Wire Size Calculator

A wire size calculator finds the smallest conductor that can carry a load without overheating and without losing too much voltage along the run. Under the 2023 National Electrical Code that means three checks: enough ampacity in Table 310.16, a breaker no larger than the small-conductor limits of 240.4(D) allow, and a voltage drop of about 3% or less. For a 30 A load at 240 V that is 100 feet away, the answer is 8 AWG copper, one size larger than the 10 AWG that ampacity alone would allow.

Four sister tools cover the rest of a circuit design. The Voltage Drop Calculator checks a gauge you have already chosen, the Watts to Amps Calculator turns an appliance rating into current, the Ohm's Law Calculator handles volts, amps, ohms and watts in general, and the Conduit Fill Calculator tells you which raceway the finished bundle fits in. Use this page when the question is which wire to buy.

How the calculator chooses a wire size

Every size from 14 AWG (12 AWG for aluminum) up to 1000 kcmil is tested against the same rules, and the smallest size that passes all of them is the answer:

L is the one-way length in feet, I the load current in amps and R the conductor resistance in ohms per 1,000 feet from NEC Chapter 9, Table 8. The 2 counts the current going out on one conductor and coming back on the other; on a balanced three-phase circuit the factor is the square root of 3. Ampacity is checked against the design current and voltage drop against the actual load current, because the 125% factor is a thermal margin, not extra current. The two checks respond to different things: ampacity depends only on the current, while voltage drop grows with every foot of wire. Short runs are therefore usually decided by ampacity and long runs by voltage drop.

Three worked examples

Example 1: a 30 A, 240 V load 100 feet away

This is the calculator's default: 30 A, 240 V single phase, 100 ft one way, copper, 75°C column and a 3% limit.

10 AWG passes the ampacity check and fails the 3% limit, so the calculator moves up to 8 AWG, which leaves 235.3 V at the load. The result card notes that 8 AWG copper could carry this load up to 154 ft before the drop reaches 3%, and the comparison card below it shows both sizes side by side. With NM-B cable the answer is the same, because the 60°C rating of 8 AWG (40 A) still covers 30 A: here voltage drop, not heat, sets the size.

Example 2: a 48 A EV charger 60 feet from the panel

Article 625 treats electric vehicle charging as a continuous load, so tick Continuous load and enter 48 A, 240 V and 60 ft:

8 AWG would drop only 1.87% on this run, but it cannot carry the 60 A design current, so ampacity decides the size: 6 AWG copper. Without the 125% factor the calculator would stop at 8 AWG, which is exactly the mistake the continuous-load rule exists to prevent. The breaker for the charger is sized at 125% as well, which makes it a 60 A device for a 48 A charger, and 6 AWG copper with 75°C terminals is rated 65 A, enough to sit behind it.

Example 3: a 100 A aluminum feeder to a detached garage

A subpanel in a garage 150 ft from the house is fed with aluminum conductors at 240 V and 75°C terminals, with a 3% target:

Over 150 feet, 1 AWG aluminum has the ampacity but drops 3.16%, so the 3% target calls for 1/0 AWG aluminum. If the garage's own branch circuits are short and you budget the feeder against the 5% combined figure instead, enter 5% and 1 AWG passes. In copper the same feeder takes 2 AWG (2.43%), since 3 AWG copper has the ampacity but drops 3.06%. A feeder also needs an equipment grounding conductor and usually a neutral, which this tool does not size.

NEC Table 310.16 ampacities and Chapter 9 resistance

These are the values built into the calculator, from 14 AWG to 500 kcmil (the calculator continues to 1000 kcmil):

Ampacities are from Table 310.16 of the 2023 NEC for not more than three current-carrying conductors in a raceway, cable or the earth at 30°C ambient; resistances are from Chapter 9, Table 8 for stranded conductors at 75°C. The asterisk on 14, 12 and 10 AWG points to 240.4(D): whatever the column says, 14 AWG copper may be protected at no more than 15 A, 12 AWG at 20 A and 10 AWG at 30 A, and 12 and 10 AWG aluminum at 15 and 25 A. Table 8 lists solid and stranded values for 14 through 8 AWG. The calculator uses stranded, which is slightly higher (1.98 against 1.93 ohms for 12 AWG), so it leans a little toward the larger size. Table 310.16 lists no 14 AWG aluminum.

