Q: Can I use the 90°C column for conductor sizing?

You can use the 90°C column as your starting point for derating calculations. However, NEC 110.14(C) requires that the final derated ampacity not exceed the ampacity in the column matching the termination temperature rating, which is usually 75°C for standard equipment.

Q: How does ampacity differ from voltage drop?

Ampacity determines the maximum safe current based on heat. Voltage drop determines the voltage lost along the conductor length. Both are separate constraints and the conductor must satisfy both. On short runs ampacity usually controls; on long runs voltage drop often requires a larger conductor.

⚠ Important Legal Notice — Please Read Before Using This Calculator

This ampacity calculator is provided for educational and estimating purposes only. It is not a substitute for professional electrical engineering design, and the results should not be used as the sole basis for selecting conductors or making installation decisions. Actual ampacity depends on many factors not fully captured by this tool, including installation method, cable grouping details, mutual heating effects, conductor termination temperature limits, and local code amendments. Base ampacity values are from NEC Table 310.16 for not more than three current-carrying conductors in raceway, cable, or earth at 30°C ambient. Temperature correction factors are from NEC Table 310.15(B)(1) and conduit fill adjustments from NEC Table 310.15(C)(1). Always have a licensed electrician or professional engineer verify conductor sizing before purchasing or installing wire and cable. Ramcorp Wire & Cable assumes no liability for errors, omissions, or any damages arising from the use of this calculator. By using this tool, you acknowledge that it is an estimation aid only and agree to consult a qualified professional for all final conductor sizing decisions.

Free Wire Ampacity Calculator with NEC Derating

Use this free ampacity calculator to look up the current-carrying capacity of copper and aluminum conductors based on NEC Table 310.16, then apply real-world derating factors for ambient temperature and conduit fill. The calculator covers wire sizes from 14 AWG through 1000 kcmil across all three standard insulation temperature ratings: 60°C, 75°C, and 90°C.

Proper conductor sizing is critical for safety and code compliance. An undersized conductor can overheat, damage insulation, and create fire hazards. The National Electrical Code (NEC) provides ampacity tables and derating requirements that electricians, engineers, and inspectors use to ensure conductors operate within safe temperature limits under actual installation conditions.

Wire Ampacity Calculator

NEC Table 310.16 with temperature & conduit fill derating

Conductor Material

Insulation Temperature Rating

Wire Gauge (AWG / kcmil)

Ambient Temperature

Current-Carrying Conductors in Raceway

Termination Temperature Rating

Per NEC 110.14(C) — caps final ampacity

Load Current (optional — for sizing check)

Amperes (A) — leave blank for lookup only

Load Type

NEC 210.20(A) — continuous loads require 125% sizing

—

Base Ampacity (A)

—

Temp Correction

—

Conduit Fill

—

Derated Ampacity (A)

—

Final Allowable (A)

NEC Reference: Base ampacity from NEC Table 310.16 (not more than 3 current-carrying conductors in raceway, cable, or earth, based on ambient of 30°C). Temperature correction per Table 310.15(B)(1). Conduit fill adjustment per Table 310.15(C)(1). Termination limits per NEC 110.14(C). Small conductor OCPD limits per NEC 240.4(D). Continuous load sizing per NEC 210.20(A). Always verify calculations with a licensed electrician for code compliance.

How to Use This Calculator

Select your conductor material (copper or aluminum), insulation temperature rating, and wire gauge. Then set the ambient temperature where the cable will be installed and the number of current-carrying conductors in the raceway. Click Calculate Ampacity to see the base ampacity from NEC Table 310.16, the applicable derating factors, and the final adjusted ampacity.

Optionally, enter your load current to check whether the selected conductor can safely carry your load after derating. If it cannot, the calculator will automatically recommend the smallest wire gauge that meets your requirements and provide a link to request a quote.

What Is Ampacity?

Ampacity is the maximum amount of electric current a conductor can carry continuously under specific conditions without exceeding its temperature rating. The word is a combination of “ampere” and “capacity.” Every conductor has an ampacity rating determined by its size, material, insulation type, and installation conditions.

When a conductor carries current, its resistance converts some electrical energy into heat. If the current exceeds the conductor’s ampacity, the heat generated can degrade the insulation, potentially causing short circuits, fires, or equipment damage. This is why the NEC provides detailed ampacity tables and requires conductors to be sized so that the operating current does not exceed the derated ampacity under the actual installation conditions.

