Southwire Handout

Low and Medium Voltage Cable Fundamentals

Southwire Company

Presented by: Sy Shaheen

1-800-444-1700

The basic function of the conductor is to carry load current. The conductor may be solid or stranded.

stranded solid

Comparison of Copper and Aluminum Conductors

COPPER ALUMINUM

Predominates in the Industrial Market place

Predominates in the Utility Market

Greater conductivity

Heavier for same ampacity

500kcmil : 1544lb/Mft

Conduct 61% of copper for a given size

Lighter in weight than copper

750kcmil : 704 lbs/Mft

More costly. Very cost effective today

COPPER AND ALUMINUM CONDUCTOR EQUIVALENT SIZES

ALUMINUM CONDUCTOR

COPPER CONDUCTOR (KCM OR AWG)

EQUIVALENT BASED ON CONDUCTIVITY (KCM)

NEAREST STANDARD SIZE

(KCM)

4/0

350

500

1000

350

575

820

1640

350

750

1000

1750

Aluminum – 61% Conductivity of Copper

American Wire Gage

The American Wire Gage is based on the following definitions: The diameter of size #0000 (often written 4/0) is chosen to be 0.4600 inch and that of size #36, 0.0050 inch; the 38 intermediate sizes are governed by geometric progression. That is, the ratio of any diameter to that of the next smaller size (i.e. the next larger gage number) is  

Circular Mil Sizes larger than 4/0 are specified in terms of the total cross-sectional area of the conductor and are expressed in circular mils. A circular mil is a unit of area equal to the area of a circle having a diameter of one mil (one mil equals 0.001 inch).

The area of a circle, in circular mils, is therefore equal to the square of its diameter, in mils. Thus a wire 10 mils in diameter has a cross-sectional area of 100 circular mils.

For convenience, sizes are usually expressed in thousands ofcircular mils (abbreviated kcmil).

 WIRE GAGE TABLE-SOLID CONDUCTORS 

 AWG

DiameterIn

Inches

 Circular Mils

Cross SectionalArea in

Sq. inches

Lb./1000 Ft.

Copper Aluminum

0000000000

.4600

.4096

.3648

.3249

211600167800133100105600

.1662

.1318

.1045.08291

640.5507.8402.8319.5

194.7154.4122.4

97.13

1234

.2893

.2576

.2294

.2043

83690663605262041740

.06573

.05212

.04133

.03278

253.3200.9159.3126.3

77.00 61.07 48.43 38.39

5678

.1819

.1620

.1443

.1285

33090262402082016510

.02599

.02061

.01635

.01297

100.279.463.

49.9

30.46 24.15 19.16 15.19

9101112

.1144

.1019

.0907

.0808

130901038082306530

.01028

.00816

.00646

.00513

39.631.424.919.8

12.04 9.55 7.57 6.02

 WIRE GAGE TABLE-SOLID CONDUCTORS, Continued 

 AWG

DiameterIn

Inches

 Circular Mils

Cross SectionalArea in

Sq. inches

Lb./1000 Ft.

Copper Aluminum

13141516

.0720

.0641

.0571

.0508

5180411032602580

.00407

.00323

.00256

.00203

15.712.4

9.87 7.81

4.77 3.77 3.00 2.37

17181920

.0453

.0403

.0359

.0320

2050162012901020

.00161

.00128

.00101.000804

6.21 4.92 3.90 3.10

1.89 1.50 1.19

.942

21222324

.0285

.0253

.0226

.0201

812640511404

.000638

.000503

.000401

.000317

2.46 1.94 1.55 1.22

.748 .599 .471 .371

25262728

.0179

.0159

.0142

.0126

320253202159

.000252

.000199

.000158

.000125

.970 .765 .610 .481

.295 .233 .185 .146

 WIRE GAGE TABLE-SOLID CONDUCTORS Continued 

29303132

.0113

.0100

.0089

.0080

12810079.264.0

.000100.0000785.0000622.0000503

.387 .303 .240 .194

.118 .0921 .0730 .0590

33343536

.0071

.0083

.0056

.0050

50.439.7

31.4025.0

.0000396

.0000312

.0000246

.0000196

.153 .120 .0949

.0757

.0465 .0365 .0233

.0230

3738

.0045

.004020.216.0

.0000159

.0000126 .0613 .0484

.0186 .0147

Stranding

• Stranded conductors were developed to overcome the stiffness of solid wires. For any given wire size, the greater the number of strands, the more flexible the conductor becomes.

