Wire gauge for amperage and run length.
Wire gauge for amperage and run length: Find the correct AWG wire gauge given amperage, run length, voltage, and allowable voltage drop.
Pick the minimum AWG that handles a given current and stays within an allowable voltage drop over the run. Uses NEC ampacity tables and standard voltage-drop math.
How this works
Two separate constraints both have to be satisfied:
- Ampacity. The wire's NEC-rated ampacity at the chosen insulation temperature must be at least the load current. This guards against overheating and is governed by NEC §310.16.
- Voltage drop. The voltage lost to the wire's resistance must stay below the allowable percentage of the supply voltage. Long runs and high currents push voltage drop up, often forcing a wire size larger than ampacity alone would require.
The voltage-drop formula
For a single-phase AC circuit or DC:
Vdrop = 2 × I × Rper ft × L
where I is current in amps, L is one-way distance in feet, and Rper ft is the conductor's resistance per foot. The factor of 2 accounts for the return path — every circuit needs a wire going out and coming back.
For 3-phase AC (208 V, 480 V), the formula uses √3 instead of 2 because of how the phases interact. This calculator handles that automatically.
Why "smaller AWG number = bigger wire"
AWG is a logarithmic gauge: each step up in number corresponds to a fixed ratio decrease in diameter (roughly 0.890× per step). So 4 AWG is bigger than 12 AWG, and 4/0 AWG is huge. This trips up almost everyone the first time. See the AWG cheat sheet for the full reference.
When to size up beyond this calculator's result
- Bundled conductors. Wires in conduit with more than 3 current-carrying conductors derate per NEC Table 310.15(B)(3)(a). Multiply ampacity by the derating factor before sizing.
- High ambient temperature. Above 30 °C (86 °F), ampacity derates per NEC Table 310.15(B)(2)(a). In an attic in Phoenix, this matters a lot.
- Continuous loads. NEC requires sizing for 125% of a load that runs more than 3 hours continuously. The result here doesn't include that — multiply your amperage input by 1.25 first if relevant.
- Motor circuits. Motors have inrush currents and different sizing rules per NEC Article 430. This calculator is for general branch and feeder circuits, not dedicated motor circuits.
- Aluminum needs slightly different terminations. Many devices are rated for "CU only" or "CU/AL"; check before substituting.
NEC continuous load and derating
Published ampacities are a starting point. Two adjustments from the National Electrical Code (NEC) routinely lower how much current a conductor may actually carry: the continuous-load rule and temperature/bundling derating. Apply them before trusting a raw ampacity number.
The 80% continuous-load rule
A continuous load is one expected to run for 3 hours or more (EV chargers, baseboard heat, lighting). The circuit and its breaker must be rated for at least 125% of the continuous load — equivalently, the load may not exceed 80% of the breaker rating.
| Breaker | Max continuous current (80%) | Max continuous watts @ 120 V |
|---|---|---|
| 15 A | 12 A | 1,440 W |
| 20 A | 16 A | 1,920 W |
| 30 A | 24 A | 2,880 W |
| 40 A | 32 A | 3,840 W |
| 50 A | 40 A | 4,800 W |
Ambient temperature correction
Ampacity tables assume a 30°C (86°F) ambient. In hot attics, rooftops, or near heat-producing equipment, multiply the conductor's ampacity by the correction factor below (75°C insulation column, NEC Table 310.15(B)(1)). A conductor in a 46–50°C attic keeps only 75% of its rated ampacity.
| Ambient temp | Correction factor (75°C) |
|---|---|
| 21–25°C (70–77°F) | 1.05 |
| 26–30°C (79–86°F) | 1.00 |
| 31–35°C (88–95°F) | 0.94 |
| 36–40°C (97–104°F) | 0.88 |
| 41–45°C (106–113°F) | 0.82 |
| 46–50°C (115–122°F) | 0.75 |
More than three conductors in a raceway
When more than three current-carrying conductors share a conduit or cable, their combined heat forces a further reduction (NEC 310.15(C)(1)): 4–6 conductors → 80%, 7–9 → 70%, 10–20 → 50%, 21–30 → 45%, 31–40 → 40%, 41+ → 35%. Temperature and bundling factors multiply: a conductor at 40°C (0.88) bundled with six others (0.70) keeps 0.88 × 0.70 = 62% of its base ampacity, which must still exceed the load.
Common questions
What is the current rating for 10 AWG wire?
In standard residential branch circuits, 10 AWG copper wire is rated for a maximum of 30 amps. Its 75°C ampacity is 35 A under NEC 310.16, but 30 A is the breaker size normally paired with it.
