Cable Rating Caculation

8/17/2019 Cable Rating Caculation

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4.3.9 - Cable rating calculation

The Regulations indicate the following symbols for use when selecting cables:

Izis the current carrying capacity of the cable in the situation where it isinstalled

Itis the tabulated current for a single circuit at an ambient temperature of30°C

Ib is the design current, the actual current to be carried by the cable

In is the rating of the protecting fuse or circuit breaer

I!is the operating current for the fuse or circuit breaer "the current at whichthe fuse blows or the circuit breaer opens#

Ca is the correction factor for ambient temperature

Cg is the correction factor for grouping

Ci is the correction factor for thermal insulation$

The correction factor for protection by a semi%enclosed "rewirable# fuse is not gi&en asymbol but has a fi'ed &alue of 0$(!)$

*nder all circumstances, the cable current carrying capacity must be e+ual to or greaterthan the circuit design current and the rating of the fuse or circuit breaer must be at leastas big as the circuit design current$ These re+uirements are common sense, because

otherwise the cable would be o&erloaded or the fuse would blow when the load isswitched an$

To ensure correct protection from o&erload, it is important that the protecti&e de&iceoperating current "I!# is not bigger than $-) times the current carrying capacity of thecable "Iz#$ .dditionally, the rating of the fuse or circuit breaer "In# must not be greaterthan the the cable current carrying capacity "Iz# It is important to appreciate that theoperating current of a protecti&e de&ice is always larger than its rated &alue$ In the case of a bac%up fuse, which is not intended to pro&ide o&erload protection, neither of thesere+uirements applies$

To select a cable for a particular application, tae the following steps: "note that to sa&etime it may be better first to ensure that the e'pected cable for the re+uired length ofcircuit will/ not result in the ma'imum permitted &olt drop being e'ceeded -$3$1#$

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1. %Calculate the e'pected "design# current in the circuit "Ib#

2. %Choose the type and rating of protecti&e de&ice "fuse or circuit breaer# to be used"In#

3. %2i&ide the protecti&e de&ice rated current by the ambient temperature%%%%% correction factor "Ca# if ambient temperature differs from 30°C

4. % urther di&ide by the grouping correction factor "Cg#

5. %2i&ide again by the thermal insulation correction factor "CI#

6. %2i&ide by the semi%enclosed fuse factor of 0$(!) where applicable

7. %The result is the rated current of the cable re+uired, which must be chosen%%%%% from the appropriate tables -$4 to -$51$

6bser&e that one should di&ide by the correction factors, whilst in the pre&ious subsectionwe were multiplying them$ The difference is that here we start with the design current ofthe circuit and ad7ust it to tae account of factors which will derate the cable$ Thus, thecurrent carrying capacity of the cable will be e+ual to or greater than the design current$ In-$3$(1 we were calculating by how much the current carrying capacity was reduced due toapplication of correction factors$

Tables -$4 to -$51 gi&e current ratings and &olt drops for some of the more commonly usedcables and sizes$ The Tables assume that the conductors and the insulation are operating

at their ma'imum rated temperatures$ They are e'tracted from the Regulations Tablesshown in s+uare bracets e$g$ 8-2./

The e'amples below will illustrate the calculations, but do not tae account of &olt dropre+uirements "see -$3$1#$

Example 4.1

.n immersion heater rated at !-0 9, 3 is to be installed using twin with protecti&econductor p$&$c$ insulated and sheathed cable$ The circuit will be fed from a ) .miniature circuit breaer type !, and will be run for much of its - m length in a roof space

which is thermally insulated with glass fibre$ The roof space temperature is e'pected torise to )0°C in summer, and where it lea&es the consumer unit and passes through a )0mm insulation%filled ca&ity, the cable will be bunched with se&en others$ Calculate thecross%sectional area of the re+uired cable$

irst calculate the design current Ib

Ib; <; 3000.

;!$).