Concentric Stranding

A concentric stranded conductor consists of a central wire or core surrounded by one or more layers of helically laid wires. Each layer after the first has six more wires than the preceding layer. Except in compact stranding, each layer is applied in a direction opposite to that of the layer under it.  If the core is a single wire and if it and all of the outer strands have the same diameter, the first layer will contain six wires; the second, twelve; the third, eighteen; etc. The following diagram shows this relationship in convenient form.

Concentric Compressed Compact Solid

3% 9% 15%

Types of Strand Construction

Approximate size comparison:

Insulation

• A material that has a high resistance to the flow of current to prevent leakage from the conductor to ground. There are two types of insulations, a thermoplastic and a thermoset insulation.

INDUSTRIAL/UTILITY PRODUCTS INSULATION TYPES

1. Thermoplastic

Definition: A classification for an insulation that is extruded and quenched. It can readily be softened and re-softened by repeated heating without a substantial change in physical and chemical properties.

Types

-High Molecular Weight Polyethylene HMPE

-Poly Vinyl Chloride (PVC)

-Polypropylene

-Thermoplastic Elastomer TPE

-Tefzel

-Teflon

Normal Temperature Rating

75ºC

60º, 75º, 90º,105ºC

80ºC

90º to 105ºC

150ºC

150º or 200ºC

INDUSTRIAL/UTILITY PRODUCTS INSULATION TYPES (Continued)

2. Thermoset

Definition: A classification for an insulation that is extruded and then, when subject to heat and pressure, undergoes a chemical change known as vulcanization or crosslinking. The process fixes or establishes the properties of the material so that when again exposed to heat, the properties are not affected.

Types

- Styrene-Butadiene Rubber (SBR or GRS)

- 0il Base Rubber

- Butyl Rubber

- Silicone Rubber

- Hypalon (Chlorosulfonated Polyethylene) (CSPE,CP)

Normal Temperature Rating

75º C

75º C

85º C

125º C

90º C

2a. CROSSLINKED Definition: A classification for an insulation which is extruded as a thermoplastic and then

converted to a thermoset compound through chemical means, through irradiation techniques, or through exposure to humidity or moisture.

Types

-Crosslinked Polyethylene (XLPE)

-Ethylene-Propylene-Rubber (EPR)

-EPM

-EPDM

Normal Temperature Rating

90º C

90ºC

The Thermoset and Thermoplastic Process

IConductorCuring Tube

Thermoset Process

ConductorI

Cooling Tube

Thermoplastic ProcessI : Insulation

600 volt Cable Designation

THHN/THWN: Thermoplastic, High Heat resistant, Dry or Wet (W), Nylon covered. Rated 90°C dry, 75°C wet.

XHHW: XL polymer (thermoset), High Heat resistant, Dry or Wet (W) locations. Rated 90°C dry, 75°C wet. (Can not be direct buried)

Note: -2 indicates 90°C wet or dry.

Example: XHHW-2 rated 90°C wet or dry

BUILDING WIRE APPLICATIONSPRODUCT TYPE DIRECT BURIAL CONDUIT CABLE TRAY

THHN NO YES 1/0 & Larger

XHHW-2 NO YES NO

XLP-USE-2 YES YES NO

TC, XHHW NO YES 1/0 & Larger

TC, XLP-USE YES YES 1/0 & Larger

UL Vertical Tray Flame Test

A large scale ribbon burner flame propagation test performed in a standard configuration flame room. An 8 foot high 12-inch wide steel ladder rack with 9-inch rung spacing is mounted vertically in the center of the flame room. Test samples usually 9/c #12 Tray cable or single conductor 1/0 awg) are mounted in the tray with ½ diameter spacing, filling the center 6-inches of the tray. A 10-inch wide ribbon burner having a theoretical heat output of 70,000 Btu/hr. is placed behind the tray in a horizontal plane, 18-inches from the tray bottom and 3-inches away from the cables in the tray. The burner is applied for 20 minutes, is then extinguished and the cable fire is allowed to burn itself out. The cables must not propagate flame to the top of the tray as defined by blistering or charring of the cables. Two test are performed to demonstrate reproducibility.

Medium Voltage, 5 and 15kv Cable Fundamentals

Conductors:

The same conductor concepts for low voltage cables do apply to medium voltage cables.

As voltage increases, projections on the surface of the conductor bundle cause concentration of electrical stress. This can lead to degradation of the insulation.

