TL;DR: Off-grid wire sizing in one minute
Wire sizing is governed by two limits. Ampacity is how much current a wire can carry without overheating. Voltage drop is how much power the wire loses over distance. Size for both. Take the continuous current, add 25 percent, and check that against the ampacity table. Then calculate voltage drop using the total circuit length, source to load and back. Keep DC charging circuits under 2 percent and AC circuits under 3 percent. Use copper. When the math lands between two gauges, choose the larger one. Undersized wire runs hot, wastes power, and starts fires.
A family outside Bozeman called me after their battery compartment nearly caught fire. Their installer had run 10 AWG wire to a 3000-watt inverter. The wire could not carry that current. It ran hot, the insulation softened, and it came close to igniting the cells beside it. The correct wire would have cost about fifty dollars more. The wrong wire almost cost them the house. Wire is not where you save money. It is the part that decides whether everything else you bought survives.
Why wire sizing decides whether your system lives or burns
Wire sizing is the foundation the whole system sits on. Get it wrong and one of two things happens. The wire overheats and becomes a fire risk. Or it bleeds voltage over distance and starves your batteries. Neither is acceptable.
Most failures I diagnose are not bad panels or bad batteries. They are bad wire. A conductor too small for its load runs hot every day. Heat ages the insulation. Aged insulation fails. The expensive components downstream were never the problem.
The National Electrical Code sets minimums, not targets. Building to the minimum means building the cheapest legal system. Off-grid systems carry your home through storms and grid failures for decades. They deserve more than the legal minimum.
WATTSON'S FIRST LAW OF WIRING: Every undersized wire is a future failure waiting for a load. Every correctly sized wire is insurance you buy once and never think about again. The wire is the cheapest part of the system and the one most likely to burn it down. Size it right the first time.
Voltage drop: the loss you cannot see
Voltage drop is the quiet thief in an off-grid system. Every foot of wire between a source and a load gives up a little power as heat. Over a long run, that loss adds up. The relationship is not gentle. As voltage falls, current rises to deliver the same power, and higher current makes more heat, which makes more loss.
A 5 percent voltage drop in a charging circuit cuts charging efficiency by 15 to 20 percent. That is energy you captured with panels and paid for, lost in the wire before it reached the battery. Over the life of a battery bank, that waste is enormous.
The limits that matter
Hold your circuits under these voltage drop limits:
| Circuit | Maximum voltage drop |
|---|---|
| Battery charging (DC) | 2% |
| Solar panel to charge controller | 2% |
| Other DC loads | 3% |
| Main panel to subpanel (AC) | 3% |
| Panel to outlet (AC, total) | 5% |
Every 1 percent of drop above these limits costs you roughly 3 to 5 percent in efficiency and shortens component life. Two percent on a 12V system is 0.24 volts. On 24V it is 0.48 volts. On 48V it is 0.96 volts. The higher your system voltage, the more room you have.
Measure the whole loop, not half of it
Here is the detail that sinks amateur installs. People measure the one-way distance and size from that. Voltage drop depends on the total circuit length, source to load and back again.
A 50-foot run from charge controller to batteries is a 100-foot circuit. Size from 50 feet and the wire is undersized by half. Always double the one-way distance before you calculate.
The voltage drop formula
Voltage Drop = (2 × K × I × L) ÷ CM
K is 12.9 for copper, 21.2 for aluminum. I is current in amperes. L is the one-way length in feet. CM is the circular mils of the wire, taken from the AWG table. The 2 accounts for the full circuit length.
Worked example. 100 amps through 50 feet of 2/0 AWG copper, which is 133,100 circular mils:
Voltage Drop = (2 × 12.9 × 100 × 50) ÷ 133,100 = 0.97 volts
On a 24V system that is 4 percent. Too high for battery charging. You would move up a size.
The AWG system: bigger numbers mean smaller wire
American Wire Gauge runs backward and it confuses everyone at first. A 10 AWG wire is smaller than a 4 AWG wire. A 4/0 wire, spoken as "four ought," is enormous. The number counts drawing operations from the 19th century. More operations made thinner wire and a higher number. It is archaic and permanent. Learn it once and move on.
