2008 28 Autumn Wiring Matters Appendix 4

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IET Wiring Matters | Autumn 08 | www.theiet.org

SELECTING THE correct cable for theapplication is imperative to ensure asatisfactory life of conductors andinsulation subjected to the thermaleffects of carrying current for prolongedperiods of time in normal service.

Choosing the minimum size cross-sectional area of conductors isessential to meet the requirements for: Protection against electric shock(Chapter 41)

Protection against thermal effects(Chapter 42)

Overcurrent protection (Chapter 43), Voltage drop (Section 525), and Limiting temperatures for terminalsof equipment to which the conductorsare connected (Section 526).

Current-carrying capacity andvoltage drop for cablesThe 17th Edition brought about somesignificant changes when calculatingthe current-carrying capacity andvoltage drop for cables. For both the16th and 17th Editions, most of thecurrent ratings have been taken fromIEC 60364-5-52 and the CENELEC HD384.5.52 +A1 1998. These IEC andCENELEC documents do not,however, provide current ratings forarmoured single-core cables,therefore, the ratings for these cablesare based on data provided by ERATechnology Ltd and the BritishCables Association.The tables have been updated toreflect present cable standards and

introduce current ratings for buriedcables. Generally, the current ratingsfor commonly used cables have notchanged between the 16th and 17thEditions.

Installation methodsThe 16th Edition recognised 20methods of installation, the 17thEdition, however, recognises 57methods in Table 4A2. It is impossible,of course, to cover every possiblemethod or installation permutationbut, with the 17th Edition recognisinga further 37 methods of installation,more possibilities are now covered.Note that all installation numbers intable 4A2 are different from those inthe 16th Edition.

Reference methodIt is impractical to calculate andpublish current ratings for everyinstallation method, since many wouldresult in the same current rating.Therefore a suitable (limited) numberof current ratings have beencalculated which cover all installationmethods, known as ReferenceMethods. There are 7 referencemethods, A to G, shown in table 1overleaf.

Appendix 4 of BS 7671by Mark Coles

Appendix 4, Current-carrying capacity and voltage dropfor cables and flexible cords, has seen significantchanges with the publishing of BS 7671:2008. Thisarticle looks at some of the changes and showsexamples of cable calculations.

ireeveRemoved PagesTo save downloading time the following pages have been removed:1) Advert pages 9 and 11

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Other methods of installationOther methods of installation arerecognised but are, essentially,variations of other methods and aretherefore allocated appropriatereference methods, shown in table 2overleaf.

It is worth noting that the 17th

Edition now references cables buriedin the ground (installation methods 70to 73). The current-carrying capacitiestabulated for cables in the ground arebased upon a soil thermal resistivity of2.5 K.m/W and are intended to beapplied to cables laid in and aroundbuildings, i.e. disturbed soil. For other

installations, where investigationsestablish more accurate values of soilthermal resistivity appropriate for theload to be carried, the values ofcurrent-carrying capacity may bederived by the methods of calculationgiven in BS 7769 (BS IEC 60287) orobtained from the cable manufacturer.

Referencemethod

Example of installation method Relevant table fromBS 7671:2008

Image

A

Non-sheathed cables in conduit in athermally insulated wall

The wall consists of an outer weatherproof skin,thermal insulation and an inner skin of wood orwood-like material having a thermal conductanceof at least 10 W/m.K. The conduit is fixed such thatit is close to, but not necessarily touching, the innerskin. Heat from the cables is assumed to escapethrough the inner skin only. The conduit can bemetal or plastic.

Installation Method 1 of Table 4A2

Multicore cables in conduit in athermally insulated wall


B

Non-sheathed cables in conduitmounted on a wooden ormasonry wall

The conduit is mounted on a wooden wall suchthat the gap between the conduit and the surface isless than 0.3 times the conduit diameter. Theconduit can be metal or plastic. Where the conduitis fixed to a masonry wall the current-carryingcapacity of the non-sheathed or sheathed cablemay be higher.


Multicore cables in conduitmounted on a wooden ormasonry wall


C(clipped direct)

Single-core or multicore cable on awooden or masonry wall

Non-sheathed cables in conduit mounted on awooden or masonry wall


D

Multicore unarmoured cable inconduit or in cable ducting in theground

Non-sheathed cables in conduit mounted on awooden or masonry wallThe cable is drawn into a100 mm diameter plastic, earthenware or metallicduct laid in direct contact with soil having a thermalresistivity of 2.5 K.m/W and at a depth of 0.8 m.The values given for this method are those stated inthis appendix and are based on conservativeinstallation parameters. If the specific installationparameters are known (thermal resistance of theground, ground ambient temperature, cable depth),reference can be made to the cable manufactureror the ERA 69-30 series of publications, which mayresult in a smaller cable size being selected.NOTE: The current-carrying capacity for cables laidin direct contact with soil having a thermal resistivityof 2.5 K.m/W and at a depth of 0.7 m isapproximately 10 % higher than the valuestabulated for Reference Method D


E, F and G

Single-core or multicore cable infree air

The cable is supported such that the total heatdissipation is not impeded. Heating due to solarradiation and other sources is to be taken intoaccount. Care is to be taken that natural airconvection is not impeded. In practice, a clearancebetween a cable and any adjacent surface of atleast 0.3 times the cable external diameter formulticore cables or 1.0 times the cable diameter forsingle-core cables is sufficient to permit the use ofcurrent-carrying capacities appropriate to free airconditions.