How many amps can a 12 gauge wire handle safely?
12 AWG copper wire is rated for 20 amps on standard residential branch circuits under the NEC.
What wire gauge size is needed for a 50 amp breaker?
A 50-amp circuit typically requires 6 AWG copper or 4 AWG aluminum conductors. Confirm against conductor temperature rating and voltage drop for long runs.
What is the difference between 12/2 and 12/3 Romex wire?
12/2 cable has one hot (black), one neutral (white) and a bare ground. 12/3 adds a second insulated hot (red) plus the neutral and ground, used for three-way switches, 240 V circuits or shared-neutral (split) circuits.
Can you run 14 gauge wire on a 20 amp breaker?
No — that is a code violation and a fire hazard. 14 AWG wire is rated for only 15 amps, so it must be on a 15-amp breaker. A 20-amp circuit requires 12 AWG wire.
What size wire do I need to run a 100 amp subpanel?
A 100-amp feeder generally uses 3 AWG copper or 1 AWG aluminum at standard ampacities. NEC 310.12 allows 4 AWG copper / 2 AWG aluminum for many 100-amp dwelling feeders — verify against your run length and local code.
How do you calculate voltage drop over a long distance?
For a single-phase run, VD = (2 × K × I × L) ÷ CM, where K is the conductor's resistivity, I is the current in amps, L is the one-way length in feet and CM is the wire's circular-mil area.
What wire size is needed for a 200 amp service entry?
A standard residential 200-amp main service typically uses 2/0 copper or 4/0 aluminum conductors (per the NEC 310.12 dwelling-service table).
How many wires can you put in a 3/4 inch EMT conduit?
Under the 40% fill rule, a 3/4-inch EMT conduit holds up to about 16 individual 12 AWG THHN conductors. Fewer if you mix in larger gauges or a ground.
What gauge wire is required for a 220V 30 amp dryer outlet?
A dedicated 30-amp, 240-volt appliance circuit uses 10 AWG wire — typically 10/3 cable with ground for a modern four-prong dryer outlet.
What is the maximum load capacity of a 15 amp circuit breaker?
For continuous loads (running more than 3 hours), a 15-amp breaker is limited to 80% of its rating — 12 amps, or about 1,440 watts at 120 V.
What size wire do I need for a 20 amp circuit run 100 feet?
To keep voltage drop under 3% over a 100-foot run at a full 20-amp load, size up from 12 AWG to 10 AWG copper.
What wire gauge is needed for an EV charger?
Most Level 2 home EV chargers draw 40 amps continuously on a 50-amp breaker, which requires 6 AWG copper wire. Always size to the charger's rated continuous current (125% rule).
What is the current capacity of 8 AWG copper wire?
8 AWG copper wire is rated for about 40 amps at the 60°C column and up to 50 amps at 75°C under NEC 310.16.
How many outlets can you put on a 20 amp breaker?
Residential code sets no strict hard limit on receptacles per circuit, but common practice is 10 to 12 outlets on a 20-amp general-purpose circuit.
What size wire is required for a 40 amp stove circuit?
A dedicated 40-amp residential range circuit needs a minimum of 8 AWG copper or 6 AWG aluminum conductors.
What does the 'THHN' label mean on an electrical wire?
THHN stands for Thermoplastic High Heat-resistant Nylon-coated. It designates individual insulated conductors rated for dry locations and approved for pulling inside conduit.
What gauge wire should be used for low voltage landscape lights?
Most low-voltage (12 V) landscape lighting uses 12 AWG or 14 AWG direct-burial cable; heavier 12 AWG is preferred on longer runs to offset voltage drop.
Can you mix copper and aluminum wires together?
Never twist them directly together — copper and aluminum corrode galvanically where they touch. They must be joined with connectors rated for both metals (marked AL-CU / CO-ALR), often with antioxidant compound.
What is the circular mil area of a conductor?
Circular mil area measures a wire's cross-sectional area: it is the diameter in mils (thousandths of an inch) squared. It is used in voltage-drop and ampacity calculations.
How many watts can a 20 amp 120 volt circuit support?
A 20-amp, 120-volt circuit supports up to 2,400 watts at its maximum (20 × 120). For continuous loads, the NEC 80% rule caps it at 1,920 watts.
What is the difference between line and load terminals?
The “line” terminal connects to the incoming power from the panel; the “load” terminal sends power downstream to other devices or outlets. Wiring them backwards is a common cause of GFCI/switch failures.
