IET Lecture LV Wiring Design September 14

8/11/2019 IET Lecture LV Wiring Design September 14

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Cable Sizing For Safe Power

System Based on IEC Standards

I r . H . P. L o o i ( m e k t r i c o n @ g m a i l . c o m )

B . E n g ( H o n s ) , F I E M , J u r u t e r a G a s

Cable Sizing Fundamentals and

E l e c t r i c a l L V S t a n d a r d

A r m a d a H o t e l , P e t a l i n g J a y a

2 3 r d A u g u s t 2 0 1 4

P a r t 2 C a b l e S i z i n g


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We had an overview of the following topics in Part 1

2 SYNOPOSIS

23 rd August 2014

1. Introduction

2. Scope

3. General Design Procedure (design road map)

4. Earthing system

5. Cable types & installation method

6. Circuit configuration

Part 2 will deal with Cable Sizing with the following:

1. Overview

2. Protection Device & Implication for cable sizing

3. Simplified Method 60364-5-52

4. Heat Flow Calculation 60297 series


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3 OVERVIEWTOCABLESIZING

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4 OVERVIEWTOCABLESIZING

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5 OVERVIEWNEHERMCGRATHAMPACITY

Cable sizing or Amps capacity calculation is basically a heat transfer problem. 1957 Neher

MacGrath equation makes the following simplification assumptions:

(a) Assumption of steady state conditions;

(b) The heat transfer media surrounding the cable system is homogeneous and the following

conditions apply:

(i) In case of cables in air, thermal transfer via convection is assumed to predominate

(direct solar radiation to be considered if relevant);

(ii) In case of cables in ground or embedded in solid media, conduction predominate;

(c) Cable diameter is assumed negligible compared to cable length, this reduces the model to2-dimensions.

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I = Ampacity (current carrying capacity), (kA)

Tc = Conductor temperature, (C)

Ta = Ambient temperature, (C)

TD= Conductor temp rise due to dielectric loss, (C)

Rdc = Conductor dc resistance, (/foot)Yc = Loss increment due to skin & proximity effects

Rca= Thermal resistance between conductor and

ambient (thermal feet)


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23 rd August 2014

6 OVERVIEW2 METHODS

Simplified 60364-5-52 Annex B to Elook up tables

IEC 60287 series of equations,


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23 rd August 2014

CircuitCapacity

CableAmpacity

VoltageDrop

I2


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23 rd August 2014

8 IEC 60364-4-43 CIRCUITCAPACITY

(1) IB< In < Iz

(2) I2< 1.45 x Iz

Calculate

current

from load

Select

protective

devices,

In > IB

Calculate cable size

based on:

Iz > In

IBdecide current

rating of circuitIB> load current

Check I2< 1.45 Iz

Electrical load

Iz = current capacity of cable.

Cables

Protection devices

In = nominal

current rating

of protective

devices

I2= current ensuring effective

operation within time

prescribed for protective

device


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23 rd August 2014

9 IEC 60364-4-43 CIRCUITCAPACITY


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23 rd August 2014

10 IEC 60364-5-52 ANNEXB

Current capacities

Tables B52.2 to B52.13

Installation method

Reference methods Table B52.1

Temperature correction factor

Table B52.14 (cables in air);B52.15 (cables in ground) and

B52.16 (cables in ducts)

Correction factor for soil

thermal resistivityTable B52.16 for method D only

where soil resistivity is other

than 2.5 K-m/W.

Reduction factor for more than one (single or multi core) cable

per circuit.Table B52.17 for installation method E to F

Table B52.18 for cables laid direct in ground

Table B52.19 for cables laid in duct in ground

Table B52.20 for multi core cables in free air (method E only)

Table B52.21 for single core cables in free air (method F only)

Annex E reduction factor due to harmonic currents

Currentcapacity

required


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12 IEC 60364-5-52 TABLEB52.16

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23 rd August 2014

13 IEC 60364-5-52EXAMPLE

EXAMPLE

1Estimate load current 0.9x25kW/(0.85x415Vx3

36A.25kW1

1 Choose rating of circuit IB= 50A.36A2

1 Choose protection device In= 63A (In>IB)60A3

1 Size cable IZ > In; choose 4x25mm PVC; IZ 90ATable B52-5

25mm4

1 Check I2 < 1.45Iz 130AFuse 63A blow in abour 200s at 130A (I2)

MCB 63A trip in about 240s at 130A (I2)

200s5


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23 rd August 2014

14 IEC 60364-5-52EXAMPLE

Melt TimeCurrent Data2-125 Amperes

TimeinSecond

Current in Amperes

Example

gG Fuse 63A

Melt in 200s for 130A

current (over-load)

Fuse 63AMelt at about 0.01s for 2kA

current (short circuit

current)


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23 rd August 2014

15 IEC 60898 BREAKERCLASS


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16 IEC 60898 FUSES

(a) The first letter indicates the breaking range of fuse-links:(i) g for full range breaking capacity; general purpose fuse link interrupts

all faults and is usually used for all overload and short circuit application.

