Calculate the correct AWG wire size for any circuit based on amperage, distance, voltage, and conductor material. Meets NEC voltage drop recommendations.
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Copper conductor for 20A at 75 ft
| Voltage Drop | 3.01V |
|---|---|
| Drop Percentage | 2.5% |
| Voltage at Load | 117V |
| Source Voltage | 120V |
| Required Circular Mils | 8,667 |
| 14 AWG | 20A |
|---|---|
| 12 AWG | 25A |
| 10 AWG | 35A |
| 8 AWG | 50A |
| 6 AWG | 65A |
| 4 AWG | 85A |
| 3 AWG | 100A |
| 2 AWG | 115A |
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Electrical wire sizing is governed by two independent requirements, and you must satisfy both:
Rule 1: Ampacity (NEC Table 310.16)
Every wire gauge has a maximum safe current rating based on its cross-sectional area and insulation temperature rating. At 75 degrees Celsius (the most common THWN/THHN insulation rating), 14 AWG copper handles 20 amps maximum, 12 AWG handles 25 amps, 10 AWG handles 35 amps, and so on. Exceeding these ratings causes the wire to overheat, potentially melting insulation and starting fires.
NEC Article 240 further limits circuit protection: 14 AWG requires a 15A breaker, 12 AWG requires a 20A breaker, and 10 AWG requires a 30A breaker. You cannot put 14 AWG wire on a 20A breaker even though it technically handles 20A because the safety margin disappears.
Rule 2: Voltage Drop (NEC 210.19 Informational Note 4)
As current flows through wire, some voltage is lost due to the wire's resistance. The NEC recommends that branch circuits should not exceed 3% voltage drop, and the total drop from service entrance to the farthest outlet should not exceed 5%.
While the 3%/5% figures are recommendations rather than hard requirements in the NEC, most jurisdictions enforce them. More importantly, excessive voltage drop causes real problems: motors run hotter and less efficiently, lights dim noticeably, electronic devices may malfunction, and you are paying for electricity that heats wire instead of powering equipment.
The circular mil (CM) formula for voltage drop is:
CM = (2 x K x I x D) / Vd
Where:
CM = Required circular mil area of the conductor
K = Resistivity constant (10.4 for copper, 17.0 for aluminum, in ohm-circular mils per foot)
I = Current in amperes
D = One-way distance from source to load in feet
Vd = Maximum allowable voltage drop in volts
The factor of 2 accounts for the round trip -- current flows out on the hot wire and returns on the neutral. For 240V circuits with no neutral, some electricians debate whether to use a factor of 2 or not; the conservative approach (factor of 2) is recommended for safety.
After calculating the required CM, you select the smallest AWG wire that meets or exceeds that CM value AND has sufficient NEC ampacity for the circuit breaker size.
Example Calculation:
20A circuit, 120V, copper conductor, 100 feet one-way, 3% max drop.
Vd = 120 x 0.03 = 3.6 volts
CM = (2 x 10.4 x 20 x 100) / 3.6 = 11,556 CM
Looking at the AWG table: 10 AWG has 10,380 CM (too small), so we need 8 AWG at 16,510 CM. The NEC ampacity for 8 AWG copper at 75 degrees C is 50A, which exceeds our 20A requirement. Therefore, 8 AWG copper is the correct wire for this circuit.
Without the voltage drop calculation, a standard 20A circuit would only require 12 AWG. But at 100 feet, 12 AWG would drop 4.8% -- exceeding the 3% recommendation and causing noticeable performance issues.
15A General Purpose Circuits (Lighting, Outlets):
NEC minimum: 14 AWG copper. Practical maximum distance at 3% drop: 70 feet one-way. For runs beyond 70 feet, upgrade to 12 AWG which extends the limit to 110 feet.
20A Kitchen, Bathroom, and Garage Circuits:
NEC minimum: 12 AWG copper. Practical maximum distance at 3% drop: 70 feet. These circuits often serve high-draw appliances (microwaves, hair dryers, shop tools) so the 20A rating is essential. For long runs to detached garages, 10 AWG may be required.
30A Dryer Circuits (240V):
NEC minimum: 10 AWG copper. At 240V, voltage drop is less problematic because the percentage is calculated against a higher base voltage. A 40-foot run on 10 AWG drops only 1.5%. Runs beyond 75 feet should consider 8 AWG.
