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Proper wire sizing must satisfy both ampacity requirements and voltage drop limits. This calculator checks both criteria and recommends the smallest wire that meets both.
Wire must safely carry the load current without overheating. Based on NEC Table 310.16 for 75°C rated conductors.
Wire must deliver adequate voltage to the load. NEC recommends maximum 3% for branch circuits, 5% total.
Allowable ampacities for insulated conductors:
| AWG | Cu 60°C | Cu 75°C | Cu 90°C | Al 75°C |
|---|---|---|---|---|
| 14 | 15A | 15A | 15A | 15A |
| 12 | 20A | 20A | 20A | 20A |
| 10 | 30A | 30A | 30A | 25A |
| 8 | 40A | 50A | 55A | 40A |
| 6 | 55A | 65A | 75A | 50A |
| 4 | 70A | 85A | 95A | 65A |
| 3 | 85A | 100A | 115A | 75A |
| 2 | 95A | 115A | 130A | 90A |
| 1 | 110A | 130A | 145A | 100A |
| 1/0 | 125A | 150A | 170A | 120A |
| 2/0 | 145A | 175A | 195A | 135A |
| 3/0 | 165A | 200A | 225A | 155A |
| 4/0 | 195A | 230A | 260A | 180A |
Based on NEC 2020 Table 310.16. Not more than 3 current-carrying conductors in raceway or cable. Ambient temperature 86°F (30°C).
Wire sizing is the process of selecting the correct American Wire Gauge (AWG) conductor for an electrical circuit based on the current it must carry, the distance of the run, the acceptable voltage drop, and the installation conditions. In the United States, wire sizes are standardized according to the AWG system, where smaller gauge numbers indicate larger wire diameters -- for example, 4 AWG is much larger than 14 AWG. Proper wire sizing is governed by the National Electrical Code (NEC), particularly Table 310.16 for ampacity ratings and informational notes for voltage drop recommendations. Selecting a wire that is too small for the load creates a fire hazard due to overheating, while an unnecessarily oversized wire wastes money on copper or aluminum. The two primary criteria for wire sizing are ampacity (the maximum current a wire can safely carry without exceeding its temperature rating) and voltage drop (the voltage lost along the wire length due to its inherent resistance).
Calculate or measure the maximum continuous current the circuit will carry. For motor circuits, include starting current and NEC multipliers (typically 125% of the full-load current for continuous loads). For branch circuits, the wire must be rated for the overcurrent protection device (breaker) size -- 15A breaker requires 14 AWG minimum, 20A requires 12 AWG minimum.
Look up the minimum wire size for your current in NEC Table 310.16, selecting the appropriate temperature column for your insulation type (60°C for TW, 75°C for THW/THWN, 90°C for THHN/XHHW). Apply derating factors for ambient temperature above 86°F and for more than 3 current-carrying conductors in a conduit.
Using the formula VD = (2 x L x I x R) / 1000 for single-phase circuits, calculate the voltage drop percentage. NEC recommends no more than 3% for branch circuits and 5% total for feeder plus branch circuit combined. If the voltage drop exceeds your limit, increase the wire size until it is within spec.
Compare the wire size required for ampacity with the wire size required for voltage drop, and choose the larger of the two. For short runs, ampacity usually governs. For long runs (over 100 feet at 120V), voltage drop often requires a larger wire than ampacity alone would dictate. Always verify that the selected wire fits the conduit and terminal connections.
Undersized wire is one of the leading causes of electrical fires. When current exceeds a wire's ampacity, the conductor overheats, melting insulation and igniting nearby combustible materials. The NEC sets minimum wire sizes to prevent this. Electrical inspectors verify wire sizing before approving installations, and using undersized wire will fail inspection and void insurance coverage.
Excessive voltage drop from undersized wire starves equipment of the voltage it needs. Motors run hotter and less efficiently, draw more current, and have shorter lifespans. LED drivers may flicker. Sensitive electronics may malfunction. Air conditioner compressors may fail to start. Properly sized wire ensures equipment receives at least 95-97% of the supply voltage.