(ii) a for partial range breaking capacity,usually associated with another

protective device in cascade for discrimination between different

segments of a circuit. It is usually used for short circuit protection only.(b) The second letter indicates the utilisation category, which defines the

accuracy of the time-current characteristics,

(i) G indicates general application;

(ii) M indicates protection of motor;

(iii) R indicates semiconductor protection;

(iv) S indicates semiconductor protection;

(v) Tr indicates for transformer protection;

(vi) D indicates time-delay (UL 248); and

(vii) N indicates non-time-delay (UL 248).

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23 rd August 2014

17 IEC 60898 FUSES


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23 rd August 2014

18 IEC 60364-5-52VOLTAGEDROP


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19 NEUTRALCABLE

Neutral (TN and TT systems) shall have the same cross sectional area andnot less than that of the line conductor in the following cases (refer IEC

60364-5-52 Clause 524.2).

(i) In all cases of single-phase, 2-wire circuits.

(ii) In polyphase and single-phase 3-wire circuit, when the size of the line

conductors is less than or equal to 16mm2 copper or 25mm2

aluminium.

Note: Some national codes require that neutral cable should be the same size

as line conductor. This requirement is mandatory in Malaysia in all cases,

except between transformer and main switch board and at the discretion

of the designer who is expected to take into account issues relating to

neutral current listed below.. 23 rd August 2014


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20 HARMONICS, ANNEXE, 5-52

Where significant harmonic current occurs in three-phase circuits, reduction

factor for conductor sizing in accordance with Table 9 (Annex E of IEC 60364-5-

52) shall be applied.

When harmonics exceed 33%, neutral currents may exceed the line current in

which case neutral conductors may be oversized compared to line conductors.

Third harmonic content of

phase current (%)

Reduction factor

Size selection is based onphase current

Size selection is based onneutral current

0 -15 1.0 -

15 - 33 0.86 -

33 - 45 - 0.86

> 45 - 1.0

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23 rd August 2014

R= Ro [1 + 20 ( 20)]

R = Rso [1 + 20 (sc 20)]X= 210-7Ln [2s/d]

1= (Rs/SR) [ 1 /(1 + (Rs/X))]


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23 VOLTAGESTANDARD

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IEC 60287 Electric cablesCalculation of the current rating

Part 1 Current rating equations (100 % load factor) and calculation oflosses:

Section 1: General;

Section 2: Sheath eddy current loss factors for two circuits in flat

formation;

Section 3: Current sharing between parallel single-core cables and -

calculation of circulating current losses;

Part 2Thermal resistance:Section 1: Calculation of thermal resistance;

Section 2: A method for calculating reduction factors for groups of cables

in free air, protected from solar radiation

Part 3Sections on operating conditions:Section 1: Reference operating conditions and selection of cable type;

Section 2: Economic optimization of power cable size;

Section 3: Cables crossing external heat sources


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23 rd August 2014


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23 rd August 2014

25 EXAMPLE1

Circuit 1 Lighting 8 nos 80W each

Circuit 2 2x13A socket outlet in radial circuit

Circuit 3 1x15A water heater

Circuit 4 1x15A, air cool split unit air conditioning

Circuit 5 4x13A socket outlet in ring circuit

Circuit 6 6x13A socket outlet in ring circuitExample 1, domestic wiring diagram with typical circuits


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23 rd August 2014

26 EXAMPLE1

Circuit 1 Lighting 8 nos 80W each

Circuit 2 2x13A socket outlet in radial circuit

Circuit 3 1x15A water heater

Circuit 4 1x15A, air cool split unit air conditioning

Circuit 5 4x13A socket outlet in ring circuit

Circuit 6 6x13A socket outlet in ring circuitExample 1, domestic wiring diagram with typical circuits


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23 rd August 2014

27 EXAMPLE1

N

o Fuse rating Conductor

(mm)

Comments

1 6 Amps 1.5 mm Lighting circuit (8 nos x 80W). Clause 524,

Table 52.2 of 60364-5-52 requires minimum

cable size at 1.5mm.

2 15 Amps 2.5 mm 2x13A socket outlet in radial circuit (2x300W)

3 20 Amps 4 mm 1x15A socket outlet for water heater

(2,000W)

4 15 Amps 2.5 mm 1x15A socket outlet for (1x750W)

5 30 Amps 2.5 mm 4x13A socket outlet in ringcircuit(4x300W)

6 30 Amps 4 mm 6x13A socket outlet in ringcircuit(6x300W)

7 60 Amps 25 mm Main feeder cables to consumer unit (total

load = 6,690W (38A).

