What wire gauge for a 30A solar charge controller over 5 meters?
For a 12V system at 30A over a 5 m run (10 m total cable), use a minimum of 10 mm² (AWG 6); 10 AWG is insufficient. Two limits decide it: ampacity and voltage drop (keep under 2.5%). At 12V, voltage drop is the binding constraint; at 24V the requirement eases.
Key figures
| Parameter | Value |
|---|---|
| Recommended minimum (12V, 30A, 5 m run) | 10 mm² / AWG 6 |
| Charge controller battery-side current (design basis) | 30 (use full rating, not typical) A |
| Continuous-load sizing factor (NEC 690.8) | 1.25× |
| Target voltage drop | ≤2.5 % |
| 12V budget for 2.5% drop | ~0.30 V |
| NEC 240.4(D) OCPD cap for 10 AWG | 30 A |
| Ampacity reference | NEC Table 310.16 (75°C column for terminations) |
Step by step
- Use the full controller current. Size to the controller's rated battery-side output (30A here), not the typical operating current. The fuse protects the wire, so the wire must handle the protection rating.
- Apply the continuous-load factor. Solar charge circuits are continuous loads; NEC 690.8 applies a 1.25× factor when sizing for ampacity.
- Check ampacity against NEC Table 310.16. Confirm the wire's ampacity (75°C column for typical terminals) meets the protected current. Note NEC 240.4(D): 10 AWG is capped at a 30A overcurrent device regardless of higher table values.
- Check voltage drop over the round-trip length. Voltage drop uses total cable length (out + back = 10 m for a 5 m run). Aim for a drop below 2.5% of system voltage. At 12V this is the binding constraint and forces a thicker wire than ampacity alone — here, a 10 mm² minimum.
- Derate for temperature if needed. In hot conduit or sun-exposed runs, derate ampacity using the NEC 310.16 correction factors for the actual ambient temperature.
The answer
For a 12V system, 30A, over a 5 m run (10 m total cable): use a minimum of 10 mm² (AWG 6). Per Victron’s cable sizing, 10 mm² carries 30A at up to 5 m total cable length within a 2.5% voltage-drop target at 12V. AWG 6 (13.3 mm²) is the nearest AWG above that minimum; AWG 4 (21.1 mm²) gives extra margin.
10 AWG (5.26 mm²) is insufficient for this run — it sits below the 10 mm² minimum. This is the common mistake: 10 AWG can legally carry 30A on ampacity grounds, but it loses too much voltage over 5 m at 12V.
The answer depends on two independent limits
Wire sizing is not one calculation but two, and you must satisfy both:
- Ampacity — can the wire safely carry the current (and the overcurrent device protecting it)?
- Voltage drop — does the wire lose too much voltage over the distance?
The required gauge is whichever of the two demands the thicker wire. At 12V over this distance, voltage drop wins, which is why the ampacity-only answer (10 AWG) is too thin.
Why the system voltage changes everything
At 12V, the 2.5% voltage-drop budget is tiny (~0.30V), so over a 5 m run at 30A the drop constraint forces a thicker wire than ampacity alone would. At 24V the same 30A carries twice the power with twice the voltage headroom, so the requirement relaxes. This is the core reason higher system voltages use thinner, cheaper copper.
Frequently asked
What gauge should I actually use for 30A at 12V over 5 m?
A minimum of 10 mm² (AWG 6). Per Victron's cable sizing, 10 mm² carries 30A at up to 5 m total cable length within a 2.5% voltage-drop target at 12V. AWG 6 (13.3 mm²) is the nearest AWG above that minimum; AWG 4 (21.1 mm²) adds margin. Note that 10 AWG (5.26 mm²) is below the minimum and should not be used for this run.
Why does voltage drop matter more than ampacity at 12V?
At 12V the 2.5% drop budget is only about 0.30V, so even a wire that can legally carry the current may lose too much voltage over distance. At 24V (and especially 48V) the same power means less current and a larger relative voltage budget, so the wire can be thinner.
Does the 5 meter run mean 5 meters of wire?
No. Voltage-drop calculations use the total cable length — current flows out and back — so a 5 m run is 10 m of conductor for the drop calculation.