Wire Size / Ampacity Calculator (NEC)

Calculate

NEC Table 310.16 lists Copper and Aluminum separately. Aluminum starts at #12 AWG.

Select a specific AWG/kcmil size for manual analysis, or Auto-size to find the smallest compliant size.

Load operating ≥ 3 hours at maximum current. NEC 210.20 applies a 125% multiplier.

Load operating < 3 hours. No 125% multiplier applies.

Conductor insulation column used for base ampacity lookup. 75°C (THWN/RHW) is default for most commercial installations.

Per NEC 110.14(C), final ampacity is capped at the terminal temperature column regardless of insulation rating. Verify actual equipment listing/marking. 90°C is uncommon — verify equipment marking.

Installation ambient temperature in °C. Default 30°C is the Table 310.16 base condition. Correction applies per Table 310.15(B)(1).

Count of current-carrying conductors sharing this raceway. EGC and unbalanced-only neutrals are not counted. Default 3 — no adjustment applies.

NEC 310.15(B)(2): raceway in direct sunlight on a rooftop adds +18°C to ambient before correction lookup. Verify installation height for exact adder.

If > 100 ft, a voltage drop advisory fires. This calculator does not compute voltage drop — verify separately per NEC 210.19.

NEC 310.10(G): each parallel conductor must be #1/0 AWG or larger, equal length, same insulation, common terminations. Calculator analyzes one conductor of the parallel set.

Required when parallel set is enabled. Minimum 2. Each conductor carries total required ampacity divided by this count.

When provided, triggers explicit NEC 240.4(D) small conductor rule check for #10 AWG and smaller.

How to Use the Wire Size / Ampacity Calculator

  1. Select Conductor Material: Copper or Aluminum. Aluminum starts at #12 AWG per NEC Table 310.16 and has lower ampacity per AWG than Copper.

  2. Select Wire Size or Auto-size. Auto-size finds the smallest compliant conductor. Manual size evaluates the specific AWG/kcmil and adds Track B sizing efficiency analysis.

  3. Enter load values. Continuous load (operating ≥ 3 hours) and non-continuous load are entered separately — the 125% multiplier on continuous load is applied automatically per NEC 210.20.

  4. Select Insulation Temperature Rating: 60°C (TW), 75°C (THWN, RHW — default), or 90°C (THHN, THWN-2, XHHW-2). This determines the base ampacity column from Table 310.16.

  5. Select Termination Temperature Rating per NEC 110.14(C). 75°C is default for most commercial equipment. Final ampacity cannot exceed the terminal temperature column from Table 310.16.

  6. Enter Ambient Temperature in °C. Leave blank or enter 30 for the Table 310.16 base condition. Enter actual installation ambient for derating.

  7. Enter Current-Carrying Conductor Count. EGC and unbalanced-only neutrals are excluded from this count. Default 3 applies no bundling adjustment.

  8. Optionally enter rooftop flag, run length, parallel set configuration, and intended OCPD. Each unlocks additional soft checks and advisory notes.

  9. Click Calculate. Results show the full 4-step derating chain, sizing classification badge, and recommended wire size when applicable.

This calculator implements NEC Table 310.16 (raceway, cable, or earth installation, ≤3 current-carrying conductors, 30°C ambient base). Free-air installations per Table 310.17, motor branch circuit sizing per NEC 430.22, and service load calculations per NEC Article 220 are out of scope.

Inputs & Outputs

Inputs

  • •Conductor Material — Options: Copper, Aluminum / Aluminum-Clad
  • •Wire Size — Options: Auto-size (smallest compliant), #14 AWG, #12 AWG, #10 AWG, #8 AWG, #6 AWG, #4 AWG, #3 AWG, #2 AWG, #1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG, 250 kcmil, 300 kcmil, 350 kcmil, 400 kcmil, 500 kcmil, 600 kcmil, 700 kcmil, 750 kcmil, 800 kcmil, 900 kcmil, 1000 kcmil
  • •Continuous Load (A)
  • •Non-Continuous Load (A)
  • •Insulation Temperature Rating — Options: 60°C — TW, 75°C — THWN, RHW, THW, USE, ZW (default), 90°C — THHN, THWN-2, RHW-2, XHHW-2, USE-2, ZW-2
  • •Termination Temperature Rating — Options: 60°C — older or certain residential equipment, 75°C — typical commercial/industrial equipment (default), 90°C — uncommon, verify equipment listing/marking
  • •Ambient Temperature (°C)
  • •Current-Carrying Conductors in Raceway
  • •Rooftop / Direct Sunlight Exposure — Options: No — standard installation, Yes — raceway on rooftop in direct sunlight
  • •Run Length (optional) (ft)
  • •Parallel Conductors — Options: No — single conductor analysis, Yes — analyzing one conductor of a parallel set
  • •Number of Parallel Conductors (per phase)
  • •Intended OCPD Rating (optional) (A)

Outputs

  • •Required Ampacity (A)
  • •Base Ampacity (Table 310.16) (A)
  • •Ambient Correction Factor
  • •Bundling Adjustment Factor
  • •Adjusted Ampacity (A)
  • •Terminal Limit (110.14(C)) (A)
  • •Final Ampacity (A)
  • •Ampacity Utilization (%)
  • •Ampacity Margin / Deficit (A)
  • •Sizing Status

Formula

NEC Wire Ampacity Formulas (Article 310)

Required Ampacity (NEC 210.20 / 215.3):

I_required = (1.25 × I_continuous) + I_non_continuous

The 125% multiplier on continuous load is mandatory per NEC 210.20. Comparing raw load amperes to conductor ampacity without this factor is a code violation.