Table 13A; Example 1, schedule of circuits


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23 rd August 2014

28 EXAMPLE1

Load EstimateSelect Earthing

System

Determine

Installation

Method

Select & Size

Wiring / Cables

Table 13A

(previous slide)TN-S

PVC single core

in conduit;

method A1

IEC60364-5-52,

method A1; Table

B52.2


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23 rd August 2014

29 EXAMPLE1

Cct

No

Load/ current(3)

IB(A)

MCB (type B) Cable Comments

Nom.rating IN

Effectivetrip I2

(1)Size mm Iz (A)

(2)

230V single phase

1 8x80W ~ 3.3A 6A 9A 1.5 14.5 Lighting circuit

2 2x300W ~ 3.3A 15A 22.5A 2.5 19.5 Socket outlet radial

circuit

3 2000W ~ 9.7A 20A 30A 4 26 Water heater

4 750W ~ 3.8A 15A 22.5A 2.5 19.5 15A outlet with airconditioning unit

5 4x300W ~ 6.5A 30A 45A 2.5 2x19.5 Ring circuit subject to

nationalconditions.6 6x300W ~ 9.8A 30A 45A 4 2x26

7 6,690W ~ 36A 60A 90A 25 80 Main power intake

(1) Assume effective trip current to be 1.5 x IN.(2) Cable installation assumes PVC insulated cables, 2 single core in conduit embedded in walls.

Current capacity referenced from IEC 60364, table B52.2, installation method A1.

(3) Load estimate are presented here without consideration of diversity as an illustration only.

The subject of load diversity can be a detail subject which may require national statistics on

average usage of electrical appliances.

Check: IB < IN< IZ I2 < 1.45 x IZ


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23 rd August 2014

Installation Config- Conductor Current capacity Group Corrected


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23 rd August 2014

31 EXAMPLE2LARGECABLESCase

Installation Config-

uration(1); XLPE cable

Conductor

size(1) (mm)

Current capacity

of cables

Group

rating factor(2) (3)

Corrected

current

capacity

Multi core cable (B52.12, column 3, method E) B52.20

(a) 3 x 4 core, 1 cable

diameter apart

3x300mm / 4

core

3x621A = 1,853A 1 1,853A

(b) 3 x 4 core, 1 cable

diameter apart

2x240mm / 4

core

3x538A = 1,614A 1 1,614A

Single core cables in trefoil (B52.12, column 5, method F) B52.21

(a) Trefoil, 2 circuit @ 2 cable

diameter apart on 1 ladder

7x400mm / 1

core (5)

2x823A = 1,646A 1 1,646A

(b) Trefoil, 3 circuit touching 11x240mm / 1core (5)

3x607A = 1,821A 0.82(4)

1,493A

(b) Trefoil, 3 circuit @ 2 cable

diameter apart on 1 ladder

11x240mm / 1

core (5)

3x607A = 1,821A 1) 1,821A

Single core cables in flat formation (B52.12, column 6 or 7,

method F or G)

B52.21

(a) Cable touching (methodF), 2 circuits, 1 ladder.

7x400mm / 1core (5)

2x868A = 1,736A 0.97 1,684A

(b) 1 cable diameter apart,

(method G), 2 circuits, 1

ladder.

7x300mm / 1

core (5)

2x902A = 1,804A 0.97 1,750A

(b) 1 cable diameter apart,

(method G), 2 circuits, 1ladder.

7x240mm / 1

core(5)

2x781A = 1,562A 0.97 1,515A


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23 rd August 2014

32 EXAMPLE2LARGECABLES

Notes:

(1) Installation configuration and conductor sizes are based on table B52.12 forXLPE insulated copper cables.

(2) Group rating factor where only 1 cable ladder is used is referenced from table

B52.17 row 5. Finer graduation in group factor can be obtained from tables

B52.20 (for multi core cables and B52.21 (for single core cables).

(3) In all cases, we assume only one cable ladder used.(4) Group rating for case 2(b). Table B52.21 for single core cables on ladder in

trefoil only considers case of cable-circuits laid 2 Deapart. To consider case of

cable touching,table B52.17 is referred for reduction factor.

(5) For single core cables, we assume neutrals to be sizes. The odd cable is

therefore used as neutral e.g. 11 core means 3x1 core for each line conductorand 2x1 core for neutral. In some national jurisdiction, size neutral may not

be allowed.

(6) Ambient temperature is assumed to be at 30C, for temperature correction

factor at 1 (from table B52.14).

Table 14A; Example 2, schedule of cable sizes and configurations


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23 r d August 2014



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Cable Sizing For Safe Power System Based

o n I E C S t a n d a r d s

I r . H . P. L o o i ( m e k t r i c o n @ g m a i l . c o m )

B . E n g ( H o n s ) , F I E M , J u r u t e r a G a s

Cable Sizing Fundamentals and

E l e c t r i c a l L V S t a n d a r d

A r m a d a H o t e l , P e t a l i n g J a y a

2 3 r d A u g u s t 2 0 1 4

P a r t 2 C a b l e S i z i n g

ThankYou for your At tent ion !

Documents

IET Lecture LV Wiring Design September 14