Useful Formulae:
Disclaimer: The information provided here should be used for reference only. These formulae do not take into account any margin of error or
environmental influence which may manifest in the working world. KT Assemblies Ltd. cannot be held responsible for any miscalculation
derived from using the information presented on this page.
NEW!! Use our realtime calculator here.
This calculator is based on
Ohm's Law.

IF YOU KNOW 
NEED TO KNOW 
VOLTS ÷ OHMS 
= AMPS 
VOLTS ÷ AMPS 
= OHMS 
VOLTS x AMPS 
= WATTS 
WATTS ÷ AMPS 
= VOLTS 
WATTS ÷ VOLTS 
= AMPS 
AMPS x OHMS 
= VOLTS 
RESISTANCE OF THE LENGTH OF CABLE

x 
CURRENT
DRAWN

= 
VOLT DROP 
 OHMS 
 AMPS 
Heat Dissipation
To work out the amount of heat radiated from a PSU (in Watts) use the following formula:
Output Current x Output Voltage x 70%
For example a 2 amp power supply at 24 volts would be:
2 x 24 x 70% =
48 x 70 % = 33 Watts
If standby batteries are being used then add 20 Watts to the total:
33 + 20 = 53 Watts.


Volt Drop Table
Current carrying capacities and associated voltage drops for twin and multicore P.V.C insulated cables, non armoured (copper conductors).
Also available as a printable PDF
BS6006 & BS6346
Conductor operating temperature 70°C
Conductor cross sectional area 
Installation methods A to C† of table 9A ('enclosed') 
Installation methods E to H of table 9A ('Clipped direct') 
Installation method K of table 9A ('Defined conditions') 
One twin cable. With or without protective conductor single phase
AC or DC.

One threecore cable, with or without protective conductor, or one four core cable phase one

One twin cable. With or without protective conductor single phase
AC or DC.

One threecore cable, with or without protective conductor, or one four core cable phase one

One twin cable. With or without protective conductor single phase
AC or DC.

One threecore cable, with or without protective conductor, or one four core cable phase one


Current carrying capacity 
Volt drop per amp per metre 
Current carrying capacity 
Volt drop per amp per metre 
Current carrying capacity 
Volt drop per amp per metre 
Current carrying capacity 
Volt drop per amp per metre 
Current carrying capacity 
Volt drop per amp per metre 
Current carrying capacity 
Volt drop per amp per metre 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
mm² 
A 
mV 
A 
mV 
A 
mV 
A 
mV 
A 
mV 
A 
mV 
1.0 
14 
42 
12 
37 
16 
42 
13 
37 




1.5 
18 
28 
16 
24 
20 
28 
17 
24 




2.5 
24 
17 
21 
15 
28 
17 
24 
15 





4 
32 
11 
29 
9.2 
36 
11 
32 
9.2 




6 
40 
7.1 
36 
6.2 
46 
7.1 
40 
6.2 




10 
53 
4.2 
49 
3.7 
64 
4.2 
53 
3.7 




16 
70 
2.7 
62 
2.3 
85 
2.7 
70 
2.3 

















25 
79 
1.8 
70 
1.6 
108 
1.8 
90 
1.6 
114 
1.8 
95 
1.6 
35 
98 
1.3 
86 
1.1 
132 
1.3 
115 
1.1 
139 
1.3 
122 
1.1 
50 




