NEC 310.16

Wire Size Calculator

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How to Use This Calculator

1. Enter your circuit's load amperage. Use the nameplate rating or calculated load, not the breaker size.
2. Check “Continuous load” if the load runs 3 or more hours without interruption (EV chargers, commercial lighting, electric baseboard heat).
3. Select your system voltage and phase configuration.
4. Enter the one-way wire run distance in feet, measured from the panel to the load, not round-trip.
5. Choose copper or aluminum conductor material.
6. Select the insulation type. THHN/THWN-2 is the most common for building wire in conduit.
7. Adjust ambient temperature if the wire passes through spaces above 86°F (attics, rooftops, mechanical rooms).
8. Select the number of current-carrying conductors sharing the same raceway or conduit.
9. Read your results on the right. The calculator handles all NEC derating, terminal temperature capping, breaker sizing, and ground wire selection automatically.

How to Size Wire Per the National Electrical Code

Selecting the correct wire size is one of the most critical decisions in any electrical installation. An undersized conductor creates a fire hazard: the wire heats beyond its insulation rating under load, degrading the insulation over time and eventually creating conditions for an arc fault or direct short. An oversized conductor wastes material cost and may not physically fit in the conduit, junction box, or equipment terminations.

The National Electrical Code (NEC) provides a systematic process for determining the minimum conductor size that safely carries the required current under the specific installation conditions. This process accounts for the conductor material (copper or aluminum), insulation temperature rating, ambient temperature, number of conductors sharing a raceway, whether the load is continuous, and the temperature rating of the equipment terminals where the wire terminates.

The wire sizing process has five key steps: calculate the design amperage, look up the base ampacity from NEC Table 310.16, apply derating factors for temperature and bundling, enforce the terminal temperature limitation per NEC 110.14(C), and verify that the final ampacity meets or exceeds the design amperage. Each step is governed by a specific NEC section, and inspectors will check that every step was followed correctly. Understanding this process is what separates a compliant installation from a code violation.

Wire sizing also affects voltage drop, though the NEC treats voltage drop as a recommendation rather than a requirement for branch circuits. NEC 210.19 Informational Note suggests limiting voltage drop to 3% on branch circuits and 5% total (feeder plus branch circuit). While not mandatory for code compliance, excessive voltage drop causes equipment malfunction, dimming lights, motor overheating, and wasted energy. After sizing wire for ampacity, always check voltage drop separately, especially for long runs.

Step 1: Calculate Design Amperage

The design amperage is the current value you use to select the wire size. For non-continuous loads, this equals the actual load current. For continuous loads (any load expected to operate at maximum current for three hours or more), NEC 210.20(A) requires multiplying the load current by 125%. This ensures the conductor and overcurrent device operate within their thermal limits during extended operation.

Common continuous loads include EV chargers (Level 2 chargers typically draw 32–48A continuously), commercial lighting systems, electric baseboard heaters, and data center power feeds. A 40A EV charger, for example, becomes a 50A design load after the 125% continuous factor. Forgetting this multiplier is one of the most common reasons electrical installations fail inspection.

Step 2: Look Up Base Ampacity from NEC Table 310.16

NEC Table 310.16 is the fundamental ampacity table for conductors rated 0–2000V in raceway, cable, or directly buried, with not more than three current-carrying conductors, based on an ambient temperature of 30°C (86°F). The table has separate columns for 60°C, 75°C, and 90°C insulation ratings, and separate sections for copper and aluminum conductors.

When derating is required (for high ambient temperatures or bundled conductors), you start with the 90°C column regardless of your wire's actual insulation rating. The 90°C column gives you the highest starting ampacity, which provides the most room for derating. This is a deliberate NEC methodology: you use the 90°C column as your derating baseline, then apply the terminal temperature cap at the end.

For example, #6 AWG copper has ampacities of 55A at 60°C, 65A at 75°C, and 75A at 90°C. If you need to derate for a hot attic, starting from 75A gives you a higher result after derating than starting from 55A or 65A. But the final result is still capped by the terminal temperature. More on that in the critical section below.

Understanding the NEC 110.14(C) Terminal Temperature Limitation

This is the single most important concept in wire sizing and the one most frequently misunderstood. NEC 110.14(C) requires that the temperature rating of the conductor be considered in conjunction with the temperature rating of the equipment terminations. In practice, this means: even with 90°C wire (THHN), your final usable ampacity is capped at the equipment termination column. NEC 110.14(C)(1) sets that column from the circuit rating: 60°C at 100 amperes or less, 75°C above it, each one unless the equipment is listed and marked otherwise. The marking on your equipment is what decides it.

Here is why this matters. A #6 AWG copper THHN conductor has a 90°C ampacity of 75A per NEC 310.16. Many online calculators simply report 75A as the ampacity and call it done. But if that #6 THHN wire terminates on a breaker marked 75°C, the connection point can only handle the heat generated by 65A, the 75°C column value. Running 75A through a 75°C-rated terminal will overheat the connection, potentially causing thermal damage, increased resistance, and eventually failure.

