Wire gauge and current chart for 12 volt DC
Size the wire for voltage drop over the run, then size the fuse to the wire.
The short answer
For a 12 volt station, size the wire so voltage drop over the whole round trip stays under about 3 percent. A 100 W HF radio drawing 23 A over a 10 ft round trip needs 10 AWG. Fuse to protect the wire rather than the load, and place the fuse at the source end where the current comes from.
- Target voltage drop
- Under 3 percent
- 23 A over 10 ft
- 10 AWG
- 23 A over 25 ft
- 8 AWG
- 11 A over 15 ft
- 12 AWG
- Fuse sized to
- The wire
- Fuse located at
- The source
Low voltage DC wiring is unforgiving in a way mains wiring is not. At 120 V, a volt of drop is under one percent and nobody notices. At 13.8 V, a volt of drop is over seven percent and your radio starts producing distorted audio and reduced output.
The current is also much higher for the same power. A 100 W transceiver draws over 20 A, which is more than most household circuits, through wire that is often much thinner than a household circuit uses.
Ampacity by gauge
Ampacity
Current capacity of copper conductors in free air
| AWG | Diameter | Chassis wiring | Power transmission |
|---|---|---|---|
| 20 | 0.032 in | 11 A | 1.5 A |
| 18 | 0.040 in | 16 A | 2.3 A |
| 16 | 0.051 in | 22 A | 3.7 A |
| 14 | 0.064 in | 32 A | 5.9 A |
| 12 | 0.081 in | 41 A | 9.3 A |
| 10 | 0.102 in | 55 A | 15 A |
| 8 | 0.128 in | 73 A | 24 A |
| 6 | 0.162 in | 101 A | 37 A |
| 4 | 0.204 in | 135 A | 60 A |
| 2 | 0.258 in | 181 A | 95 A |
Chassis wiring figures assume short runs in free air with the conductor allowed to warm. Power transmission figures assume long runs and a conservative temperature rise. For station wiring, voltage drop rather than ampacity is almost always the binding constraint, so use the table below.
Voltage drop, which is what actually decides the gauge
Gauge selection
Minimum gauge for under 3 percent drop at 13.8 V, by current and one-way run length
| Current | 5 ft | 10 ft | 15 ft | 25 ft | 40 ft |
|---|---|---|---|---|---|
| 5 A | 16 | 16 | 14 | 12 | 10 |
| 10 A | 14 | 12 | 12 | 10 | 8 |
| 15 A | 12 | 10 | 10 | 8 | 6 |
| 20 A | 12 | 10 | 8 | 8 | 6 |
| 25 A | 10 | 8 | 8 | 6 | 4 |
| 30 A | 10 | 8 | 6 | 6 | 4 |
| 40 A | 8 | 6 | 6 | 4 | 2 |
Run length is one way; the calculation accounts for the return conductor. Going one gauge heavier than the table costs very little and removes any argument. Work out a specific case on the power supply sizing calculator.
Fusing
- Size the fuse to protect the wire, not the load. The fuse exists so the cable cannot overheat and catch fire. A 30 A fuse on 10 AWG is correct even if the radio only draws 5 A.
- Put the fuse at the source end. At the battery terminal, or at the supply output. A fuse at the far end leaves the whole cable unprotected against a short at the source end.
- Fuse both conductors on a radio power lead. Radios ship with fuse holders in both the positive and negative leads for exactly this reason.
- Use a distribution panel rather than splices. A Powerwerx PD-5F fused distribution block gives each device its own fuse and makes a fault obvious.
- Never bypass a blown fuse. It opened for a reason and the reason is still present.
- Carry spares. A blown fuse ends a field activation otherwise.
Fuse sizing
Maximum fuse rating by conductor gauge
| AWG | Typical maximum fuse | Common station use |
|---|---|---|
| 18 | 7.5 A | Accessory leads, small devices |
| 16 | 10 A | Handheld chargers, small radios |
| 14 | 15 A | Mobile VHF radios up to 25 W |
| 12 | 20 A | Mobile VHF radios at 50 W |
| 10 | 30 A | 100 W HF transceivers |
| 8 | 40 A | Main feeds and distribution panels |
| 6 | 60 A | Battery to distribution runs |
These are conservative figures for continuous duty in a warm enclosure. Automotive and marine standards vary slightly, and where a published standard applies to your installation, follow it rather than this table.