163 
0.92 
140 
0.81 
172 
0.92 
148 
0.81 

AC / DC 

AC / DC 

70 




207 
0.65/0.64 
176 
0.57 
218 
0.65/0.64 
186 
0.57 
95 




251 
0.48/0.46 
215 
0.42 
265 
0.48/0.46 
227 
0.42 

120 




290 
0.40/0.36 
251 
0.34 
306 
0.40/0.36 
265 
0.34 
150 




330 
0.32/0.25 
287 
0.29 
348 
0.32/0.25 
302 
0.29 
185 




380 
0.29/0.23 
330 
0.24 
400 
0.29/0.23 
348 
0.24 
240 




450 
0.25/0.18 
392 
0.20 
474 
0.25/0.18 
413 
0.20 
300 




520 
0.23/0.14 
450 
0.18 
548 
0.23/0.14 
474 
0.18 
400 




600 
0.22/0.11 
520 
0.17 
632 
0.22/0.11 
548 
0.17 
† For installation Method C, the tabulated values are
applicable only to the range up to and including 35mm².
For larger sizes in this installation method, see ERA report
6930. For cables in ducts in the floor of a building, the
ERA ratings must be adjusted by the appropriate factor for
the ambient temperature.
The current carrying capacities in columns 6 and 8 are applicable
to flexible cables to BS 6004 Table 1(b) where the cables
are used in fixed installations.
Correction Factors
For Ambient Temperature
Ambient temperature 
25°C 
35°C 
40°C 
45°C 
50°C 
55°C 
60°C 
65°C 
Correction factor 
1.06 
0.94 
0.87 
0.79 
0.71 
0.61 
0.50 
0.35 
Sample Formulae for the Volt Drop Table
FORMULA = LENGTH X VOLT DROP X AMPS = VOLT DROP OVER LENGTH

1.0 mm TWIN CABLE FORMULA = 42 MV PER AMP PER METER




10 metres at 5 amp with 1.0mm twin =

10 x 5 x 42 =

2100 mv
1000

2.1 volts

20 metres at 5 amp with 1.0mm twin =

20 x 5 x 42 =

4200 mv
1000

4.2 volts

30 metres at 5 amp with 1.0mm twin =

30 x 5 x 42 =

6300 mv
1000

6.3 volts

50 metres at 5 amp with 1.0mm twin =

50 x 5 x 42 =

10500 mv
1000

10.5 volts

75 metres at 5 amp with 1.0mm twin =

75 x 5 x 42 =

15750 mv
1000

15.7 volts

100 metres at 5 amp with 1.0mm twin =

100 x 5 x 42 =

21000 mv
1000

21 volts


1.5 mm TWIN CABLE FORMULA = 28 MV PER AMP PER METER




10 metres at 5 amp with 1.5mm twin =

10 x 5 x 28 =

1400 mv
1000

1.4 volts

20 metres at 5 amp with 1.5mm twin =

20 x 5 x 28 =

2800 mv
1000

2.8 volts

30 metres at 5 amp with 1.5mm twin =

30 x 5 x 28 =

4200 mv
1000

4.2 volts

50 metres at 5 amp with 1.5mm twin =

50 x 5 x 28 =

7000 mv
1000

7.0 volts

75 metres at 5 amp with 1.5mm twin =

75 x 5 x 28 =

10500 mv
1000

10.5 volts

100 metres at 5 amp with 1.5mm twin =

100 x 5 x 28 =

14000 mv
1000

14.0 volts


2.5 mm TWIN CABLE FORMULA + 18mv PER AMP PER METER




10 metres at 5 amp with 2.5mm twin =

10 x 5 x 17 =

850 mv
1000

0.85 volts

20 metres at 5 amp with 2.5mm twin =

20 x 5 x 17 =

1700 mv
1000

1.7 volts

30 metres at 5 amp with 2.5mm twin =

30 x 5 x 17 =

2500 mv
1000

2.5 volts

50 metres at 5 amp with 2.5mm twin =

50 x 5 x 17 =

4250 mv
1000

4.2 volts

75 metres at 5 amp with 2.5mm twin =

75 x 5 x 17 = 
6375 mv
1000

6.3 volts

100 metres at 5 amp with 2.5mm twin =

100 x 5 x 17 =

8500 mv
1000

8.5 volts







MINIATURE CIRCUIT BREAKERS FOR USE IN CONJUNCTION WITH MOTOR STARTERS AND TRANSFORMERS

Table 21 phase 240V AC DOL starting

Motor starters
In general miniature circuit breakers can give only short
circuit protection to motor loads due to high starting
currents which may be encountered: typically 3 to 12 times
full load current (FLC)
Assumptions
The tables give recommended mcb ratings for motors up
to 37kW based on the following assumptions:
Direct online starting
starting current = 7 x FLC
run up time =
6 seconds, motors < 3 kW
10 seconds, motors < 22 kW
running currents = average values only
(individual manufacturers figures may vary)
four pole motors i.e. speed approx.
1500 rev/min.
For Higher inertia loads i.e. hoists and fans run up
times maybe considerably longer than those assumed above.
The rating of the mcb must take account of the greater
runup time and starting current. The required mcb rating
can be determined by reference to time/current curves
(consult us)
Star/ delta starting
Since, during the changeover from star to delta, a high
current surge in the order of DOL values may be met,
the mcb rating selected should be the same as that
recommended for DOL starting