NEC Table 310.16 Explained

NEC Table 310.16 is the most commonly referenced ampacity table in the National Electrical Code. It lists the allowable ampacities of insulated conductors rated up to 2000 volts, with not more than three current-carrying conductors in a raceway, cable, or earth (directly buried), based on an ambient temperature of 30°C (86°F). The table provides values for both copper and aluminum conductors across three insulation temperature ratings: 60°C, 75°C, and 90°C.

The values in Table 310.16 represent the base ampacity — the starting point before any derating. In real-world installations, you almost always need to adjust these values downward based on the actual ambient temperature and the number of conductors sharing a raceway.

Temperature Correction (Derating for Ambient Temperature)

NEC Table 310.15(B)(1) provides correction factors for ambient temperatures other than 30°C. When conductors are installed in environments hotter than 30°C — such as attics, boiler rooms, rooftops, or industrial facilities — the ampacity must be reduced because the conductor starts closer to its maximum temperature limit and has less thermal headroom.

The correction factor depends on both the ambient temperature and the insulation temperature rating. Higher-rated insulation (90°C) retains more of its ampacity at elevated temperatures than lower-rated insulation (60°C). For example, at 40°C ambient, a 90°C-rated conductor retains 91% of its base ampacity, while a 60°C-rated conductor drops to only 82%. In cooler environments (below 30°C), the correction factor is greater than 1.0, meaning the conductor can actually carry more current than its base rating.

Conduit Fill Adjustment (Derating for Multiple Conductors)

When more than three current-carrying conductors share a raceway or cable, heat dissipation is reduced because the conductors warm each other. NEC Table 310.15(C)(1) requires the ampacity to be reduced based on the number of current-carrying conductors. For 4–6 conductors, the adjustment factor is 80%; for 7–9 conductors, 70%; and for 10–20 conductors, only 50% of the base ampacity is allowed.

The count includes only current-carrying conductors — the equipment grounding conductor and neutral conductors that carry only unbalanced current from other conductors are generally not counted. However, if the neutral carries harmonic currents (common with nonlinear loads like LED drivers and VFDs), it must be counted as a current-carrying conductor.

Insulation Temperature Ratings

The insulation material around a conductor determines its maximum operating temperature, which directly affects its ampacity. The three standard ratings in NEC Table 310.16 are:

60°C insulation (TW, UF): The lowest rating, used primarily in older installations and some residential applications. Provides the least ampacity for a given wire size. Most modern installations have moved to higher-rated insulation types.

75°C insulation (THW, THWN, XHHW, USE): The most commonly specified rating for general commercial and industrial work. This is also the maximum temperature rating for most standard terminations and circuit breakers per NEC 110.14(C), which means that even if you use 90°C-rated wire, the ampacity is often limited to the 75°C column based on the termination temperature rating.

90°C insulation (THHN, THWN-2, XHHW-2, USE-2): The highest standard rating in NEC Table 310.16. While the base ampacity is higher, the primary advantage of 90°C insulation is that it provides more thermal headroom for derating. After applying temperature and conduit fill corrections, a 90°C conductor may still meet the load requirement where a 75°C conductor would not — as long as the derated ampacity does not exceed the 75°C column value at the termination.

High-Temperature Insulation (105°C – 250°C)

For industrial, mining, and high-heat applications, insulation ratings above 90°C are available. This calculator supports ratings from 105°C through 250°C, using the NEC 310.15(B) correction factor formula: CF = √((T_rating − T_ambient) / (T_rating − 30)). The base ampacity comes from the 90°C column of NEC Table 310.16, but the higher insulation rating dramatically improves the temperature correction factor in elevated-ambient environments.

105°C (MV-105): Common for medium voltage cables. Rated for continuous operation at 105°C, making these cables standard for utility and industrial power distribution.

125°C (SIS, SA, silicone rubber): Used in switchboard wiring, industrial control panels, and applications near moderate heat sources. Silicone rubber insulation provides excellent flexibility at both high and low temperatures.

150°C (FEP, ECTFE): Fluoropolymer-insulated cables for demanding environments — chemical plants, steel mills, glass manufacturing, and anywhere ambient temperatures regularly exceed 90°C. FEP (fluorinated ethylene propylene) offers excellent chemical resistance, while ECTFE (ethylene chlorotrifluoroethylene) provides outstanding mechanical toughness alongside thermal performance. Ramcorp stocks a range of high-temperature cables in these insulation types.