Base Ampacity:

I_base = NEC Table 310.16 [size][material][insulation temperature column]

4-Step Derating Chain:

Step Formula Reference
1 Ambient correction: I_base × ambient_correction Table 310.15(B)(1)
2 Bundling adjustment: I_base × ambient × bundling Table 310.15(C)(1)
3 Adjusted ampacity: I_adjusted = I_base × ambient × bundling Combined
4 Terminal limit: I_terminal = Table 310.16 at terminal temperature column NEC 110.14(C)

Final Ampacity (the controlling formula):

I_final = MIN(I_adjusted, I_terminal_limit)

Termination temperature is the step most simplified calculators miss. When 90°C insulation is paired with 75°C terminals, the terminal column caps final ampacity at the 75°C base value — the higher insulation rating provides no benefit for the final ampacity calculation.

Ampacity Utilization:

utilization = (I_required / I_final) × 100%

Ampacity Margin (ADEQUATE / AT-LIMIT):

I_margin = I_final − I_required

Ampacity Deficit (UNDERSIZED / INFEASIBLE):

I_deficit = I_required − I_final

Track A — Sizing Adequacy (priority order, mutually exclusive):

Priority Class Condition
1 INFEASIBLE I_required > I_final at largest wire size of selected material
2 UNDERSIZED I_required > I_final at selected wire size
3 AT-LIMIT I_required ≤ I_final AND utilization > 95%
4 ADEQUATE I_required ≤ I_final AND utilization ≤ 95%

Track B — Sizing Efficiency (manual size, ADEQUATE or AT-LIMIT only):

Class Condition
OPTIMAL Selected size = smallest size of selected material where I_required ≤ I_final
OVERSIZED A smaller size of the same material also satisfies I_required ≤ I_final

What Is Conductor Ampacity?

Conductor ampacity is the maximum current a wire can carry continuously without exceeding its temperature rating under defined installation conditions. NEC Table 310.16 is the primary reference for insulated conductors in conduit, cable, or directly buried — values assume not more than three current-carrying conductors in a raceway at 30°C (86°F) ambient. These base conditions almost never describe an actual installation without adjustment, which is why the derating chain exists. The NEC 310.16 base values are derived from heat balance — the conductor generates I²R heat, the insulation conducts it outward, and ampacity is the current at which the heat balance lands at the conductor's rated temperature. Ambient temperature shifts the equilibrium; bundling reduces the heat that the conduit air can carry away. Both adjustments directly affect how much current the conductor can safely handle. Ampacity limits are not safety margins in the colloquial sense. A conductor operating at exactly its rated ampacity is operating at its rated temperature. Exceeding the ampacity accelerates insulation degradation, increases failure probability over time, and eventually causes insulation failure. The NEC ampacity limits encode the thermal balance point; exceeding them is a code violation with real engineering consequences.

The Four-Step Derating Chain

Field ampacity calculations require four steps applied in a specific order. Skipping or combining steps is one of the most common errors in field electrical design. Step one is base ampacity from NEC Table 310.16. The lookup requires conductor size (AWG or kcmil), material (Copper or Aluminum), and insulation temperature rating (60°C, 75°C, or 90°C column). The insulation rating column must match the actual conductor marking, not the termination rating. Step two is ambient temperature correction per NEC Table 310.15(B)(1). The correction factor multiplies the base ampacity and varies by actual ambient minus the 30°C base condition. Factors below 1.0 reduce ampacity; factors above 1.0 (for cool environments) increase it. The lookup uses the actual conductor insulation column — not the terminal rating — because insulation temperature governs how much heat the conductor can tolerate before the insulation begins to degrade. Step three is bundling adjustment per NEC Table 310.15(C)(1). When more than three current-carrying conductors share a raceway, heat accumulation reduces the effective ampacity of each conductor. The factor applies to conductors that actually carry current — equipment grounding conductors and neutrals that carry only unbalanced current are excluded from the count. Step four — the step most frequently missed — is the NEC 110.14(C) termination temperature limit. The final ampacity cannot exceed the ampacity at the terminal temperature column from Table 310.16, regardless of how high the conductor insulation rating is. If a circuit uses 90°C THHN but terminates in equipment rated for 75°C, the final ampacity is capped at the 75°C table value. Upgrading the insulation does not help if the controlling step is the termination.