KW 
Hp 
Running 
C60H 
C60HC 
C60HD 
NC100C 
NC100D 
0.12 
0.166 
0.55 
2 
1 
1 


0.18 
0.25 
0.7 
2 
1 
1 


0.25 
0.33 
0.87 
2 
2 
1 


0.37 
0.5 
1.35 
4 
2 
2 


0.55 
0.75 
1.55 
4 
2 
2 


0.75 
1 
1.93 
6 
4 
2 


1.1 
1.5 
2.5 
6 
4 
4 


1.5 
2 
3.5 
10 
5 
6 


2.2 
3 
4.8 
16 
10 
10 
10 
10 
3 
4 
6.4 
16 
16 
10 
16 
10 
3.75 
5 
7.8 
20 
20 
16 
20 
16 
4 
5.5 
8.1 
25 
20 
16 
20 
16 
5.5 
7.5 
11 
25 
25 
16 
25 
16 
7.5 
10 
14.4 
32 
25 
20 
25 
20 
9.33 
12.5 
17.3 
40 
32 
20 
32 
20 
11 
15 
21 
50 
40 
25 
40 
25 
13 
17.5 
25 
63 
50 
32 
50 
32 
15 
>20 
28 
63 
50 
40 
50 
40 
18.5 
25 
35 

63 
50 
63 
50 
22 
30 
40 

63 
50 
63 
50 
30 
40 
54 


63 
80 
63 
37 
50 
65.5 



100 
80 
Table 21 phase 240V AC DOL starting

KW 
Hp 
Running 
C60H 
C60HC 
C60HD 
NC100C 
NC100D 
0.12 
0.166 
0.95 
2 
2 
1 


0.18 
0.25 
1.5 
4 
2 
2 


0.25 
0.33 
1.7 
6 
2 
2 


0.37 
0.5 
3 
10 
6 
4 


0.55 
>0.75 
4.5 
16 
10 
6 
10 

0.75 
1 
5.5 
16 
16 
10 
16 
10 
1.1 
1.5 
8.5 
20 
20 
16 
20 
16 
1.5 
2 
10.5 
25 
25 
20 
25 
20 
2.2 
3 
15.5 
32 
32 
25 
32 
25 
3 
4 
20 
40 
40 
32 
40 
32 
3.75 
5 
24 
50 
50 
40 
50 
40 
5.5 
7.5 
34 
63 
63 
50 
63 
50 
6.3 
8.5 
36.5 

63 
63 
63 
63 
7.5 
10 
45 


63 
80 
63 
11 
15 
66.5 



100 
80 

VA 
Primary
in (A)

C60H 
C60HC 
C60HD 
NC100C 
NC100D 
Transformers
High inrush currents are also produced when transformers
are switched on. Typically 1015 times full load current.
Assumptions
The tables give recommended mcb ratings for single phase
transformers up to 12500 VA and three phase transformers
up to 30000 VA on the following formula.
Mcb rating
15 x normal current of transformer
min instantaneous tripping
cc efficient of mcb

500 
0.7 
4 
2 
1 


750 
1.04 
6 
4 
2 


1000 
1.39 
6 
4 
2 


2000 
2.78 
10 
10 
6 
10 

5000 
6.95 
32 
16 
10 
16 
10 
10000 
13.89 
50 
32 
20 
32 
20 
15000 
20.84 

50 
32 
50 
32 
20000 
27.78 

53 
40 
63 
40 
25000 
34.73 


50 
80 
50 
30000 
41.67 


63 
80 
63 
Table 4  1 phase transformers 240V AC supply

VA 
Primary
in (A) 
C60H 
C60HC 
C60HD 
NC100C 
NC100D 
50 
0.21 
1 




100 
0.42 
2 
1 
1 


250 
1.04 
6 
4 
2 


500 
2.08 
10 
6 
4 


1000 
4.17 
20 
10 
10 
10 
10 
2500 
10.42 
40 
25 
16 
25 
16 
5000 
20.84 

50 
32 
50 
32 
10000 
41.67 


63 
80 
63 