%* !-0

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The ambient temperature correction factor is found from Table -$31 to be 0$($ The groupcorrection factor is found from Table -$-1 as 0$)!$ "The circuit in +uestion is bunched withse&en others, maing eight in all#$

The thermal insulation correction factor is already taen into account in the current rating

table "-2!. ref$ method -/ and need not be further considered$ This is because we canassume that the cable in the roof space is in contact with the glass fibre but not enclosedby it$ hat we must consider is the point where the bunched cables pass through theinsulated ca&ity$ rom Table -$)1 we ha&e a factor of 0$=5$

The correction factors must now be considered to see if more than one of them applies tothe same part of the cable$ The only place where this happens is in the insulated ca&itybehind the consumer unit$ actors of 0$)! "Cg# and 0$=5 "CI# apply$ The combined &alueof these "0$-43#, which is lower than the ambient temperature correction factor of 0$(,and will thus be the figure to be applied$ >ence the re+uired current rating is calculated:%

Table 4.6 - Current ratings and olt drops !or uns"eat"ed single core p..c. insulatedcables

Crosssectional

area

In conduitin thermalinsulation


Inconduiton wall

Inconduiton wall

Clippeddirect

Clippeddirect

9olt drop 9olt drop

"mm?# ".# ".# ".# ".# ".# ".# "m9@.@m# "m9@.@m#

% ! cables3 or -cables

!cables

3 or -cables

!cables

3 or -cables

! cables3 or -cables

$0 $0 0$) 3$) !$0 )$) -$0 --$0 3=$0

$) -$) 3$) ($) )$) !0$0 =$0 !5$0 !)$0

!$) 5$) =$0 !-$0 !$0 !($0 !)$0 =$$0 )$0

-$0 !4$0 !-$0 3!$0 !=$0 3($0 33$0 $0 5$)

4$0 3-$0 3$0 -$0 34$0 -($0 -3$0 ($3 4$-

0$0 -4$0 -!$0 )($0 )0$0 4)$0 )5$0 -$- 3$=

4$0 4$0 )4$0 (4$0 4=$0 =($0 (5$0 !$= !$-

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Table 4.7 - Current ratings and olt drops !or s"eat"ed multi-core p..c.-insulated cables

Crosssectional

area



Inconduiton wall

Inconduiton wall

Clippeddirect

Clippeddirect

9olt drop 9olt drop

"mm?# ".# ".# ".# ".# ".# ".# "m9@.@m# "m9@.@m#

% ! core3 or -core

! core3 or -core

! core3 or -core

! core3 or -core

$0 $0 0$0 3$0 $) )$0 3$) --$0 3=$0

$) -$0 3$0 4$) )$0 5$) ($) !5$0 !)$0

!$) =$) ($) !3$0 !0$0 !($0 !-$0 =$0 )$0

-$0 !)$0 !3$0 30$0 !($0 34$0 3!$0 $0 5$)

4$0 3!$0 !5$0 3=$0 3-$0 -4$0 -$0 ($3 4$-

0$0 -3$0 35$0 )!$0 -4$0 43$0 )($0 -$- 3$=

4$0 )($0 )!$0 45$0 4!$0 =)$0 (4$0 !$= !$-

Iz ; in ; ) . ; 3!$- .Cg ' Ci 0$)! ' 0$=5

rom Table -$(1, 4 mm? p$&$c$ twin with protecti&e conductor has a current rating of 3! .$

This is not +uite large enough, so 0 mm?with a current rating of -3 . is indicated$ Aotonly would this add considerably to the costs, but would also result in difficulties due toterminating such a large cable in the accessories$

. more sensible option would be to loo for a method of reducing the re+uired cable size$or e'ample, if the eight cables left the consumer unit in two bunches of four, this wouldresult in a grouping factor of 0$4) "from Table -$-1#$ Before applying this, we must chec thatthe combined grouping and thermal insulation factors "0$4) ' 0$=5 ; $0$)=# are still lessthan the ambient temperature factor of 0$(, which is the case$

Table 4.# - Current ratings o! mineral insulated cables clipped direct

Cross%sectional

area9olt

p$&$c$sheath

! ' singleor twin

p$&$c$heath 3

core

p$&$c$heath 3 'single or

twin

Baresheath !' single

Baresheath

3 'single

"mm?# ".# ".# ".# ".# ".#

$0 )00& =$) 4$) 4$) !!$0 !$0

$) )00& !-$0 !$0 !$0 !=$0 !($0

!$) )00& 3$0 !=$0 !=$0 3=$0 34$0

-$0 )00& -!$0 3($0 3($0 )$0 -($0

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$0 ()0& !0$0 ($) ($) !-$0 !-$0