What size wire do I need? Copper by load and distance

At 240 V

Each cell is the smallest copper size that passes both checks for a non-continuous single-phase load at 240 V, using the 75°C column and a 3% limit. The distance is one way, panel to load:

Read across a row and you can see where voltage drop takes over. A 40 A load needs 8 AWG up to 100 ft, 6 AWG at 150 ft and 3 AWG at 300 ft, so the same appliance can need three different wire sizes depending on where it sits. For a continuous load, look up 125% of the current: a 48 A charger belongs in the 60 A row.

At 120 V: distance matters twice as much

At 120 V the same current produces the same drop in volts but twice the percentage, so every run length that works at 240 V is cut in half. Copper, 75°C column, 3% limit:

A 15 A load that needs only 14 AWG for ampacity needs 8 AWG at 100 ft: 14 AWG would drop 7.85%, 12 AWG 4.95% and 10 AWG 3.10%. The next table turns the question around and gives the longest one-way run each copper size can serve at its full allowable current (75°C column, with the 240.4(D) limits) before the drop reaches 3%:

The run grows in proportion as the load falls, so 12 AWG serving 10 A instead of 20 A reaches about 91 ft at 120 V. Fully loaded, almost every size runs out of voltage-drop room somewhere between about 40 and 100 ft on 120 V, which is why long 120 V runs are so often upsized.

Copper or aluminum?

Aluminum has about 64% more resistance and a lower ampacity than copper of the same size, so it usually runs about two gauge numbers larger: 1 AWG aluminum against 3 AWG copper for 100 A. It is lighter and is the common choice for feeders and service conductors, where larger sizes are routine. Aluminum at 240 V, 75°C column, 3% limit:

Terminals and connectors must be identified for aluminum (NEC 110.14), and the connections need the torque the equipment label calls for. The calculator treats copper-clad aluminum like aluminum for ampacity, as Table 310.16 does; Table 8 lists no resistance for copper-clad aluminum, so the voltage drop uses the aluminum values.

Choosing the 60°C, 75°C or 90°C column

The column is not simply the insulation printed on the wire. NEC 110.14(C) ties it to the terminals at each end: equipment for circuits of 100 A or less, or marked for 14 through 1 AWG conductors, is treated as 60°C unless it is listed and marked for 75°C, and larger equipment is treated as 75°C. NM-B and UF cable are limited to the 60°C ampacity by 334.80 and 340.80, even though the conductors inside NM-B are rated 90°C. The 90°C column applies to THHN, THWN-2 and XHHW-2, but its main use is as the starting point when ampacity must be reduced for a hot attic or roof, or for more than three current-carrying conductors in one raceway. If you pick 90°C for an ordinary circuit, check that the size also carries the load in the 75°C column.

How to use this wire size calculator

1. Enter the load in amps, or switch to watts and the calculator converts at the selected voltage with a power factor of 1.0.
2. Choose the system voltage and phase. 120 V is always single phase; 208, 240 and 480 V can be single or three phase.
3. Enter the one-way length from the panel to the load along the route the wire will actually take.
4. Pick copper or aluminum and the ampacity column. Use 60°C for NM-B, UF and terminals without a 75°C marking, 75°C for THWN or THHN on 75°C terminals.
5. Set the voltage-drop limit. 3% is the default; the buttons switch to 2% or 5%.
6. Tick Continuous load for loads that run three hours or more, such as EV chargers and tank water heaters.
7. Read the answer. The orange card gives the minimum size, the check that decided it, the allowable current, the voltage drop and the voltage at the load. Below it you see the recommended size next to one size smaller, and a table of the sizes around the answer.

Who this calculator is for

• Homeowners and DIYers planning a permitted circuit for a shop, a hot tub, a well pump or an EV charger, who want to know what to buy before the electrician or inspector visits.
• Electricians and apprentices who want a quick second check of ampacity and voltage drop on a long run.
• Solar, generator and RV installers sizing feeders and inlets where distance is the main problem.
• Estimators pricing a takeoff, where one wire size up across a long feeder changes the material cost noticeably.
• Students working through Article 310 and Chapter 9 problems.

