Posted by Fred C (W6BSD) on Feb 05 2022

People think that all wires are the same and that any cable should be OK to power their rig. This is far from true, especially when you want to power your radio using a battery. This article focuses on installing a mobile radio or installing a battery backup for your shack, but everything discussed here can be applied to any situation.

Safety

When you buy a new radio, review the manual provided by the manufacturer and find a schematic like this one:

In this schematic, you can see a fuse next to the transceiver. The purpose of this fuse is to protect the radio. There are also two fuses close to the battery. These two fuses are here to protect your car. If there is a short, the battery will drop 600, 700, or 800 Amps1 on these wires, depending on the battery's capacity. This much power will melt the cables and potentially set your car on fire. Having these two fuses close to the battery is essential for your safety.

The transceiver

When you flip through a few pages of your radio's manual, you will see a page containing general pieces of information about your rig. You will see the operating frequency, modes, and dimensions, but the most important information for today's topic is Power supply and Current drain. These two pieces of information will help us calculate the gauge of the wires we need to power our radio.

As shown on the page above, our transceiver nominal voltage is 13.8V DC ±15%. It means that our radio is guaranteed to operate without issues if we provide a voltage between 11.7V and 15.8V. Outside that voltage range, our radio will either shut down or drop its output power. Older rigs might have an undetermined behavior.

The battery

When the car is running or at home with a power supply, the voltage supplied will be around 13.8V. The 13.82 corresponds to a lead-acid battery's charging voltage. When the car is not running or the power goes down, the battery is not charging anymore. The voltage at the battery will quickly drop and stabilize at3 12.5V. The voltage will stay constant at 12.5V for a while. The time will depend on the capacity of your battery. Then the voltage will start to drop as the battery discharges slowly. See the discharge rate graph on the side.

The wires

Wires are lossy. The thinner and the longer the wire, the more losses you will have. This means that the resistance of the wire will increase with its length.

According to Ohm's law, the voltage drop across the wire will be proportional to the current and the resistance: $$V = R \times I$$

To help you calculate the voltage drop, you can use an online calculator. Search online for voltage drop calculator, and your favorite search engine will return dozens of links.

The graph on the right shows the voltage drop for a 20 Amp load for different lengths of wires and gauges.

For example, to install the radio above in an RV using a 20 feet gauge 14 wire to power the rig, the total voltage drop will be 1V.

The radio will work without issues as long as the engine is running and the alternator provides 13.8V. The voltage at the radio, with a 1V drop, will be inside of our 15% bracket.

$$13.8V_{alternator}-1V_{drop}=12.8V$$

As soon as the engine stops running, the voltage at the battery will quickly drop to its nominal voltage of 12.5V. We will probably be able to listen for a short while but not transmit. The voltage at the radio will be under the 15% threshold.

$$12.5V_{battery} - 1V_{drop} = 11.5V$$

The table below shows the voltage drop, for different wire gauges, for the following values:

• Current: 20A
• Volts: 12.50V
• Wire Length: 20'
AWG V-Drop V-End Good? Max Len Watts
16 1.61 10.89 4 32
14 1.01 11.49 7 20
12 0.64 11.86 11 12
10 0.40 12.10 18 7
8 0.25 12.25 Yes 29 5
6 0.16 12.34 Yes 47 3
4 0.10 12.40 Yes 75 1

As you can see on the table above, the smallest gauge we should use to power our rig is an 8 AWG wire. With a battery fully charged, the voltage at the transceiver will be 12.25V. The rig will work fine until the battery voltage drops under 11.95V.

If you use a 6 AWG, the radio will continue to work without issues until the voltage at the battery drops to 11.87V. There is no point in using a thicker wire. We have reached the point of diminishing returns.

The column Watts on the table shows the power dissipated by the wire. It is the power converted into heat.

Conclusion

For your safety, use fuses. A battery can dump several hundred, up to a thousand amps when you have a short. With that much power, the wire can become red hot and melt. It can ignite a fire in your car or your house. Fuses are lossy as well. You can have a voltage drop between 0.05 and 0.09 volt on each fuse.

Choose the proper wire gauge to avoid losses. Correctly sized wires will allow you to use more of the power available in your battery and use your radio for a longer time.

1. Diesel trucks' batteries can dump more than 1000 Amps.

2. The voltage will be higher for the Lithium Iron Phosphate (LiFePo4) battery.

3. Lead-acid battery's nominal voltage.