200°C (FEP, PTFE, PFA): For extreme environments such as furnace wiring, heat treating equipment, and engine compartments. PTFE (polytetrafluoroethylene) and PFA (perfluoroalkoxy) maintain their dielectric properties and flexibility at sustained high temperatures, making them the go-to insulation for aerospace, semiconductor manufacturing, and industrial oven wiring.

250°C (PFA, nickel-plated copper): The highest commercially available ratings, used in foundries, kilns, and other applications where conductors are exposed to extreme radiant heat. PFA insulation maintains integrity at temperatures that would decompose standard fluoropolymers. Nickel-plated copper conductors prevent oxidation at these extreme temperatures, maintaining conductivity over the life of the installation. Our High Temperature Wire & Cable Guide covers selection and specification in detail.

Copper vs. Aluminum Ampacity

Copper conductors have higher ampacity than aluminum at every wire gauge because copper has lower electrical resistance, which means less heat generation for the same current. For example, 4 AWG copper at 75°C is rated for 85A, while 4 AWG aluminum at 75°C is rated for only 65A. To get equivalent ampacity, aluminum typically needs to be one to two sizes larger — 2 AWG aluminum (90A at 75°C) is the approximate equivalent of 4 AWG copper (85A at 75°C).

Despite the lower ampacity, aluminum is widely used for larger feeder and service entrance conductors because it costs significantly less per foot and weighs about one-third as much as copper. Our How to Choose the Right Cable guide covers the copper vs. aluminum decision in detail. Ramcorp stocks both copper building wire and aluminum conductors in all standard sizes.

Termination Temperature Limits (NEC 110.14(C))

One of the most important — and most overlooked — rules in conductor sizing is NEC 110.14(C). Even if you install 90°C-rated wire, the termination temperature rating of the equipment (circuit breakers, lugs, disconnects) determines the maximum ampacity you can actually use. Most standard equipment terminations are rated for 75°C, which means the final allowable ampacity cannot exceed the 75°C column value from NEC Table 310.16, regardless of the insulation rating. This calculator applies this cap automatically based on your termination selection.

The 90°C column is still valuable because it gives you a higher starting point for derating calculations. After applying temperature and conduit fill corrections, the derated 90°C value may still be within the 75°C termination limit where the derated 75°C value would not be. Select “No limit” in the termination dropdown for educational calculations or when equipment is listed for higher termination temperatures.

Small Conductor Protection (NEC 240.4(D))

NEC 240.4(D) imposes hard limits on overcurrent protection for small conductors, regardless of calculated ampacity. These limits cannot be exceeded even when derating produces a higher number. The maximum overcurrent protection device (fuse or breaker) ratings are:

Copper: 14 AWG — 15A, 12 AWG — 20A, 10 AWG — 30A

Aluminum: 12 AWG — 15A, 10 AWG — 25A

This rule exists because small conductors are more susceptible to damage from overcurrent and are commonly used in branch circuits where the risk of contact with combustible materials is highest. The calculator displays a warning and caps the final ampacity when 240.4(D) applies.

Continuous Load Rule (NEC 210.20(A))

A continuous load is any load where the maximum current is expected to continue for three hours or more — common examples include commercial lighting, data center equipment, and industrial process controls. NEC 210.20(A) requires that the conductor be sized for 125% of the continuous load. This means a 100A continuous load requires a conductor rated for at least 125A after all derating. The calculator applies this multiplier automatically when you select “Continuous load” in the load type dropdown.

NEC Table 310.16 — Ampacity Reference (Copper)

Wire Gauge	60°C (A)	75°C (A)	90°C (A)
14 AWG	15	20	25
12 AWG	20	25	30
10 AWG	30	35	40
8 AWG	40	50	55
6 AWG	55	65	75
4 AWG	70	85	95
2 AWG	95	115	130
1/0 AWG	125	150	170
4/0 AWG	195	230	260
250 kcmil	215	255	290
500 kcmil	320	380	430

For the complete table including all wire gauges and aluminum values, refer to NEC Table 310.16 in the current edition of the National Electrical Code. Our AWG Wire Gauge Guide also provides a helpful cross-reference of common wire sizes and their properties.

Frequently Asked Questions

What is the ampacity of 12 AWG copper wire?

Per NEC Table 310.16, 12 AWG copper has a base ampacity of 20A at 60°C, 25A at 75°C, and 30A at 90°C. In typical residential installations with 3 or fewer conductors at 30°C ambient, 12 AWG copper THHN (90°C rated) is often used on 20A circuits. However, the ampacity is usually limited to 25A based on the 75°C termination temperature rating of standard circuit breakers per NEC 110.14(C).

