In some situations a long extension cord or power cable will be needed to reach the generator or other power source. Voltage drop is also an important consideration when selecting or fabricating a power cable for DC-powered equipment. We’ll provide advice on DC power cables later in this article. For now, let’s focus on power cables for AC-powered equipment.
Many radio amateurs are familiar with the National Electrical Code (NEC) rule that 14 AWG conductors may be used to deliver up to 15 A to the load, and 12 AWG can carry to 20 A. But these ampacity ratings are based solely on limiting heat stress to the power cable. If the cable run is long, the resistance of the wire may result in excessive drop in the voltage delivered to the load. In such cases, you will need to select a larger wire size than required by the NEC ampacity tables in order to reduce the voltage drop seen at the load end of the cable.
The following table lists recommended wire sizes (in AWG) for a given length and load current, based on voltage drop as well as the ampacity of the wire. The table also shows the reduction in voltage drop obtained by going to the next larger wire size. The recommendations are based on a nominal line voltage of 120 V and a maximum voltage drop of 6 V, which corresponds to 5% of the nominal line voltage. (The actual value of the voltage drop will be the same when working with a 240 V system, but the percentages will be halved.)
This table is a useful tool for any time you need to select a power cable. Start by selecting the maximum expected load current and cable length in the first two columns. The third column lists the smallest wire gauge that will limit the voltage drop to 5% or less. Columns 4 and 5 present the resulting voltage drop. The remaining columns show the improvement to be gained by going to the next larger wire gauge. Keep in mind that there will be additional voltage drop in the load equipment’s internal wiring as well as the source wiring.
A note of caution: the voltage drops shown in the table are approximate. They are based on nominal resistance values taken from the National Electrical Code. The actual resistance of a given manufacturer’s wire may be slightly different. In addition, the resistance of copper wire increases with temperature. The resistance values used to construct this table are based on a wire temperature of 90 °F, but on a hot summer day in the sun, a power cord’s temperature might be higher than that, resulting in a higher voltage drop than shown. Also note that the table takes into account the resistance of both the supply and return path through the power cord, assuming that both conductors have the same wire gauge. Finally, the table neglects the influence of reactance and power factor, which have relatively little effect on the voltage drop for portable cordage smaller than 2 AWG.
If you want an estimate of voltage drop for an arbitrary cable length and load current, use the following calculator.
Considerations for DC Power Cables
This calculator works for both AC and DC power supply cables. However, whereas a voltage drop of 6 V might be acceptable in a 120 VAC circuit, if you are powering a radio from a 12 VDC power supply, a voltage drop of a couple of tenths of a volt may be significant. For that reason, we want DC power cables to be as short and direct as practical. For example, consider a 100-watt HF radio that has a 12 AWG power cord 1 m (3.28 ft) long. The calculator shows that the voltage drop is 270 mV when the radio is drawing 25 A at full power, and only about 30 mV in receive mode when the radio’s current draw is closer to 3 A.
How important is that voltage drop of a quarter-volt? It represents roughly 2% of the radio’s nominal supply voltage of 13.8 VDC. One likely effect is that the radio’s display will dim slightly on voice peaks, or when the key is down in CW mode. And you are effectively giving up several watts of the radio’s available output power, since the maximum output power of many HF radios is proportional to the square of the supply voltage.
But the biggest adverse impact occurs if you are running on battery power. Consider a typical radio whose data sheet specifies an operating supply voltage range of 13.8 VDC ± 15%. Outside that range, the radio’s performance may degrade, and it might even shut down. As a practical matter, although your battery has useful capacity down to about 10.5 V, the radio stops operating when the battery discharges down to 11.7 V. At that point, any energy remaining in the battery is useless to you.
And here is the key point: if the radio’s power cord is losing 250 mV, you have to stop operating when the battery voltage reaches 11.95 V to ensure that the radio’s minimum operating voltage of 11.7 V is maintained when transmitting. In general, the higher the voltage drop in the power cord, the less of the battery’s stored energy is available to you. So excessive voltage drop in the DC power cable can really eat into your run time.
Choose Your Power Cables Wisely
For maximum performance, safety, and reliability, if power cable is to be used outdoors, we recommend sticking with portable cordage rated for rough duty (National Electrical Code type SOOW or SEOOW) and having at least 14 AWG conductors.
Some things to look for:
- Type SOOW cable has a rubber (usually EPDM rubber) jacket.
- Type SEOOW cable has a thermoplastic elastomer jacket that provides better flexibility and a wider temperature range.
- Type STOOW cable has a thermoplastic jacket that is significantly less flexible than the other types and has a more limited temperature range. It is a budget choice that may not pay off in the long run.
- Better cables have higher strand counts, which improves flexibility.
- Better cable types have longitudinal filler elements, typically made from paper or plastic, that help separate and cushion the conductors at the cost of increasing the overall cable diameter. These filler materials help to prevent the conductors from being twisted inside the jacket. Twisting prevents the cable from laying flat, creating a tripping hazard and preventing one from neatly coiling the cable. This is often seen with inexpensive orange-jacketed extension cords. Twisting also creates mechanical stresses that lead to premature failure.
Connectors should also be suitable for outdoor use. Supplemental weatherproofing may be indicated in some situations, and consider elevating connections if standing water is a possibility. Also consider the need to strain-relieve connections when running cables.
Fully uncoil the cable if it will be delivering significant current. Consider that a 50-foot 14 AWG cable carrying 15 A is producing about 60 watts of heat – the same as a small soldering iron. As long as the cable is stretched out, the heat can escape to the environment. When the cable is tightly bundled or coiled on a reel, some of that heat is trapped in the middle of the bundle, and the temperature rise can be high enough to melt insulation, potentially leading to a short-circuit and even starting a fire.
Content on this page is Copyright © Al Taylor, KN3U, 2022. Used by permission.