TL;DR: Off-grid wire sizing in one minute
Wire has two jobs, and two ways to fail. It has to carry the current without overheating, and it has to deliver the power without losing too much over distance. Pick a wire too thin and it runs hot, which starts fires. Pick a wire too thin for a long run and it bleeds away voltage, which starves your batteries. The fix is simple. Take your current, add 25 percent, and look up the gauge. Then check the distance. Keep charging circuits under 2 percent loss and AC circuits under 3 percent. Use copper. When the answer lands between two sizes, pick the bigger one. The calculator does all of this for you.
A family outside Bozeman called me after their battery compartment nearly caught fire. Their installer ran thin wire to a big inverter. The wire could not carry that much current, so it ran hot, the insulation softened, and it almost set the batteries beside it on fire. The right wire would have cost about fifty dollars more. The wrong wire nearly cost them the house. Wire is not where you save money. It is the part that decides whether everything else you bought survives.
Wire is the cheapest part that can burn your house down
Here is the thing nobody tells beginners. You can buy the best panels and the best batteries, then ruin the whole system with wire someone guessed at.
Wire has to do two things. It has to carry the electricity without getting hot, and it has to carry it the distance without losing much along the way. Get either one wrong and you have a problem. Too thin for the current, and it overheats. That is a fire risk. Too thin for the distance, and it loses voltage. That starves your batteries.
Most off-grid failures I get called about are not bad gear. They are bad wire. The good news is that wire sizing is not hard once someone explains it in plain language. That is what this guide does.
WATTSON'S FIRST LAW OF WIRING: Every wire that is too thin is a future failure waiting for a load. Every wire sized right is insurance you buy once and never think about again. Wire is the cheapest part of the system and the one most likely to burn it down. Size it right the first time.
The two things every wire has to handle
Two ideas run this whole topic. Learn these two and the rest is lookups.
Ampacity is how much current a wire can carry before it overheats. Think of it like the width of a pipe. A wider pipe carries more water without bursting. A thicker wire carries more current without cooking.
Voltage drop is how much power a wire loses over distance. Picture a long garden hose. The farther the water travels, the weaker it comes out the far end. Electricity does the same thing. The longer the wire, the more voltage leaks away as heat before it reaches your battery.
Every wire you size has to pass both tests. Thick enough to carry the current. Thick enough to deliver it over the distance. The calculator checks both at once. The rest of this guide shows you how it works so the numbers make sense.
Voltage drop, explained simply
When voltage drops over a long wire, your battery gets a weak trickle instead of a full charge. A 5 percent drop in a charging circuit can cut your charging by 15 to 20 percent. That is energy your panels captured and you paid for, lost in the wire before it ever reached the battery.
So we set limits. Keep your circuits under these and you are fine:
| Circuit | Keep voltage drop under |
|---|---|
| Battery charging | 2% |
| Solar panel to charge controller | 2% |
| Other DC loads | 3% |
| Main panel to subpanel (AC) | 3% |
| Outlet circuits (AC, total) | 5% |
The higher your system voltage, the more room you have. Two percent of 12 volts is only a quarter of a volt. Two percent of 48 volts is almost a full volt. This is one reason 48V systems are easier to wire than 12V systems.
Always measure the round trip
Here is the mistake that trips up almost everyone. People measure how far the wire runs one way, then size from that. But electricity has to travel down the wire and back. So the distance that matters is the round trip.
A 50-foot run from your charge controller to your batteries is really a 100-foot circuit, because the current goes down and comes back. Forget this and your wire ends up half the size it should be.
WATTSON'S RULE OF THUMB: When in doubt, go one size thicker. The extra copper costs a few dollars. The wire that is one size too thin costs you a battery bank, or worse. Nobody ever called me because their wire was too thick.
The AWG system, in plain English
Wire size in America uses a system called AWG, American Wire Gauge. It runs backward, and it confuses everyone at first, so let me just say it plainly.
Bigger number means thinner wire. Smaller number means thicker wire.
