Cable sizing is the part of a solar or backup power system that's easiest to get wrong, because an undersized cable won't fail outright the way an undersized inverter would — it just quietly wastes power as heat, and in bad cases, creates a genuine fire risk. Picking the right cable size mostly comes down to one thing: keeping voltage drop within an acceptable limit over the distance the current has to travel.
Why voltage drop matters
Every conductor has resistance, and pushing current through that resistance over a distance causes some voltage to be lost along the way — this is voltage drop. The longer the cable run and the higher the current, the more voltage is lost before it reaches your load. Left unchecked, this means:
- Appliances and inverters receive less voltage than they need, which can cause poor performance, overheating, or premature failure.
- The lost energy is converted to heat in the cable itself, which is wasted power and — in more severe cases — a fire hazard.
- In battery-based systems, excessive voltage drop between the battery and the load or charge controller can trick the system into inaccurate readings, since voltage is often used to estimate battery state.
Industry practice keeps voltage drop under about 3% for most circuits, and often tightens that to 1–2% for long cable runs or highly sensitive equipment.
The formula behind the calculator
The calculator uses the standard voltage drop formula for a round-trip circuit:
Cross-sectional area (mm²) = (2 × Length × Current × 0.0175) ÷ Allowed voltage drop (V)
The factor of 2 accounts for the fact that current has to travel out to the load and back to the source — the full round-trip distance, not just the one-way distance you measured. The constant 0.0175 is the resistivity of copper in ohm·mm²/m. Once the required area is calculated, the tool rounds up to the next standard cable size, since cables are only manufactured in fixed increments.
Cable sizing works differently on the DC side versus the AC side of a system. On the DC side, string cables connecting panels to the inverter or combiner box are typically 8 mm² or 10 mm², depending on the string voltage. On the AC side, the cable size depends on the connected load or, for larger-scale projects, the transformer rating. I've seen this distinction hold consistently across both small residential systems and larger installations in different locations across Jordan — DC and AC sizing simply follow different logic, and mixing up the two is a common mistake on smaller projects.
Standard cable sizes: mm² and AWG
| mm² | Closest AWG | Typical use |
|---|---|---|
| 1.5 mm² | 16 AWG | Low-power sensors, small DC loads |
| 2.5 mm² | 14 AWG | Lighting circuits, small appliances |
| 4 mm² | 12 AWG | General household circuits |
| 6 mm² | 10 AWG | Medium loads, short-to-medium runs |
| 10 mm² | 8 AWG | Higher current loads, panel-to-inverter runs |
| 16 mm² | 6 AWG | Battery interconnects, longer array runs |
| 25 mm²+ | 4 AWG and larger | High-current main feeders, long battery-to-inverter runs |
mm² and AWG don't convert to each other with a clean formula — the pairings above are the closest standard equivalents commonly used interchangeably in the industry, which is why the calculator shows both side by side.
Copper vs. aluminum cable
This calculator assumes copper, which is the standard choice for most solar and battery wiring due to its lower resistance and better reliability at connection points. Aluminum cable is lighter and cheaper for a given current rating, and is common in large-scale utility wiring, but it requires roughly 1.6 times the cross-sectional area of copper to carry the same current with the same voltage drop, along with special connectors and antioxidant compound to prevent corrosion at terminals. For most home and small commercial solar systems, copper remains the simpler and more reliable choice.
A worked example
With the calculator's default values — a 1,000W load at 24V, 10 meters one-way, and a 3% allowed voltage drop — the current works out to about 41.7A. Applying the formula: (2 × 10 × 41.7 × 0.0175) ÷ 0.72V (3% of 24V) gives a required area of roughly 20.3 mm², which rounds up to the next standard size of 25 mm² (4 AWG). At that size, the actual voltage drop comes out slightly under the 3% target — exactly what the calculator returns.
Already have a cable in mind? Check its actual voltage drop with the voltage drop calculator. You'll also want to size your panels and charge controller to know the current your cables must carry.
Solar panel and inverter cable sizing
The two runs people ask about most are the panel cables and the inverter cable. For solar panel cable size, the current is set by your array — the calculator handles it once you enter the panel or string current and the run length; longer roof-to-controller runs need a bigger gauge to hold the voltage drop down. For inverter cable size, the current is usually much higher because it's on the low-voltage battery side (inverter power ÷ battery voltage), so inverter cables are typically the thickest in the system. Either way the method is the same: work out the current, set your allowable voltage drop, and let the cable sizing calculator return the mm² and AWG. For a fuller walk-through with a worked example, see our guide on what size cable you need for solar panels and inverters.
Frequently asked questions
Does this calculator work for AC circuits too?
The core voltage drop formula is the same for both DC and single-phase AC circuits. For three-phase AC systems, the calculation differs slightly due to how phases interact, so treat this tool as accurate for DC and single-phase AC only.
Should I always round up to the next cable size?
Yes. Cables are manufactured in fixed standard sizes, and rounding down would put you over your target voltage drop. When in doubt, sizing one step larger than the calculated minimum adds a safety margin and reduces long-term heat buildup in the cable.
Does cable length include both the positive and negative runs?
Enter only the one-way distance from source to load — the calculator already doubles it internally to account for the return path, since current has to travel both directions to complete the circuit.