Cable Capacity Of Conduits And Trunking 4j35k

Cable capacity of conduits and trunking

The following single-core p.v.c. insulated cables are to be run in a conduit 6 m long with a double set: 8 x 1,4 x 2.5 and 2 x 6 mm². Choose a suitable size. Table 1- Cable factors () for conduit and trunking Type of conductor

Conductor c.s.a. (mm²)

Factor for Factor for trunking pvc conduit insulation

Factor for trunking thermosetting insulation

Solid

1.0

16

3.6

3.8

Solid

1.5

22

8.0

8.6

Stranded

1.5

22

8.6

9.1

Solid

2.5

30

11.9

11.9

Stranded

2.5

30

12.6

13.9

Stranded

4.0

43

16.6

18.1

Stranded

6.0

58

21.2

22.9

Stranded

10.0

105

35.3

36.3

Stranded

16.0

145

47.8

50.3

Stranded

25.0

217

73.9

75.4

Table 2- Cable factors () for straight runs up to 3m. Type of conductor

Conductor c.s.a. (mm²)

Cable factor

Solid

1.0

22

Solid

1.5

27

Solid

2.5

39

Stranded

1.5

31

Stranded

2.5

43

Stranded

4.0

58

Stranded

6.0

88

Stranded

10.0

146

Table 3- Conduit factors () Length of run between boxes (m) 1

2

3

Conduit, straight

4

5

6

8

10

-

16mm

290

290

290

171

171

167

158

150

20mm

460

460

460

286

278

270

256

244

25mm

800

800

800

514

500

487

463

442

32mm

1400

1400

1400

900

878

857

818

783

Conduit, one bend

-

16mm

188

177

167

158

150

143

130

120

20mm

303

286

270

256

244

233

213

196

25mm

543

514

487

463

442

422

388

258

32mm

947

900

857

818

783

750

692

643

Conduit, two bends

-

16mm

177

158

143

130

120

111

97

86

20mm

286

256

233

213

196

182

159

141

25mm

514

463

422

388 358

333

292

260

32mm

900

818

750

692

600

529

474

643

For 38mm conduit use the 32mm factor x 1.4. For 50mm conduit use the 32mm factor x 2.6. For 63mm conduit use the 32mm factor x 4.2.

Table 4 Trunking factors () Dimensions of trunking (mm x mm)

Factor

37.5 x 50

767

50 x 50

1037

25 x 75

738

37.5 x 75

1146

50 x 75

1555

75 x 75

2371

25 x 100

993

37.5 x 100

1542

50 x 100

2091

75 x 100

3189

100 x 100

4252

Consulting {Table 1} gives the following cable factors: 16 for 1 mm², 30 for 2.5 mm² and 58 for 6 mm² Total cable factor is then

(8 x 16) + (4 x 30) + (2 x 58)

= 128 + 120 + 116 = 364 The terrn "bend" means a right angle bend or a double set. {Table 3} gives a conduit factor for 20 mm conduit 6 m long with a double set as 233, which is less than 364 and thus too small. The next size has a conduit factor of 422 which will be acceptable since it is larger than 364. The correct conduit size is 25 mm diameter.

The first conduit from a distribution board will be straight and 10 m long. It is to enclose 4 x 10 mm² and 8 x 4 mm² cables. Calculate a suitable size. From {Table 1}, cable factors are 105 and 43 respectively. Total cable factor: =(4 x 105) + (8 x 43) = 420 + 344 = 764 From ({Table 2}, a 10 m long straight 25 mm conduit has a factor of 442. This is too small, so the next size, with a factor of 783 must be used. The correct conduit size is 32 mm diameter.

A 1.5 m straight length of conduit from a consumer's unit encloses ten 1.5 mm² and four 2.5 mm² solid conductor p.v.c. insulated cables. Calculate a suitable conduit size. From ({Table 2} (which is for short straight runs of conduit) total cable factor will be: = (10 x 27) + (4 x 39) = 426 Table 3

shows that 20 mm diameter conduit with a factor of 460 will be necessary.

Example

A length of trunking is to carry eighteen 10 mm², sixteen 6 mm², twelve 4 mm², and ten 2.5 mm² stranded single p.v.c. insulated cables. Calculate a suitable trunking size. The total cable factor for trunking is calculated with data from {Table 1}. 18 x 10mm² at 36.3

= 18 x 36.3

= 653.4

16 x 6mm² at 22.9

= 16 x 22.9

= 366.4

12 x 4mm² at 15.2

= 12 x 15.2

= 182.4

10 x 2.5mm² at 11.4

= l0 x ll.4

= 114.0

Total cable factor

= 1316.2

-

From the trunking factor {Table 4. }, two standard trunking sizes have factors slightly greater than the cable factor, and either could be used . They are 50 mm x 75 mm at 1555, and 37.5 mm x 100 mm at 1542.

