How this is calculated
Volts lost equals twice the run length, times the current, times resistivity, divided by the conductor’s area. The doubling is there because current goes out on one conductor and comes back on the other.
Turn it round to size the wire: required area equals twice the length, times current, times resistivity, divided by the volts you will give up. Copper is 0.017241 ohm·mm²/m at 20°C; aluminum is about 0.0282, roughly 61% as conductive, which is where the 1.64× area factor comes from.
Worked example, the defaults above. A 1,200W inverter on a 24V bank draws 50A over 20 ft (6.10 m) one way, and 3% of 24V is 0.72V. Required area is 2 × 6.10 × 50 × 0.017241 ÷ 0.72 = 14.6 mm²: 4 AWG, or 16 mm² metric.
Put 4 AWG (21.2 mm²) back in and the drop is 0.50V, about 2.1%, with 24.8W of heat in the cable. Ampacity alone would have passed 8 AWG, so voltage drop binds here and costs two sizes.
Worked example, the defaults above. A 1,200W inverter on a 24V bank draws 50A over a 6 m one-way run, and 3% of 24V is 0.72V. Required area is 2 × 6 × 50 × 0.017241 ÷ 0.72 = 14.4 mm²: 16 mm², or 4 AWG in the American sizes.
Put 16 mm² back in and the drop is 0.65V, 2.7%, with 32W of heat in the cable. Voltage drop binds here and costs two sizes over what the heat limit alone would have asked for.
Temperature moves the answer. Copper’s resistance rises about 0.393% per °C, so a conductor at 45°C is roughly 10% more resistive than at 20°C, and at 75°C about 22% more. A cable in a hot loft or a sealed battery box deserves the next size up.
Ampacity and voltage drop are two separate limits
Ampacity is a heat rule: the current a conductor carries before its insulation cooks, read from a code table rather than a formula. Voltage drop is a performance rule about how much of the battery reaches the load. Satisfy both, and take whichever demands the bigger conductor.
Short, fat runs — battery to inverter, battery to busbar — are usually decided by ampacity. Anything that travels is decided by voltage drop, often by two or three sizes.
The ampacity numbers here are NEC Table 310.16, 75°C column, at 30°C ambient with three or fewer current-carrying conductors together. The 3% and 5% drop figures are Informational Note No. 4 to NEC 210.19(A) and Informational Note No. 2 to 215.2(A)(1) in the 2023 code, which is guidance rather than an enforceable article, though some jurisdictions adopt the limits as a local amendment. Bundling, heat and conduit fill all derate the table value.
The equivalent capacities come from IEC 60364-5-52 and harmonized HD 60364-5-52, tabulated by installation method rather than one column. Annex G gives the drop limits normally applied: 3% for lighting and 5% for other uses, measured from the origin of the installation. Grouping and ambient factors derate them.
Sizing the whole system rather than one cable? Start at the off-grid solar sizing calculator, which fixes the array, bank and system voltage this page then wires up.
Why 48V uses a quarter of the copper
Two effects stack. Double the voltage and the same watts need half the current; a fixed percentage drop at that voltage also allows twice as many volts. Half the current with double the allowance is a quarter of the conductor area.
Carry 1,200W over a 10 m one-way run at 3% and it reads 96 mm² of copper at 12V, 24 mm² at 24V, 6 mm² at 48V. Sixteen times the metal for the same job.
So 12V suits a van or a small cabin where nothing travels far. Once a run crosses a yard, 24V is the least painful choice and 48V is usually cheaper once cable is priced.
Reference chart: minimum copper at 3% drop
Every cell is the smallest copper conductor that holds a 3% drop over that one-way run, in AWG and the nearest metric size above it. Ampacity floor and fuse rating come from the 75°C copper column and the 125% rule. Cells marked >4/0 need paralleled cable or a higher voltage.
Showing 24V. Click a column heading to sort.