Table 1: Reference methods

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In locations where the effective soilthermal resistivity is higher than2.5 K.m/W, an appropriate reduction incurrent-carrying capacity should bemade or the soil immediately aroundthe cables should be replaced by amore suitable material, i.e. such casescan usually be recognised by very dryground conditions. Rating factors forsoil thermal resistivities other than2.5 K.m/W are given in Table 4B3.Flat, twin and earth cables installed

in thermal insulation are alsorecognised by installation methods 100to 103.

Sizing of cablesRelationship of current-carryingcapacity to other circuit parametersThe relevant symbols used in theRegulations are shown in Table 3:

The rated current or current setting of theprotective device (In) must not be less thanthe design current (Ib) of the circuit, and therated current or current setting of theprotective device (In) must not exceed thelowest of the current-carrying capacities (Iz)of any of the conductors of the circuit,see fig. 1.

Iz It Ib

The current-carrying capacityof a cable for continuousservice, under the particularinstallation conditionsconcerned.

The value of current tabulated inthis appendix for the type ofcable and installation methodconcerned, for a single circuit inthe ambient temperature statedin the current-carrying capacitytables.

The design current of the circuit, i.e.the current intended to be carriedby the circuit in normal service. Inthe rated current or current settingof the protective device.

In I2 C

The rated current or currentsetting of the protectivedevice.

The operating current (i.e. thefusing current or tripping currentfor the conventional operatingtime) of the device protecting thecircuit against overload.

A rating factor to be applied wherethe installation conditions differfrom those for which values ofcurrent-carrying capacity aretabulated in this appendix.

The various rating factors are identified as follows:

Ca Cg Ci

For ambient temperature. For grouping. For thermal insulation.

Ct Cc

For operating temperature ofconductor.

For the type of protective deviceor installation condition.

Example ofinstallationmethod

Reference method

Cable on a floor Reference Method C applies for current rating purposes.

Cable under aceiling

This installation may appear similar to Reference Method C but because of the reduction in natural airconvection, Reference Method B is to be used for the current rating.

Cable tray systems A perforated cable tray has a regular pattern of holes that occupy at least 30% of the area of the base ofthe tray. The current-carrying capacity for cables attached to perforated cable trays should be taken asReference Methods E or F. The current-carrying capacity for cables attached to unperforated cable trays(no holes or holes that occupy less than 30% of the area of the base of the tray) is to be taken asReference Method C.

Cable laddersystem

This is a construction which offers a minimum of impedance to the air flow around the cables, i.e.supporting metalwork under the cables occupies less than 10% of the plan area. The current-carryingcapacity for cables on ladder systems should be taken as Reference Methods E or F.

Cable cleats, cableties and cablehangers

Cable supports hold the cable at intervals along its length and permit substantially complete free air flowaround the cable. The current-carrying capacity for cable cleats, cable ties and cable hangers should betaken as Reference Methods E or F.

Cable installed in aceiling

This is similar to Reference Method A. It may be necessary to apply the rating factors due to higherambient temperatures that may arise in junction boxes and similar mounted in the ceiling.

NOTE: Where a junction box in the ceiling is used for the supply to a luminaire, the heat dissipation fromthe luminaire may provide higher ambient temperatures than permitted in Tables 4D1A to 4J4A (seealso Regulation 522.2.1). The temperature may be between 40 C and 50 C, and a rating factoraccording to Table 4B1 must be applied.

Table 2: Other recognised methods of installation

Table 3: Symbols used in the Regulations

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Where the overcurrent device isintended to afford protection againstoverload, I2 must not exceed 1.45 Iz andIn must not exceed Iz, see fig. 2.Where the overcurrent device is

intended to afford fault currentprotection only, In can be greater thanIz and I2 can be greater than 1.45 Iz.The protective device must be selectedfor compliance with Regulation 434.5.2.

Determination of the size of cableto be usedWhen overcurrent protection is to beprovided, the current-carryingcapacity of the cable, Iz, is determinedby applying correction factors to thetabulated cable ratings, It, fromAppendix 4 of BS 7671.

