Cable Size Calculator

About the Cable Size Calculator

The Cable Size Calculator determines the correct wire gauge (AWG) for electrical circuits based on current, voltage, distance, and allowable voltage drop. Proper wire sizing ensures safety, code compliance, and efficient power delivery.

Undersized wires create fire hazards, trip breakers, and waste energy through excessive resistance heating. Oversized wires waste money on unnecessary copper or aluminum. This calculator finds the optimal gauge by checking both ampacity (current-carrying capacity per NEC Table 310.16) and voltage drop limits (typically 3% for branch circuits, 5% total including feeder).

Enter your circuit parameters — current, voltage, one-way distance, conductor material, and installation conditions — to get the minimum wire size that satisfies both ampacity and voltage drop requirements. It gives you a practical starting point for choosing a safe conductor before you buy the wire. That is especially useful when you are comparing a short run to a long one, or deciding whether a larger gauge is worth it.

When This Page Helps

Use this calculator when you need a wire size that satisfies both ampacity and voltage-drop limits instead of checking only one side of the problem. It is useful for branch circuits, feeders, and longer runs where the code-minimum gauge may not be the performance-minimum gauge. That matters most when distance starts to erode usable voltage at the load.

How to Use the Inputs

1. Enter the circuit current in amps (or load in watts with voltage).
2. Select the system voltage (120V, 208V, 240V, 480V, etc.).
3. Enter one-way wire run distance in feet.
4. Select conductor material: copper or aluminum.
5. Set maximum allowable voltage drop percentage.
6. Review the recommended AWG gauge and supporting calculations.
7. Check the wire comparison table for alternatives.

Example Calculation

Tips & Best Practices

• Always check BOTH ampacity AND voltage drop — the larger wire size wins.
• For 120V circuits, voltage drop is twice as critical as 240V for the same current and distance.
• Use 90°C rated wire (THWN-2, XHHW-2) for ampacity but derate to 75°C for terminations.
• Underground runs in conduit may need additional derating for soil temperature and fill.
• When in doubt, go one size larger — the cost difference is small compared to a redo.

NEC Ampacity Tables

NEC Table 310.16 provides the foundation for wire sizing. For 75°C-rated copper wire in normal conditions: 14 AWG = 20A, 12 AWG = 25A, 10 AWG = 35A, 8 AWG = 50A, 6 AWG = 65A, 4 AWG = 85A, 3 AWG = 100A, 2 AWG = 115A, 1 AWG = 130A, 1/0 = 150A, 2/0 = 175A, 3/0 = 200A, 4/0 = 230A.

These values assume no more than 3 current-carrying conductors, 30°C ambient temperature, and a single conduit or cable. Derating is required for higher temperatures, more conductors, or bundled cables.

Voltage Drop Calculations

For single-phase circuits: VD = 2 × I × R × D / 1000 (where R is resistance per 1000ft). For three-phase circuits: VD = √3 × I × R × D / 1000. Using circular mils directly: VD = 2 × K × I × D / cmil for single-phase.

The factor of 2 accounts for the round trip (hot and neutral). In three-phase balanced circuits, √3 replaces 2 because neutral current is zero under balanced conditions.

Common Circuit Sizes

15A circuit (typical lighting): 14 AWG copper, up to 50ft. 20A circuit (general receptacles): 12 AWG, up to 50ft. 30A circuit (dryer, water heater): 10 AWG, up to 50ft. 40A circuit (range, large appliance): 8 AWG. 50A circuit (ranges, subpanels): 6 AWG. 100A subpanel: 3 AWG copper or 1 AWG aluminum. 200A service: 2/0 copper or 4/0 aluminum.

Frequently Asked Questions

• What wire size for a 30-amp circuit?

  NEC requires minimum 10 AWG copper (or 8 AWG aluminum) for 30-amp circuits. However, voltage drop over long runs may require larger wire. A 30A circuit at 240V running 100ft may need 8 AWG copper for acceptable voltage drop.

• What is the 3% voltage drop rule?

  NEC recommends (not requires) that voltage drop on branch circuits not exceed 3%, and total drop (feeder + branch) not exceed 5%. For a 120V circuit, 3% = 3.6V. Exceeding this causes dim lights, slow motors, and equipment problems.

• Copper vs aluminum wire — which should I use?

  Copper has lower resistance (smaller wire for same ampacity), better connections, and is standard for branch circuits. Aluminum is cheaper and lighter, used mainly for large feeders and service entrance cables (4 AWG and larger).

• Does conduit type affect wire sizing?

  Conduit fill affects the number of conductors, which requires ampacity derating per NEC 310.15(C). More than 3 current-carrying conductors in a conduit reduces allowable ampacity by 20-50%. The wire gauge itself doesn't change, but you may need a larger size to compensate.

• How do I size wire for a motor?

  Motor circuits require wire sized for 125% of full-load current (NEC 430.22). Use the NEC motor FLA tables (not the nameplate) for sizing. A 10HP 240V motor has 50A FLA, so wire for 62.5A — typically 6 AWG copper.

• What about temperature derating?

  Wire ampacity decreases in high-temperature environments. Standard NEC tables assume 30°C (86°F) ambient. Above 30°C, apply correction factors from NEC Table 310.15(B)(1). In a 45°C attic, 90°C-rated wire capacity drops by about 13%.