Ham Radio Power Supply Sizing Calculator
Calculate the DC power supply current rating for an amateur radio station from transmitter power and accessory draw, with wire gauge and headroom advice.
Sizing a power supply for amateur radio
Most HF (high frequency, the shortwave bands) and VHF (very high frequency) amateur radio transceivers run on 13.8 VDC. The power supply must provide enough current (amps) for peak transmit load plus any accessories. An undersized power supply causes voltage sag during transmit, distorting your signal and potentially damaging the radio.
Basic formula:
Current draw (amps) = Transmit power (watts) ÷ (Supply voltage × Efficiency)
The efficiency here is the whole-rig figure: RF watts out versus DC watts in, including the receiver, display, and fans that run the whole time. For a modern 100 W HF transceiver that works out to only ~33% (an IC-7300 draws about 21-23 A at 13.8 V), even though the power amplifier stage alone manages 40-60%.
Total current = Radio current + Accessory current
Recommended PSU = Total current × 1.25 (25% headroom)
Typical current draw by radio power class:
| Radio Class | TX Power | Approx TX Current at 13.8V | Receive Current |
|---|---|---|---|
| QRP (5W) | 5 W | 2–3 A | 0.5–1 A |
| Mobile (50W) | 50 W | 10–12 A | 1–2 A |
| HF 100W | 100 W | 20–23 A | 2–3 A |
| HF 200W | 200 W | 35–40 A | 2–3 A |
Common accessories and their current draw:
| Accessory | Current Draw |
|---|---|
| Antenna tuner (automatic) | 0.5–1 A |
| External speaker | negligible |
| Digital interface (SignaLink, etc.) | 0.1–0.3 A |
| Cooling fan (external) | 0.5–1.5 A |
| LED desk lamp | 0.3–0.5 A |
| Rotor controller | 1–3 A (when moving) |
| Linear amplifier bias | varies (check manual) |
Worked example — 100W HF station:
Radio: Icom IC-7300 (100W HF, draws 23A peak on SSB transmit) Accessories: Auto-tuner (1A) + digital interface (0.3A) + cooling fan (1A)
Total peak current: 23 + 1 + 0.3 + 1 = 25.3 A With 25% headroom: 25.3 × 1.25 = 31.6 A
Round up to a 35A-class supply, and watch the fine print on ratings: an Astron RS-35A is 35A intermittent but only 25A continuous, so this station really wants a unit rated at least 30A continuous (Astron RS-50A class, or a 30A+ continuous switching supply).
Continuous vs. peak ratings:
Power supplies are rated for both continuous and peak (surge) current. Always size based on the continuous rating. A “30A” supply with 30A peak but 25A continuous is really a 25A supply for your purposes. SSB voice transmit creates repetitive peaks that are effectively continuous load during a conversation.
Linear vs. switching power supplies:
Linear supplies (like Astron RS-series) produce very clean DC with minimal RF noise, but are heavy and less efficient. Switching supplies (like Samlex, MFJ, Meanwell) are lighter and cheaper but can introduce RF noise on HF bands. If using a switching supply, look for models specifically designed for amateur radio with good RF filtering.
Wire gauge for DC connections:
| Distance (one way) | Up to 20A | Up to 35A | Up to 50A |
|---|---|---|---|
| Under 1m (3ft) | 12 AWG | 10 AWG | 8 AWG |
| 1–3m (3–10ft) | 10 AWG | 8 AWG | 6 AWG |
| 3–5m (10–16ft) | 8 AWG | 6 AWG | 4 AWG |
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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