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