A 10 AWG wire is thin. A 4 AWG wire is much thicker. A 4/0 wire, which you say out loud as "four ought," is as thick as your thumb. The numbering is a leftover from the 1800s and it is never going to change, so just remember the rule: the smaller the number, the beefier the wire.
Why a wire's rating shrinks in the real world
A wire's rating assumes cool air and open space. Real installs are hotter and more crowded, so the wire can safely carry less than its sticker says. This is called derating, and skipping it is how beginners cook their wire.
| Where the wire runs | Lower its rated capacity by |
|---|---|
| Hot climate (90°F and up) | 20–30% |
| Inside an enclosed battery box | 30–40% |
| Bundled in conduit (4–6 wires) | 20% |
| Bundled in conduit (7+ wires) | up to 40% |
| Buried in the ground | 10–15% |
A wire rated for 100 amps in open air might only handle 60 amps safely inside a hot, closed battery box. Plan for where the wire actually lives, not a lab.
Copper, not aluminum
Use copper. Aluminum is cheaper, but it has more resistance, it loosens at the connections as it heats and cools, and it corrodes in a way that makes connections worse over time. Some insurers will not even cover it. The extra cost of copper is cheap insurance. This is not the place to save money.
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 →Why DC wiring needs thicker wire than AC
This surprises people, so here is the simple version. The thicker wire on an off-grid system is almost always the DC side, the part between your batteries, charge controller, and inverter.
Why? Because off-grid DC runs at low voltage, and low voltage means high current for the same amount of power. And current is what forces you to use thick wire.
Watch what happens to the same 3000 watts at different voltages:
| Same 3000 watts at... | Current it draws | Wire needed |
|---|---|---|
| 120V AC | 25 amps | Modest |
| 48V DC | 63 amps | Thicker |
| 24V DC | 125 amps | Much thicker |
| 12V DC | 250 amps | Very heavy |
Same power, wildly different current. That is why a 12V system needs such heavy, expensive cable, and why most serious systems are built at 48V. Higher voltage, lower current, thinner and cheaper wire.
How the sizing actually works
You do not have to do this by hand. The calculator does it. But here is what is happening under the hood, so the numbers are not a mystery.
You take your current, add a 25 percent safety cushion, factor in the distance, and the formula spits out a minimum wire size. Then you round up to a real gauge. The formula, for the curious:
Circular mils needed = (2 × 12.9 × Current × Length) ÷ Allowed voltage drop
The 2 is for the round trip. The 12.9 is just a fixed number for copper. Length is the one-way distance in feet. That is it.
Worked example: a 2000W inverter on a 24V bank
Say you are wiring a 2000-watt inverter to a 24V battery bank, 8 feet away. Here is every step, with the answer at each stage:
| Step | What you do | The math | Result |
|---|---|---|---|
| 1 | Find the current | 2000W ÷ 24V | 83.3 amps |
| 2 | Add 25% safety margin | 83.3 × 1.25 | 104 amps |
| 3 | Find the wire size needed | (2 × 12.9 × 104 × 8) ÷ 0.48V | 44,600 circular mils |
| 4 | Pick the gauge | 4 AWG is too small; 2 AWG fits | 2 AWG copper |
| 5 | Double-check the drop | (2 × 12.9 × 104 × 8) ÷ 66,360 | 1.3% ✓ |
The answer is 2 AWG copper. Notice step 5: we go back and confirm the drop is under our 2 percent limit. It comes out at 1.3 percent, so we are good.
A builder in Billings ran 4 AWG for that exact job to save money. His voltage drop came out at 2.8 percent. It does not sound like much. But his inverter kept seeing low voltage, the low-voltage shutoff kept tripping, the batteries never fully charged, and the whole system ran 15 percent below where it should. The fix was 2 AWG wire, eighty dollars, and one afternoon. The shortcut cost him six months of bad performance first.
A simple rule for your battery bank
When you connect batteries together, keep every connecting cable between them the exact same length and the same gauge. Even a six-inch difference makes some batteries work harder than others, and that shortens the life of the whole bank. Same length, same size, every time.
