How to Size Electrical Wire for Branch Circuits
Start With the Load, Not the Wire
Wire sizing begins with the load. Before you look at any table, establish three things: the circuit amperage, the voltage (120V, 240V, 277V, or 480V), and the installation method (conductors in conduit or free air).
The NEC determines minimum conductor sizes primarily through Table 310.16 (conductors in raceway, cable, or earth) and Table 310.17 (single conductors in free air). Both tables are organized by insulation temperature rating — 60°C, 75°C, and 90°C columns — and list the maximum ampacity for each conductor size.
The Standard Residential Wire Sizes
For residential work at 120/240V with copper conductors, the standard sizes cover most circuits:
14 AWG handles 15 amps and is used for lighting circuits and lightly loaded receptacle circuits. 12 AWG handles 20 amps and is the standard for general-purpose receptacle circuits, kitchen countertop circuits, and bathroom circuits. 10 AWG handles 30 amps and serves clothes dryers, water heaters, and other dedicated 30A loads. 8 AWG handles 40 amps. 6 AWG handles 55 amps (75°C column) and is common for electric ranges and large appliances.
These are ampacity values at standard conditions — 30°C ambient temperature, no more than three current-carrying conductors in a raceway. Real-world conditions often require adjustment.
Derating: When Standard Tables Aren’t Enough
Two common conditions reduce conductor ampacity below the table values.
Ambient temperature correction. NEC Table 310.16 assumes a 30°C (86°F) ambient temperature. If conductors run through an attic in summer where ambient reaches 50°C, the ampacity must be reduced using correction factors from the table footnotes. Higher ambient temperatures increase conductor resistance, which increases heat generation at a given current.
Conductor bundling. When more than three current-carrying conductors share a raceway, each conductor’s ability to dissipate heat decreases. NEC Table 310.15(C)(1) provides adjustment factors: 4-6 conductors derate to 80% of table ampacity, 7-9 conductors to 70%, and so on. This matters most in commercial installations with large conduit runs.
When both conditions apply simultaneously, you multiply the correction factors together. A 12 AWG THHN conductor rated at 30A (90°C column) in a 40°C ambient with 6 conductors bundled: 30A × 0.88 (temp correction) × 0.80 (bundling) = 21.1A effective ampacity.
Copper vs Aluminum
Aluminum conductors have lower ampacity than copper at the same gauge. Aluminum is roughly 61% as conductive as copper, so you typically need to go up one or two wire sizes when using aluminum.
Aluminum is most common in larger feeders (service entrance cables, subpanel feeds) where the cost savings from cheaper material outweigh the need for a larger conductor. For branch circuits 30A and under, copper dominates.
When sizing aluminum, use the aluminum column in NEC Table 310.16 — don’t try to convert from copper sizes manually.
The Voltage Drop Check
Wire sizing based on NEC tables ensures the conductor won’t overheat. But a conductor can be thermally safe and still have excessive voltage drop on a long run. After selecting a wire size from the ampacity tables, calculate voltage drop to confirm it stays within the NEC’s recommended 3% for branch circuits.
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If voltage drop exceeds 3%, upsize the conductor regardless of what the ampacity tables allow. This is common on long residential runs — a 12 AWG wire is thermally fine for 20A at 200 feet, but the voltage drop may push you to 10 AWG.
Quick Reference: Common Circuits
15A lighting circuit (120V): 14 AWG copper minimum. Use 12 AWG if the run exceeds 50 feet to manage voltage drop.
20A receptacle circuit (120V): 12 AWG copper minimum. Check voltage drop on runs over 75 feet.
30A dryer circuit (240V): 10 AWG copper minimum. 240V circuits have half the voltage drop percentage of 120V circuits at the same amperage and distance, so long runs are less problematic.
50A range circuit (240V): 6 AWG copper minimum at 75°C rating.
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