Article 310 Conductors for General Wiring

310.1

Scope

Article 310 is to be used for general requirements for wiring, but not in areas where it is part of an integral device such as a motor, motor controller, or where covered in another part of the NEC®.

310.2

Conductors

Conductors must be insulated and made of copper, copper-clad aluminum, or aluminum unless otherwise specified in the NEC®.

310.3

Stranded Conductors

Any conductor installed in a raceway must be stranded if it is 8 AWG or larger. This is not required if it is permitted or required in another part of the NEC®.

310.4

Conductors in Parallel

Conductors 1/0 AWG or larger and having each phase, polarity, neutral, or grounded circuit conductor can be run in parallel if they are the same length, the same size, and of the same material with the same insulation and terminated the same way. They must be electrically joined at both ends. The cables or raceways must have the same physical characteristics if the conductors are run in separate cables or raceways. The same numbers of conductors have to be used in each raceway or cable. The conductors of one phase, neutral, or ground do not have to have the same physical characteristics as those of another to achieve balance. There are four exceptions to this:

1. Section 620.12(A)(1), Exception.

2. If smaller than 1/0 AWG, used for control power to indicating instruments, relays, and similar devices, and in the same raceway, each individual conductor can carry the entire load and the overcurrent rating is not larger than the ampacity of any single conductor.

3. Sizes smaller than 1/0 AWG are allowed for frequencies of 360 Hz and higher provided that Exception 2 is met.

4. With engineering supervision 2 AWG or larger grounded neutral conductors can be run parallel in existing installations.

310.5

Minimum Size of Conductors

Minimum sizes are shown in Table 310.5 except as permitted in other sections of the Code®. For voltages up to 2000, it is 14 AWG for copper and 12 AWG for aluminum or copper-clad aluminum.

310.6

Shielding

310.7

Direct Burial Conductors

310.8

Locations

In dry locations any type of insulated conductor or cable noted in the Code can be used. Use only Type RHW, TW, THW, MTW, RHW-2, THW-2, THHW, THHW-2, THWN, THWN-2, XHHW, XHHW-2, ZW, or a type listed as such for a wet location, or a moisture impervious metal-sheathed conductor. For dry and damp locations use Types FEP, FEPB, MTW, PFA, RHW, RHH, RHW-2, SA, XHH, XHHW, XHHW-2, THHN, THW-2, THW, THHW, THHW-2, THWN, THWN-2, TW, Z, or ZW. If conductors are exposed to direct sunlight the cables must be listed as sunlight resistant; or conductors must be listed and identified as sunlight resistant; or insulated conductors can be covered with tape, sleeving, or similar material that is listed for the application and identified as sunlight resistant.

310.9

Corrosive Conditions

310.10

Temperature Limitations of Conductors

310.11

Marking

All conductors and cables must have marked on them the maximum rated voltage, the proper letter type, the manufacturer's name or trademark, the AWG size or circular mil area, and cable assemblies where the neutral conductor is smaller than the ungrounded conductor. The size markings should be on the surface and spaced not more than 610 mm (24 in.) for eight types of cables. Other markings may be spaced 1.0 m (40 in.) apart. Certain cables can use marker tape inside the cable. Refer to this section in the NEC® for specific types where marker tape is permitted, which cables can have the size located elsewhere, and suffix designations for number of conductors. Special characteristics can be surface-marked on conductors listed in Chapter 3.

310.12

Conductor Identification

(A) Grounded Conductors. Insulated grounded conductors must be identified as noted in Section 200.6.

(B) Equipment Grounding Conductors. Equipment grounding conductors must be as noted in Section 250.119.

(C) Ungrounded Conductors. These must be distinguished from grounded and grounding conductors.

310.13

Conductor Constructions and Applications

Insulated conductors must be in accordance with Tables 310.13 and 310.61 through 310.64.

310.14

Aluminum Conductor Material

310.15

Ampacities for Conductors Rated 0-2000 Volts

(A) General

(1) Tables or Engineering Supervision. Ampacities can be determined by tables as provided in (B) or under engineering supervision as noted in 310.15(C).

(2) Selection of Ampacity. If more than one ampacity is determined from the tables or calculations, the lowest value must be used. There is one exception for adjacent portions of a circuit under certain circumstances.

