Volt Drop and Cable Selection

8/18/2019 Volt Drop and Cable Selection

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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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Ib @ In


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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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conditions


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conditions


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conditions


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conditions


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Volt Drop and Cable Selection