Energy lost in wiring is converted to waste heat (I²R losses). For a 200-foot, 30-amp circuit, upgrading from 10 AWG to 8 AWG reduces line losses by approximately 40%. In commercial buildings where circuits carry heavy loads for extended hours, properly sized wiring saves significant energy costs over the building's lifetime.
| AWG | Diameter (inches) | Area (kcmil) | Resistance (ohms/1000ft) | Typical Use |
|---|---|---|---|---|
| 14 | 0.0641 | 4.11 | 3.14 | 15A branch circuits, lighting |
| 12 | 0.0808 | 6.53 | 1.98 | 20A branch circuits, outlets |
| 10 | 0.1019 | 10.38 | 1.24 | 30A circuits, dryers, AC units |
| 8 | 0.1285 | 16.51 | 0.778 | 40-50A circuits, ranges, large AC |
| 6 | 0.1620 | 26.24 | 0.491 | 55-65A circuits, sub-panels |
| 4 | 0.2043 | 41.74 | 0.308 | 70-85A circuits, feeders |
| 3 | 0.2294 | 52.62 | 0.245 | 85-100A circuits, feeders |
| 2 | 0.2576 | 66.36 | 0.194 | 95-115A circuits, large feeders |
| 1 | 0.2893 | 83.69 | 0.154 | 110-130A circuits, service entrance |
| 1/0 | 0.3249 | 105.6 | 0.122 | 125-150A, 100A service entrance |
| 2/0 | 0.3648 | 133.1 | 0.0967 | 145-175A, 150A service |
| 3/0 | 0.4096 | 167.8 | 0.0766 | 165-200A, 200A service |
| 4/0 | 0.4600 | 211.6 | 0.0608 | 195-230A, 200A service entrance |
Resistance values are for solid copper conductors at 68°F (20°C). Stranded conductors have slightly higher resistance. Ampacity ratings vary by insulation type and installation conditions.
The NEC requires that continuous loads (operating for 3 hours or more) must not exceed 80% of the overcurrent protection device rating. This means a 20A breaker on a continuous circuit should only carry 16A, and the wire must be sized for the full breaker rating. This rule ensures thermal safety margins for sustained operation.
When more than 3 current-carrying conductors share a conduit, each conductor's ampacity must be derated per NEC Table 310.15(C)(1). For 4-6 conductors, derate to 80%; for 7-9, derate to 70%; for 10-20, derate to 50%. This derating means you may need to go up one or two wire sizes to safely carry the required current.
Solid wire (single conductor) is used for 14 AWG through 10 AWG in residential branch circuits. It is easier to terminate under screw terminals but is stiffer and harder to pull through conduit. Stranded wire (multiple thin conductors twisted together) is required for 8 AWG and larger because solid wire in these sizes is too stiff to bend and install properly.
THHN is rated for 90°C in dry locations, while THWN is rated for 75°C in wet locations. Most modern wire sold as THHN/THWN-2 carries both ratings -- 90°C dry and 75°C wet. The 90°C rating is primarily used for derating calculations, not for higher ampacity in standard installations, because terminal ratings limit the usable ampacity.
Calculate the total connected load on the sub-panel, apply NEC demand factors where applicable, and size the feeder wire for the calculated demand load plus any continuous load multipliers. The wire must also be large enough to limit voltage drop to 3% on the feeder. Common sub-panel feeders include 6 AWG for 60A, 4 AWG for 100A, and 2/0 for 150A, but always verify for your specific run length.
Look up the ampacity of any wire gauge based on insulation type, conductor material, and installation conditions with NEC derating factors applied.
Calculate the voltage drop for a specific wire gauge, length, and current to verify your installation meets the NEC 3% and 5% recommendations.
Calculate the current drawn by a load from voltage and power. Essential for determining the load current before selecting the appropriate wire size.