Electrical stress lines

The semi-conductive layer between conductor and insulation compensates for air voids that exist between conductor and insulation. Air is a poor insulator, having a nominal dielectric strength of only 55 volts per mil, while most cable insulations have dielectric strengths over 700 volts/mil. Without strand shielding an electrical potential exists that will over-stress these air voids. As air breaks down or ionizes, it causes corona (partial discharges). This forms ozone, which chemically deteriorates cable insulations. The semi-conductive strand shielding eliminates this potential by simply “shorting out” the air. Modern cables are generally constructed with an extruded strand shield.

Medium Voltage CableConductor (Strand) Shielding

The same insulating concepts for low voltage cables do apply to medium voltage cables. ie. thermoset, thermoplastic, crosslinking.

Medium Voltage Insulation

100% Insulation Level – 133% Insulation Level(Grounded Neutral) (Ungrounded Neutral)

For shielded medium voltage power cables, the phase-to-phase voltage rating of the cable is specified along with an insulation level category – 100% Insulation Level (IL) or 133% Insulation Level (IL). (Several years ago, these categories were referred to as Grounded Neutral (GN) and Ungrounded Neutral (UN), respectively. Some customers still use these terms.) The Insulation Level (IL) category is used to define what happens to a cable during failure conditions and determines the proper insulation thickness for the cable.

100% IL Cable in this category are used on electrical systems with relay protection such that ground faults (cable failure) will be cleared within 1-minute (i.e., fault current is transmitted to circuit breaker which opens, removing all three phases from the circuit). A normal insulation thickness can be used for these cables because no exposure to over- voltages occurs during the failure.

133% IL Cables in this category are used on electrical systems where a ground fault (cable failure) cannot be cleared in 1-minute but the faulted cables will be de-energized within 1 hour. These cables are often used on delta connected circuits or ungrounded neutral circuits. When one phase fails, the two remaining phases continue to operate but with a higher than normal voltage applied across the insulation. A greater insulation thickness is required on some cables to withstand this higher voltage.

CABLE INSULATION THICKNESS COMPARISON CHART

Industry Standard 5kV– 100% IL 5kV – 133% IL 15kV-100% IL 15kV-133% IL

UL MV-90, ICEA .090 “ .090” .175” .220”

NOTE: Some customers specify a 133% IL Cable even though their system complies with 100% IL Definition: i.e., desire over insulated cable for lower electrical stress operation and perceived longer life.

• EPR - Ethylene Propylene Rubber– Good Insulating Properties– Flexible; Easier to Work With– Tree Retardant– Premium Cost

• Low Voltage - Occasionally Used

• Medium Voltage - Used by Some Utilities and Most Industrial Plants

Medium Voltage EPR

Medium Voltage XLPE

• XLPE – Cross-Linked Polyethylene– Excellent Electrical Properties

– Less Expensive than EPR

– Physically Tough Insulation

– Good Aging Characteristics

– Stiff

• Low Voltage - Widely Used• Medium Voltage - Typically Used by Utilities

EPR -vs- XLPE for Medium Voltage Applications

Consideration………...………….Typical Cable Choice

• Thermal Overload….………………………....EPR

• Flexibility…………...…………..……………...EPR

• Aerial Installation….………………..EPR or XLPE

• Dry Conditions….…………………..EPR or XLPE

• Wet Conditions.…………………….EPR or XLPE

• Initial Cost…………...…………....………… XLPE

• Lower Electrical Losses…….…………..…. XLPE

* This is only a general Guideline. Multiple factors affect most applications. select Materials on a case-by-case basis

EPR• Base Filler (EPDM)

• Clay Filler

• Silane

• Zinc oxide

• Paraffin Wax

• Polyethylene

• Red Lead

• Peroxide

TR-XLP or XLP• Polyethylene

• Anti-oxidant

• Peroxide

• Tree-retardant Additive for TRXLP

Ingredients

INSULATION Th

erm

al

Sta

bili

ty

S

Mo

istu

re R

esi

sta

nce

Ru

gg

ed

ne

ss

Fle

xib

ility

Sp

lice

ab

ility

Oil

Re

sist

ance

Ch

em

ica

l Re

s.

Fla

me

Re

s.

