Wire Gauge Calculator

This wire gauge calculator finds the smallest AWG wire that can safely carry your load and still deliver enough voltage at the far end. Enter the current in amps, or the load in watts plus the circuit voltage, then the one-way length of the run, copper or aluminum, and the maximum voltage drop you will accept.

Two checks decide the answer. The wire must be rated for the current under NEC Table 310.16, and the voltage lost along the run must stay within your limit, 3% by default. The calculator shows which check set the size, the resulting voltage drop, and how nearby sizes compare, so you can see why a long run needs heavier wire.

How to Use the Wire Gauge Calculator

1
Enter the load
Use the "I know the amps" tab to type the load current, or switch to "I know the watts" and enter the wattage. Watts are divided by the circuit voltage to get amps.
2
Set voltage and length
Choose the circuit voltage (12, 24 or 48 V DC, 120, 208, 240 or 277 V, or another value) and type the one-way distance from the panel or battery to the load in feet or meters.
3
Pick the wire and limits
Choose copper or aluminum, the 60°C or 75°C ampacity column, and the maximum voltage drop. Turn on the continuous-load switch for loads that run three hours or more.
4
Read the result
The result shows the minimum AWG size, why it was chosen, the allowed ampacity, the voltage drop in volts and percent, and a short table of neighboring sizes.

How the wire size is chosen

The calculator walks up the AWG sizes from 14 AWG to 4/0 AWG and stops at the first one that passes both tests. Ampacity comes from NEC Table 310.16 for no more than three current-carrying conductors at a 30°C (86°F) ambient. Small conductors are also capped by NEC 240.4(D): 15 A for 14 AWG copper, 20 A for 12 AWG and 30 A for 10 AWG, and 15 A and 25 A for 12 and 10 AWG aluminum.

Voltage drop uses the DC resistance of stranded conductors at 75°C from NEC Chapter 9, Table 8, and assumes a two-wire circuit (single-phase or DC), so current flows out and back over twice the one-way length.

Voltage drop (V) = 2 x one-way length (ft) x current (A) x resistance (ohms per 1,000 ft) / 1,000; drop % = drop / circuit voltage x 100

Resistance used, copper: 14 AWG 3.14, 12 AWG 1.98, 10 AWG 1.24, 8 AWG 0.778, 6 AWG 0.491 ohms per 1,000 ft
Resistance used, aluminum: 12 AWG 3.25, 10 AWG 2.04, 8 AWG 1.28, 6 AWG 0.808 ohms per 1,000 ft
Continuous loads (three hours or more) are checked against 125% of the load current, as NEC 210.19(A) requires for branch circuits
Watts are converted with amps = watts / volts, which assumes a power factor of 1 (heaters, incandescent lights)

Worked example: a 16 A load 100 feet away

Say you are running a 120 V circuit to a detached shop 100 feet from the panel for a 16 A load, using copper NM-B cable (60°C column) and a 3% drop limit. By ampacity alone, 12 AWG (20 A) is enough. Voltage drop tells a different story.

12 AWG: 2 x 100 x 16 x 1.98 / 1,000 = 6.34 V, or 5.28% of 120 V. Too much drop.
10 AWG: 2 x 100 x 16 x 1.24 / 1,000 = 3.97 V, or 3.31%. Still over 3%.
8 AWG: 2 x 100 x 16 x 0.778 / 1,000 = 2.49 V, or 2.07%. Passes both checks.
Result: 8 AWG copper, chosen because of voltage drop, not ampacity.
A second case: a 40 A continuous EV charger at 240 V, 100 ft of copper. At 125% the wire must carry 50 A. In the 60°C column that is 6 AWG (55 A) with a 1.64% drop. If the wire and both terminals are rated 75°C, 8 AWG (50 A) with a 2.59% drop also works.

Maximum run length at a 3% voltage drop (copper)

This table shows the longest one-way run each copper size can handle at full load before the drop passes 3%, using the same resistance values as the calculator. Past these distances, go up one size. A dash means the wire is not allowed to carry that current.