Temperature derating: the factor amateurs skip
Ampacity ratings assume 30°C ambient air and wire in free space. Real installations are hotter and more crowded. You must derate.
| Condition | Derate capacity by |
|---|---|
| Hot climates (90°F+) | 20–30% |
| Enclosed battery compartments | 30–40% |
| Conduit, 4–6 conductors | 20% |
| Conduit, 7–24 conductors | up to 40% |
| Direct burial, no conduit | 10–15% |
A wire rated for 100 amps in open air might safely carry only 60 amps inside a hot battery box. Ignore derating and you have built a heat source.
Copper, not aluminum
Aluminum costs less and tempts budget builds. I do not use it off-grid. It has higher resistance, so it needs one or two sizes larger for the same job. It expands and contracts more, which loosens connections over time. Its oxide is an insulator, so corroded connections gain resistance. Some insurers will not cover it. The copper premium is small insurance against connection failures and fire. Use copper.
Stop guessing. Run the numbers.
Wattson's Wire Sizing Helper does the voltage drop and ampacity math for your exact run. Length, current, voltage, and gauge. No spreadsheet required.
OPEN THE WIRE SIZING HELPER →DC wire sizing: where most systems fail
DC is where off-grid systems fail hardest. AC runs at 120 or 240 volts. DC runs at 12, 24, or 48 volts, so it pushes far more current for the same power. More current means bigger wire.
A 3000-watt load at 120V AC draws 25 amps. The same 3000 watts at 24V DC draws 125 amps. Five times the current. That is why DC wire sizing is not optional precision. It is the difference between a system that runs and one that overheats.
The sizing formula
CM = (2 × K × I × L) ÷ VD
CM is the required circular mils. K is 12.9 for copper. I is current with a 25 percent safety margin added. L is one-way length in feet. VD is allowable voltage drop in volts, usually 2 percent of system voltage. After you find CM, pick the next wire size above your number from the AWG table.
Worked example: a 2000W inverter on 24V
You are wiring a 2000-watt inverter to a 24V bank. The run is 8 feet. Allowable drop is 2 percent, which is 0.48 volts.
Step 1. Base current: 2000W ÷ 24V = 83.3 A
Step 2. Add 25% margin: 83.3 × 1.25 = 104 A
Step 3. Required CM: (2 × 12.9 × 104 × 8) ÷ 0.48 = 44,600 CM
Step 4. Pick the wire: 4 AWG = 41,740 CM (too small)
2 AWG = 66,360 CM (correct)
Step 5. Verify drop: (2 × 12.9 × 104 × 8) ÷ 66,360 = 0.32 V
0.32 ÷ 24 = 1.3% (acceptable)
Use 2 AWG copper.
A builder in Billings ran 4 AWG for that exact job to save money. His drop was 2.8 percent. It does not sound like much. But at full load his inverter saw 23.3 volts instead of 24, the low-voltage protection kept tripping, the batteries never charged fully, and the system ran 15 percent below spec. The fix was 2 AWG, eighty dollars, and four hours. The shortcut cost him six months of bad performance first. Always round up to the next size. The extra copper is trivial next to years of loss.
Battery bank wiring
How you connect the bank changes the wire you need. Series wiring raises voltage and keeps capacity, which lowers current and shrinks the wire. Parallel wiring keeps voltage and adds capacity, which raises current and grows the wire. Most systems use a series-parallel mix.
One rule does not bend. Every interconnecting cable between batteries must be the same length and gauge. A six-inch difference creates a voltage imbalance that shortens the life of the whole bank.
AC wire sizing for inverter output
AC follows different rules. Size the wiring for 125 percent of the inverter's continuous output, per NEC 690.8(B). Size for the continuous rating, not the surge rating.
| Inverter | Minimum wire | Breaker |
|---|---|---|
| 1000W | 12 AWG | 20A |
| 2000W | 10 AWG | 30A |
| 3000W | 8 AWG | 40A |
| 4000W | 6 AWG | 50A |
| 5000W | 6 AWG | 60A |
| 6000W | 4 AWG | 70A |
These are minimums for runs under 50 feet. Longer runs need larger wire to hold voltage drop in check. Always check drop for your actual distance.