| Current | Ampacity floor | Fuse | 10 ft / 3 m | 20 ft / 6 m | 30 ft / 9 m | 50 ft / 15 m | 100 ft / 30 m |
|---|---|---|---|---|---|---|---|
| 5A | 14 | 10A | 14 / 1.5 | 12 / 4 | 10 / 6 | 8 / 10 | 4 / 16 |
| 10A | 14 | 15A | 12 / 4 | 8 / 6 | 6 / 10 | 4 / 16 | 2 / 35 |
| 15A | 14 | 20A | 10 / 6 | 6 / 10 | 6 / 16 | 3 / 25 | 1/0 / 50 |
| 20A | 14 | 25A | 8 / 6 | 6 / 16 | 4 / 25 | 2 / 35 | 2/0 / 70 |
| 30A | 10 | 40A | 6 / 10 | 4 / 25 | 3 / 35 | 1/0 / 50 | 4/0 / 95 |
| 40A | 8 | 50A | 6 / 16 | 3 / 25 | 1 / 35 | 2/0 / 70 | >4/0 / 120 |
| 50A | 8 | 70A | 4 / 16 | 2 / 35 | 1/0 / 50 | 3/0 / 95 | >4/0 |
| 60A | 6 | 80A | 4 / 25 | 1 / 35 | 1/0 / 70 | 4/0 / 95 | >4/0 |
| 80A | 4 | 100A | 3 / 25 | 1/0 / 50 | 3/0 / 70 | >4/0 / 120 | >4/0 |
| 100A | 3 | 125A | 2 / 35 | 2/0 / 70 | 4/0 / 95 | >4/0 | >4/0 |
| 125A | 1 | 175A | 1 / 50 | 3/0 / 95 | >4/0 / 120 | >4/0 | >4/0 |
| 150A | 1/0 | 200A | 1/0 / 50 | 4/0 / 95 | >4/0 | >4/0 | >4/0 |
| 200A | 3/0 | 250A | 2/0 / 70 | >4/0 / 120 | >4/0 | >4/0 | >4/0 |
| 5A | 14 | 10A | 14 / 1.5 | 14 / 1.5 | 12 / 2.5 | 10 / 4 | 8 / 10 |
| 10A | 14 | 15A | 14 / 1.5 | 12 / 4 | 10 / 6 | 8 / 10 | 4 / 16 |
| 15A | 14 | 20A | 12 / 2.5 | 10 / 6 | 8 / 10 | 6 / 16 | 3 / 25 |
| 20A | 14 | 25A | 12 / 4 | 8 / 6 | 6 / 10 | 4 / 16 | 2 / 35 |
| 30A | 10 | 40A | 10 / 6 | 6 / 10 | 6 / 16 | 3 / 25 | 1/0 / 50 |
| 40A | 8 | 50A | 8 / 6 | 6 / 16 | 4 / 25 | 2 / 35 | 2/0 / 70 |
| 50A | 8 | 70A | 8 / 10 | 4 / 16 | 3 / 25 | 1 / 50 | 3/0 / 95 |
| 60A | 6 | 80A | 6 / 10 | 4 / 25 | 3 / 35 | 1/0 / 50 | 4/0 / 95 |
| 80A | 4 | 100A | 6 / 16 | 3 / 25 | 1 / 35 | 2/0 / 70 | >4/0 / 120 |
| 100A | 3 | 125A | 4 / 16 | 2 / 35 | 1/0 / 50 | 3/0 / 95 | >4/0 |
| 125A | 1 | 175A | 4 / 25 | 1 / 50 | 2/0 / 70 | 4/0 / 95 | >4/0 |
| 150A | 1/0 | 200A | 3 / 25 | 1/0 / 50 | 2/0 / 70 | >4/0 / 120 | >4/0 |
| 200A | 3/0 | 250A | 2 / 35 | 2/0 / 70 | 4/0 / 95 | >4/0 | >4/0 |
| 5A | 14 | 10A | 14 / 1.5 | 14 / 1.5 | 14 / 1.5 | 14 / 2.5 | 10 / 4 |
| 10A | 14 | 15A | 14 / 1.5 | 14 / 1.5 | 12 / 2.5 | 10 / 4 | 8 / 10 |
| 15A | 14 | 20A | 14 / 1.5 | 12 / 2.5 | 12 / 4 | 8 / 6 | 6 / 16 |
| 20A | 14 | 25A | 14 / 1.5 | 12 / 4 | 10 / 6 | 8 / 10 | 4 / 16 |
| 30A | 10 | 40A | 12 / 2.5 | 10 / 6 | 8 / 10 | 6 / 16 | 3 / 25 |
| 40A | 8 | 50A | 12 / 4 | 8 / 6 | 6 / 10 | 4 / 16 | 2 / 35 |
| 50A | 8 | 70A | 10 / 4 | 8 / 10 | 6 / 16 | 4 / 25 | 1 / 50 |
| 60A | 6 | 80A | 10 / 6 | 6 / 10 | 6 / 16 | 3 / 25 | 1/0 / 50 |
| 80A | 4 | 100A | 8 / 6 | 6 / 16 | 4 / 25 | 2 / 35 | 2/0 / 70 |
| 100A | 3 | 125A | 8 / 10 | 4 / 16 | 3 / 25 | 1 / 50 | 3/0 / 95 |
| 125A | 1 | 175A | 6 / 10 | 4 / 25 | 2 / 35 | 1/0 / 50 | 4/0 / 95 |
| 150A | 1/0 | 200A | 6 / 16 | 3 / 25 | 2 / 35 | 2/0 / 70 | >4/0 / 120 |
| 200A | 3/0 | 250A | 4 / 16 | 2 / 35 | 1/0 / 50 | 3/0 / 95 | >4/0 |
Cells read as AWG / mm², copper at 20°C and 3% drop. The AWG size is worked on the foot figure in the heading and the mm² size on the meter figure, which is why a few cells step differently. For aluminum, multiply the area by about 1.64.