2 - Ducting and trunking Metal and plastic trunkings are very widely used in electrical installations. They must be manufactured to comply with the relevant British Standards, and must be installed so as to ensure that they will not be damaged by water or by corrosion (see {4.2.5}). Table 4.13 spacings for trunking Typical trunking size (mm)

Metal

Insulating

Horizontal

Vertical

Horizontal

Vertical

Up to 25 x 25

0.75

1.0

0.5

0.5

Up to 50 x 25

1.25

1.5

0.5

0.5

Up to 50 x 50

1.75

2.0

1.25

1.25

Up to 100 x 50

3.0

3.0

1.75

2.0

If it is considered necessary to provide an additional protective conductor in parallel with steel trunking, it must be run inside the trunking or the presence of steel between the live and protective cables will often result in the reactance of the protective cable being so high that it will have little effect on fault loop impedance. Trunking must be ed as indicated in {Table 4.13}. The table does not apply to special lighting trunking which is provided with strengthened couplers. Where crossing a building expansion t a suitable flexible t should be included. Where trunking or conduit es through walls or floors the hole cut must be made good after the first fix on the construction site to give the partition the same degree of fire protection it had before the hole was cut. Since it is possible for fire to spread through the interior of the trunking or conduit, fire barriers must be inserted as shown in {Fig 4.18}. An exception is conduit or trunking with a crosssectional area of less than 710 mm², so that conduits up to 32 mm in diameter and trunking up to 25 mm x 25 mm need not be provided with fire barriers. During installation, temporary fire barriers must be provided so that the integrity of the fire prevention system is always maintained.

Since trunking will not be solidly packed with cables (see {4.5.3}) there will be room for air movement. A very long vertical trunking run may thus become extremely hot at the top as air heated by the cables rises; this must be prevented by barriers as shown in {Fig 4.19}. In many cases the trunking will through floors as it rises, and the fire stop barriers needed will also act as barriers to rising hot air. Lighting trunking is being used to a greater extent than previously In many cases, it includes copper conducting bars so that luminaires can be plugged in at any point, especially useful for display lighting. The considerably improved life, efficiency and colour rendering properties of extra-low voltage tungsten halogen lamps has led to their increasing use, often fed by lighting trunking. It is important here to that whilst the voltage of a 12 V lamp is only one twentieth of normal mains potential, the current for the same power inputs will be twenty times greater. Thus, a trunking feeding six 50 W 12 V lamps will need to he rated at 25 A.

Breaker or Fuse Type Fuses and FixedTrip Circuit Breakers**

Conditions

Rating The standard ampere ratings for fuses and inverse time circuit breakers shall be considered as follows: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601.

Adjustable-Trip Circuit Breakers**

Breakers have external means for adjusting the current setting (long-time pickup setting), not meeting

= the maximum setting possible.

the requirements of Restricted Access breakers in below. Restricted Access Adjustable-Trip Circuit Breakers **(see fig.1) 

 

Breakers that have restricted access to the adjusting means. Restricted access shall be defined as located behind one of the following: Removable and sealable covers over the adjusting means, Bolted equipment enclosure doors, Locked doors accessible only to qualified personnel.

= the adjusted current setting (long-time pickup setting).

Notes: * The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted. ** A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit. - The set long-time pickup rating is the opposite to the instantaneous trip rating. Multipole or Single-Pole Circuit breakers Circuit breakers (as Overcurrent Device) shall open all ungrounded conductors of the circuit both manually and automatically. (see fig.2) Exception: single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor as in the blow table.

Case Multiwire Branch Circuits(see fig.3) Grounded Single-Phase  Alternating-Current Circuits (see fig.4) 

3-Phase and 2-Phase Systems (see fig.4)

 

Condition Serve only single-phase line-to neutral loads. individual single-pole circuit breakers rated 120/240 volts ac, For line-to-line connected loads for singlephase circuits.

For line-to-line loads in 4-wire, 3-phase systems or 5-wire, 2-phase systems, The systems have a grounded neutral point and the voltage to ground does not exceed 120

volts.

3-Wire Direct-Current  Circuits 

Individual single-pole circuit breakers rated 125/250 volts dc, For line-to-line connected loads for 3-wire, direct current circuits supplied from a system with a grounded neutral where the voltage to ground does not exceed 125 volts.

Circuit breaker used as a switch   

15- or 20-ampere Circuit breakers used as switches in 120-volt and 277-volt fluorescent lighting circuits shall be listed and shall be marked SWD. 15- or 20-ampere Circuit breakers used as switches in high-intensity discharge lighting circuits shall be listed and shall be marked as HID. Circuit breakers marked “HID” can be used for switching both high-intensity discharge and fluorescent lighting loads; however, a circuit breaker marked “SWD” can be used only as a switching device for fluorescent lighting loads. Permissible Usages for Fuses fuses shall be permitted to be used in certain circuits as per the following Table:

Fuse Type Plug fuses of the Edison-base type(see fig.5)

Type S fuses (see fig.5)

Cartridge fuses and fuseholders of the 300-volt type (see fig.6)

Ratings

Permissible Usages

Shall be classified at not over 125 volts and 30  amperes and below.