AC wire sizing for your inverter output
The AC side, the household 120V or 240V wiring coming out of your inverter, is simpler. Match the wire to the inverter's continuous output, with a 25 percent cushion. Here are the common sizes for short runs:
| Inverter size | 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 work for runs under 50 feet. Longer than that, you bump up a size to keep the voltage from dropping too much. Here is what that looks like.
Worked example: a long run to the workshop
You want a 20-amp circuit to reach a workshop 75 feet away. Does the standard 12 AWG wire work?
| Wire tested | Voltage drop over 75 ft | Verdict |
|---|---|---|
| 12 AWG | 4.9% | Too much. Lights will dim. |
| 10 AWG | 3.1% | Acceptable. Use this. |
So that long run needs 10 AWG, one size up from the usual 12 AWG, or your workshop lights flicker and your motors struggle to start.
Grounding, in plain language
Grounding keeps you from getting shocked and keeps your house from catching fire. It is not optional, and it is worth getting right. Here is the plain-English version of the two jobs grounding does.
One job is to connect all the metal parts, like the inverter case and the panel frames, so that if a wire ever comes loose inside, the metal you might touch never becomes live. The other job is to give a stray current a safe, easy path back so it trips the breaker instead of traveling through you.
The grounding mistakes that actually hurt people
These three are the ones that cause real harm, so I am calling them out plainly:
| Mistake | Why it is dangerous |
|---|---|
| Grounding the DC negative on a system with ground-fault protection | Disables the safety system. Read your charge controller manual. |
| Leaving the inverter case unbonded | The case can become live at full voltage with no way to trip the breaker. Shock hazard. |
| Using random hardware-store wire for grounding | It can burn up during a fault and leave the system ungrounded at the worst moment. Use listed grounding wire only. |
The simplest rule for grounding: bond everything to ground at one single point, at your main disconnect. Doing it in more than one place creates problems. One point, done right.
Ground rods, the short version
If you are driving ground rods, they need to be at least 8 feet long, driven straight down, and if you use more than one, space them at least 6 feet apart. Use a proper clamp, never a twist of bare wire. In rocky or sandy soil, grounding is weaker, so you may need extra rods.
Protect your wire from damage and rodents
Bare wire does not survive outdoors or in a crawlspace. Sun, weather, and especially rodents will destroy it. Mice and rats love to chew wiring, and they will chew right through plastic conduit. Where rodents are a problem, run your DC wire in steel conduit, not plastic.
A few simple protection habits:
| Practice | What to do |
|---|---|
| Support the conduit | Every 3 feet for metal, every 4 feet for PVC |
| Do not overstuff conduit | Keep it under 40% full when running several wires |
| Bury it deep enough | 18 inches in a trench, 6 inches under concrete |
| Protect every entry point | Use bushings where conduit meets a box, screen openings against rodents |
Do you have to follow the electrical code?
The National Electrical Code, the NEC, is the rulebook. Even if you are off-grid in the middle of nowhere with no inspector, it is worth following, because every rule in it exists because someone once got hurt or burned down a building. It is hard-won best practice, not red tape.
Following it also protects you in practical ways. Work that does not meet code can void your insurance and cause headaches if you ever sell the place. The two sections that matter most for you are Article 690, which covers solar, and Article 250, which covers grounding.
Label every disconnect
Wherever you put a switch to shut off part of the system, label it clearly and permanently, with weatherproof labels. Say what it controls and warn of the shock hazard. This helps any firefighter or future helper who has to work on your system know what they are touching.
Connections are where systems actually fail
More systems fail at the connections than anywhere else. Every place two wires join is a possible weak spot and a possible source of heat. Make each one properly:
| Do this | Not this |
|---|---|
| Strip to the exact length, no bare copper showing | Leaving exposed strands outside the terminal |
| Clean the copper bright and add anti-corrosion compound | Connecting dull or corroded wire |
| Tighten to the maker's spec with a torque wrench | Tightening "by feel" |
| Cover with heat-shrink and label it | Leaving it bare |
| Use proper connectors and terminal blocks on DC | Wire nuts on DC circuits |
Tightness matters in both directions. Too loose causes arcing and heat. Too tight damages the terminal. A small torque wrench is cheap and prevents the exact kind of heat that starts fires.
