Enter the load in amps or watts, the distance and the metal. You get the AWG gauge, the breaker size and the voltage drop, from code ampacity tables.
How it is calculated
The calculator works in five steps. The ampacity values come from Table E3705.1 of the 2021 International Residential Code, published by the International Code Council, which reproduces the National Electrical Code table of allowable current for copper and aluminum conductors.
- Load current. If you enter amps, that is the current. If you enter watts, the tool divides by the voltage for a single-phase circuit, or by the voltage times 1.732 for a three-phase circuit, and assumes a power factor of 1.
- Required current. A continuous load is one expected to run at its maximum current for three hours or more. The code asks for 125 percent of a continuous load, so the tool multiplies by 1.25 when you answer yes.
- Breaker size. The tool picks the smallest standard rating that covers the required current: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350 or 400 amps.
- Wire gauge. It picks the smallest conductor whose table ampacity, in the temperature column you choose, is at least the breaker rating. Small conductors also have a ceiling on the breaker that may protect them: copper 14 AWG 15 A, 12 AWG 20 A and 10 AWG 30 A, and aluminum 12 AWG 15 A, 10 AWG 25 A and 8 AWG 30 A. The 60 °C column is the default for circuits of 100 amps or less unless the equipment is marked for a higher rating.
- Voltage drop. The tool computes the drop over the distance you enter and moves up to a larger wire until it is under your limit.
| Size | Copper 60 °C | Copper 75 °C | Aluminum 60 °C | Aluminum 75 °C | Circular mils |
|---|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | not listed | not listed | 4,110 |
| 12 AWG | 20 A | 25 A | 15 A | 20 A | 6,530 |
| 10 AWG | 30 A | 35 A | 25 A | 30 A | 10,380 |
| 8 AWG | 40 A | 50 A | 35 A | 40 A | 16,510 |
| 6 AWG | 55 A | 65 A | 40 A | 50 A | 26,240 |
| 4 AWG | 70 A | 85 A | 55 A | 65 A | 41,740 |
| 3 AWG | 85 A | 100 A | 65 A | 75 A | 52,620 |
| 2 AWG | 95 A | 115 A | 75 A | 90 A | 66,360 |
| 1 AWG | 110 A | 130 A | 85 A | 100 A | 83,690 |
| 1/0 AWG | 125 A | 150 A | 100 A | 120 A | 105,600 |
| 2/0 AWG | 145 A | 175 A | 115 A | 135 A | 133,100 |
| 3/0 AWG | 165 A | 200 A | 130 A | 155 A | 167,800 |
| 4/0 AWG | 195 A | 230 A | 150 A | 180 A | 211,600 |
The voltage-drop formula is the one published by IAEI Magazine, and the circular-mil values come from a Michigan State University conductor tech note. The tool also carries copper sizes from 250 to 500 kcmil for large circuits. Drop is shown as a percentage of the circuit voltage. The article calls 5 percent the maximum generally suggested, and it notes that the local authority may set its own limit, such as 3 percent, so the tool defaults to 3 percent and lets you choose 5 percent or no limit.
Required current = current × 1.25 if the load runs 3 hours or more
Voltage drop (V) = 2 × K × I × L ÷ CM (single-phase), or 1.732 × K × I × L ÷ CM (three-phase)
K = 12.9 for copper and 21.2 for aluminum at 75 °C. I = amps, L = one-way distance in feet, CM = circular mils of the wire.
Voltage drop (%) = voltage drop ÷ circuit voltage × 100
Worked examples
A 240 V, 30 A load, 100 ft from the panel, copper
Breaker: 30 A. In the 60 °C column 10 AWG carries 30 A, so ampacity alone allows 10 AWG.
Voltage drop with 10 AWG (10,380 CM): 2 × 12.9 × 30 × 100 ÷ 10,380 = 7.5 V, which is 3.1 percent of 240 V and just over a 3 percent limit.
With 8 AWG (16,510 CM) the drop is 2 × 12.9 × 30 × 100 ÷ 16,510 = 4.7 V, or 1.95 percent. Result: 8 AWG copper with a 30 A breaker.
A 32 A load that runs for hours, 10 ft from the panel
Continuous, so 32 × 1.25 = 40 A required. The breaker is 40 A.
Copper 8 AWG carries 40 A in the 60 °C column. Drop = 2 × 12.9 × 32 × 10 ÷ 16,510 = 0.5 V, about 0.2 percent. Result: 8 AWG copper with a 40 A breaker.
Copper, aluminum and temperature columns
Aluminum needs a larger gauge than copper for the same current. A 30 A breaker takes 10 AWG copper but 8 AWG aluminum, and aluminum also has a higher voltage-drop constant. The two temperature columns show what the wire may carry when the connected equipment is rated for 60 °C or 75 °C terminals. If you are unsure, leave the 60 °C default, which is the more conservative result. The tool does not apply the code’s correction factors for hot locations or for many cables bundled together, so real installations can need a larger wire than shown.
Safety and limits
Frequently asked questions
What wire size do I need for a 30 amp breaker?
In the 60 °C column, 10 AWG copper carries 30 amps, and 8 AWG aluminum carries 35 amps within its 30 amp breaker limit. A long run can need a larger wire to keep voltage drop down: at 240 V and 100 ft the tool moves 30 amps up to 8 AWG copper.
Why does a longer run need thicker wire?
Resistance grows with length, so more voltage is lost along the wire. The tool computes the drop as a percentage of the circuit voltage and picks a larger gauge when it exceeds your limit. For 15 A at 120 V over 100 ft, 14 AWG would lose almost 8 percent, so the tool asks for 8 AWG at a 3 percent limit.
What is a continuous load?
A load whose maximum current is expected to continue for three hours or more. The code asks for the circuit to be sized at 125 percent of it. Whether a given dwelling load counts as continuous is often left to the local authority, so ask your inspector if you are unsure.
Can 14 AWG wire be used on a 20 amp circuit?
No. Even in the 75 °C column 14 AWG copper lists 20 amps, but the small-conductor rule limits it to a 15 amp breaker. The tool therefore moves up to 12 AWG for a 20 amp breaker.
Can I use this to change my own wiring or breaker?
No. The tool estimates sizes for planning and for talking to an electrician. Wiring and panel work must be done or checked by a licensed electrician following your local code.
Related calculators
Sources
Every figure on this page comes from the sources below. They were opened and checked while building the tool.
- International Code Council (2021): International Residential Code, Chapter 37, Branch Circuit and Feeder Requirements (Tables E3705.1 and E3705.5.3)
- IAEI Magazine (2012 to 2019): Voltage Drop Formulas; Voltage Drop Calculations; Focus on the Code: continuous loads (2012)
- Michigan State University (2009): Biosystems and Agricultural Engineering Electrical Tech Note 212, Conductor Properties