Worked example: a long AC run
A 20-amp circuit runs 75 feet to a workshop.
12 AWG (6,530 CM): (2 × 12.9 × 20 × 75) ÷ 6,530 = 5.9 V = 4.9% (too high)
10 AWG (10,380 CM): (2 × 12.9 × 20 × 75) ÷ 10,380 = 3.7 V = 3.1% (acceptable)
That 75-foot run needs 10 AWG, not the standard 12 AWG, or the lights dim and motors struggle to start.
Balance the legs
On a split-phase 120/240V system, spread loads evenly across both legs. Unbalanced loads push current down the neutral, which cuts capacity and efficiency. Aim for within 10 percent balance.
Grounding and bonding: the part that protects people
Grounding and bonding are not optional. They protect your family from shock and your home from fire. Most builders confuse the two. They are different.
Equipment grounding connects metal enclosures so a fault cannot energize a case you might touch. System grounding references one conductor to earth. Bonding ties all the metal together so fault current has a low-resistance path back to trip the breaker.
Mistakes that kill
Do not ground the negative DC bus on a system with ground-fault protection. It creates ground loops that disable the safety system. Modern charge controllers expect an ungrounded negative. Read the manual.
Always bond the inverter case to the equipment ground. An unbonded inverter case can become live at full voltage with no path to trip the breaker. That is a lethal shock hazard.
Never use random wire for grounding. Only listed grounding conductors are rated to carry fault current. Hardware-store wire can vaporize in a fault and leave the system ungrounded at the worst moment.
NEC grounding components (Article 690)
| Component | Requirement |
|---|---|
| Equipment Grounding Conductor | Size per NEC 250.122. Often 10 AWG copper minimum. |
| Grounding Electrode Conductor | Size per NEC 250.66 by largest conductor. |
| Grounding electrodes | 2+ ground rods, 6+ feet apart; UFER if available. |
| Main bonding jumper | Bonds grounded conductor to ground at ONE point only. |
| Grounding bushings | Where metal conduit enters enclosures (NEC 250.92). |
Bond at one point only, at the main disconnect. Multiple bonds create ground loops and safety problems.
Ground rods
Rods must be at least 8 feet, driven vertically. Multiple rods sit at least 6 feet apart. The top of the rod should be flush with or below grade. Use listed clamps or exothermic welds, never a twist of wire. Rocky or sandy soil grounds poorly. Test for under 25 ohms and add rods or ground enhancement if you read higher.
Conduit and protection
Exposed wire does not last. Protect every run from physical damage, UV, and rodents. Cheap protection is expensive over a decade.
Rodents seek out DC wiring. Use steel conduit for DC runs where rodents are active, since they chew through PVC. Screen conduit entries with hardware cloth, use steel junction boxes, and seal entries.
| Practice | Spec |
|---|---|
| Support intervals | Every 3 ft (EMT), every 4 ft (PVC) |
| Expansion fittings | PVC runs over 30 ft |
| Conduit fill | Never over 40% for 3+ conductors (NEC 310.15) |
| Burial depth | 18 in for PVC in trench, 6 in under concrete |
| Entry protection | Bushings on all conduit entries |
NEC compliance
The code is written from decades of fires and deaths. Each rule exists because someone paid for the lesson. Article 690 covers solar systems specifically. Even off-grid in an unincorporated area, the code is the best practice you want to follow. Non-compliant work can void insurance, fail a future sale, and remove your legal footing if the system ever causes harm.
Key articles: 690 (solar PV), 705 (interconnected sources), 250 (grounding and bonding), 310 (conductor ampacity and derating), 240 (overcurrent protection), 110 (general requirements).