Connectors
- Amateur radio has standardised on Powerpole style connectors. They are genuinely good, rated well above what a station needs, and every field site you visit will accept them.
- A 30 A quick-disconnect power connectors, 40-piece assortment covers a whole station and lets any device plug into any supply.
- Match the contact size to the wire. Using a 30 A contact on 10 AWG is right; crushing 10 AWG into a 15 A contact is not.
- Crimp properly with the correct tool. A soldered Powerpole contact is brittle at the transition and fails from vibration.
- Keep polarity consistent, red on the right with the hood tongue up, which is the convention that makes cross-site interchange work.
- Avoid cigarette lighter plugs for anything above about 10 A. The contact resistance is high and the fusing is not yours to control.
What to buy
DC wiring supplies
Distribution, connectors, supplies and the packs they connect to.
Powerwerx
Powerwerx PD-5F fused distribution block
A fused distribution panel so every device has its own protection and a fault is obvious.
Check price$50.85
smseace
30 A quick-disconnect power connectors, 40-piece
The connector standard. Buy an assortment once and stop thinking about adapters.
Check price$9.99
Powerwerx
Powerwerx SS-30DV desktop power supply
A 30 A supply with Powerpole outputs already fitted, which removes one wiring step.
Check price$164.17
Astron
Astron RS-35M-AP linear power supply
A 35 A linear supply with binding posts, for a station where switching noise is not welcome.
Check price$551.00
GOLDENMATE
GOLDENMATE 12 V 20 Ah LiFePO4 battery
A LiFePO4 pack for portable and backup use. Fuse it at the terminal.
Check price$67.49
BuyUneed
Tinned copper ground strap braid, 15 ft
Bonding braid for a vehicle installation, where the return path is the body rather than a wire.
Check price$22.99
Common questions
Questions people ask about this
What gauge wire do I need for a 100 watt HF radio?
Ten AWG for a run up to about 10 ft one way, and 8 AWG beyond that. A 100 W transceiver draws 20 to 23 A on transmit peaks, and at 13.8 V even a small voltage drop is a large percentage. Undersized DC wire produces exactly the symptoms people blame on the radio: distorted audio at the far end, reduced output on long transmissions and occasional resets.
Where should the fuse go?
At the source end, meaning the battery terminal or the supply output, as close as physically possible. The fuse protects the cable, so a fuse at the far end leaves the whole run unprotected against a short near the source. Size it to the wire rather than to the load, and fit fuses in both the positive and negative conductors as radios are supplied.
How much voltage drop is acceptable?
Under about 3 percent, which at 13.8 V is around 0.4 V total for the round trip. Radios are specified at 13.8 V and produce reduced output and degraded audio as supply voltage falls. Because low voltage systems carry high current for a given power, drop is almost always the binding constraint rather than the ampacity of the conductor.
Can I use automotive wire from a car parts shop?
Yes, provided it is genuinely the gauge it claims. Some inexpensive wire is sold by outside diameter including a thick jacket, so the actual conductor is two gauges thinner than the label. Buy from a supplier that states the conductor size, and prefer finely stranded wire, which is more flexible and easier to terminate reliably.
Why does my radio work on the bench but not in the car?
Almost always the power feed. An accessory socket is fused at 10 to 15 A through thin wiring with a poor contact, which is inadequate for anything above about 25 W. Run the power lead directly to the battery with correctly sized wire and fuses in both conductors near the terminals, which is covered in the [mobile radio installation guide](/guides/mobile-radio-installation/).
Keep going
Related on this site
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Disclaimer Researched guidance for planning purposes, not professional engineering, electrical or legal advice. Antenna work, RF exposure, mast and tower work, grounding and battery handling all carry real risk that depends on your specific site and installation. Verify anything safety-critical against current FCC rules, the National Electrical Code and the manufacturer's own documentation, and hire a professional where the job calls for one. We research equipment from published specifications, regulatory documents and verified owner reviews rather than claiming hands-on testing we have not done.