(B) Tables. Ampacities are noted in the Allowable Ampacities Tables 310.16 through 310.19 and 310.20 through 310.23 for conductors rated 0 to 2000 volts. They can be modified by paragraphs (1) through (6).

Table 310.16. Allowable Ampacities of Insulated Conductors, Rated 0 through 2000 Volts, 60° through 90°C (140° through 194°F) Not More than Three Current Conductors in Raceway, Cable, or Earth (Directly Buried) Based on Ambient Temperature of 30°C (86°F)

Size AWG or kcmil

Temperature Rating of Conductor (See Table 310.13)

Size AWG or kcmil

60°C (140°F)

75°C (167°F)

90°C (194°F)

60°C (140°F)

75°C (167°F)

90 °C (194°F)

Types TW, UF

Types RHW, THHW, THW, THWN, XHHW, USE, ZW

Types TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2, THWN-2 USE-2, XHH, XHHW, XHHW-2, ZW-2

Types TW, UF

Types RHW, THHW, THW, THWN, XHHW, USE,

Types TBS, SA, SIS, THHN, THHW, THW-2, THWN-2, RHH, RHW-2, USE-2, XHH, XHHW, XHHW-2, ZW-2

Copper

Aluminum or Copper-Clad Aluminum

18

. . . .

. . . .

14

. . . .

. . . .

. . . .

. . . .

16

. . . .

. . . .

18

. . . .

. . . .

. . . .

. . . .

14[*]

20

20

25

. . . .

. . . .

. . . .

. . . .

12[*]

25

25

30

20

20

25

12[*]

10[*]

30

35

40

25

30

35

10[*]

8

40

50

55

30

40

45

8

6

55

65

75

40

50

60

6

4

70

85

95

55

65

75

4

3

85

100

110

65

75

85

3

2

95

115

130

75

90

100

2

1

110

130

150

85

100

115

1

1/0

125

150

170

100

120

135

1/0

2/0

145

175

195

115

135

150

2/0

3/0

165

200

225

130

155

175

3/0

4/0

195

230

260

150

180

205

4/0

250

215

255

290

170

205

230

250

300

240

285

320

190

230

255

300

350

260

310

350

210

250

280

350

400

280

335

380

225

270

305

400

500

320

380

430

260

310

350

500

600

355

420

475

285

340

385

600

700

385

460

520

310

375

420

700

750

400

475

535

320

385

435

750

800

410

490

555

330

395

450

800

900

435

520

585

355

425

480

900

1000

455

545

615

375

445

500

1000

1250

495

590

665

405

485

545

1250

1500

520

625

705

435

520

585

1500

1750

545

650

735

455

545

615

1750

2000

560

665

750

470

560

630

2000

Correction Factors

Ambient Temp. °C

For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities shown above by the appropriate factor shown below.

Ambient Temp. °F

21-25

1.08

1.05

1.04

1.08

1.05

1.04

70-77

26-30

1.00

1.00

1.00

1.00

1.00

1.00

78-86

31-35

0.91

0.94

0.96

0.91

0.94

0.96

87-95

36-40

0.82

0.88

0.91

0.82

0.88

0.91

96-104

41-45

0.71

0.82

0.87

0.71

0.82

0.87

105-113

46-50

0.58

0.75

0.82

0.58

0.75

0.82

114-122

51-55

0.41

0.67

0.76

0.41

0.67

0.76

123-131

56-60

. . . .

0.58

0.71

. . . .

0.58

0.71

132-140

61-70

. . . .

0.33

0.58

. . . .

0.33

0.58

141-158

71-80

. . . .

. . . .

0.41

. . . .

. . . .

0.41

159-176

For table headings see page 94.

[*] See Section 240.4(D).

