Voltage Drop

Picking the Right Cable for the Long Run

Reliability may not be a tangible item that’s installed alongside a new furnace or wired into a dock-side crane, but it is nonetheless an essential “accessory” that can mean the difference between overtime and lost time; in-stock and out-of-stock; perfect fits and refits. Being labeled “unreliable” can mean ruin for a business, regardless of what you’re making, installing or servicing. That’s why it is vital to understand simple, yet often overlooked problems, like voltage drop in product applications.

What is Voltage Drop?

Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. Wires carrying electricity have inherent resistance, or impedance, to current flow. Voltage drop is the amount of voltage loss that occurs through a circuit due to this impedance.

For equipment to operate properly, it must be supplied with the right amount of power, which is measured in watts which is calculated by multiplying current (amps) x voltage (volts). Motors, generators, tools — anything that runs on electricity — is rated for power. The correct amount of power enables equipment to meet its designed power rating and operate efficiently. Too much or insufficient amounts of power can result in inefficient operation, wasteful power usage, and even equipment damage. That is why understanding voltage drop calculations and selecting the correct cable for each application is so important.

The National Electrical Code (NEC) catalogs the requirements for safe electrical installations and represents the primary document for guidance in the United States. Providing direction for both trained professionals and end users, these codes set the foundation for the design and inspection of electrical installations. So how does the Code treat voltage drop issues? For branch circuits, look to NEC (NFPA 70) Section 215.2(A)(3) footnote 2 and Section 210.19(A)(1) footnote 4. Both advise that conductors for feeders to dwelling units should be sized to prevent voltage drop exceeding 3% and maximum total voltage drop on both feeders and branch circuits should not exceed 5% for “reasonable efficiency of operation.”

In addition, look to NEC (NFPA 70) Section 647.4 (D) when dealing with sensitive electronic equipment. It states that voltage drop on any branch circuit shall not exceed 1.5% and the combined voltage drop on branch-circuit and feeder conductors shall not exceed 2.5%. It is important to note that much of the equipment manufactured today contains electronics that are particularly sensitive to excessive voltage drop.

Ampacity, a cable’s electric current-carrying capacity, is also connected to voltage drop. The Code stresses the importance of accounting for voltage drop when considering a cable’s ampacity rating and the need to satisfy both requirements. NEC Section 310.15 (A)(1) states that ampacity tables do not take voltage drop into consideration.

How is Voltage Drop Calculated?

For DC current, voltage drop is proportional to the amount of current flow and wire resistance. In AC circuits, total impedance and power factor (power loss ratio) also need to be considered. Since wire resistance is a factor of wire size, material and length of run, it is important to choose the proper wire size for the length of run to keep voltage drop at the desired level.

Use the following voltage drop calculation tale to simplify your voltage drop calculations.

This table makes calculating project voltage drop straightforward and easy. For example, let’s say your project involves a 100-foot run of 12/3 SOOW wire, 12 amps line current for equipment, line circuit of 120 volts AC, 3 phase, 100% power factor. According to the calculation table, the factor is 3190. Next, multiply current x distance (feet) x factor: 12 x 100 x 3190 = 3,828,000. Finally, place a decimal in front of the last six figures, and the result is the volts lost, or voltage drop, which equals3.8 volts in this example (3.2% of overall voltage).

So to ensure the reliability of your products, installations, or service calls, be sure to account for voltage drop when selecting cable. While it is primarily a nuisance issue, voltage drop can affect equipment efficiency, power consumption and cause potential damage to sensitive electronics and other systems. Fortunately, these issues are easily avoided, especially when you rely on the NEC codes and standards that relate to voltage drop: each of which provides useful guidance in ensuring the success of your application.

By selecting a cable with the correct voltage drop characteristics, you will optimize the operation of your connected equipment, increase your efficiencies and prevent equipment damage. And that’s a pretty good payoff, in the short term or the long run.

Wire Size Power Factor % 90AC Single Phase80 AC Three Phase DC
14 AWG 100 5880 5090 5880
90 5360 4640
80 4790 4150
70 4230 3660
60 3650 3160
12 AWG 100 3690 3190 3690
90 3380 2930
80 3030 2620
70 2680 2320
60 2320 2010
10 AWG 100 2320 2010 2820
90 2150 1861
80 1935 1675
70 1718 1487
60 1497 1296
8 AWG 100 1462 1265 1462
90 1373 1189
80 1248 1081
70 1117 969
60 981 849
6 AWG 100 918 795 918
90 882 764
80 812 703
70 734 636
60 653 565
4 AWG 100 578 501 578
90 571 494
80 533 462
70 489 423
60 440 381
2 AWG 100 367 318 363
90 379 328
80 361 313
70 337 292
60 309 268
1 AWG 100 291 252 288
90 311 269
80 299 259
70 284 246
60 264 229
1/0 AWG 100 233 202 229
90 257 222
80 252 218
70 241 209
60 227 106
2/0 AWG 100 187 162 181
90 213 184
80 212 183
70 206 178
60 196 169
3/0 AWG 100 149 129 144
90 179 155
80 181 156
70 177 153
60 171 148
4/0 AWG 100 121 104 114
90 152 131
80 156 135
70 155 134
60 151 131
250 kcmil 100 102 89 97
90 136 117
80 143 123
70 143 124
60 141 122
300 kcmil 100 86 75 81
90 121 104
80 128 111
70 131 113
60 130 113
350 kcmil 100 74 64 69
90 109 95
80 118 102
70 122 105
60 122 106
400 kcmil 100 66 57 60
90 101 88
80 111 96
70 115 99
60 116 101
500 kcmil 100 54 47 48
90 89 78
80 99 86
70 105 91
60 108 93
600 kcmil 100 47 41 40
90 83 72
80 93 81
70 99 86
60 103 89
750 kcmil 100 39 34 32
90 75 65
80 86 75
70 93 81
60 97 84
1000 kcmil 100 31 27 24
90 67 58
80 79 68
70 86 75
60 91 78
 

This resource compliments of General Cable

General Cable is headquartered in Highland Heights, Kentucky, and is a global leader in the development, design, manufacture, marketing and distribution of aluminum, copper and fiber optic wire and cable products for the energy, communications, transportation, industrial, construction and specialty segments. General Cable is one of the largest wire and cable manufacturing companies in the world, operating manufacturing facilities in its core geographical markets, and has sales representation and distribution worldwide.