Amp to Wire Size Calculator
Enter the load current, system voltage, one-way run length, and conductor type to instantly find the minimum safe wire gauge per NEC Table 310.15(B)(16). The calculator applies temperature-correction and conduit-fill derating, then checks the recommended gauge against your allowable voltage-drop limit. Both AWG and metric mm2 sizes are shown, with a full show-your-work panel.
How to use this wire size calculator
Enter the load current (amps) and select the system voltage, circuit type, and conductor material. Then set the one-way run length and the maximum voltage drop you can accept. For most household branch circuits, 3% voltage drop is the standard target, while 5% is the NEC-permitted ceiling for the combined feeder and branch. The calculator applies the 125% multiplier for continuous loads (loads energized for more than 3 hours), reads the correct ampacity column for your insulation temperature rating, corrects for ambient temperature, and corrects again for the number of current-carrying conductors sharing the same conduit. Both the minimum ampacity gauge and the minimum voltage-drop gauge are found, and the larger of the two is shown as the recommendation.
Wire sizing: ampacity vs. voltage drop
Two separate rules govern wire size selection. Ampacity (NEC Table 310.15(B)(16)) ensures the wire does not overheat by limiting current density. Voltage drop ensures the load receives enough voltage to operate correctly. On short runs, ampacity is usually the binding constraint. On long runs, especially at lower voltages such as 12 V or 24 V, voltage drop becomes the binding constraint and drives you to a larger wire than ampacity alone would require. This calculator checks both and reports whichever is the larger (more conservative) result.
Temperature derating and conduit fill
The NEC ampacity table is set for a 30 C ambient temperature and up to three current-carrying conductors in a raceway. If the ambient temperature is higher, the wire dissipates heat less efficiently and its allowable current must be reduced. If more than three conductors share a conduit, they heat each other and the bundle must be further derated. This calculator applies both correction factors automatically based on NEC Table 310.15(B)(2)(a) and 310.15(C)(1). A typical outdoor installation on a hot roof may need 40-50% derating compared with the base table values.
Copper vs. aluminum conductors
Aluminum conductors have higher resistivity than copper and require a larger AWG to carry the same current. The NEC does not permit aluminum in 14 AWG or 12 AWG sizes for most applications. For 10 AWG and larger, aluminum is widely used for feeders and service entrance conductors because it is significantly cheaper by weight. However, aluminum is prone to oxidation and requires anti-oxidant compound and AL/CU-rated terminations. The voltage-drop formula in this calculator uses K = 12.9 for copper and K = 21.2 for aluminum.
NEC Table 310.15(B)(16) - copper conductor ampacity (up to 4/0 AWG)
| Wire Size | mm2 | 60 C (A) | 75 C (A) | 90 C (A) | Typical Use |
|---|---|---|---|---|---|
| 14 AWG | 2.08 | 15 | 20 | 25 | Lighting, outlets (15A circuit) |
| 12 AWG | 3.31 | 20 | 25 | 30 | Outlets, small appliances (20A) |
| 10 AWG | 5.26 | 30 | 35 | 40 | Dryers, A/C (30A) |
| 8 AWG | 8.37 | 40 | 50 | 55 | Ranges, EV chargers (40-50A) |
| 6 AWG | 13.3 | 55 | 65 | 75 | Sub-panels, water heaters (60A) |
| 4 AWG | 21.2 | 70 | 85 | 95 | Large HVAC, feeders (70-85A) |
| 2 AWG | 33.6 | 95 | 115 | 130 | Service entrance, large motors |
| 1/0 AWG | 53.5 | 125 | 150 | 170 | Service entrance, large feeders |
| 2/0 AWG | 67.4 | 145 | 175 | 195 | Service entrance |
| 3/0 AWG | 85.0 | 165 | 200 | 225 | Service entrance |
| 4/0 AWG | 107.2 | 195 | 230 | 260 | Service entrance, 200A service |
Allowable ampacities for not more than three current-carrying conductors in a raceway, cable or buried in earth. Based on 30 C ambient temperature.
Frequently asked questions
What wire gauge do I need for 20 amps?
For a 20-amp branch circuit in a standard household installation (copper, 75 C insulation, 30 C ambient, up to 3 conductors in conduit), 12 AWG copper is the NEC minimum. If the run is long, voltage drop may require 10 AWG. Always check voltage drop for runs over 50 feet at 120 V or over 25 feet at 12-24 V.
What wire gauge do I need for 30 amps?
10 AWG copper is the NEC minimum for a 30-amp circuit at 75 C. For 240 V circuits such as dryers and A/C units this is usually acceptable on runs under 75-100 feet. On longer runs you may need 8 AWG to keep voltage drop below 3%.
What is the 125% continuous load rule?
NEC 210.20(A) requires that conductors and over-current protection devices be rated at least 125% of the load current when the load is "continuous" - energized for 3 hours or more at a time. For a 16-amp load that is on continuously, the design current becomes 16 x 1.25 = 20 A, and you size the wire and breaker to that higher value.
How do I calculate voltage drop?
For single-phase circuits, voltage drop (V) = 2 x K x I x L / CM, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way length in feet, and CM is the circular mil area of the wire. For three-phase, replace the factor 2 with 1.732. The percentage drop is (VD / system voltage) x 100.
Does wire size depend on AC or DC?
Ampacity is the same for AC and DC at residential frequencies. Voltage drop uses the same formula for both, but the NEC tables are calibrated for AC circuits. At higher DC voltages (EV, solar, battery systems), the same formulas apply. At very low DC voltages like 12 V or 24 V, voltage drop becomes critical much more quickly because even 0.5 V is a large percentage of the supply.
What does derated ampacity mean?
The NEC table lists base ampacity values for standard conditions (30 C, 3 conductors). Derating reduces these values to account for heat buildup in hotter environments or densely filled conduits. For example, a 12 AWG copper wire rated at 25 A (75 C column) derated by 0.88 for 40 C ambient and 0.80 for 4-6 conductors in conduit becomes 25 x 0.88 x 0.80 = 17.6 A - meaning a 15-amp load is fine but a 20-amp load would require a larger wire.
Can I use aluminum wire for household wiring?
Aluminum is not permitted in 14 AWG or 12 AWG by the NEC for most applications due to its higher resistance and susceptibility to connection failure (cold creep). For 10 AWG and above, aluminum is widely used for service entrance conductors, feeders to sub-panels, and larger circuits. All devices and terminations must be rated for aluminum (marked AL or AL/CU) and the connections must be made with anti-oxidant compound.