Answer / amarnath

To select the cable size we need to know the ampere rating
of the load and normally 1 sq.mm of Aluminium cable can
carry upto 1.2Amp and 1 sq.mm of Copper cable can carry
4.3Amp of Current but we should also consider the voltage
drop when the load is far away using the formula
VD=2KIL/cable size in CM
where VD is Voltage drop, K is the losses normally 12 for
copper and 19 for aluminium, L is the length from the
source to the load and CM is centimiles 1sq.mm in centimil
is 1943.5252.

Answer / rajeshwar paste

Practically there is no formula, but u can refer various
cable supplier booklets in which they specify cable size
against current

Example

Conductor Requirements for ISO 10133 and ISO 13297
This appendix is reproduced from Annex 'A' (normative) of
both ISO 10133 and 13297. Both
ISOs are supporting Standards to the Recreational
Directive. The use of these recommendations
can be used to demonstrate compliance to this Directive.
Current Ratings
Table A1 gives allowable continuous current ratings in
amperes determined for an ambient
temperature of 30°C and the minimum number of strands for
conductors.
Table A1 - Conductor cross-sectional area, allowable
continuous current and stranding.
Maximum Current in amps for a single conductor at
insulation temperature ratings
Cross sectional
Area mm2
60°C 70°C 85 to 90°C 105°C 125°C 200°C
Minimum number of strands
Type A* Type B*
0.75 6 10 12 16 20 25 16
1 8 14 18 20 25 35 16
1.5 12 18 21 25 30 40 19 26
2.5 17 25 30 35 40 45 19 41
4 22 35 40 45 50 55 19 65
6 29 45 50 60 70 75 19 105
10 40 65 70 90 100 120 19 168
16 54 90 100 130 150 170 37 266
25 71 120 140 170 185 200 49 420
35 87 160 185 210 225 240 127 665
50 105 210 230 270 300 325 127 1064
70 135 265 285 330 360 375 127 1323
95 165 310 330 390 410 430 259 1666
120 190 360 400 450 480 520 418 2107
150 220 380 430 475 520 560 418 2107
NOTE: Conductor current ratings may be interpolated for
cross sectional areas between those shown in Table A1.
* Conductors with at least a Type A stranding shall be used
for general wiring of craft.
Conductors with Type B stranding shall be used for any
wiring where frequent flexing is involved
during use.
For conductors in engine rooms (60c
ambient), the maximum current rating in
tableA1 shall be de-rated by the factors:-
Temperature rating of conductor insulation °C Multiply
maximum current from table A1 by
70 0.75
85 to 90 0.82
105 0.86
125 0.89
200 1.0
BUNDLING (For a.c. only
When more than three a.c. conductors are bundled together
the maximum current ratings in
Table A1 shall be de-rated by the factor below:-
Number of conductors bundled Multiply maximum current from
A1 by
4 to

Answer / raul

cable size depends upon THE MOST ECONOMICAL CONDUCTOR SIZE
concept in which the size depends upon the max. dielectric
stress on the conductor..........i.e. g(max.)= 2V/dlogD/d,
where V=voltage, x= distance from the conductor centre,
D=sheath diameter, d=conductor diameter........by
differentiating above relation n equating to zero......we
get logD/d=1 i.e. d=D/2.718...........

Answer / anil

The cable is depend upon the connected load and type of
conductor. there is no thumb rule of choosing conductor size.
1 sq mm cu carries 11A current. whereas Al cond. 1 sqmm
carries only 3 amp current. this can only be choose from
companies catalogoues.

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