A 50 watt GMRS mobile or a 100 watt HF rig raises the same questions on myGMRS, QRZ and the off-road boards: what fuse, what wire, which battery, and how to build a lithium go-box. Every answer starts with one number: the current the radio draws on transmit.
Start with the transmit current
We list only figures we could read in each maker's own manual or spec page.
| Radio | Rated output | Supply the maker specifies | Transmit current | Fuse the maker specifies |
|---|---|---|---|---|
| Icom IC-7300 | 100 W | 13.8 V DC ±15%, at least 21 A | 21 A at 100 W | ATC 25 A or 30 A in the DC cable, by version |
| Yaesu FT-891 | 100 W | 13.8 V DC ±15%, at least 25 A | Not in the pages we checked | 25 A |
| Wouxun KG-1000G (GMRS) | Up to 50 W | 13.8 V DC ±15%, 20 A or more | Not listed in the manual | 15 A |
A few details matter:
- Plus or minus 15 percent of 13.8 V is 11.7 to 15.9 V. That is the window the radio needs at its own connector, while transmitting.
- Wouxun disables transmit above 16 V and shuts the radio off below 11.5 V.
- Yaesu says to wire straight to the vehicle battery, not to ignition or accessory circuits, for less noise and steadier voltage.
- Midland's MXT500 page lists 50, 25 and 5 watt settings but no fuse or current figure. Read the rating off the fuse holder on its cord.
Fuse size: the maker's fuse, plus one at the battery
- Never fit a bigger fuse than the maker specifies in the radio's cord.
- Fuse every extension at the battery. When you add cable to reach the battery, fit a fuse within a few inches of the battery positive. That fuse protects the wire, so choose one no larger than the wire can safely carry and no smaller than the radio's own fuse.
- Keep both leads fused if the cord came that way. One NC4x4 member noted their 50 W GMRS cord came with 15 amp fuses on both positive and negative.
- Run the negative back to the battery too, alongside the positive.
For one heavier feed to several radios, a MIDI fuse in a holder at the battery does the job. Our cable and fuse sizing chart pairs fuses with cable sizes.
Wire gauge and voltage drop on long runs
The fuse is about safety; the gauge is about voltage. Here is the drop in copper wire, counting both leads, at 21 A (an IC-7300 at 100 W) and at 15 A, the KG-1000G's fuse used as a worst case.
| Wire | 21 A, 10 ft run | 21 A, 20 ft run | 15 A, 10 ft run | 15 A, 20 ft run |
|---|---|---|---|---|
| 12 AWG | 0.67 V | 1.33 V | 0.48 V | 0.95 V |
| 10 AWG | 0.42 V | 0.84 V | 0.30 V | 0.60 V |
| 8 AWG | 0.26 V | 0.53 V | 0.19 V | 0.38 V |
| 6 AWG | 0.17 V | 0.33 V | 0.12 V | 0.24 V |
"Run" means the one-way distance from battery to radio.
Try it. A healthy lead-acid starting battery rests at about 12.6 V with the engine off. Put an HF radio 20 feet back on 10 AWG and it sees about 11.8 V on transmit. That is right at the bottom of Icom's window, and one tired crimp puts it below. On 6 AWG it sees about 12.3 V.
A common target is 3 percent, about 0.4 V. That means roughly 8 AWG for a 10 foot HF run and 6 AWG for 20 feet. A 50 W GMRS radio gets there on 10 AWG at 10 feet and 8 AWG at 20. Check the result with a meter at the radio's connector while transmitting into a dummy load. Our 12 V voltage drop guide goes further.
Which battery in a dual-battery vehicle
- The starting battery is fine for a radio used mostly while driving. The risk is a long net with the engine off. Wouxun's 11.5 V cut-off protects the radio, not your ability to start the engine; a lead-acid battery that low is close to flat.
- An auxiliary battery charged by a DC-DC charger is better for camp and nets. The starting battery stays reserved for starting. A lithium auxiliary sits around 13.2 to 13.4 V, well inside the window. While charging it rises to its absorption voltage, 14.2 to 14.4 V on Victron's SuperPack, still under 15.9 V and under Wouxun's 16 V limit.
- Mount the auxiliary battery near the radio if you can. A shorter run beats thicker wire.
The Orion XS and the 30 amp Orion-Tr Smart both charge an auxiliary battery from the alternator. Chargers and solar controllers are switching converters, and some operators hear them on HF. If you do, switch the charger off to confirm, and route the radio's power leads away from its cables.
A LiFePO4 go-box
Pick a battery whose continuous rating beats the transmit draw. Victron's 12.8 V Lithium SuperPack in 20 Ah and 60 Ah sizes is rated at 30 A continuous and 80 A for 10 seconds. That covers a 21 A HF rig.
Size it for your operating. An example: 12 hours on an IC-7300, transmitting 10 percent of the time. Receive is 10.8 hours at Icom's 1.25 A maximum, or 13.5 Ah. Transmit is 1.2 hours at 21 A, or 25.2 Ah. The total is about 39 Ah. SSB voice averages far below key-down current; digital modes run close to it. A 60 Ah SuperPack covers this even in the cold, where Victron rates it at 48 Ah at 32 °F. The 20 Ah suits a GMRS mobile or short activations.
Inside the box:
- A MIDI fuse at the battery positive, sized to the internal cable.
- A small positive and negative busbar, with each output fused for its own lead.
- A SmartShunt on the negative. Lithium voltage won't tell you what is left; the shunt will.
Connectors. Most hams standardize on Anderson Powerpole. We don't stock Powerpole connectors, so buy those from a ham radio supplier. We do stock a 50 amp SB50-style Anderson plug with pre-wired cable. It suits the heavy link between the go-box and a vehicle charging lead. It does not mate with Powerpoles.
Charger. Match the charger to the battery's charge limit: 15 A for the 20 Ah SuperPack, 30 A for the 60 Ah. The Blue Smart IP22 comes in 15, 20 and 30 A versions with a Li-ion preset. The sealed Blue Smart IP67 suits field use.
Can the charger run the radio? The IP22 has a power supply mode that runs equipment with or without a battery connected, at a voltage and current limit you set. A 15 or 20 A charger can't carry a 21 A transmit peak alone, but with the battery connected it covers the average and the battery covers the peaks. Set the voltage no higher than the battery's float voltage, 13.5 V on the SuperPack.
Cold. The SuperPack blocks charging below about 32 °F. Charge a winter go-box indoors.
Solar for portable operation
A folding panel and a SmartSolar MPPT keep a go-box topped up. Victron rates the 75/15 at 15 A and 220 W of panel at 12 V, a neat match for the 20 Ah SuperPack's 15 A charge limit. The 100/20 handles 20 A and 290 W. The panel must be at least 5 V above the battery before the controller starts. If it adds noise, disconnect the panel while you work a weak signal. Our MPPT pairing chart matches panels to controllers.
Related guides
Send us the radio, its fuse rating and the cable run, and we will help size the wire and battery. Everything ships from Upland, California, and the counter is open for Inland Empire go-kit builders.