Fusing the cable you just sized
The fuse protects the wire. Rate it at 125% of the continuous current, round up to the next standard rating, then check the conductor’s ampacity is at least that rating. Fifty amps continuous becomes 62.5A: a 70A device on wire good for 70A.
A load drawing its maximum for three hours or more counts as continuous, and the 125% keeps the device off its own limit. Solar strings take the multiplier twice in the American code: once on short-circuit current for irradiance above test conditions, again for continuous duty.
Standard ratings, NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225 and 250A, with 1, 3, 6 and 10A standard for fuses but not for breakers. Battery-side DC uses Class T, ANL or MRBF fuses, which interrupt the tens of thousands of amps a lithium bank delivers into a dead short.
gG fuse-links to IEC 60269 are stocked at 2, 4, 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200 and 250A, and some body styles add 8, 12, 13 and 35A. DC circuits need devices rated for DC at the working voltage; an AC-only breaker has no arc-quenching zero crossing and can fail to clear a battery fault.
MC4 leads, battery cable and the joints that get hot
Panel leads terminate in MC4 connectors on double-insulated PV cable: 10 AWG in the United States, 4 or 6 mm² H1Z2Z2-K in Europe.
Stäubli’s Original MC4 catalog (11014112-en, June 2026) rates the open-crimp connector 30A on 14 AWG, 35A on 12 AWG and 50A on 10 AWG to UL 6703 at 1500V DC, and 39A on both 4 and 6 mm² to IEC 62852 at 1100V DC.
The higher 1500V IEC numbers quoted around the trade belong to the MC4-Evo 2, a different part.
Mixing brands of MC4-alike connector is the quiet fault on an array: tolerances differ just enough that the contact warms, oxidizes and fails open mid-string on a wet day.
Battery cable is a different animal. Fine-stranded welding or marine cable takes vibration and tight bends that building wire will not, and every joint wants a hex crimp and adhesive-lined heatshrink. A poor lug is a resistor: 0.5 milliohm at 200A burns 20W in a lump the size of a thumb.
- Keep the battery-to-inverter pair short and equal in length; it is almost always the ampacity-limited run.
- Size charge controller to battery at 2%: drop there fools the controller’s voltage sensing and undercharges the bank.
- Torque every lug to the maker’s figure, then measure the real drop under load with a meter across each end.
Quick answers
What size wire do I need for a 12V system?
At 3% drop, 20A over 10 ft (3 m) wants 8 AWG or 6 mm²; the same 20A over 50 ft (15 m) wants 2 AWG or 35 mm². Distance decides it, more than the appliance does.
How do you calculate DC voltage drop?
Volts lost = 2 × run length × current × resistivity ÷ conductor area. Copper is 0.017241 ohm·mm²/m at 20°C, aluminum about 0.0282.
Ampacity or voltage drop?
Both, taking the larger conductor. Ampacity is a fire limit from the code tables; voltage drop is a performance limit that usually wins past a few meters.
Why does 48V need so much less copper?
A quarter of the area per voltage doubling: half the current for the same watts, twice the volts allowed for the same percentage. From 12V to 48V, sixteen-fold.
What fuse goes with the wire?
125% of the continuous current, rounded up to the next standard rating, with the conductor rated at or above that number. Fifty amps continuous means a 70A device.
Can I use aluminum instead of copper?
For long fixed runs, yes, at about 1.64 times the area and with terminations rated for aluminum. For battery interconnects, use fine-stranded copper.
Sources
Ampacities from NEC Table 310.16, 75°C copper and aluminum columns, 2023 edition. Drop guidance from Informational Note No. 4 to NEC 210.19(A) and Informational Note No. 2 to 215.2(A)(1). Overcurrent ratings from NEC 240.6(A); the 125% multipliers from NEC 210.20(A) and 690.8(A)(1) and (B). Metric practice from IEC 60364-5-52 and its Annex G, and gG ratings from IEC 60269. Resistivity of 0.017241 ohm·mm²/m and aluminum at 61% from the International Annealed Copper Standard; temperature coefficients 0.00393 per °C for copper and 0.00403 for aluminum. Connector figures from the Stäubli Original MC4 catalog 11014112-en, June 2026.
Checked September 2026. Code editions move; check the ampacity and overcurrent tables against the book your inspector is working from.
Go deeper
Fix the array, bank and system voltage first with the off-grid solar sizing calculator, then check the battery-to-inverter pair against the surge in inverter and charger sizing. The lithium against lead-acid comparison and the battery bank calculator settle the voltage this page assumes.