Wire size	15 A at 120 V	20 A at 120 V	30 A at 240 V
14 AWG	38 ft	-	-
12 AWG	60 ft	45 ft	-
10 AWG	96 ft	72 ft	96 ft
8 AWG	154 ft	115 ft	154 ft
6 AWG	244 ft	183 ft	244 ft

60°C or 75°C: which ampacity column applies

Wire insulation has a temperature rating, but the whole circuit is only as good as its weakest part. NM-B (Romex-type) cable is limited to the 60°C column by the NEC even though its conductors are rated 90°C. For circuits of 100 A or less, terminations on breakers and devices are also treated as 60°C unless they are marked for 75°C.

Use the 75°C column only when the wire type (for example THHN in conduit) and every termination are rated for it. When in doubt, the 60°C column is the safer choice and is the default here.

Tips and common mistakes

Enter the one-way distance, not the round trip. The formula already doubles it.
Measure the actual cable path, including runs up walls and across ceilings, not the straight-line distance.
The breaker protects the wire. Never put a larger breaker on a wire just because the load is small.
Hot attics, conduits with more than three current-carrying wires and buried runs can require derating that this tool does not apply.
Motors draw a surge at startup. Size motor circuits from the nameplate and NEC Article 430, not from running watts.
For 12 V and 24 V systems in boats and vehicles, the drop limit matters more than ampacity, and marine and automotive wiring follows its own standards.

Safety and code note

This calculator gives a planning estimate. It does not replace the National Electrical Code as adopted in your area, your local inspector, or a licensed electrician. Many jurisdictions require a permit for new circuits, and some work must be done by a licensed professional. Always turn off power and verify it is off before touching any wiring.

Features

Checks NEC Table 310.16 ampacity (60°C or 75°C column) and voltage drop together
Applies the 15 A / 20 A / 30 A limits for 14, 12 and 10 AWG copper
Enter amps directly or watts plus voltage, with length in feet or meters
Copper or aluminum, with an optional 125% continuous-load setting
Explains whether ampacity or voltage drop set the size and shows the drop for nearby sizes

Frequently Asked Questions

What size wire do I need for 20 amps?

For a 20 A circuit, 12 AWG copper is the minimum because NEC 240.4(D) limits 12 AWG copper to 20 A. At a full 20 A load on 120 V, 12 AWG stays within a 3% voltage drop up to about 45 feet one way. For longer runs, use 10 AWG (about 72 feet) or 8 AWG (about 115 feet).

What size wire do I need for 30 amps?

For 30 A, 10 AWG copper is the minimum, since NEC 240.4(D) caps 10 AWG copper at 30 A. At 240 V and a full 30 A load, 10 AWG keeps the drop within 3% up to about 96 feet one way. In aluminum, 8 AWG is the smallest size rated for 30 A in the 60°C column (35 A).

How much voltage drop is acceptable?

An informational note in the NEC recommends no more than 3% drop on a branch circuit and 5% total for feeder plus branch circuit. It is a recommendation for good performance rather than a strict rule in most places, although some energy codes and equipment makers require it. This calculator uses 3% by default, and you can change it.

Does wire length change the wire gauge I need?

Yes. Resistance adds up with length, so a longer run loses more voltage. A 16 A load at 120 V only needs 12 AWG copper for ampacity, but at 100 feet 12 AWG drops 5.28%. To stay under 3% you need 8 AWG, which drops 2.07%. Heavier wire lowers the drop.

Can I use aluminum wire instead of copper?

Aluminum conducts less well than copper, so you generally need one or two sizes larger for the same current and drop. For example, 8 AWG copper and 6 AWG aluminum are both rated 40 A in the 60°C column of Table 310.16. Aluminum terminations must be rated for it (marked AL or CU-AL), and small aluminum branch circuits deserve a licensed electrician's review.

Is this wire gauge calculator a substitute for an electrician?

No. It checks ampacity and voltage drop, which covers the two most common sizing questions, but it does not apply derating for heat or bundling, motor rules, conduit fill or local amendments. Use the result to plan and price materials, then confirm the design with your local code, inspector or a licensed electrician before installing.

Last reviewed September 21, 2026