$) ()0& !)$0 !!$0 !!$0 3$0 30$0

!$) ()0& 3-$0 30$0 30$0 -!$0 -$0

-$0 ()0& -)$0 -0$0 -0$0 ))$0 )3$0

4$0 ()0& )($0 )$0 )$0 (0$0 4($0

0$0 ()0& (=$0 45$0 45$0 54$0 5$0

4$0 ()0& 0-$0 5!$0 5!$0 !($0 5$0

Aote that in "Tables -$= and -$5# D


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This time reference method I "clipped direct# will apply to the current rating Table -$(1$ ora two core cable, -$0 mm?, two core has a rating of 34 ., so this ill be the selected size$

It is of interest to notice how +uite a minor change in the method of installation, in thiscase clipping the cable to the 7oists or battens clear of the glass fibre, has reduced the

acceptable cable size$

Example 4.3 .ssume that the immersion heater indicated in the two pre&ious e'amples is to beinstalled, but this time with the protection of a ) . rewirable "semi%enclosed# fuse$Calculate the correct cable size for each of the alternati&es, that is where firstly the cableis in contact with glass fibre insulation, and secondly where it is held clear of it$

This time the &alue of the acceptable current carrying capacity Iz will be different becauseof the need to include a factor for the rewirable fuse as well as the new ambienttemperature and grouping factors for the rewirable fuse from Tables -$3 and -$-1$

Iz ; in ; ) . ; -)$( . Cg ' Ca ' 0$(!) 0$)! ' 0$=( ' 0$(!)

In this case, the cable is in contact with the glass fibre, so the first column of Table -$(1 ofcurrent ratings will apply$ The acceptable cable size is 4 mm? which has a current ratingof )( .$

This cable size is not acceptable on the grounds of high cost and because the conductorsare liely to be too large to enter the connection tunnels of the immersion heater and its

associated switch$ If the cables lea&ing the consumer unit are re%arranged in two groupsof four, this will reduce the grouping factor to 0$4), so that the newly calculated &alue of Izis 34$4 .$ This means using 0 mm? cable with a current rating of -3. "from Table -$(1#,since 4 mm? cable is shown to ha&e a current rating in these circumstances of only 3! .$By further rearranging the cables lea&ing the consumer unit to be part of a group of onlytwo, Cg is increased to 0$=, which reduces Iz to !5$( . which enables selection of a 4mm? cable$

hould it be possible to bring the immersion heater cable out of the consumer unit on itsown, no grouping factor would apply and Iz would fall to !3$= ., allowing a - mm? cable tobe selected$

here the cable is not in contact with glass fibre there will be no need to repeat thecalculation of Iz, which still has a &alue of !5$( . pro&ided that it is possible to group theimmersion heater cable with only one other where it lea&es the consumer unit$ This timewe use the DReference Hethod "clipped direct#D column of the current rating Table -$(1,which shows that - mm? cable with a current rating of 34 . will be satisfactory$

F'amples -$, -$! and -$3 show clearly how forward planning will enable a moreeconomical and practical cable size to be used than would appear necessary at first$ It is,of course, important that the design calculations are recorded and retained in theinstallation manual$

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Example 4.4 . -) 9 )0 >z three%phase motor with an output of (,) , power factor 0$= andefficiency =) is the be wired using )00 9 light duty three%core mineral insulated p$&$c$sheathed cable$ The length of run from the >BC protecting fuses is !0 m, and for abouthalf this run the cable is clipped to wall surfaces$ or the remainder it shares a cable tray,

touching two similar cables across the top of a boiler room where the ambienttemperature is )0°C$ Calculate the rating and size of the correct cable$

The first step is to calculate the line current of the motor$

Input ; output ; ($) ' 00 ; =$=! efficiency =)

Eine current Ib ; < ; =$=! ' 03 . ; )$3 . %%%%%%%%%%%%%%%%%%%%%%%%%J3 ' *E ' cosK J3 ' -) ' 0$=

e must now select a suitable fuse$ ig 3$)1 for B == fuses shows the 4 . size to bethe most suitable$

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Cable Rating Caculation