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DC power wiring

A 100 W transceiver such as the Icom IC-7300 draws 21 A on transmit, so a few meters of thin wire can starve it. See where the volts go, choose a wire size and estimate battery run time.

Voltage drop

V
A
V
± %

Defaults are the Icom IC-7300’s published specifications: 13.8 V DC ±15 %, 21 A transmitting at 100 W and 0.9 A receiving.

°C

Copper resistance rises with temperature. 20 °C is the table’s reference.

13.8 V at the supply. Each 10 AWG lead of 3 m drops 0.206 V, leaving 13.39 V at the radio, at or above its 11.73 V minimum.Supply13.8 VRadio13.39 Vdrops 0.206 Vdrops 0.206 V10 AWG, 3 m+ and - leadsVoltsmin 11.7313.810

The radio sees 13.39 V while transmitting, 1.66 V above its 11.73 V minimum.

Drop in the cable
0.413 V (2.99 %)
Both conductors
19.66 mΩ
Heat in the cable
8.67 W

By voltage drop alone, the thinnest wire that keeps the radio within its specification over this run is 16 AWG, which would turn 34.9 W into heat. With 10 AWG the run can be up to 15 m each way.

Voltage drop is not the only limit. Wire carrying more current than it is rated for heats up, so check the current rating the wire’s maker gives and fuse it to protect the wire.

10481214
Each line is one wire size carrying 21 A. The dashed red line is the radio’s minimum supply voltage; where a line crosses it is the longest run for that gauge. The amber line is the gauge you chose. Solid copper at 20 °C from NBS Handbook 100; stranded wire is slightly worse.

Battery run time

Ah
A

Lower this if your battery’s datasheet limits how deeply it may be discharged.

4 h 4 min

Average current: 21 A × 20 % + 0.9 A × 80 % = 4.92 A. Run time is the usable 20 Ah divided by that average.

Listening is cheap and transmitting is expensive: the first few percent of transmit time cost the most hours.

Try this

  1. At 21 A the two 6 m conductors drop about 2.1 V, leaving the radio just under the 11.73 V its ±15 % specification allows. Choose 10 AWG and it stays well inside.

  2. The cable now drops over 2.6 V and turns more than 50 W into heat while you transmit: it behaves like a resistor in series with the radio.

  3. Copper resistance rises about 0.39 % per °C, so 40 °C of warming adds about 16 % to the drop (NBS Handbook 100).

  4. FT8 alternates 15 second transmit and receive periods, so calling continuously means transmitting half the time. Compare 50 % with a listening-only day at 0 %.

Apply it

  • Use short, heavy-gauge wires for the radio’s DC power: they minimize voltage drop when transmitting (Technician pool T4A03).
  • For a 100 W HF mobile radio, connect directly to the vehicle battery with heavy-gauge wire, fused (General pool G4E03), and return the negative lead to the battery chassis ground (T4A11).
  • A fuse removes power in an overload. Never replace a fuse with a higher-rated one: excessive current could cause a fire (T0A04, T0A05).
  • Shorting the terminals of a 12 V battery without protection circuitry can cause burns, fire or an explosion (T0A01). For maximum life, do not discharge a standard 12 V lead-acid battery below 10.5 V (General pool G6A01).

How it works

  • Wire resistance is from NBS Handbook 100, Copper Wire Tables, Table 5 (annealed copper at 20 °C), adjusted by 0.393 % per °C. Stranded wire of the same gauge has slightly higher resistance.
  • Current flows out on one conductor and back on the other, so the drop is the current times the resistance of twice the one-way length (Ohm’s law).
  • A radio’s minimum supply voltage is its rated voltage less its tolerance. The Icom IC-7300 is specified at 13.8 V DC ±15 %, or 11.73 V minimum, and draws 21 A transmitting at 100 W and 0.9 A receiving.
  • Battery run time is the ampere-hour rating divided by the average current; the average weights transmit and receive current by the share of time spent transmitting.
  • FT8 uses 15-second sequences that alternate between transmitting and receiving, so calling continuously means transmitting about half the time.

Learn more: Choosing your first radio

Sources: Copper Wire Tables (Handbook 100) (National Bureau of Standards (NIST)); IC-7300 specifications (Icom America); WSJT-X User Guide 2.7.0 (WSJT Development Group); Radio Mathematics (ARRL Handbook supplement) (ARRL); 2026-2030 Technician question pool (NCVEC Question Pool Committee); 2023-2027 General question pool (NCVEC Question Pool Committee); 2024-2028 Extra question pool (NCVEC Question Pool Committee)

Practice the exam questions: T4 Amateur Radio Practices (Technician), T0 Safety (Technician), G4 Amateur Radio Practices (General)

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