The 90°C column value IS used, but only as the starting point for derating calculations. After applying temperature correction and bundling adjustment factors to the 90°C value, the result is compared to the straight 75°C column value. The final usable ampacity is the lesser of these two numbers. This means the 90°C rating provides benefit only when derating reduces the 90°C value below what the 75°C column allows. In effect, it gives you extra headroom for derating without needing to upsize the conductor.

When is the 90°C column value used directly without a 75°C cap? Only when both ends of the conductor terminate on equipment specifically listed and marked for 90°C terminations. This is uncommon in residential and light commercial work. Some industrial motor controllers, high-temperature-rated disconnects, and specialty equipment carry 90°C terminal ratings, but you must verify this on the equipment label. Never assume.

This calculator enforces NEC 110.14(C) automatically. The “Terminal Cap (75°C)” line in the results shows the final ampacity after applying this critical limitation. This is our key differentiator from calculators that simply report the 90°C ampacity without terminal temperature enforcement.

Temperature Derating per NEC Table 310.15(B)(1)(1)

NEC Table 310.16 assumes conductors operate in an ambient temperature of 30°C (86°F). When the ambient temperature exceeds this baseline, the conductor's ampacity must be reduced because the conductor cannot dissipate heat as effectively. The correction factors come from NEC Table 310.15(B)(1)(1), which provides multipliers for each temperature range and insulation rating.

Common scenarios requiring temperature derating include attics (which routinely reach 120–150°F in summer), rooftop conduit runs exposed to direct sunlight, mechanical rooms adjacent to boilers or furnaces, and conduit runs near steam pipes or hot water lines. Even a relatively moderate increase to 104°F (40°C) reduces 90°C wire ampacity by 9% (factor of 0.91).

To apply temperature derating, multiply the base ampacity from the 90°C column by the correction factor for your ambient temperature. For example, #6 THHN copper in a 122°F attic: 75A (90°C base) × 0.82 (correction factor) = 61.5A derated ampacity. Compare this to the 75°C column value of 65A. Since 61.5A is less than 65A, the terminal cap does not further reduce the ampacity. The final usable ampacity is 61.5A, which means #6 is only good for about 61A in that attic. For a 50A continuous load (62.5A design), you would need to upsize to #4 AWG.

This is exactly where the 90°C insulation rating provides its benefit. If you used the 75°C column as your derating starting point instead: 65A × 0.75 (75°C factor at 122°F) = 48.75A, well below the 61.5A you get starting from the 90°C column. The 90°C rating gives you both a higher base ampacity and a gentler correction factor, which is why THHN is the standard choice for hot locations. The 75°C terminal cap still applies at the end, but after real derating the corrected value usually governs.

Bundling Adjustment per NEC Table 310.15(C)(1)

When more than three current-carrying conductors share a raceway, cable, or conduit, each conductor's ability to dissipate heat is reduced because the adjacent conductors are also generating heat. NEC Table 310.15(C)(1) provides adjustment factors based on the number of current-carrying conductors in the raceway.

The adjustment factors are: 4–6 conductors at 80%, 7–9 at 70%, 10–20 at 50%, 21–30 at 45%, 31–40 at 40%, and 41 or more at 35%. These factors multiply with the temperature correction factor, so a conductor in a hot attic inside a conduit with six other current-carrying conductors gets hit with both derations.

An important nuance: the count of “current-carrying conductors” does not always include every wire in the conduit. Equipment grounding conductors are excluded. Neutral conductors that carry only unbalanced load in a balanced system are typically excluded. However, in a three-phase, four-wire wye system supplying nonlinear loads (like LED drivers or computer power supplies), the neutral carries harmonic currents and must be counted. When in doubt, count the neutral. Oversizing is safer than undersizing.

Both the temperature correction factor and bundling adjustment factor are applied to the 90°C base ampacity before the terminal temperature cap. The formula is: Derated Ampacity = Base Ampacity (90°C) × Temperature Factor × Bundling Factor. The final usable ampacity is then the lesser of this derated value and the 75°C column value.

Copper vs Aluminum: When to Use Each

Copper and aluminum are the two conductor materials used in building wiring, and each has distinct advantages. Aluminum has only about 61% of copper's conductivity, meaning a smaller copper wire carries the same current as a larger aluminum wire. Copper is also more ductile, easier to terminate, and resists oxidation better than aluminum.

Aluminum, however, is lighter and less expensive than copper for the same ampacity, and the cost gap widens on larger feeders. A 200A dwelling service entrance in copper requires #2/0 AWG under the NEC 310.12 dwelling rule (#3/0 where that rule does not apply); the equivalent aluminum installation uses #4/0 AWG. Conductor prices track a commodity market and move continuously, so price both materials at your own supplier on the day rather than working from a fixed percentage or a rule of thumb.

In practice, aluminum is the standard choice for service entrance conductors, large feeder runs (60A and above), and sub-panel feeders. Copper is the standard for branch circuits (15–50A), device connections, and smaller wire sizes where the cost difference is minimal and the superior workability of copper saves labor time.