Catching a problem before it becomes a disaster
Even a good install drifts over time from heat, vibration, and corrosion. The most common thing I find is a connection that has gone bad and started heating up. Here is what to watch for:
| Warning sign | What it means |
|---|---|
| A terminal warmer than the air around it | Resistance building up. Catch it now. |
| Discolored or melted insulation | The wire has been running hot. |
| Voltage fine at rest but drops under load | A weak connection or undersized wire. |
| Batteries that never fully charge | Voltage being lost in the wiring. |
| Green or white powder at a terminal | Corrosion. Clean and re-treat it. |
If you find a bad connection: shut off the power, let it cool, clean the contact until it is bright, add anti-corrosion compound, reconnect it to the proper tightness, then turn it back on and make sure it stays cool under load.
When you test for voltage drop, always test under full load, never at rest. A reading with nothing running tells you nothing useful.
The bottom line
Wire sizing is not the exciting part of going off-grid. It is the part that decides whether the exciting parts survive. People spend thousands on batteries and panels, then choke the whole system with wire they guessed at.
The rules are simple. Size the wire for the current, with a little cushion. Check it for the distance. Use copper. When the answer falls between two sizes, pick the bigger one. Do that and the wire becomes the part of your system you never have to think about again.
You do not have to do the math by hand. The calculator does it for you. But now you know what it is doing, and why it matters.
Get your exact wire size from Wattson's AI
The chart and the calculator cover the common cases. If your setup is unusual, or you just want a second opinion, ask Wattson's AI tool directly. You do not need to know how to write a prompt. Copy the one below, fill in your numbers, and paste it in.
Copy this, fill in the blanks, and paste it into Wattson's AI tool:
I need help sizing wire for an off-grid circuit. Here are my details:
- What the wire connects: ____ (example: inverter to battery bank)
- System voltage: ____ (12V, 24V, or 48V)
- The load or current: ____ watts, or ____ amps if I know it
- One-way distance of the run: ____ feet
- Where the wire runs: ____ (open air, inside a battery box, in conduit, buried)
- My climate: ____ (hot, mild, cold)
Please tell me: the wire gauge I need in AWG, the voltage drop I should expect, whether I should size up for the distance, the right breaker or fuse size, and any safety issue I should watch for. Explain it simply, like I am new to this.
Wattson's AI will walk you through the answer and explain the why behind it.
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 your continuous current and add 25 percent. Look that up on the wire chart, after lowering the rating for heat and crowding. Then check the voltage drop over your round-trip distance. Keep charging circuits under 2 percent and AC circuits under 3 percent. When the answer lands between two sizes, pick the bigger one. The calculator does all of this for you.
Why does DC need thicker wire than AC? Because off-grid DC runs at low voltage, and low voltage means high current for the same power. The same 3000 watts that draws 25 amps at 120V AC draws 125 amps at 24V DC. More current means thicker wire.
What is the most voltage drop I should allow? Keep battery charging and solar circuits under 2 percent. Other DC loads can go to 3 percent. AC circuits should stay under 3 percent, and the total to an outlet under 5 percent. Lower is always better.
Should I use copper or aluminum wire? Copper. Aluminum costs less but has more resistance, loosens at connections over time, and corrodes in a way that makes things worse. The small extra cost of copper is cheap insurance against fire and failed connections.
Do I measure the wire one way or round trip for voltage drop? Round trip. A 50-foot run from controller to battery is a 100-foot circuit, because the current goes down and comes back. Forget this and your wire ends up half the size it needs to be.
Do off-grid systems really have to follow the electrical code? Even where no inspector will ever visit, the code is the accumulated best practice for not starting fires or shocking anyone. Following it also protects your insurance and your home's resale. The key sections are Article 690 for solar and Article 250 for grounding.
What size wire does a 2000W inverter need? On the AC output, 10 AWG on a 30-amp breaker for short runs. On the DC side of a 24V bank over a short run, 2 AWG copper after adding the safety margin and checking the voltage drop. Longer runs need a thicker wire.