12V wire size calculator

What wire size means on a 12 volt circuit

Wire size in the United States is written in AWG, short for American Wire Gauge, and the number runs backwards from the thickness. A 14 AWG conductor is thinner than a 12 AWG, which is thinner than an 8 AWG. Each drop of three gauge steps roughly doubles the copper area, so 9 AWG would carry about twice the current of 12 AWG if it were a standard size. The sizes you can buy in a hardware store step 14, 12, 10, 8, 6, 4, 2, 1, then 1/0 through 4/0.

A 12 AWG copper conductor measures 0.0808 inch across, and 14 AWG measures 0.0641 inch. That small change in diameter is a large change in metal: going from 14 to 12 AWG adds about 59 percent more copper area, and it cuts resistance by 37 percent, from 3.07 to 1.93 ohms per 1,000 feet.

Low voltage systems feel every ohm. A 12 volt circuit has two failure modes, and they pull in opposite directions. Too small a conductor overheats and can start a fire. Too small a conductor also drops voltage along the run, so a pump or an inverter sees less than 12 volts and works harder. On a 120 volt branch circuit a 3 percent drop costs 3.6 volts and nobody notices. On a 12 volt run the same percentage costs 0.36 volts, and an inverter that cuts off at 10.5 volts notices plenty.

The runs that need this math are common: a trolling motor on a boat, a solar panel feeding a battery, a diesel heater in a van, a fridge in an RV, a winch on a truck, or a run of LED strip lights across a basement ceiling. They all share one property, which is a low supply voltage and a load that pulls real current.

Two limits: ampacity and voltage drop

The first limit is ampacity, the current a conductor can carry for hours without damaging its insulation. The second is voltage drop, the energy lost in the conductor itself. Both come from the same resistance, and both must pass before a size is safe.

Ampacity comes from NEC 2023 Table 310.16, the National Electrical Code published by NFPA, and the NFPA ampacity workflow sheet reproduces the table and its correction factors. For a 12 AWG copper conductor with 75 C insulation, the table lists 25 A; for 10 AWG it lists 35 A; for 8 AWG, 50 A. The table assumes no more than three current-carrying conductors in a raceway or cable, an ambient temperature of 30 C, and a 75 C termination.

A second rule trims the small sizes. NEC 240.4(D) limits the overcurrent device on 14, 12 and 10 AWG copper branch circuits to 15, 20 and 30 A, with a few listed exceptions. A 12 AWG wire may be able to survive 25 A, but a 30 A breaker protecting it is a code violation in ordinary branch circuit work. That cap is why a household 20 A circuit uses 12 AWG rather than 14 AWG.

Voltage drop comes from an informational note under NEC 210.19(A), which suggests keeping the drop to 3 percent at the farthest outlet of a branch circuit and to 5 percent from the service to the load. NFPA prints those figures as notes rather than requirements, and NEC 90.5(C) says informational notes are not enforceable. Many jurisdictions, inspectors and equipment makers still treat 3 percent as the working target, and this page uses it as the default for that reason.

At 12 volts the two limits rarely compete. Ten amps through 12 AWG copper is 40 percent of its 25 A ampacity, so the wire stays cool, but at 20 feet the same run drops 0.772 V, which is 6.43 percent. The drop limit calls for a larger wire while the ampacity table is still comfortable. That is the pattern on almost every 12 volt run longer than a few feet.

The voltage drop formula for a DC run

For direct current the arithmetic is one line: VD = 2 x L x I x R / 1000, where VD is the drop in volts, L is the one-way length of the run in feet, I is the current in amps, and R is the conductor resistance in ohms per 1,000 feet. The result is the volts lost in the whole loop.

The factor of 2 is the part people forget. Current leaves the battery on one conductor and returns on the other, so it crosses the full length twice. A 20 foot run uses 40 feet of copper. A formula without the 2 will understate the drop by half, which is the same error as using half the run length.

