Wire Size Calculator

About the Wire Size Calculator

Choosing the correct wire size is one of the most important decisions in electrical design. A wire that is too small can overheat, cause excessive voltage drop, or even start a fire. A wire that is too large wastes money unnecessarily. The National Electrical Code (NEC) provides ampacity tables and derating rules that form the foundation of wire sizing in North America.

Wire size selection must satisfy two independent criteria simultaneously: the wire must have enough ampacity to carry the current without overheating (NEC Article 310), and it must be large enough to keep the voltage drop within acceptable limits (NEC recommends 3% for branch circuits, 5% total including feeder). The stricter of the two requirements determines the final size.

This calculator handles both criteria: it checks the NEC ampacity tables (for 60°C, 75°C, and 90°C insulation ratings), applies the 125% continuous load rule, applies conduit fill derating factors, and simultaneously computes the minimum area needed for voltage drop compliance. It then recommends the larger of the two required sizes, showing which criterion was limiting.

When This Page Helps

Manual wire sizing requires cross-referencing NEC tables, applying multiple derating factors, and calculating voltage drop — a process prone to errors. This calculator automates the entire workflow: ampacity lookup, continuous load adjustment, conduit fill derating, and voltage drop calculation. It shows both the ampacity-required and voltage-drop-required gauges so you can verify which criterion controls.

How to Use the Inputs

1. Enter the load current in amps and the system voltage.
2. Select single-phase or three-phase, and set the maximum voltage drop percentage (typically 3%).
3. Enter the one-way wire length (the calculator handles round-trip automatically).
4. Select copper or aluminum conductor and insulation temperature rating.
5. Set whether the load is continuous (NEC 125% rule) and enter the number of conductors in conduit.
6. Read the recommended AWG gauge, which criterion limited the choice, and the actual voltage drop.
7. Check the gauge comparison visual and NEC ampacity table for adjacent sizes.

Example Calculation

Tips & Best Practices

• NEC Article 210.19(A): Branch circuit conductors must have ampacity ≥ 125% of the continuous load plus 100% of the non-continuous load.
• The 3% voltage drop recommendation is a guideline, not a hard requirement in the NEC. However, exceeding it can cause equipment issues — motors start slowly, lights dim, and electronic devices may malfunction.
• For conduit runs with many conductors, don't forget derating: 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%. This can significantly upsize your wire.
• Aluminum wire requires one AWG size larger than copper for equivalent ampacity. Use only AL/CU rated terminations.
• Three-phase circuits have a √3 (1.732) factor advantage: lower voltage drop per amp than single-phase for the same wire.
• For long runs (>100 ft / 30 m), voltage drop almost always controls the wire size, not ampacity.

NEC Wire Sizing Workflow

Professional wire sizing follows a structured process: (1) Determine the design current (apply continuous load factor), (2) apply ambient temperature derating if above 30°C, (3) apply conduit fill derating if more than 3 current-carrying conductors, (4) select the smallest AWG from the appropriate table column (60/75/90°C) that meets all derating conditions, (5) separately calculate minimum area for voltage drop, (6) use the larger of the two sizes, (7) verify the result against available conduit size. This calculator automates steps 1 through 6.

Common Circuit Sizes in Residential Wiring

Most North American homes use AWG 14 (15 A) or AWG 12 (20 A) for general branch circuits, AWG 10 (30 A) for dryers and water heaters, AWG 8 (40 A) or AWG 6 (50-60 A) for ranges, and AWG 4/0 or larger for 200 A service entrances. EV chargers typically use AWG 8 (40 A) or AWG 6 (50 A). HVAC condensers range from AWG 12 to AWG 8 depending on tonnage.

Metric Wire Sizes (IEC 60228)

Outside North America, wire sizes are specified in mm² cross-sectional area. Common sizes: 1.5 mm² (≈ AWG 16), 2.5 mm² (≈ AWG 13), 4 mm² (≈ AWG 11), 6 mm² (≈ AWG 9), 10 mm² (≈ AWG 7), 16 mm² (≈ AWG 5), 25 mm² (≈ AWG 3). IEC standards use different ampacity ratings based on installation method (buried, in trunking, in free air, etc.) rather than insulation temperature alone.

Frequently Asked Questions

• Why does continuous load require 125% sizing?

  NEC defines continuous load as one expected to run 3+ hours. Sustained current generates more heat than intermittent current because equipment has no time to cool. The 125% rule ensures the wire, breaker, and connections stay within safe temperature limits during extended operation.

• What voltage drop is acceptable?

  NEC recommends (not mandates) 3% for branch circuits and 5% total including feeder. For sensitive electronics, 2% may be preferred. For motors, excessive drop causes higher starting current and slower acceleration. For LED drivers, most tolerate 5-10% drop without visible effect.

• How does insulation temperature affect wire sizing?

  Higher-rated insulation allows more current: AWG 12 at 60°C = 20 A, at 75°C = 25 A, at 90°C = 30 A. However, NEC requires that the conductor be sized at the lower temperature rating of any connected terminal. Most residential breakers and receptacles are rated 75°C, so that column typically applies.

• When should I use aluminum instead of copper?

  Aluminum is 60% lighter and much cheaper — advantageous for large feeders and service entrances. AWG 1 aluminum (42 mm²) costs roughly 40% less than AWG 2 copper (34 mm²) at similar ampacity. For branch circuits below AWG 6, copper is standard because aluminum requires special anti-oxidant compounds and torque specifications.

• What is conduit fill derating?

  When multiple conductors share a conduit, each wire's heat adds up and reduces cooling efficiency. NEC 310.15(C) requires derating: 4-6 conductors = 80% of table ampacity, 7-9 = 70%, 10-20 = 50%. This often results in upsizing the wire, especially in large commercial conduit runs.

• Does wire size for a 3-phase circuit differ from single-phase?

  For the same power, three-phase uses less current (by factor of √3), so wires can often be smaller. For voltage drop, the factor is √3 instead of 2, giving 13.4% less drop. Three-phase also avoids the 2× round-trip multiplier of single-phase — only the √3 factor applies for line-to-line loads.