40A/50A Range and Sub-Panel Circuits (240V):
NEC minimum: 8 AWG (40A) or 6 AWG (50A) copper. These high-amperage circuits generate more heat, so proper sizing and connection torque are critical. Sub-panel feeds to detached garages or workshops often need 4 AWG or larger due to distance.
100A/200A Service Entrance:
Main service entrance cables are typically 2/0 AWG copper or 4/0 AWG aluminum for 200A service. These are sized primarily by ampacity, as the distance from meter to panel is usually short.
Copper has been the standard residential wiring material for decades, but aluminum is gaining ground in specific applications:
Resistivity: Copper at 10.4 ohm-CM/ft vs aluminum at 17.0 ohm-CM/ft. Aluminum has 63% higher resistance, meaning it needs a larger gauge to carry the same current with equivalent voltage drop.
Ampacity: For the same gauge, aluminum carries approximately 78% of copper's ampacity. An 8 AWG copper wire rated at 50A corresponds to approximately 6 AWG aluminum at 50A.
Cost: Aluminum wire costs roughly 25-35% of copper's price per ampere of capacity. For a 200A service entrance, aluminum saves $500-$1,000 in material cost.
Weight: Aluminum weighs 30% of copper's weight per unit length, making installation easier for large cables.
Connection Issues: Aluminum wire requires special connectors and anti-oxidant compound at all terminations. Aluminum expands and contracts more than copper with temperature changes, which can loosen connections over time. All termination points must be rated for aluminum use (marked AL-CU or CO/ALR).
The practical recommendation: use copper for branch circuits (15A-50A) where connection reliability and smaller conduit sizes matter. Use aluminum for large feeder circuits (100A+) and service entrances where the cost savings are substantial and connections are fewer. To compare the long-term financial impact of copper vs aluminum wiring in a renovation, our home improvement ROI calculator can help quantify the investment.
Wire ampacity ratings assume specific conditions. When actual conditions differ, you must derate (reduce) the allowable amperage:
Ambient Temperature: Standard ratings assume 30 degrees C (86 degrees F) ambient temperature. In attics, rooftops, or industrial environments where ambient temperature exceeds this, ampacity must be reduced per NEC Table 310.15(B)(1). At 40 degrees C, multiply ampacity by 0.82. At 50 degrees C, multiply by 0.58.
Conduit Fill: When more than three current-carrying conductors share a conduit, each wire must be derated because they share a confined cooling space. Four to six conductors: multiply ampacity by 0.80. Seven to nine conductors: multiply by 0.70. This is why commercial installations often use larger conduit or separate circuits.
Continuous Loads: If a circuit operates at maximum load for three hours or more continuously, NEC requires sizing the wire and breaker for 125% of the continuous load. A 16A continuous load requires a 20A breaker and 12 AWG wire.
These derating factors compound. A wire in a hot attic (0.82 factor) with six conductors in conduit (0.80 factor) has an effective ampacity of only 65.6% of its table rating. This is why professional electrical design involves more than just picking a wire size from a chart.
All electrical work must comply with local building codes, which may be stricter than the NEC. Key safety points:
Always verify local code requirements before purchasing materials. Permits are required for most electrical work, and inspections verify compliance. DIY electrical work is prohibited in some jurisdictions.
Never use extension cords as permanent wiring. Never splice wire without an accessible junction box. Always use wire nuts or other listed connectors, never tape alone. Ensure all connections are tight -- loose connections cause arcing and fires.
This calculator provides guidance for planning purposes. A licensed electrician should perform or verify all electrical installations. The cost of hiring an electrician ($50-$100/hour) is trivial compared to the cost of an electrical fire.
Every electrical conductor has resistance. When current flows through that resistance, some electrical energy converts to heat instead of reaching the load. This energy loss manifests as a reduction in voltage at the load end of the circuit, called voltage drop.
Ohm's Law governs this relationship: V = I x R, where V is voltage drop, I is current, and R is the total wire resistance. For a round-trip circuit (hot wire out, neutral wire return), the total resistance is twice the one-way wire resistance.