Required disconnects
| Disconnect | Location |
|---|---|
| PV array DC | At or near the combiner, before the building |
| Charge controller input | Between array and controller |
| Charge controller output | Between controller and battery |
| Battery bank | Accessible, marked, rated for DC |
| Inverter DC input | Within sight of the inverter |
| Inverter AC output | Within sight of the inverter |
| Main AC | At the distribution panel |
Every disconnect needs a permanent, weatherproof label showing its ratings, what it controls, and a shock-hazard warning per NEC 690.56. Labels help emergency crews and anyone working on the system later.
Connections: where systems actually fail
Bad connections kill more systems than weather ever will. Every junction is a possible failure point and heat source. Make each one right.
Strip to the exact length with no exposed copper outside the terminal. Clean the copper bright. Apply antioxidant compound. Seat the wire fully with no loose strands. Torque to the manufacturer's spec with a real torque wrench, not by feel. Cover with adhesive-lined heat shrink. Label it.
Torque matters in both directions. Too loose creates resistance and arcing. Too tight damages the terminal. A torque wrench costs little and prevents the kind of heat that starts fires. In DC circuits use listed connectors and terminal blocks, never wire nuts.
Troubleshooting before it becomes a disaster
Even good installs drift over time from vibration, heat cycling, and corrosion. The most common failure I find is a high-resistance connection.
Signs a connection is failing
A terminal warmer than the air around it. Discolored or melted insulation. Voltage that reads fine at rest but drops under load. Batteries that will not fully charge. Intermittent faults. Green or white powder at a terminal.
If you find one: kill the power, let it cool, clean the contact bright, apply antioxidant, reseat and torque to spec, then test under load and confirm the temperature stays normal.
Voltage drop diagnosis
Test under full load, not at rest. A no-load reading tells you nothing. Measure at the source, mid-run, and load end to find where the loss happens. If the measured drop is more than 50 percent above your calculated value, you have a problem to find and fix.
The bottom line
Wire sizing is not the glamorous part of off-grid power. It is the part that decides whether the rest of it survives. Builders spend thousands on batteries and panels, then choke the whole system with wire someone guessed at.
Physics does not negotiate. Voltage drop does not care about your budget. The ampacity tables make no exceptions. Size it for the load, check it for distance, use copper, and round up when the math sits between two gauges. Do that and the wire becomes the part of the system you never think about again.
Run your exact numbers before you buy wire.
Enter your current, voltage, and run length. Wattson's Wire Sizing Helper returns the correct gauge with voltage drop and ampacity already checked.
OPEN THE WIRE SIZING HELPER →Frequently asked questions
How do I size wire for an off-grid system? Take the continuous current and add 25 percent for the safety margin. Check that against the ampacity table after derating for temperature and conduit. Then calculate voltage drop using the total circuit length, source to load and back. Keep DC charging under 2 percent and AC under 3 percent. Choose the larger gauge when the result falls between sizes.
Why does DC need bigger wire than AC? DC off-grid systems run at low voltage, so they carry much more current for the same power. A 3000-watt load draws 25 amps at 120V AC but 125 amps at 24V DC. More current requires larger wire.
What is the maximum voltage drop for off-grid wiring? Keep battery charging and solar input circuits under 2 percent. Other DC loads can go to 3 percent. AC feeders should stay under 3 percent, and total AC drop to an outlet under 5 percent. Lower is always better for efficiency.
Should I use copper or aluminum wire? Copper for off-grid systems. Aluminum has higher resistance, loosens at connections as it expands and contracts, and corrodes in a way that raises resistance. The small copper premium is cheap insurance against connection failures and fire.
Do I measure one-way or round-trip wire length for voltage drop? Round trip. A 50-foot run from controller to battery is a 100-foot circuit. The voltage drop formula already includes a factor of 2 for the full loop, so use the one-way length in the formula but never forget the current travels both directions.
Do off-grid systems still have to follow the NEC? Even where no inspection is required, the code is the accumulated best practice for not starting fires or electrocuting people. Non-compliant work can also void insurance and complicate a future sale. Follow Article 690 for solar and Article 250 for grounding.
What size wire does a 2000W inverter need? For the AC output, 10 AWG minimum on a 30-amp breaker for short runs. For the DC side on a 24V bank over a short run, 2 AWG copper after adding the safety margin and checking voltage drop. Longer runs need larger wire.