When do I need to apply derating factors?

Derating is required whenever the installation conditions differ from the base assumptions of NEC Table 310.16: ambient temperature above 30°C (86°F), or more than three current-carrying conductors in a single raceway or cable. If both conditions apply, you multiply both factors together. For example, 6 AWG copper THHN at 40°C ambient with 6 conductors in a conduit: 75A base × 0.91 (temp) × 0.80 (fill) = 54.6A adjusted ampacity.

Can I use the 90°C column for conductor sizing?

You can use the 90°C column from Table 310.16 as your starting point for derating calculations. However, NEC 110.14(C) requires that the final derated ampacity not exceed the ampacity listed in the column matching the termination temperature rating. Most standard terminations (circuit breakers, lugs, disconnects) are rated for 75°C, so the derated 90°C ampacity is often capped at the 75°C value. The 90°C rating is most valuable when you need the extra thermal headroom to survive derating and still have a usable ampacity.

Does the equipment grounding conductor count toward conduit fill?

No. Equipment grounding conductors are not counted as current-carrying conductors for conduit fill derating purposes per NEC 310.15(E). Similarly, a neutral conductor that carries only the unbalanced current from other conductors of the same circuit is generally not counted. However, if the neutral carries harmonic currents from nonlinear loads (such as electronic ballasts, LED drivers, VFDs, or computer power supplies), it must be counted as a current-carrying conductor.

How does ampacity differ from voltage drop?

Ampacity and voltage drop are two separate constraints on conductor sizing, and the conductor must satisfy both. Ampacity determines the maximum safe current based on heat — exceeding it risks fire or insulation damage. Voltage drop determines the voltage lost along the length of the conductor — exceeding it wastes energy and causes equipment performance issues. On short runs, ampacity is usually the controlling factor. On long runs, voltage drop often requires a larger conductor than ampacity alone would dictate.

How does high-temperature insulation (105°C–250°C) affect ampacity?

High-temperature insulation does not increase the base ampacity from NEC Table 310.16 — that is determined by conductor size and material. What it does is dramatically improve the temperature correction factor at elevated ambient temperatures. For example, at 100°C ambient, a 90°C-rated conductor cannot be used at all, but a 200°C-rated conductor retains 77% of its base ampacity. This means in high-heat industrial environments, high-temp insulation lets you use the same (or smaller) wire size where standard insulation would fail or require excessive upsizing.

What is the difference between NEC Table 310.16 and Table 310.15?

Table 310.16 contains the base ampacity values for conductors in raceway, cable, or earth. Table 310.15 contains the adjustment and correction factors — specifically, Table 310.15(B)(1) for temperature correction and Table 310.15(C)(1) for conduit fill adjustment. You start with the base value from 310.16 and multiply it by the applicable factors from 310.15 to get the adjusted ampacity for your actual installation conditions.

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Disclaimer: This ampacity calculator and the accompanying guide are provided for informational and educational estimation purposes only. This tool is not installation advice and does not constitute professional electrical, engineering, or code-compliance advice. The results produced by this calculator are approximations based on NEC Table 310.16, Table 310.15(B)(1), and Table 310.15(C)(1) — actual allowable ampacity in an installed circuit may differ due to factors including but not limited to installation method, termination temperature ratings, mutual heating from adjacent circuits, cable tray fill, and local code amendments. Installing wire & cable can be dangerous and pose a risk of possible electric shock, fire, or other hazards. Building codes, NEC editions, and local amendments change periodically. Always have conductor sizing verified by a licensed electrician or professional engineer and consult your local authority having jurisdiction (AHJ) before specifying or installing cable. Ramcorp Wire & Cable makes no warranties, express or implied, regarding the accuracy of this calculator and assumes no liability for errors, omissions, or any losses or damages arising from its use. Images are for illustration purposes and may not reflect actual installed products.

The information on this page is provided for general reference only and may contain errors or omissions. NEC® is a registered trademark of the National Fire Protection Association (NFPA®). UL® is a registered trademark of Underwriters Laboratories. All other trademarks, product names, and brand names referenced on this page are the property of their respective owners. Ramcorp Wire & Cable is not affiliated with or endorsed by these organizations unless explicitly stated.

Free Wire Ampacity Calculator — NEC Table 310.16 with Derating | Ramcorp Wire & Cable