Wire Size Calculator

Wire Size Calculator returns the smallest copper or aluminum conductor that passes the voltage drop limit and NEC Table 310.16 ampacity on single-phase, three-phase or DC circuits.

How the Wire Size Calculator Picks a Conductor

The Wire Size Calculator runs two separate tests on every circuit. The first finds the conductor area that keeps voltage drop inside your limit over the one-way distance. The second finds the smallest size whose NEC Table 310.16 ampacity covers the load.

The larger of the two answers becomes the result, shown as an AWG or kcmil size in copper or aluminum. The line under it names the test that set the size, so you can see whether distance or current is driving the wire.

Choosing the correct wire size comes down to those two numbers. A voltage drop formula on its own can return a wire that holds voltage on a short run but is too small for the current, and the ampacity test catches that case.

Distance is the one-way length from the panel or battery to the load. The formula already doubles it for the return wire on single-phase and DC circuits, so entering the round trip would count the run twice.

Load can be typed in amps, kilowatts or watts. To work it as a wire size calculator in kW, switch the Load unit to kW and the tool converts to amps from the voltage, the phase and the power factor field that opens under Ampacity and Temperature.

Wire Size Formula for Voltage Drop

Voltage drop sizing uses the circular mil formula that electricians work by hand. For single-phase AC and DC circuits, the required conductor area is found with the equation below.

$$A=\frac{2\times K\times I\times L}{V_{drop}}$$

Three-phase circuits replace the 2 with 1.732, because the current returns on the other phase conductors and not on a dedicated return wire, and the voltage is entered line to line.

$$A=\frac{1.732\times K\times I\times L}{V_{drop}}$$

A is the area in circular mils, I is the load current in amps, L is the one-way distance in feet and V drop is the allowed loss in volts. K is the resistance of one circular mil-foot of conductor, which IAEI Magazine gives as 12.9 for copper and 21.2 for aluminum at 75°C from NEC Chapter 9 Table 8.

Take the circuit the Wire Size Calculator loads with, a 30 A load on 120 V single-phase, 100 ft from the panel, with a 3% limit. Three percent of 120 V is 3.6 V of allowed drop.

$$A=\frac{2\times 12.9\times 30\times 100}{3.6}=21500\text{ cmil}$$

That area equals 10.89 mm². The first standard size at or above it is 6 AWG at 26,240 circular mils, which drops 2.46% and leaves 117.05 V at the load. One size down, 8 AWG would drop 3.91% and miss the limit.

K rises with conductor temperature, so the Wire Temperature for Drop field matters on long runs. Copper works out to about 11.0 at 30°C, 12.9 at 75°C and 13.5 at 90°C. Leave it at 75°C to match a hand calculation, or lower it for a lightly loaded wire that runs cool.

The area card shows the mm² the formula calls for, with the matching bare wire diameter in millimeters and inches. Anyone after a wire size calculator in inches can read the diameter there, such as 3.724 mm (0.147 in) for the circuit above.

When the math lands on a gauge that Table 310.16 does not list, such as 7 or 9 AWG, a Gauge by Formula Alone row shows it. A 16 A load at 100 ft on 120 V works out to 9 AWG by formula and comes back as 8 AWG.

To test a wire you already have, the Voltage Drop Calculator runs the same math from a known size. The Wire Resistance Calculator and Resistivity Calculator cover the resistance behind the K value.

Ampacity, Breaker and Terminal Limits

Ampacity is the current a conductor can carry continuously without overheating its insulation. The Wire Size Calculator reads it from NEC Table 310.16, which covers up to three current-carrying conductors in a raceway, cable or the earth at 86°F (30°C).

A continuous load, one that runs for three hours or more, is sized at 125% of its current. The 30 A example therefore needs 37.5 A of ampacity and a 40 A breaker, the next standard rating above that figure.

The drop itself is still figured on the actual 30 A and not on the 125% figure. That matches the practice electricians describe on the Mike Holt forum, where the extra 25% applies to ampacity only.

Terminals often limit a wire before its insulation does. On Auto, the Terminal Rating follows NEC 110.14(C) and uses the 60°C column for circuits of 100 A or less and the 75°C column above that, a rule EC&M walks through in detail.

That is why 6 AWG THHN shows 55 A of usable ampacity in the example and not its 75 A insulation rating. If the breaker and equipment are marked for 75°C, select 75°C Terminals and the same wire reads 65 A.