What color wire is hot in a 240V circuit?
In US residential 240-volt single-phase circuits, both the black and the red conductors are energized hot legs; white is neutral (where present) and green/bare is ground.
What is the standard height for an electrical wall outlet box?
By convention, duplex receptacles are set about 12 to 16 inches from the finished floor (often 12 inches to the bottom of the box). This is a common practice, not a strict code dimension, so match existing outlets and accessibility needs.
How many continuous watts can you run on a 15A breaker?
Under the NEC 80% continuous-load rule, a 15-amp, 120-volt circuit is limited to 1,440 continuous watts (12 amps).
What size conduit is required for three 4 AWG wires?
Three 4 AWG THHN conductors need at least 1-inch EMT or rigid conduit to stay within the 40% fill limit; 3/4-inch is too small.
What causes a GFCI outlet to trip repeatedly without a load?
Recurring no-load trips usually point to a real ground-fault leakage path — moisture in a box, a neutral shared or crossed with another circuit downstream, or a worn-out GFCI sensor.
What is the difference between a neutral wire and a ground wire?
The neutral is the normal return path that carries current back to the panel during operation. The ground carries current only during a fault, providing a safe path that trips the breaker. They are bonded together only at the main service.
What size breaker do I need for a 1500 watt heater?
A 1,500-watt heater draws 12.5 amps at 120 V. Because that exceeds the 12-amp continuous limit of a 15-amp circuit, it should run on a dedicated 20-amp circuit.
How do you calculate the circular mil area of a stranded wire?
Square the diameter of one strand in mils (thousandths of an inch), then multiply by the number of strands. The result is the total circular-mil area used in ampacity and voltage-drop math.
What gauge wire do I need to run a 240V 40 amp hot tub?
A 40-amp, 240-volt hot tub needs a minimum of 8 AWG copper conductors, run in liquid-tight conduit and protected by a GFCI breaker.
What is the maximum distance you can run wire before voltage drop matters?
As a rule of thumb, keep branch-circuit runs under about 100 feet before stepping up a wire size to offset voltage drop; the exact limit depends on the load and target drop (usually ≤3%).
Can you run ethernet in the same conduit as 120V wire?
No — codes keep Class 2 low-voltage cabling like ethernet out of the same raceway as line-voltage wiring, to avoid interference and shock hazards. Run them in separate conduits or maintain the required separation.
What does 'NEMA 3R' mean for electrical enclosures?
A NEMA 3R enclosure is rated for outdoor use, protecting the contents against falling rain, sleet and ice formation. It is not rated for washdown or dust-tight service.
How many amps does a 5 HP single-phase motor pull?
At full load on a 230-volt single-phase line, a 5-horsepower motor draws roughly 28 amps (NEC Table 430.248). Size conductors and overloads to the motor rules, not just the running current.
What is the difference between AWG and kcmil wire?
AWG (American Wire Gauge) sizes run up through 4/0. Conductors larger than 4/0 are measured in kcmil (thousand circular mils), which increases linearly with area.
What size fuse is needed for a 12V 120W accessory?
120 W ÷ 12 V = 10 amps, and adding a 125% margin gives about 12.5 amps — so a 15-amp automotive blade fuse is the right choice.
What is the purpose of an isolated ground receptacle?
An isolated-ground receptacle runs a separate ground conductor straight back to the panel, reducing electrical noise for sensitive electronics. It is identified by an orange face or triangle marking.
Can you use a 20A rated receptacle on a 15A circuit?
No. Putting a 20-amp receptacle on 15-amp wiring lets someone plug in a 20-amp load the circuit can't safely carry. The reverse — 15-amp receptacles on a 20-amp circuit — is allowed when multiple receptacles share it.
What wire size is needed for a 30 amp RV hookup service line?
A 30-amp RV service (TT-30) runs on a single 120-volt leg and needs a minimum of 10 AWG copper wire.
Sources
- Ampacity: NFPA 70 (NEC), Table 310.16, copper and aluminum at 60/75/90 °C insulation, ≤ 3 current-carrying conductors, 30 °C ambient.
- Resistivity: Annealed copper 1.7241 × 10⁻⁸ Ω·m (IEC 60228). Aluminum (1350 alloy) 2.65 × 10⁻⁸ Ω·m. Both at 20 °C.
- Geometry: ASTM B258 AWG diameter formula.
Disclaimer. This calculator is for reference and educational use. Final wire sizing for any installation must be done by a qualified electrician and must comply with the NEC and local codes.