Iz = It Ca Cg Ci Cc

where:

Ca is rating factor for ambienttemperature, see Table 4B1 of BS 7671Cg is rating factor for grouping see,Table 4C1 of BS 7671Ci is rating factor for conductorssurrounded by thermal insulationCc is a rating factor applied whenoverload protection is being providedby an overcurrent device with a fusingfactor greater than 1.45, e.g. Cc = 0.725for semi-enclosed fuses to BS 3036, andwhen the cable is laid in the ground.

By referring to fig. 1, the current-carrying capacity of the cable, Iz, mustequal or exceed the circuit overcurrentdevice rated current, In.

Iz In

and, hence, by combining the twoequations above, we get:

This equation can be read as, whenovercurrent protection is to beprovided, the tabulated cable ratingsfrom Appendix 4 of BS 7671 must equalor exceed the circuit overcurrentdevice rating corrected for ambienttemperature, grouping, thermalinsulation and the use of a rewirablefuse if applicable.

Example 1A circuit supplying a shower with aloading of 6 kW would have a designcurrent Ib given by:

The nominal current rating in amps,In, of the protective device (fuse orcircuit-breaker) for a circuit is selected

so that In is greater than or equal tothe design current, Ib, of the circuit.

In Ib

So, in the example of the 6 kW showerIn must be 26;

select say a 32 A circuit-breaker, that is

In = 32 A

A cable must now be selected so that itsrating, Iz, in the particular installationconditions exceeds the design currentof the load, Ib.

Iz Ib

In the example of the 6 kW showerload, Ib = 26, so Iz 26.

Where overload protection of the cableis to be provided, as is usual, the cableis also selected so that its rating in itsinstalled conditions Iz exceeds thecurrent rating of the circuit protectivedevice.

Iz In

In the example of the 6 kW showercircuit with overload protection,Iz 32. It may be argued that nooverload need be provided for a showeras the load is fixed.

Fig. 1: Coordination of load, device and cable characteristics Fig. 2: Coordination for overload protection

InIt

CaCgCiCc

6 x 1000 WIb = = 26 A230 V

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Therefore, Iz In Ib

In the example of the 6 kW showerwith overload protection, thisrelationship will be satisfied ifIb = 26 A, In = 32 A and the circuitconductors are sized such that Iz 32 A.To calculate the tabulated cable

ratings, It, the following formula(from Appendix 4 of BS 7671)is used.

Iz = It Ca Cg Ci Cc

Overload protection is provided inpractically all circuit designs in orderto protect the cable should the load beincreased, e.g. by adding further lightsto a lighting circuit or changing ashower for one of a higher ratingwithout proper checks being made.However, for a fixed load e.g. a

shower circuit, this is not an actualrequirement of BS 7671.Note that the term overcurrentincludes both overload current andfault current.Where protection is being provided

against overload, protection will alsobe provided against fault currents,however, the reverse is not trueKnowing Iz, it is necessary to select a

tabulated cable rating such that

Example 2For the shower circuit above:the ambient temperature is assumed(as is usual) to be 30 C, so Ca = 1the cable is not grouped so Cg = 1the cable is not installed in thermalinsulation so Ci = 1, andit is not a semi-enclosed (rewirablefuse) so Cc = 1.

Hence

For a thermoplastic (PVC) insulatedand sheathed flat cable with protectiveconductor from Table C.1 or Table4D5A of BS 7671 or Table 6F of theOn-Site Guide, installed in an insulatedwall, 6 mm2 cable is adequate as it hasa tabulated rating of 32 A forinstallation method A.

Further informationFurther information and reading canbe found in the following publications: BS 7671:2008 Requirements forElectrical Installations, IEE WiringRegulations, Seventeenth EditionGuidance Note 3 - Inspection andTesting

Electrical Installation Design Guide -Calculations for Electricians andDesigners (2008), IET publicationISBN 978-0-86341-550-0

IET Guidance Note 6 - Protection

against overcurrent BS 7769 (BS IEC 60287) Electric cables.Calculation of the current rating.Thermal resistance. A method forcalculating reduction factors forgroups of cables in free air, protectedfrom solar radiation

IEC 60502-1 Power cables withextruded insulation and theiraccessories for rated voltages from1 kV (Um = 1,2 kV) up to 30 kV(Um = 36 kV) Part 1: Cables for ratedvoltages of 1 kV (Um = 1,2 kV) and3 kV (Um = 3,6 kV)

BS EN 60228 Conductors of insulatedcables

IEC 364-5-523: 1983 Electricalinstallations of buildings - Part 5:Selection and erection of electricalequipment. Chapter 52: Wiringsystems. Section 523 - Current-carryingcapacities

IzIt

CaCgCiCc

32It 1 x 1 x 1 x 1

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2008 28 Autumn Wiring Matters Appendix 4