Example: 12 AWG copper has a resistance of approximately 1.59 ohms per 1,000 feet. For a 75-foot one-way run (150 feet round trip), the total resistance is 0.24 ohms. At 15 amps: V = 15 x 0.24 = 3.56 volts drop. On a 120V circuit, that is 3.0% -- right at the NEC recommendation.
At the same distance with 20 amps: V = 20 x 0.24 = 4.75 volts = 4.0% drop. Now we exceed the 3% recommendation and need to upgrade to 10 AWG to reduce resistance.
The key insight is that voltage drop scales linearly with both current and distance. Higher loads and longer runs compound the problem. This is why detached garages, workshops, barn feeds, and landscape lighting circuits frequently need oversized wire.
The NEC's 3% branch circuit and 5% total recommendations exist for practical reasons:
Motor Performance: Electric motors are particularly sensitive to voltage drop. A motor rated for 120V receiving only 114V (5% drop) draws higher current to compensate, increasing operating temperature by 10-15%. This accelerates winding insulation breakdown, reducing motor life by 30-50%. HVAC compressors, well pumps, refrigerators, and garbage disposals are all motor-driven loads affected by voltage drop.
Lighting Quality: Incandescent bulbs dim proportionally to voltage reduction -- a 5% voltage drop reduces light output by approximately 10%. LED drivers compensate for moderate voltage drop but may flicker or fail below their minimum input voltage (typically 100-110V). Fluorescent ballasts can overheat with sustained under-voltage.
Electronic Equipment: Most electronics have switching power supplies that accept a wide voltage range (100-240V). However, cheap power supplies, older equipment, and sensitive instruments may malfunction below 110V. Computer UPS systems will switch to battery mode if input voltage drops too low, defeating their purpose.
Energy Waste: Voltage drop converts electricity to heat in the wire. A 5% drop means 5% of your electricity bill for that circuit is literally heating your walls instead of powering equipment. For a heavily loaded circuit, this can be $50-$100 per year in wasted electricity.
Scenario 1: Detached Garage Sub-Panel
60A sub-panel, 100 feet from main panel, 240V single-phase, copper wire.
Max voltage drop: 240 x 0.03 = 7.2V
Required CM: (2 x 10.4 x 60 x 100) / 7.2 = 17,333 CM
8 AWG copper (16,510 CM) is slightly under. Use 6 AWG (26,240 CM).
Actual drop with 6 AWG: (2 x 10.4 x 60 x 100) / 26,240 = 4.75V = 2.0%
Note: NEC ampacity for 6 AWG copper is 65A, sufficient for a 60A sub-panel. The 6 AWG satisfies both voltage drop and ampacity requirements.
Scenario 2: Landscape Lighting
Low-voltage landscape lighting at 12V, 200 watts total, 150 feet from transformer.
Amps: 200W / 12V = 16.7A
Max drop at 12V: 12 x 0.03 = 0.36V (at 12V, even small drops are significant)
Required CM: (2 x 10.4 x 16.7 x 150) / 0.36 = 144,667 CM
This exceeds 4/0 AWG (211,600 CM would work but is impractical for landscape wire). The solution: use a higher-voltage system (if available), split into multiple shorter runs from the transformer, or use thicker landscape wire. This example illustrates why low-voltage systems over long distances are problematic.
Scenario 3: EV Charger
Level 2 EV charger, 40A continuous load (50A circuit), 240V, 50 feet, copper.
For continuous loads, wire sizing must be based on 50A breaker (125% of 40A = 50A).
Max drop: 240 x 0.03 = 7.2V
Required CM: (2 x 10.4 x 50 x 50) / 7.2 = 7,222 CM
10 AWG (10,380 CM) satisfies voltage drop.
But NEC ampacity for 10 AWG is only 35A -- insufficient for 50A circuit.
Need 6 AWG (65A ampacity, 26,240 CM).
This scenario illustrates how ampacity (not voltage drop) often governs wire sizing for short, high-amperage circuits. Always check both requirements. For EV charger installation costs, our EV charging cost calculator provides comprehensive cost analysis.
Increase Wire Gauge: The most straightforward solution. Going up one gauge (e.g., 12 to 10 AWG) reduces resistance by approximately 37% and costs $0.20-$0.40 per foot more. For critical circuits, the extra cost is minimal relative to the total project.
Increase Voltage: 240V circuits can be twice as long as 120V circuits with the same percentage drop, because the allowable drop in volts is doubled. Where possible, use 240V for distant loads (workshops, well pumps, HVAC equipment).