Table 310.20. Ampacities of Not More than Three Single-Insulated Conductors, Rated 0 through 2000 Volts, Supported on a Messenger, Based on Ambient Air Temperature of 40°C (104°F)

Size AWG or kcmil

Temperature Rating of Conductor (See Table 310.13)

Size AWG or kcmil

75°C (167°F)

90°C (194°F)

75°C (167°F)

90°C (194°F)

TYPES RHW, THHW, THW, THWN, XHHW, ZW

TYPES MI, THHN, THHW, THW-2, THWN-2, RHH, RWH-2, USE-2, XHHW, XHHW-2, ZW-2

TYPES RHW, THW, THWN, THHW, XHHW

TYPES THHN, THHW, RHH, XHHW, RHW-2, XHHW-2, THW-2, THWN-2 USE-2, ZW-2

Copper

Aluminum or Copper-Clad Aluminum

8

57

66

44

51

8

6

76

89

59

69

6

4

101

117

78

91

4

3

118

138

92

107

3

2

135

158

106

123

2

1

158

185

123

144

1

1/0

183

214

143

167

1/0

2/0

212

247

165

193

2/0

3/0

245

287

192

224

3/0

4/0

287

335

224

262

4/0

250

320

374

251

292

250

300

359

419

282

328

300

350

397

464

312

364

350

400

430

503

339

395

400

500

496

580

392

458

500

600

553

647

440

514

600

700

610

714

488

570

700

750

638

747

512

598

750

800

660

773

532

622

800

900

704

826

572

669

900

1000

748

879

612

716

1000

CORRECTION FACTORS

Ambient Temp. (°C)

For ambient temperatures other than 40° (104°F), multiply the ampacities shown above by the appropriate factor shown below.

Ambient Temp. (°F)

2125

1.20

1.14

1.20

1.14

7077

2630

1.13

1.10

1.13

1.10

7986

3135

1.07

1.05

1.07

1.05

8895

3640

1.00

1.00

1.00

1.00

97104

4145

0.93

0.95

0.93

0.95

106113

4650

0.85

0.89

0.85

0.89

115122

5155

0.76

0.84

0.76

0.84

124131

5660

0.65

0.77

0.65

0.77

133140

6170

0.38

0.63

0.38

0.63

142158

7180

0.45

0.45

160176

(1) General. For explanation of Type Letters, and for recognized size of conductors for the various conductor insulations, see Section 310.13. For installation requirements, see Sections 310.1 through 310.10, and the various articles of this Code. For flexible cords, see Tables 400.4, 400.5(A), and 400.5(B).

(2) Adjustment Factors

(a) More than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where multi-conductor cables or single conductors are bundled or stacked longer than 600 mm (24 in.) without proper spacing and not in a raceway, the ampacities shall be reduced as shown in the following table Table 310.15(B)(2)(a). In a parallel set of conductors each conductor is counted as a current-carrying conductor. The exceptions to this are listed on the bottom of the table.

Table 310.15(B)(2)(a). Adjustment Factors for More than Three Current-Carrying Conductors in a Raceway or Cable

Number of Current-Carrying Conductors

Percent of Values in Tables Number of 310.16 through 310.19 Current-Carrying as Adjusted for Ambient Conductors Temperature if Necessary

4 through 6

80

7 through 9

70

10 through 20

50

21 through 30

45

31 through 40

40

41 and above

35

FPN Note 1: See Appendix B, Table B.310.11 for adjustment factors for more than three current-carrying conductors in a raceway or cable with load diversity.

FPN Note 2: For correction factors for conductors in sheet metal auxiliary gutters see 366.23(A) and 376.22 for conductors in metal raceways.

Exception No. 1. When conductors of different systems, as provided in Section 300.3, are installed in a common raceway or cable the derating factors shown in Table 310.15(B)(2)(a) shall apply to the number of power and lighting (Articles 210, 215, 220, and 230) conductors only.

Exception No. 2. For conductors installed in cable trays, the provisions of Section 392.11 shall apply.

Exception No. 3. Derating factors shall not apply to conductors in nipples having a length not exceeding 610 mm (24 in.).

Exception No. 4. Derating factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit or rigid nonmetallic conduit having a length not exceeding 3 mm (10 ft.) and the number of conductors does not exceed 4.

Exception No. 5. Adjustment factors shall not apply to Type AC cable or to Type MC cable without an overall outer jacket under the following conditions:

(a) Each cable has not more than three current carrying conductors.

(b) The conductors are 12 AWG copper.

(c) Not more than 20 current carrying conductors are bundled, stacked, or supported on "bridle rings."

A 60 percent adjustment factor shall be applied where the current carrying conductors in these cables that are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing exceeds 20.

