivity (Rho) and Backfill Rho
• Duct ID (if not Direct Buried)
• Dielectric Losses Insulation
• Jacket and Conduit Rho
• Proximity to other heat sources (cables, steam pipes, furnaces…)
• Current and load cycle shape with time – Load Factor
Key Factors that Affect Ampacity (cont.)
CABLE INSTALLATION TOPICS
ELECTRICAL:
Cable Design
Cable Configuration
DB or Duct
Rho
Current
PHYSICAL:Reel HandlingCable ArrangementDB or Duct CalcsPulling Tension
AttachmentsSidewall PressureJamming
Electrical Aspects
ELECTRICAL:
Cable Design
Cable Configuration
DB or Duct
Rho
Current
Cable Design
• Cable should be selected or designed with application in mind.
• Single phase URD cable requires a neutral with the most common
being a concentric neutral around the insulated conductor.
• Concentric neutral cable can be applied used for 3-phase but with
a 1/3 neutral the cable has a lower ampacity than a 5 mil tape
shielded cable due to circulating currents. (Choice may be made
based upon common practice at the utility.)
• More than normal shielding may be required to handle fault current
which will reduce ampacity.
• Refer to cable designs from previous lectures.
Cable Design for Direct Buried, 1/Duct, 3/Duct
1 2 3 4 5 6
7 8 9 10 11 12
Cable Configuration
• Installation configuration determines ampacity
and method of installing
• Direct burial will provide highest ampacity but
could be vulnerable to dig-ins.
• 3-Conductor cable may require more splices
than single conductors for given circuit length
• Large conductors in triangular form result in
the highest ampacity.
U. G. INSTALLATION METHODS
- DIRECT BURIAL
TRENCHING
PLOWING
- DUCT
DIRECT BURIED
CONCRETE ENCASED
- TUNNELING
UNGUIDED
GUIDED
Heat Sources
• Conductor
• Neutral or Shield
• Insulation (dielectric loss)
• Surrounding-nearby facilities
WC = Watts Generated in
Conductor (Losses)
WD = Watts Generated
Insulation (Dielectric Losses)
WS = Watts Generated in
Shield/Neutral(Losses)
RI = Thermal Resistance
-cInsulation
RJ = Thermal Resistance - Jacket
RSD = Thermal Resistance - Air in
Conduit
RD = Thermal Resistance - NM Duct
RE = Thermal Resistance - Earth
Ta - Ambient
Tc - Conductor
Thermal Circuit:
Conductor to Air
Soil Rho
• Quartz Sand - 40
• Sand (wet) - 50
• Controlled (Thermal) Backfills - 50 to 70
• Concrete - 85
• Soil - 80 to 150
• Average Soil - 90
• Sand (dry) - 120
• Hostile Earth - 120 to 220+
(certain clays, cinders, high content
organic material)
• Water - 60
Depth of Burial-”It’s Not Cool” to go Deeper 3-1/C 500 Kcmil Cu, 15 kV, 90C in 4” PVC Duct
Depth 1 Duct 3 Ducts (7.5” apart)
9” 547 427
18” 528 398
36” 509 370
48” 501 359
60” 495 351
72” 490 345
96” 482 336
120” 476 (71 diff) 329 (88 diff)
Effect of Duct Size
3-1/C 500 kcmil Cu, 90C in 4”
PVC, 36” to Top of Duct
Insulation
Thickness
175 mils 508 amps
220 mils 509 amps
580 mils 510 amps
Other Ampacity Considerations
• Transient Ampacity – Time and Load Variables
• Emergency Ampacity (140ºC)
• Vertical Risers
• Adjacent Circuits
• Road Bores
• Submarine Crossings
• Steam Pipes and other Heat Sources
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