A municipal voltage drop worksheet from Chino, California, runs the same check at 3 percent on a 120 volt circuit. Work the standard case. A 10 amp load sits 20 feet from a 12 volt battery, wired in 12 AWG copper at 1.93 ohms per 1,000 feet. The drop is 2 x 20 x 10 x 1.93 / 1000 = 0.772 V. As a share of 12 volts that is 0.772 / 12 = 0.0643, or 6.43 percent, well past the 3 percent target of 0.36 V.

Two assumptions sit inside those numbers. The resistance is the 75 C value from NEC Chapter 9 Table 8, which is the temperature a conductor reaches under load in a warm raceway. At 20 C the same 12 AWG conductor measures about 1.59 ohms per 1,000 ft, roughly 18 percent lower, so the 75 C figure is the conservative choice. And the formula ignores reactance, which is fair for DC because there is no alternating field to fight. On a 60 Hz AC circuit you would use the impedance column in Table 9 instead, and the answer would come out slightly worse.

The formula also gives you a quick ceiling for any size. Divide the allowed volts by the drop per foot and you have the longest run that size can serve: 0.36 V allowed, 10 amps, 12 AWG, and the answer is 0.36 x 1000 / (2 x 10 x 1.93), which lands at 9.3 feet. Section 7 turns that into a table.

Three worked runs, start to finish

Run 1: a 10 amp pump, 20 feet, 12 AWG in mind. The drop is 0.772 V, or 6.43 percent, so 12 AWG fails the 3 percent target even though its ampacity is fine. Step up to 10 AWG at 1.21 ohms per 1,000 feet: 2 x 20 x 10 x 1.21 / 1000 = 0.484 V, which is 4.03 percent. Still over. Step up again to 8 AWG at 0.764: 2 x 20 x 10 x 0.764 / 1000 = 0.306 V, which is 2.55 percent, and the run passes. The answer is 8 AWG copper.

Run 2: a 100 watt LED bar, 10 feet, on 12 volts. Watts become amps first: 100 / 12 = 8.33 A. In 14 AWG at 3.07 ohms per 1,000 feet the drop is 2 x 10 x 8.33 x 3.07 / 1000 = 0.511 V, or 4.26 percent, which fails. In 12 AWG the drop is 2 x 10 x 8.33 x 1.93 / 1000 = 0.322 V, or 2.68 percent, which passes, and the 20 A cap on a 12 AWG branch circuit covers the 8.33 A load with room. The answer is 12 AWG copper.

Run 3: a 30 amp inverter feed, 5 feet. Short runs look safe, and this one is close. In 10 AWG the drop is 2 x 5 x 30 x 1.21 / 1000 = 0.363 V, or 3.03 percent, a hair over the line, and the 30 A cap on 10 AWG leaves no margin for the inverter's start-up surge. In 8 AWG the drop is 2 x 5 x 30 x 0.764 / 1000 = 0.229 V, or 1.91 percent, with 50 A of ampacity behind it. The answer is 8 AWG copper.

The converter above runs the same steps for any input and returns the first size that passes both tests, along with the drop it produces and the next size up for comparison. When no listed size fits, it says so rather than printing a number that cannot meet the limit.

Ampacity: NEC 2023 Table 310.16

The table below lists copper and aluminum ampacities from NEC 2023 Table 310.16 for sizes used on 12 volt runs, with the 240.4(D) cap in the last column. A conductor sizing design guide from Seattle Public Utilities works the same table through real examples. Read across a row to see how much the insulation rating matters: 12 AWG jumps from 20 A at 60 C to 30 A at 90 C, a 50 percent gain from the same copper.

Two corrections can lower those numbers in a real install. Ampacity falls when the ambient temperature climbs above 30 C, and the correction factors in Table 310.15(B)(1) cut the figure for a conductor in a hot engine bay or on a roof. Ampacity also falls when more than three current-carrying conductors share a raceway, through the adjustment factors in Table 310.15(C)(1). A 12 volt bundle of six positive leads in one conduit is a common place to lose capacity that the plain table promises.