Sm

oke

Evo

lve

men

t

Aci

d G

as

Ge

ne

ratio

n

Dis

sip

atio

n F

act

or

SIC

Die

lect

ric

Str

en

gth

Impu

lse

Str

en

gth

Co

ron

a R

esis

tan

ce

E

lect

rica

l Sta

bili

ty –

H20

Die

lect

ric

Lo

sse

s

Co

st

XLPE

3

5

4

1

3

2

3

1

2

4

5

5

4

5

4

4

5

5

TR-XLPE

3

5

4

1

3

2

3

1

2

4

4

5

4

5

4

4

4

5

EPR

5

4

3

3

4

2

4

3

3

4

3

4

3

4

5

5

3

4

Ratings: 5 = Exceptional 4 = Excellent

3 = Good 2 = Fair 1 = Poor

Medium Voltage Insulation Compound Comparisons

This is What We Have Built This Far

Conductor shield

Conductor

Insulation

Ground

Proximity of cable to ground can cause electrical stress concentration on the outer surface of the insulation.

Electrical stress linesgo to ground

Insulation

Electrical Stress

Electrical stress is defined as the electrical force acting on a unit positive charge.

The average radial stress is determined by the ratio of the applied voltage to the total insulation thickness:

S = E / t

At 15kv, 220 mils insulation, S = 15 = 68 Volts per mil220

S = Average stress in volts per milE = Voltage applied in voltst = Insulation thickness in mils

Electrical Stress Cont.

The stress at any point in a homogeneous cable insulation is given by:

S = E 2.303 r log D/d

S = Stress in volts per milE = Voltage to ground in voltsd = Diameter of conductor in mils (over strand shield)D = Diameter over insulation in milsr = Distance of point from axis in mils

Electrical Stress Cont.

As the radius of the conductor increases the electrical stresses decreases.

Example: The stress at the surface of the conductor interface operating at 8660 volts to ground:

Conductor Conductor Insulation Stress Size Diameter “d” Diameter “D” v/mil

4/0 .528 1.00 51.4500 kcmil .813 1.29 46.2750 kcmil .998 1.48 44.0

Insulation Shield

• The function of the insulation shield is to:– Confine the electrical field within the cable– Obtain symmetrical radial distribution of

voltage stress within the insulation.– To limit radio interference.– To reduce the hazard of shock. The shield

must be grounded

Equal Electrical Stress Line Distribution

Electrical stress lines

InsulationConductorshield

Insulationshield

The Capacitor Affect

V

The insulation shield does not confine the voltage to the insulation.If the insulation shield is not grounded, the cable acts like a long line capacitor and presents a serious shock hazard.

C = 7.35 Log D/d

: dielectric constant of insulation. SICD : OD. of insulation. (in)d : OD. of conductor including shield material (in)

Metallic Shield

• The metallic shield can either be a wire shield or tape shield.– The metallic shield is used to ground the non-

metallic insulation shield.– To prevent shock hazard– To provide fault current path– Sometimes to provide a system neutral (URD

cable)

Medium Voltage Cable with Wire Shield

conductor

conductor shield

Insulation

Wire shield

Insulation shield

Jacket

• The purpose of the jacket is to: - p: 58

– Protect the underlying cable from physical abuse

– Protect the underlying cable from water ingress

– Protect the underlying cable from chemicals – Protect the underlying cable from corrosion

JacketsHypalon CSPE: Chlorosulfonated polyethylene, DuPont polymer - Primarily used as a thermoset jacket in 600 and Medium Voltage cables.

CPE: Chlorinated polyethylene - Primarily Used as a thermoplastic jacket. Unlike Hypalon has no sulfur

LSZH: Low Smoke Zero Halogen - Primarily used as a thermoset jacket or insulation for 600 volt and a thermoplastic jacket for Medium Voltage. LSZH is a Polyolefin compound.

FR-PVC: Fire Retardant PVC - Primarily used as a thermoplastic insulation on 600 volt cables and a jacket on 600 volt medium voltage cables. Does have chlorine,

PVC CPE Hypalon SOLONON

PHYSICAL PROPERTIES

Tensile Strength (PSI)

Elongation (%)

1500

100

1400

150

1800

300

2000

160

AGING CHARACTERISTICS

Air Oven, 121C, 168 Hrs.