Reduce Load: If a circuit serves multiple outlets, splitting it into two circuits reduces the current on each, reducing voltage drop proportionally. This also provides better capacity for future loads.
Relocate the Panel: For new construction, positioning the electrical panel centrally in the house minimizes maximum wire distances. A panel in the center of a 60-foot-wide house has a maximum run of 30 feet vs 60 feet for a panel at one end.
Use Parallel Conductors: For very large loads, two smaller conductors in parallel (each carrying half the current) can be more practical than one very large conductor. NEC allows parallel conductors only for 1/0 AWG and larger.
If you suspect voltage drop in an existing circuit, measure it directly with a multimeter:
Step 1: Measure voltage at the panel with the circuit loaded (all lights on, equipment running).
Step 2: Measure voltage at the farthest outlet on the same circuit, under the same load.
Step 3: The difference is the actual voltage drop.
If the panel reads 122V and the far outlet reads 115V, the drop is 7V (5.7%) -- exceeding recommendations. The solution is to re-wire with larger gauge, reduce load on the circuit, or split the circuit.
Also check for hot connections: use an infrared thermometer or thermal camera on breaker connections, wire nuts, and receptacle terminals. A connection more than 20 degrees F above ambient indicates high resistance -- usually a loose or corroded connection that adds to voltage drop.
DC solar and battery circuits follow the same physics but with important differences:
Solar panel strings operate at 30-600V DC depending on configuration. Higher voltage means lower current for the same power, reducing wire size requirements. This is why modern solar inverters prefer higher string voltages.
Battery bank connections operate at 12V, 24V, or 48V -- very low voltages where even small resistance causes significant percentage drops. Battery cables must be oversized and kept as short as possible. A 48V battery system at 100A needs 4/0 AWG for just a 10-foot run to maintain 1% drop.
For solar installation economics including wiring costs, use a dedicated solar ROI calculator to evaluate the full financial picture.
For a 20A circuit, NEC requires 12 AWG copper minimum. However, for runs longer than 70 feet at 120V, you should upgrade to 10 AWG to keep voltage drop within the recommended 3%. At 240V, 12 AWG handles up to about 140 feet before needing an upgrade.
Voltage drop = (2 x K x I x D) / CM, where K is the resistivity constant (10.4 for copper, 17.0 for aluminum), I is current in amps, D is one-way distance in feet, and CM is the wire's circular mil area. NEC recommends keeping branch circuit voltage drop at or below 3%.
At 120V with 3% maximum voltage drop, 12 AWG copper can run approximately 70 feet one-way for a 20A circuit. At 240V, the distance extends to approximately 140 feet because the allowable voltage drop in volts doubles.
Use copper for branch circuits (15A-50A) where connection reliability matters and space is limited. Use aluminum for large feeder circuits (100A+) and service entrances where cost savings are significant. Aluminum requires AL-CU rated connectors and anti-oxidant compound at all terminations.
Most Level 2 EV chargers draw 32-40A continuously, requiring a 40-50A circuit. At 240V with copper wire, use 8 AWG for runs up to 50 feet and 6 AWG for runs up to 100 feet. Remember the NEC 125% continuous load rule: a 40A continuous load needs a 50A breaker.
Yes. Voltage drop converts electricity to heat in the wire. A 5% voltage drop means 5% of the electricity for that circuit heats your walls instead of powering equipment. For heavily loaded circuits running many hours daily, this can cost $50-$100 per year in wasted energy.
NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total from service entrance to the farthest outlet. While technically advisory in the NEC, most local jurisdictions enforce these limits. Exceeding them causes motor overheating, light dimming, and energy waste.
AWG (American Wire Gauge) uses counterintuitive numbering: smaller numbers mean larger wire. 14 AWG is thin household wire; 4/0 AWG is thick service cable. Each drop of 3 gauge numbers approximately doubles the wire's cross-sectional area and current capacity.
CM = (2 x K x I x D) / Vd
K = Resistivity (10.4 copper, 17.0 aluminum), I = Amps, D = One-way distance (ft), Vd = Max voltage drop (volts).
The calculator selects the smallest AWG meeting both the voltage drop and NEC ampacity (75C) requirements.
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