Hot spaces and crowded conduits lower ampacity further. THHN in a 113°F attic is corrected by 0.87, and four to six current-carrying conductors in one raceway take another 0.80. Together they bring 6 AWG THHN from 75 A down to 52.2 A.

The breaker row respects the small conductor rule, which caps 14 AWG copper at 15 A, 12 AWG at 20 A and 10 AWG at 30 A. Motors and air conditioners follow their own NEC articles, so size those from the minimum circuit ampacity on the nameplate.

Breaker for This Load lists the next standard US rating. For a wire size calculator MCB check outside North America, take the Ampacity needed line from the step table and choose the next MCB rating at or above it, such as 16 A, 20 A or 32 A.

When voltage drop pushes the wire up a size or two, the breaker does not change. The 16 A load at 100 ft needs 8 AWG for drop, yet it stays on a 20 A breaker. Wire that large may not fit a receptacle terminal, so check the device's wire range or plan a short pigtail.

Larger conductors also need more room. Run the Box Fill Calculator before closing up a device box, and the Pull Box Sizing Calculator once the run reaches 4 AWG or larger. The Wire Ampacity Calculator looks up the ampacity of a size you already know.

How Far Each Wire Size Can Run

The Max One-Way Run card turns the question around and gives the longest distance the chosen size can feed at your drop limit. In the 30 A example, 6 AWG reaches 122 ft, and the Next Size Up row shows 4 AWG reaching 194 ft.

Power Lost in the Wire is the heat the run gives off at full load, 88.5 W in that example. It is the number to weigh against the price of a larger conductor on loads that run for hours every day.

The table below gives the longest one-way run at a 3% drop for common copper circuits, with the load equal to the breaker rating and the wire at 75°C.

Past those lengths the wire still carries the current, but the voltage at the load sags. The NEC lists 3% on a branch circuit and 5% overall in informational notes, which makes it a recommendation and not a rule, as Mike Holt explains in EC&M.

Open Wire Size Chart for This Circuit to see every size from 14 AWG to 2000 kcmil with its drop, ampacity, longest run and a pass or fail result. It works as a wire size chart built for your own load and distance. The step-by-step panel above it lists each number the result came from.

Wire Size Calculator for AC and DC Circuits

The Electrical System menu sets the tool up as a wire size calculator for AC single-phase or three-phase loads, and the DC option covers battery and solar circuits. Used as a wire size calculator for DC, it keeps the single-phase formula and only removes the power factor field.

Low voltage is where distance hurts most. A 20 A load on a 12 V battery system only 15 ft away needs 6 AWG to stay within 3%. That is the same size as the 120 V example at 100 ft, because 3% of 12 V is just 0.36 V.

For vehicle, solar or equipment wiring thinner than 14 AWG, set Smallest Size to Use to 18 AWG. Building wire stays at the NEC minimum of 14 AWG in copper, which is the default setting.

On three-phase, enter the line-to-line voltage. A 50 kW load at 480 V and 0.85 power factor draws 70.8 A, and 300 ft away the result is 2 AWG copper. Ampacity sets that size, since voltage drop alone would allow 4 AWG.

Aluminum needs more area for the same job. An 80 A continuous load on 240 V at 150 ft comes back as 1 AWG in copper and 1/0 AWG in aluminum. Loads too large for one 2000 kcmil conductor return as parallel sets of 1/0 AWG or larger.

On AC conductors of 1/0 AWG and larger, the result adds a note that reactance raises the drop beyond what a resistance formula shows. Solar and generator runs that push power back toward the panel can be checked from the other end with the Voltage Rise Calculator.

Southwire publishes a Voltage Drop Calc app for phones that also limits wire size by voltage drop and NEC ampacity, in AWG and kcmil sizes only. People searching for a wire size calculator app or the Southwire one want that same pair of checks. This one runs in a phone browser and adds the metric size and run length cards.

Metric Sizes: 1.5, 2.5 and 4 mm² Wire

Every result includes the nearest metric size up. To set it up as a wire size calculator India users can work with, enter 230 V, switch the distance to meters and the load to kW, then read the mm² line under the result.

Metric cable is rated by its maker and not by Table 310.16. The figures below come from the Finolex FR data sheet for single-core copper wire, two wires on single phase, with the load worked out at 230 V from the conduit rating.

What gauge is 2.5 mm wire?