The “aluminum wiring scare” of the 1960s and 1970s stemmed from a specific problem: solid aluminum branch circuit wiring (sizes #12 and #10) used with devices designed for copper only. Aluminum expands and contracts more than copper with temperature cycling, and it oxidizes when exposed to air. These properties caused loose connections at receptacles, switches, and splice points, leading to overheating and fires. The problem was never the aluminum conductor itself. It was the termination compatibility.

Modern aluminum installations use stranded aluminum conductors in larger sizes (#6 and above) with anti-oxidant compound (NoAlox or equivalent) on every termination, and all breakers and panel lugs carry AL/CU ratings. These practices eliminate the historical termination issues. For sub-panel feeders and service entrance conductors, aluminum remains a safe, code-compliant, and cost-effective choice when installed correctly.

5 Wire Sizing Mistakes That Fail Inspection

Electrical inspectors see the same wire sizing errors repeatedly. Understanding these common mistakes helps you avoid costly rework and failed inspections.

1. Using the 90°C column without applying the terminal temperature cap. This is the most common error on online calculator results that homeowners bring to permit offices. A #8 THHN copper shows 55A in the 90°C column, but with standard 75°C equipment terminals, the usable ampacity is only 50A (the 75°C value). For a 50A continuous load (62.5A design), you need #6 AWG, not #8.
2. Forgetting the 125% continuous load factor for EV chargers. A 48A EV charger is a continuous load. The design amperage is 48 × 1.25 = 60A. Many DIY installations are wired with #6 AWG (65A at 75°C) on a 50A breaker, but the breaker should be 60A, and some inspectors require the wire to support the full 60A design load. Always size for the continuous-adjusted amperage, then select the breaker.
3. Ignoring temperature derating for attic runs. Wire routed through an attic must be derated for the elevated ambient temperature. An attic or rooftop run at 150°F reduces #12 THHN copper from 30A (90°C) to about 17A after derating (0.58 factor). This means a standard 20A circuit through a hot attic may require #10 AWG instead of #12.
4. Using the wrong NEC table column for the insulation type. TW insulation is rated 60°C, THW and THWN are 75°C, and THHN and THWN-2 are 90°C. Using the 90°C column for TW wire overstates the ampacity by 50–70% and creates a direct fire hazard. Always verify the insulation marking on the conductor jacket.
5. Undersizing the equipment grounding conductor. The EGC must be sized per NEC Table 250.122 based on the overcurrent protective device rating, not the wire size. A 100A sub-panel feeder requires a #8 AWG copper ground regardless of whether the hot conductors are #3 copper or #1 aluminum. Some installers mistakenly match the ground to two sizes below the hot conductors. This is not a valid method.

Common Wire Size Requirements

These are typical values for standard conditions. Always verify with the actual nameplate amperage, wire run length, ambient temperature, and number of conductors in conduit.

NEC Table 310.16 Copper Ampacities

Worked Examples

Example 1: EV Charger, 40A Continuous, 50 ft, Copper THHN

A Level 2 EV charger draws 40A continuously (charges for 8+ hours overnight). Per NEC 625.42, the branch circuit and OCPD must be sized at 125% of the continuous load. Design amps: 40A × 1.25 = 50A, so the breaker is 50A. Look up #8 THHN copper: 90°C ampacity = 55A. No derating needed (standard conditions). Apply terminal cap: min(55A, 50A at 75°C) = 50A. Since 50A ≥ the 50A design load, #8 AWG copper passes (many installers upsize to #6 for margin). A 48A charger steps up one size: 48 × 1.25 = 60A breaker with #6 AWG.

Example 2: Sub-Panel 100A, 75 ft, Copper THHN

A 100A sub-panel feeder is a non-continuous load (the diversified load rarely exceeds 80A in practice). Design amps: 100A. Look up wire sizes in the 90°C column: #3 copper = 115A, #4 copper = 95A. #4 is insufficient (95A < 100A after terminal cap of 85A). #3 THHN: 90°C ampacity = 115A. Terminal cap: min(115A, 100A at 75°C) = 100A. Since 100A ≥ 100A, #3 AWG copper passes. Breaker = 100A. Ground wire per NEC 250.122: #8 AWG.

Example 3: Kitchen Circuit 20A, 60 ft in Attic (150°F Ambient)

A 20A kitchen branch circuit routed through an attic that reaches 150°F under a summer roof deck. Design amps: 20A (non-continuous). At 150°F (150–158°F range), the 90°C correction factor is 0.58. Check #12 THHN copper: 90°C ampacity = 30A. Derated: 30A × 0.58 = 17.4A. Terminal cap: min(17.4A, 25A at 75°C) = 17.4A. Since 17.4A < 20A, #12 fails. Try #10 THHN: 40A × 0.58 = 23.2A. Terminal cap: min(23.2A, 35A) = 23.2A. Since 23.2A ≥ 20A, #10 AWG passes. The hot attic run requires upsizing from #12 to #10.

Wire Sizing FAQ

Wire Size for a Specific Breaker

Quick-answer pages for the most common breaker ratings, with the copper and aluminum conductor, ground wire, and code basis for each.

Related Calculators

Long run? Check voltage drop before you pull wire.

Ampacity is only half the sizing question. Past 100 feet, voltage drop often forces the gauge up before the ampacity table does. Run the numbers before you buy the wire, not after.