The temperature column also depends on the terminals, not only the wire. A 90 C conductor landed on a 75 C breaker or lug must use the 75 C column, per NEC 110.14(C). Most 12 volt gear, from busbars to fuse blocks, is rated at 75 C, so the 75 C column is the safe default even when the wire itself is THHN rated for 90 C.

Resistance per 1,000 feet, the number in the formula

The R in the drop formula is direct-current resistance at 75 C, tabulated in NEC Chapter 9 Table 8. It is the same table an electrician uses for AC work, because at direct current the resistance is the whole story: no skin effect worth counting, no reactance, no power factor.

Every three gauge steps cuts resistance by about half, which is why a run that fails at 12 AWG usually passes two sizes up. It also explains why the last step from 2 AWG to 1/0 AWG buys less than the first step from 14 to 12: the curve flattens as the copper piles up, and cost climbs faster than the drop falls.

Stranded wire carries the same tabulated resistance as solid wire of the same gauge, so a flexible 12 AWG boat cable matches the 1.93 figure. Tinned marine cable runs about 5 percent higher because tin is a worse conductor than copper, a difference small enough to ignore in the drop formula but easy to cover by choosing one size up on a long saltwater run.

How far 12 volts can go at 10 amps

A single table answers the question people ask most: how long a run can I make before the drop gets out of hand. Each figure is the longest one-way length that keeps a 10 amp load inside 3 percent of 12 volts, which is 0.36 V. The rule behind the table is the same formula solved for L.

Double the current and the run halves, because the drop is linear in amps. Double the voltage and the same wire reaches twice as far, because the allowed volts double while the drop stays the same. That is the argument for a 24 volt system on a long solar run: 500 W at 12 V pulls 41.7 A, while 500 W at 24 V pulls 20.8 A, so the copper can be two sizes smaller for the same percentage loss.

Read the table as a planning tool rather than a rule. A trolling motor that pulls 40 A for a minute at a time sits in a different category from a fridge that runs for hours, and a momentary surge can drop the volts far enough to reset a device without overheating anything. Give the important loads a size of headroom, and check the device manual, since makers of inverters, motors and radios often print their own minimum gauge and maximum run.

Copper or aluminum, and where 12 volt rules differ

Copper wins on conductivity, and on a 12 volt run the difference is large enough to matter. Aluminum at 8 AWG carries 1.26 ohms per 1,000 feet against 0.764 for copper, so an aluminum conductor needs roughly one size larger to match the drop and often two to match the ampacity. Aluminum also needs terminals rated for it and a joint compound on the strands, which is why it shows up on service entrance cable and rarely on a boat or an RV.

Building code is not the only rulebook. Boats follow ABYC E-11, which sets its own ampacity table and calls for tinned copper and 3 percent drop on critical circuits. Cars and trucks follow the OEM harness design, where a conductor in open air under the hood can carry more current than Table 310.16 allows because heat escapes freely; SAE J1127 and J1128 describe the wire itself. Solar arrays follow NEC Article 690, which adds requirements for the DC conductors on the roof side of the charge controller.

Several habits carry across all of them. Fuse for the wire, not the load: a 12 AWG run protected by a 20 A fuse is safe, and the same run behind a 50 A fuse is a heater waiting for a short. Keep the return conductor the same size as the supply, since the formula assumes equal resistance in both directions. Put the fuse or breaker at the battery end, where a short in the run is still protected. And torque the terminals, because a loose lug adds resistance at the one spot the formula does not model.

One more difference belongs here. Automotive and marine DC systems often run at 13.8 to 14.4 volts while the engine or charger is on, which buys a little drop headroom, and sag toward 12.0 V or lower when the battery is half empty. Size the run at the low end, not the charging voltage, or the load will brown out on the last hour of a trip.