One of the problems we frequently encounter in the field is that power supply users fail to take into account the voltage drop in the wires connecting a power supply to a device under test (DUT) or other electronic system. When a load draws a high current, the voltage drop across the power leads could be high enough to cause a device under test to fail or cause a system to malfunction.

The voltage drop across the power leads is actually very easy to calculate:

E = 2 x I x R

This is basically Ohm's Law, but because there are two wires that connect the power supply to the load, the voltage drop, E, across the power leads is twice the value you'd normally calculate using Ohm's Law. I is the current drawn by the device under test or load, and R is the resistance of one of the power leads. When performing this calculation, you should use the maximum current that your DUT or load will draw. That will give you the worst case voltage drop for your system.

 Wire Size (AWG) Ω/100 ft. (one-way) Ampacity 14 0.257 15 12 0.162 20 10 0.102 30 8 0.064 40 6 0.043 55 4 0.025 70 2 0.015 95 1/0 0.010 125 3/0 0.006 165

Once you know the maximum current, you then need to figure out the resistance of the power leads. You do this by referring to the table at right. It lists the resistance per 100 ft. for a variety of popular copper wire sizes. To calculate the resistance of your power leads, you divide the length of the leads by 100 and then multiply by the value in the table. For example, the resistance of a 10-ft. length of #12 wire would be 0.162/10 or 0.016 Ω. The ampacity column gives the maximum current value that the given wire size can safely handle.

Let's look at an example. If your system uses 10-ft., 12-ga. power leads and the load draws 20 A, the voltage drop across the power leads is:

E = 2 x I x R = 2 E = 2 x 20 x (10/100 x 0.162) = 0.65 V

Once you've performed this calculation, you can decide whether or not this voltage drop is acceptable in your application. If not, you have several options. You can increase the wire size of the power leads or use a supply that uses remote sensing to compensate for the voltage drop.

Knowing the voltage drop across your power leads will help you achieve better results with your test or electronic system. For more information on this and other power supply topics, contact AMETEK Programmable Power. You can send e-mail to sales.ppd@ametek.com or phone 800-733-5427.