Key terms

• AWG: American Wire Gauge. Smaller numbers are thicker wires; after 1 AWG come 1/0 to 4/0.
• kcmil: thousands of circular mils, the size name for conductors larger than 4/0.
• Ampacity: the current a conductor can carry continuously without exceeding its temperature rating, from Table 310.16.
• Overcurrent protective device: the breaker or fuse. 240.4(D) caps it at 15, 20 and 30 A for 14, 12 and 10 AWG copper.
• Continuous load: a load expected to run at its maximum current for three hours or more; the conductor is sized at 125% of it.
• Voltage drop: the voltage lost in the conductors between the source and the load, equal to current times the round-trip resistance.
• Termination rating: the temperature rating of the lugs and terminals the wire lands on, which can limit the usable ampacity column.
• Current-carrying conductor: a conductor that carries load current; grounding conductors do not count, and more than three in one raceway require derating.

What changes the answer

• Distance: doubling the run doubles the drop. A 30 A, 240 V load needs 10 AWG at 50 ft, 8 AWG at 100 ft and 4 AWG at 300 ft.
• Voltage: 7,200 W at 120 V draws 60 A and needs 3 AWG over 100 ft; at 240 V it draws 30 A and needs 8 AWG.
• Continuous duty: the 125% factor moves a 48 A charger from 8 AWG to 6 AWG.
• Column: a 50 A load is 8 AWG at 75°C but 6 AWG at 60°C.
• Material: aluminum runs about two gauge numbers larger than copper, 1 AWG against 3 AWG at 100 A.
• Phase: three phase uses 1.732 instead of 2, so the drop is about 13% lower: 2.25% against 2.59% for 40 A on 8 AWG over 200 ft at 480 V.
• Heat and bundling: hot locations and more than three current-carrying conductors in a raceway reduce ampacity under 310.15. The calculator does not apply those factors.

Practical tips

• Measure the run along the real route, up walls and around obstacles, and add the vertical drops at both ends.
• Use the nameplate current of motors, compressors and welders rather than watts divided by volts.
• When a run sits near the edge of a voltage-drop limit, the next size up costs little extra and leaves room for a future load.
• Check the conduit size for the final bundle with the conduit fill tool before buying pipe.
• Remember the equipment grounding conductor and, on most 240 V feeders, the neutral; neither is sized here.

Limitations and code edition

The calculator uses the 2023 edition of NFPA 70, the National Electrical Code. The 2026 edition has been published, and some states already enforce it: Texas adopted it as the state electrical code effective September 1, 2026. Other jurisdictions are still on the 2023 or 2020 edition, often with local amendments, so check which edition and amendments apply where you work.

Ampacities assume not more than three current-carrying conductors at 30°C ambient, without the temperature correction and adjustment factors of 310.15. Voltage drop uses DC resistance at 75°C and ignores reactance and power factor, which matter on large conductors and motor loads. Motor and air-conditioning circuits, tap rules, the 83% dwelling service rule of 310.12, conduit fill, grounding conductors and parallel conductor sets are outside its scope. The result is a planning estimate: local code and a licensed electrician decide the final size, and permitted work must pass inspection.

Related calculators

• To check the drop of a gauge you have already chosen, use the Voltage Drop Calculator.
• To turn an appliance rating in watts into amps for AC or DC, use the Watts to Amps Calculator.
• For volts, amps, ohms and watts in any combination, use the Ohm's Law Calculator.
• To find the raceway the conductors fit in, use the Conduit Fill Calculator.
• For transformer and generator ratings in kVA, use the kVA to Amps Calculator.
• To see what the circuit costs to run, use the Electricity Cost Calculator.

Sources

• National Fire Protection Association, NFPA 70, National Electrical Code, 2023 edition: Table 310.16 (ampacities), 240.4(D) (small-conductor overcurrent protection), 110.14 and 110.14(C) (terminals and termination temperature), 310.15(B) and (C)(1) (correction and adjustment), 310.12 (dwelling services), 334.80 and 340.80 (NM and UF cable), 422.13 (water heaters), Article 625 (EV charging), the voltage-drop informational notes to 210.19 and 215.2, and Chapter 9, Table 8 (conductor properties).
• Texas Department of Licensing and Regulation, Commission Adopts Rules: the 2026 NEC adopted as the state electrical code, effective September 1, 2026.