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Electrical Science
Steve Sayles
Cable selection
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What factors may have to be considered when
selecting cables?
Thermal effectsAmbient Temperature (Ca)
Grouping of cables (Cg)
Current carrying capacity of conductor(Iz)
Operating temperature of conductor (Ct)
Design current of te circuit! I!e te current
intended to be carried by te circuit in normalser"ice (Ib)
Termal insulation (Ci)
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I = V / R
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Transposing the formla we can find any
vale if we !now the other two vales"
I = V / R
R = V / I
V = I R
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#$ % &$' "olt drop
ere as te oter &' " gone
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#oes the above circit meet the re$irements of %S &'&(
Reglation )*)+,(+,* ?
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-ne consideration when selecting cables is
that of volt drop" %S&'&( contains Crrent+carrying tables .appendi 0 1" These tables
also inclde vales from which a cable volt
drop can be calclated per ampere per metre .
in millivolts1"
To calculate te "olt drop! te figure from te
tables must be multiplied by te lengt of te
cable (in metres) and te design current on fullload* Te final product must be di"ided by $''' to
gi"e te ans+er in "olts*
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,-C Cables!
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Te simplest metod to adopt is to determine te
ma.imum m-/Am +ic +ill comply +it te
ma.imum "olt drop allo+ed*
&0' 1 $'' . 2 % 3*&
2aimm volt drop
allowed = ((") v
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3ample
4 *5,v single+phase coo!er circit has a5,m length of rn in 0mm6 T73 8VC
inslated cable" The circit has a design
crrent .Ib1 of 0)4 " Calclate the voltdrop"
$$ . 42 . 0' % 55555555555555555
$'''
$4*62 -
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What si9e cable wold meet the
re$irements of the reglation for the
circit in the last eample?
4 *5,v single+phase coo!er circit has a
5,m length of T73 8VC inslated cable"
The circit has a design crrent of 0)4 "
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E.ample
In a large ouse! a ligting circuit supplies a ban7 of $'
8' fluorescent luminaries" Te lengt of cable run is
48m* at is te "olt drop using te minimum ,-C cable
csa allo+ed
Ib = . ', (, (": 1; *5, = 0"'
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Remember compliance
with Volt drop reglations
does >-T mean
compliance with other
reglations"
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3ample
4 small hotel has a *5,v single+phase(,")w 3lectric shower circit that is
the smallest of its five shower" It has a
0)m rn of pvc cable" Calclate cable
csa and volt drop in order to meet volt
drop reglations compliance"
Ib % $'2'' 1 &0' " % 42 A
Apply di"ersity of &2: % $$*&2 A
(("*) 0) (: =
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Remember compliance
with Volt drop reglations
does >-T mean
compliance with other
reglations"
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Cable Selection 8rocess
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Cable Selection 8rocess
Ib @ In
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Cable Selection 8rocess
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Cable Selection 8rocess
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Cable Selection 8rocess
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Cable Selection 8rocess
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Cable Selection 8rocess
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Shoc! 8rotection
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Shoc! 8rotection
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Shoc! 8rotection
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Thermal Constraints-erification of te +itstand capabilities of cables under sort;circuit
conditions
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Thermal Constraints-erification of te +itstand capabilities of cables under sort;circuit
conditions
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Thermal Constraints-erification of te +itstand capabilities of cables under sort;circuit
conditions
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Thermal Constraints-erification of te +itstand capabilities of cables under sort;circuit
conditions
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Thermal Constraints-erification of te +itstand capabilities of cables under sort;circuit
conditions
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