Wire Size Calculator
Calculate wire size accurately for your build. Get material quantities, waste allowances, and project cost breakdowns. Free to use with no signup required.
Formula
Min R = (V x Drop%) / (I x PhaseFactor x Length) | Ampacity per NEC 310.16
Wire size is determined by the larger of two requirements: the NEC ampacity rating (based on current and insulation temperature) and the maximum resistance allowed to keep voltage drop within limits. The maximum allowable resistance per 1,000 feet is calculated from the voltage, allowable drop percentage, current, phase factor (2 for single-phase, 1.732 for three-phase), and distance.
Worked Examples
Example 1: Residential 240V Circuit
Problem: Size copper wire for a 40A, 240V single-phase load at 120 feet from the panel with 3% max voltage drop.
Solution: Ampacity: 8 AWG copper rated 50A at 75C (meets 40A)\nVoltage drop check for 8 AWG: 2 x 40 x (0.778/1000) x 120 = 7.47V = 3.11%\nExceeds 3%, try 6 AWG: 2 x 40 x (0.491/1000) x 120 = 4.71V = 1.96%\nUse 6 AWG (determined by voltage drop)
Result: 6 AWG copper | 1.96% voltage drop | 65A ampacity
Example 2: Commercial 3-Phase Feeder
Problem: Size aluminum wire for 175A, 480V 3-phase feeder at 250 feet with 3% max drop.
Solution: Ampacity: 3/0 AWG aluminum rated 155A (too small), need 4/0 at 180A\nVoltage drop for 4/0 Al: 1.732 x 175 x (0.100/1000) x 250 = 7.58V = 1.58%\n4/0 works for both ampacity and voltage drop.
Result: 4/0 AWG aluminum | 1.58% voltage drop | 180A ampacity
Frequently Asked Questions
How do I determine the correct wire size?
Wire size is determined by two independent requirements: ampacity (the maximum current the wire can safely carry without overheating) and voltage drop (keeping the voltage loss within acceptable limits). You must check both and use the larger wire. NEC Table 310.16 provides ampacity ratings for different wire sizes, insulation types, and installation conditions. For voltage drop, the NEC recommends no more than 3% for branch circuits and 5% total. Long runs often require upsizing wire beyond the minimum ampacity requirement to keep voltage drop acceptable.
When should I use aluminum instead of copper wire?
Aluminum wire is commonly used for service entrance conductors, large feeders (4 AWG and larger), and utility connections because it costs significantly less than copper for the same current capacity. However, aluminum must be sized approximately two AWG sizes larger than copper to carry the same current. Aluminum requires anti-oxidant compound at all connections and terminals specifically rated for aluminum (marked AL or AL/CU). Aluminum is not recommended for branch circuits due to historical connection problems with smaller sizes. For 100A and larger feeders, aluminum offers substantial cost savings with proper installation.
How does conduit fill affect wire sizing?
When multiple current-carrying conductors are installed in the same conduit, their ampacity must be derated per NEC Table 310.15(C)(1). For 4-6 conductors, ampacity is reduced to 80%. For 7-9 conductors, it drops to 70%. For 10-20 conductors, only 50% of the table ampacity can be used. This is because conductors bundled together cannot dissipate heat as effectively as individual conductors in free air. Additionally, the conduit must be sized per NEC Chapter 9 to ensure proper fill percentages, typically 40% maximum for three or more conductors.
How do I size an HVAC system for a building?
HVAC sizing uses Manual J calculations considering square footage, insulation, window area, climate zone, and occupancy. A rough estimate is 1 ton of cooling per 400-600 square feet. Oversized systems short-cycle and waste energy; undersized systems cannot maintain comfort.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Can I use the results for professional or academic purposes?
You may use the results for reference and educational purposes. For professional reports, academic papers, or critical decisions, we recommend verifying outputs against peer-reviewed sources or consulting a qualified expert in the relevant field.