*     Tensile Strength

(% of Unaged Value)

* Elongation (% of Unaged Value)

85

60

85

50

85

65

111

63

MOISTURE RESISTANCE

7 Days in 70C Water

* Mg Absorbed/Square Inch

8 25

 35 14.3

LIMITING OXYGEN INDEX (LOI) (%) 28 28 30 40

FLAME RESISTANCE

20 Min. at 70,000 BTU/Hr.

* Vertical Ladder Tray Test

PASS PASS PASS PASS

LOW TEMPERATURE RATING

Cold Bend Test Temperature Passed (C)

-10 -35 -30 -40

HALOGEN CONTENT (Chlorine) (%) 29 19 13.5 0.0

COEFFICIENT OF DYNAMIC

FRICTION () in PVC Duct 0.35 0.35 0.5 0.35

LIMITED SMOKE TEST

(PASS/FAIL) UL 1685

Fail Fail Fail Pass

Complete Medium Voltage Cable

conductor

conductor shield

Insulation

Wire shield

Insulation shield

jacket

The Manufacturing Process of 15kv Cable

CSIISConductorCuring Tube

Thermoset Process

Assembled Cable

PVCJacketCooling Tube

Thermoplastic Process

CS: Conductor ShieldI : InsulationIS : Insulation Shield

TERMINOLOGY• AWM Appliance Wiring Material

• TC Tray Cable

• TFFN Fixture Wire

• PLTC Power Limited Tray Cable

• MC Metal Clad (IA)

• MTW Machine Tool Wire

• MV-90 Medium Voltage 90 C

• MV-105 Medium Voltage 105 C

• FOR CT USE Cable Tray Flame Test Designation

• VW-1 Flame Test Designation For Wire

• SUN RES Sunlight Resistant

• DIR BUR Direct Burial

• OPEN WIRING TC Optional Listing

• LS Limited Smoke

Dielectric Loss

DL = .00276 V tan log D/d

V : phase to neutral in kV : dielectric constant, SIC (specific inductive capacity)tan : power factor of insulation in decimalsD : outside diameter of insulation. (in)d : outside diameter of conductor including shield material (in)

2

SIC tanTRXLP 2.4 .0008EPR 2.9 .0030

Megger Testing

Megger Testing 

The megger test is a common electrical test performed on low (600 volt) cables after they have been pulled in to raceways or conduit. A megger test is performed with a low voltage instrument that measures the leakage current through insulation. The megger test procedure consists of applying a dc voltage to the cable being tested, with all other cables connected together and grounded. The test voltage is usually between 500 and 1,000 volts. The minimum acceptable value for insulation resistance per ICEA (S-73-532) is given as: IR > 2000/ L Mega-ohms

IR: Insulation ResistanceL: Length of circuit in feet

Southwire’s “2 to 50 Megohm Rule”

Acceptable: A megohm reading of 50 megohm or higher

Investigate: A megohm reading between 2 – 50 megohm

Unacceptable: A megohm reading less than 2 megohm

High Potential Hi-Pot Testing

DC Installation Testing

DC installation testing is accomplished by employing high voltage, low current dc power to the cable. Installation testing is important in that it provides assurance that no damage has occurred during installation or in handling after leaving the factory. If the cable is installed by a contractor, the test can serve as an acceptance test and assure the owner that the cable has not been damaged and should perform satisfactorily.

Recommended dc Test Voltages for Shielded Power Cable Systems From 5 - 35KV

System Voltage Accceptance Test Voltage Maintenance Test VoltageKV Phase to Phase (KV dc, Cond-gnd) (KV dc, Cond-gnd)

5 28 238 36 2915 56 4625 75 6128 85 6835 100 75

Acceptance test voltage duration is normally 15 minutes. Maintenance test voltage duration is normally not less than 5 minutes or more than 15 minutes

Three Components of dc Current

The output current of the test set into the cable is not the true leakage current. It is based on the sum of the three components below. The absolute value of the output current is not of primary importance as there is no set current value for a good/bad insulation.

• Capacitive charging current required to charge up the capacitance between the conductor and the shield. Current decreases with an RC time constant depending on the cable length and the resistance of the set.

• Dielectric absorption current required to polarize the molecules in the cable dielectric.

• Leakage current a relatively constant value at a fixed temperature and dc voltage. A plot of leakage current and vs. test voltage should result in a straight line for good insulation.

Leakage Current (IL)

Capacitive Charging Current (Ic)

Dielectric Absorption Current (Ia)

Total dc Test Current It = (Ic)+I(a)+I(l)

Time

Components of dc Proof-Test Current

Cur

rent

Behavior of dc Current Good and Bad Insulation

Good Insulation

• The total test current should never increase appreciably at a constant dc test voltage.

• There should be a fairly close match in the leakage currents between the three phases at a given voltage.

• A drop of current with respect to time. A falling leakage curve is indicative of good insulation.

Bad Insulation

• A rising current at a steady voltage is an indication of questionable condition

• Leakage current between phases differ greatly.

• A rise in current with respect to time indicates a bad insulation.