How to identify perfect electrical cable size

Selecting cable size is a very important matter to providing electrical connection in any machinery or home appliances. Most people don’t know how to identify perfect electrical cable size for any electrical device. The right capacity electric cable can ensure safety from an unexpected fire. Moreover, it is cost-effective and ensures the longevity of electrical equipment. It is easy to identify the perfect electrical cable size for power lines for either home or industrial equipment purposes. In addition, manufacturers maintain standard specified dimensions to make quality cables. The cable size is determined after calculating how much power is to be passed through the cable. Literally people want to use reliable cable to reduce risk factor. 

A better example can provide a clear concept of cable size identification. Inside the house is used 4 fans, 6 energy lights, 1 desktop pc,1 refrigerator, 1 iron and 2 HP motor. In this circumstance, it is required to calculate cable size identification.

Total power = 4 * 80 + 6 * 15 +1* 200 + 100 + 1500 + 1492 = 3702 watt

Consider additional load 20% = 3702 * 1.20 = 4443 watt

Current I = P / V = 4443/ 220 = 20.2 amp

Cable Size – Table 1

Current carrying capacity ( Amp )	Number of wire and thickness of each wire	Area in mm2
11	1/1.13	1
13	1/1.38	1.5
18	1/1.78	2.5
24	7/0.85	4
31	7/1.04	6
42	7/1.35	10
56	7/1.70	16
73	7/2.14	25
90	19/1.53	35
145	19/1.78	50
185	19/2.14	70
230	19/2.52	95

In this case, as per table 1 cable is to be selected (nearest maximum 24 amp) 7/0.85 mm.

If the ambient temperature become 35 degrees centigrade then temperature factor is considered 0.97 as pre table 2

Temperature Factor – Table 2

Temperature factor	1.02	1	0.97	0.94	0.91	0.88	0.77	0.63
Temperature C	25	30	35	40	45	50	55	60

Thus, Current standard = 24 * 0.97 = 23.28 amp, which is greater than 20.2 amp

Now It is necessary to check voltage drop for the 20m length cable.

As per table 3 the voltage drop is 7 volt per 20 m cable for 28 amp current. So 7 * 20/30 = 4.66 volt.

Voltage Drop According to Cable Length-Table 3

Single AC And DC Voltage drop	Phase Cable Both per 30 m / 100 ft	Three phase Voltage drop	Cable per 30 m/100 ft
Current rating	Voltage drop	Current rating	Voltage drop
Amp	Volt	Amp	Volt
11	14	9	9.8
13	12	11	9.1
16	11	14	7.7
21	8.4	18	6.4
28	7	23	5.3
34	5.7	28	4.1
43	5.5	36	4
56	4.8	48	3.5
66	4.3	56	3.2
77	3.6	65	2.7
105	3.4	88	2.5

According to IEEE rule B-23, Voltage drop should be maximum 2.5 % of the provided voltage. Thus, provided voltage 220 v. so maximum voltage drop = 220 * 2.5% = 5.5 volt

The actual voltage drop is below the approved voltage drop. So, the cable size is perfect and not necessary to increase the cable size.

If the voltage drop is more than the approved voltage, it is necessary to select the next upper capacity cable size.