A 2.5 mm² wire sits between 14 AWG at 2.08 mm² and 12 AWG at 3.31 mm², closest to the rarely used 13 AWG. Its copper is about 1.78 mm across. If 2.5 mm is the diameter and not the area, that is close to 10 AWG at 2.59 mm.

A 1.5 mm² wire is about 1.38 mm across and falls between 16 AWG and 14 AWG. The AWG to mm² Calculator converts any size in either direction when a drawing mixes the two systems.

How much load can a 2.5 mm cable carry?

By the Finolex figures, a 2.5 mm² copper wire carries 18 A in conduit and 22 A clipped in the open. At 230 V that is about 4.1 kW in conduit. A 1.5 mm² wire carries 13 A in conduit, or about 3.0 kW.

That gap is the difference between the two sizes in practice. A 1.5 mm² wire usually goes on lighting and fan circuits, and a 2.5 mm² wire is used for socket outlets and single appliances such as a room air conditioner or a small water heater.

Can 1.5 mm cable take 16A or 20A?

In conduit it cannot. The 13 A rating is below both, so a 1.5 mm² wire belongs on a 6 A or 10 A MCB and covers a 13 A load only at its limit. Clipped in open air it is rated 16 A with no margin left.

A 2.5 mm² cable is fine for 16 A, with 18 A available in conduit. A 20 A circuit needs 4 mm² when the wires run in conduit, because 2.5 mm² stops at 18 A there.

Can a 2.5 mm cable take 32A?

Not as a single run, since its 22 A open-air rating is well short of 32 A. On the same data sheet, 6 mm² clipped in the open at 37 A is the first size that covers 32 A. In conduit 6 mm² is rated 31 A, so the next size up is needed there.

Can you mix 1.5 mm and 2.5 mm cable?

The two can share a circuit only if the breaker protects the smaller wire. A run that is part 2.5 mm² and part 1.5 mm² has to be treated as 1.5 mm² throughout, so it stays on a 10 A MCB. The NEC small conductor rule works the same way, and a circuit with any 14 AWG copper in it stays on a 15 A breaker.

AC Units, Geysers and House Wiring

Can 1.5 ton AC run on 2.5 mm wire?

A 1.5 ton air conditioner moves 18,000 BTU per hour. At an EER of 10 it draws 1,800 W, which is 7.8 A at 230 V. A 2.5 mm² wire rated 18 A in conduit covers that with room to spare, so 2.5 sq mm wire is good for an AC of this size.

Entered in the Wire Size Calculator as 1.8 kW at 230 V over 20 m, the result is 14 AWG with 2.5 mm² as the metric size and a 1.40% drop. Check the nameplate current before buying wire, because an older or less efficient unit draws more.

Can we run 2 AC on 4mm wire?

Two such units together draw about 15.7 A, inside the 24 A conduit rating of 4 mm² wire. For 3.6 kW at 230 V over 25 m the Wire Size Calculator gives 12 AWG, or 4 mm² in metric, with a 2.20% drop.

Sharing one wire also means sharing one breaker, so a fault on either unit shuts off both. Follow the installation manual if it calls for a dedicated circuit for each unit.

Can I use a 2.5 mm cable for a geyser?

A 2 kW geyser draws 8.7 A at 230 V and a 3 kW unit draws 13.0 A. Both sit inside the 18 A conduit rating of 2.5 mm² wire, so the cable can carry either one.

The Wire Size Calculator is stricter on the 3 kW unit. It sizes a continuous load at 125% and reads the 60°C column, which returns 12 AWG and 4 mm² over a 15 m run. Switch Load Type to Non-Continuous for a heater that runs in short cycles and the result drops to 14 AWG, or 2.5 mm².

How much wire for a 1000 sq ft house?

Wire length depends on the layout and not just the floor area, so it is taken off circuit by circuit. Measure each run from the panel to its last outlet along the route the conduit or cable will follow. That one-way length is also the distance the Wire Size Calculator asks for.

In conduit with single wires, multiply the run by the number of conductors. A 60 m run with phase, neutral and earth needs 180 m of wire. Finolex supplies 2.5 mm² wire in 90 m coils, so that run takes two coils, and the price of 60 meters of 2.5 mm wire is the dealer's per-meter rate times 60.

Add at least 6 in. (150 mm) of free conductor at every box for connections. If the conduit has to step around beams or other pipes, the Offset Bend Calculator and Saddle Bend Calculator give the bend marks for the route.