(b) More than One Conduit, Tube, or Raceway. Spacing between conduits, tubing, or raceways shall be maintained.

(3) Bare Conductors or Covered Conductors

Where bare or covered conductors are used with insulated conductors, their allowable ampacities shall be limited to that permitted for the adjacent insulated conductors.

Table 310.21. Ampacities of Bare or Covered Conductors in Free Air, Based on 40°C (104°F) Ambient, 80°C (176°F) Total Conductor Temperature, 610 mm/sec (2 ft/sec) Wind Velocity

Copper Conductors

AAC Aluminum Conductors

Bare

Covered

Bare

Covered

AWG or kcmil

Amperes

AWG or kcmil

Amperes

AWG or kcmil

Amperes

AWG or kcmil

Amperes

8

98

8

103

8

76

8

80

6

124

6

130

6

96

6

101

4

155

4

163

4

121

4

127

2

209

2

219

2

163

2

171

1/0

282

1/0

297

1/0

220

1/0

231

2/0

329

2/0

344

2/0

255

2/0

268

3/0

382

3/0

401

3/0

297

3/0

312

4/0

444

4/0

466

4/0

346

4/0

364

250

494

250

519

266.8

403

266.8

423

300

556

300

584

336.4

468

336.4

492

500

773

500

812

397.5

522

397.5

548

750

1000

750

1050

477.0

588

477.0

617

1000

1193

1000

1253

556.5

650

556.5

682

636.0

709

636.0

744

795.0

819

795.0

860

954.0

920

1033.5

968

1033.5

1017

1272

1103

1272

1201

1590

1267

1590

1381

2000

1454

2000

1527

(4) Neutral Conductor

  1. A neutral conductor that carries only the unbalanced current from other conductors of the same circuit need not be counted when applying the provisions of Section 310.15(B)(2)(a).
  2. In a three-wire circuit consisting of two-phase wires and the neutral of a four-wire, three-phase wye-connected system, a common conductor carries approximately the same current as the line to neutral load currents of the other conductors and shall be counted when applying the provisions of Section 310.15(B)(2)(a).
  3. On a four-wire, three-phase wye circuit where the major portion of the load consists of nonlinear loads there are harmonic curents present in the neutral conductor and the neutral shall be considered to be a current-carrying conductor.

(5) Grounding or Bonding Conductor

A grounding or bonding conductor shall not be counted when applying the provisions of Section 310.15(B)(2)(a).

(6) 120/240 Volts, Three-Wire, Single-Phase Dwelling Services and Feeders

For individual dwelling units of one family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted to be utilized as 120/240-V, three-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For applications of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch circuit panelboard(s). The feeder conductors to a dwelling unit shall not be required to be larger than its service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided that the requirements of Sections 215.2, 220.61, and 230.42 are met.

Table 310.15(B)(6). Conductor Types and Sizesfor 120/240-Volt, Three-Wire, Single-Phase Dwelling Services and Feeders

Conductor (AWG or kcmil)

 

Copper

Aluminum or Copper-Clad Aluminum

Service or Feeder Ratings (Amperes)

4

2

100

3

1

110

2

1/0

125

1

2/0

150

1/0

3/0

175

2/0

4/0

200

3/0

250

225

4/0

300

250

250

350

300

350

500

350

400

600

400

(C) Engineering Supervision.

With engineering supervision the following formula can be used to calculate ampacities:

where

TC

= conductor temperature in degrees Celsius (°C)

TA

= ambient temperature in degrees Celsius (°C)

DELTA TD

= dielectric loss temperature rise

RDC

= dc resistance of conductor at temperature TC

YC

= component ac resistance resulting from skin effect and proximity effect

RCA

= effective thermal resistance between conductor and surrounding ambient

FPN: See Annex B for examples of formula application.

310.60

Conductors Rated 2001 to 35,000 Volts.

(A) Definitions.

(B) Ampacities of Conductors Rated 2001 to 35,000 Volts.

(C) Tables.





Pocket Guide to the National Electrical Code 2005
Pocket Guide to the National Electrical Code(R), 2005 Edition (8th Edition)
ISBN: 0131480014
EAN: 2147483647
Year: 2004
Pages: 120
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