Wire Size Calculator
Calculate recommended AWG wire gauge, circular mils, voltage drop, and NEC ampacity ratings for 12V, 24V, 120V, and 240V copper and aluminum circuits.
Calculation Results
Minimum conductor cross-section (circular mils) to stay within the allowable voltage drop percentage.
Minimum conductor cross-section (circular mils) mandated by NEC Table 310.16 to prevent wire overheating.
The strict governing circular mil requirement satisfying both NEC fire safety ampacity and voltage drop.
Standard American Wire Gauge (AWG). Value of 0 represents 1/0 AWG or larger.
Actual voltage lost across the wire run based on the selected wire size.
Usable operating voltage delivered at the receptacle or appliance terminals.
How This Calculator Works
Size electrical cables correctly in four simple steps:
- Enter Load Amps: Input the circuit breaker rating or the continuous nameplate running current of the equipment.
- Select Voltage: Pick 120V for standard wall outlets, 240V for electric dryers/EV chargers/water heaters, or 12V/24V for solar systems.
- Measure One-Way Distance: Enter the actual length of the wire run from the main circuit breaker panel to the outlet or junction box.
- Select Conductor Material: Copper is standard for branch circuits; Aluminum is common for long exterior subpanels and feeder lines.
Formula & Calculations
Safe electrical conductor sizing must satisfy both National Electrical Code (NEC) thermal ampacity limits and Ohm's law voltage drop formulas:
1. Voltage Drop Formula (Circular Mils)
Required CMIL = (2 × K × I × Distance) / Max Permissible Voltage Drop
Where:
K: Specific electrical resistivity (12.9 Ω·cmil/ft for Copper; 21.2 Ω·cmil/ft for Aluminum).I: Circuit current in Amperes.Distance: One-way wire distance in feet (the2 ×multiplier accounts for the round-trip neutral/return conductor).Max Permissible Drop:System Voltage × (Allowable Drop% / 100).
2. NEC Ampacity Overheating Safeguard (NEC Table 310.16)
Wire cannot be arbitrarily downsized for short distances. The governing conductor size is max(Ampacity CMIL, Voltage Drop CMIL):
- 15 Amp Breaker: #14 AWG minimum (4,110 circular mils)
- 20 Amp Breaker: #12 AWG minimum (6,530 circular mils)
- 30 Amp Breaker: #10 AWG minimum (10,380 circular mils)
- 40-50 Amp Breaker: #8 AWG minimum (16,510 circular mils)
- 60 Amp Breaker: #6 AWG minimum (26,240 circular mils)
Step-by-Step Calculation Examples
Garage Subpanel Run (50A, 240V, 100ft, Copper)
Sizing a 50-amp feeder cable from the basement to a detached garage at 240 volts with max 3% voltage drop.
- current: 50
- voltage: 240
- distance: 100
- material: 12.9
- allowable_drop: 3
- v_drop_cmil: 17916
- thermal_cmil: 26240
- governing_cmil: 26240
- recommended_awg: 6
- actual_voltage_drop: 4.92 V
- voltage_at_load: 235.08 V
Frequently Asked Questions
Why does the National Electrical Code recommend a 3% voltage drop limit?
NEC Section 210.19(A) Informational Note 4 recommends that voltage drop on branch circuits should not exceed 3%, and total drop from service panel to the farthest outlet should not exceed 5%. Excessive voltage drop causes electric motors to overheat, LED lights to flicker, and computers to reboot.
Why is low voltage (12V / 24V) so sensitive to wire length?
At 12 volts, a 3% allowable voltage drop is only 0.36 volts. Because low-voltage circuits carry high current to deliver equivalent wattage (P = V x I), resistive voltage drop occurs rapidly over just 15 to 20 feet, requiring surprisingly thick 8 AWG or 6 AWG cables for modest 15-amp loads.
Can I use aluminum wire instead of copper for home wiring?
Modern aluminum alloy conductors (AA-8000 series) are safe and code-compliant for large feeders (such as 100A or 200A main service entrance cables and subpanel feeds) and offer significant cost savings. However, aluminum is prohibited for standard 15A and 20A residential branch receptacles unless specialized CO/ALR terminals are used.
What is the difference between 60°C, 75°C, and 90°C wire ratings?
Standard NM-B Romex cable uses 90°C conductors, but NEC Section 334.80 mandates sizing Romex ampacity using the conservative 60°C column of NEC Table 310.16. Commercial THHN/THWN-2 wires pulled in conduit can often be sized using the higher-capacity 75°C or 90°C columns if terminal lugs permit.
When should I upsize a wire by one AWG gauge?
You should upsize one wire gauge whenever your one-way distance exceeds 100 feet on 120V circuits, whenever ambient temperatures exceed 86°F (30°C) in hot attics, or when more than 3 current-carrying conductors are bundled inside the same raceway or conduit.
How do AWG numbers work?
American Wire Gauge uses an inverse logarithmic scale: smaller numbers denote physically thicker conductors. A #12 AWG wire has roughly 60% more copper cross-sectional area and lower electrical resistance than a #14 AWG wire.