Plug fuses shall be permitted to be used in the following circuits: Circuits not exceeding 125 volts between conductors (such as circuits supplied by 120/240-volt, single-phase, 3-wire systems and by Shall be classified 208Y/120-volt, 3-phase, 4-wire at not over 125 systems), volts and 0 to 15  Circuits supplied by a system amperes, 16 to having a grounded neutral point 20 amperes, and where the line-to-neutral voltage 21 to 30 does not exceed 150 volts. amperes. Shall be classified shall be permitted to be used in the at not over 300 following circuits: volts.  Circuits not exceeding 300 volts between conductors,  Single-phase line-to-neutral circuits supplied from a 3-phase, 4-

wire, solidly grounded neutral source where the line-to-neutral voltage does not exceed 300 volts

Rule#1: Location of overcurrent protection devices (ODs) Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except for the following: Branch-Circuit Conductors. Feeder Taps. Transformer Secondary Conductors. Service Conductors. Busway Taps. Motor Circuit Taps. Conductors from Generator Terminals. Battery Conductors.

Protection systems usually comprise five components: 

Current and voltage transformers to step down the high voltages and currents of the electrical power system to convenient levels for the relays to deal with



Protective relays to sense the fault and initiate a trip, or disconnection, order;



Circuit breakers to open/close the system based on relay and autorecloser commands;



Batteries to provide power in case of power disconnection in the system.



Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment.

For parts of a distribution system, fuses are capable of both sensing and disconnectingfaults.

Where a branch circuit supplies continuous (runs all the time) loads, or a combination of continuous and non-continuous (intermittent) loads, the rating of the overcurrent device shall not be less than the non-continuous load plus 125% of the continuous load. Branch circuit conductors shall be protected .. Flexible cords and fixture wires shall be protected in accordance with Article 240.5.

Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment. For parts of a distribution system, fuses are capable of both sensing and disconnectingfaults.

set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in the following conditions: 1. 2. 3. 4. 5. 6.

Protection by Primary Overcurrent Device. Transformer Secondary Conductors Not over 3 m (10 ft) Long. Industrial Installation Secondary Conductors Not over 7.5 m (25 ft) Long. Outside Secondary Conductors. Secondary Conductors from a Feeder Tapped Transformer. Secondary Conductors Not over 7.5 m (25 ft) Long.



To protect Overcurrent protection Devices (ODs) from physical damage, overcurrent devices can be installed in enclosures, cabinets, cutout boxes or equipment assemblies. They can also be installed in boards or control boards that are in rooms or enclosures free from dampness, easily ignitable material and accessible only to qualified personnel. Also, the operating handle shall be accessible without opening the door or cover.



Fuses and circuit breakers should be located or shielded so that people will not be burned or otherwise injured by their operation. Handles or levers of circuit breakers and similar parts that may move suddenly in such a way that persons in the vicinity are likely to be injured by being struck by them shall be guarded or isolated. Outside Taps of Unlimited Length.

Where the conductors are located outdoors of a building or structure, except at the point of load termination, and comply with all of the following conditions: 1- The conductors are protected from physical damage in an approved manner. 2- The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side. 3- The overcurrent device for the conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.

4- The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following: a. Outside of a building or structure b. Inside, nearest the point of entrance of the conductors c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors Load The current-carrying capacity (In, A) of the breaker should be higher than the expected load in the circuit. MCCBs are available up to 4000A from Terasaki, but become less cost-effective for very large ratings (2000A and above). The advantage of MCCBs for very large ratings is their compact size. An ACB is physically larger, but more cost-effective for higher ratings. In a short circuit the s of Terasaki MCCBs open before the first peak of the current waveform (within five milliseconds in a 50 Hz system). The fault current flowing through the MCCB never reaches its peak, and the fault energy allowed downstream is limited. This fault limitation protects sensitive equipment which is not rated to withstand faults. ACBs are selected for their ability to withstand fault current rather than limit it - (see Discrimination - Selectivity). A typical ACB will open a short-circuit in between twenty-five and thirty milliseconds, allowing between one and two cycles of fault current through before opening. The load protected by an ACB (transformers, busbars for example) should be rated to withstand fault current for a short duration. Fault Limitation In a short circuit the s of Terasaki MCCBs open before the first peak of the current waveform (within five milliseconds in a 50 Hz system). The fault current flowing through the MCCB never reaches its peak, and the fault energy allowed downstream is limited. This fault limitation protects sensitive equipment which is not rated to withstand faults. ACBs are selected for their ability to withstand fault current rather than limit it - (see Discrimination - Selectivity). A typical ACB will open a short-circuit in between twenty-five and thirty milliseconds, allowing between one and two cycles of fault current through before opening. The load protected by an ACB (transformers, busbars for example) should be rated to withstand fault current for a short duration.

Fault Level Circuit breakers must be capable of safely interrupting the maximum potential short-circuit current at their location in the circuit. The circuit breaker must have a breaking capacity higher than the potential short-circuit current. Note that the cost of circuit breakers becomes lower with lower breaking capacity. Potential short-circuit current is determined by: 1. The available power from the transmission network 2. Transformer characteristics 3. Impedance of conductors in the distribution system. A fault level study which s for

transformer characteristics and conductor impedance at all circuit breaker installation points will allow selection of breakers with optimum breaking capacity, saving money. Terasaki's Application Team provide this service.