Why Termination Temperature Controls More Than Engineers Expect

The 110.14(C) terminal limit is the single most commonly overlooked step in field ampacity calculations. Most engineers focus on ambient and bundling derating and apply the 90°C insulation column to get maximum base ampacity — but when commercial equipment has 75°C-rated terminals (the norm for most breakers, panelboards, switchgear, and motor control centers under 100A), the 90°C base ampacity is immediately capped back to the 75°C value. This creates a paradox: specifying 90°C THHN for a circuit feeding standard 75°C-rated equipment provides no ampacity benefit over 75°C THWN. The benefit of 90°C insulation comes in two scenarios: (1) when the ambient temperature is high enough that the higher insulation column retains more ampacity than 75°C (because Table 310.15(B)(1) has a less aggressive correction factor for 90°C), and (2) when both terminations are genuinely listed and marked for 90°C operation. This calculator explicitly identifies whether termination temperature is the binding constraint. When it is, the result explanation notes that insulation upgrades alone cannot resolve an UNDERSIZED condition — only a terminal upgrade or wire upsize can help.

Continuous Loads and the 125% Multiplier

NEC 210.20 and 215.3 require that conductors serving continuous loads (defined as loads expected to operate at maximum current for three or more hours) be sized at 125% of the continuous current component. This is applied before the ampacity comparison, not after. The error pattern is consistent: an engineer calculates load current as 40A, selects #8 AWG copper THWN (75°C ampacity 50A), and concludes the circuit complies because 40A < 50A. If the load is continuous, the required ampacity is 1.25 × 40 = 50A, which exactly meets the conductor ampacity with zero margin. If any ambient correction or bundling applies, the circuit fails immediately. The 125% multiplier is not optional for continuous loads — it is a mandatory code requirement per NEC 210.20(A). This calculator applies the 125% multiplier to the continuous load component before comparing to final conductor ampacity. The result explanation always notes the calculation explicitly when a continuous load is present, because this is the most common field calculation error.

Key Facts

  • NEC 110.14(C) termination temperature limit frequently controls final ampacity — 90°C THHN paired with 75°C-rated terminals (standard for most commercial equipment) is capped at the 75°C ampacity column.
  • The 125% continuous load multiplier (NEC 210.20 / 215.3) is the most common field calculation error — omitting it from required ampacity before comparing to conductor ampacity is a code violation.
  • INFEASIBLE is scoped to the selected conductor material — switching from Aluminum to Copper typically increases ampacity by 24–30% per AWG, often resolving the condition without parallel conductors.
  • Track B (OPTIMAL / OVERSIZED) runs only when manual wire size is selected and Track A is ADEQUATE or AT-LIMIT. AT-LIMIT / OVERSIZED is valid — AWG steps are non-linear and the binding constraint can shift between adjacent sizes.
  • Aluminum conductors smaller than #12 AWG are not in NEC Table 310.16 for general branch/feeder wiring — the calculator rejects these combinations with a typed error.
  • Bundling adjustment counts only current-carrying conductors — equipment grounding conductors and neutrals carrying only unbalanced current are excluded from the count per NEC 310.15(C)(1).
  • Rooftop conduit in direct sunlight adds +18°C (33°F) to ambient before the Table 310.15(B)(1) correction lookup per NEC 310.15(B)(2). The actual adder varies with raceway height above the roof.

Applications

  • Branch circuit conductor sizing for residential, commercial, and industrial installations
  • Feeder conductor sizing per NEC 215.3 with continuous load 125% requirement
  • Derating analysis for high-ambient installations (mechanical rooms, rooftops, attics)
  • Bundled conductor ampacity verification in conduit with multiple circuits
  • Termination temperature compatibility check (90°C insulation vs 75°C terminals)
  • Aluminum conductor sizing for cost-optimized feeder and service entrance applications
  • Parallel conductor feasibility screening per NEC 310.10(G)
  • Auto-sizing for minimum compliant conductor selection on new circuit designs
  • Small conductor OCPD compliance check per NEC 240.4(D) for #10 AWG and smaller
  • Rooftop conduit ambient temperature adder verification per NEC 310.15(B)(2)

Example Calculation

Example Calculations

Example 1: Standard Branch Circuit — ADEQUATE / OPTIMAL

Inputs: 14 A continuous + 4 A non-continuous. Copper, 75°C THWN, 75°C terminals, 30°C ambient, 3 current-carrying conductors. Manual size: #12 AWG.

I_required = 1.25 × 14 + 4 = 21.5 A
I_base (#12 AWG Cu, 75°C) = 25 A
Ambient correction (30°C, 75°C) = 1.00
Bundling (3 CCC) = 1.00
I_adjusted = 25 × 1.00 × 1.00 = 25 A
I_terminal_limit (75°C, #12 AWG Cu) = 25 A
I_final = MIN(25, 25) = 25 A
utilization = 21.5 / 25 = 86.0%
I_margin = 25 − 21.5 = 3.5 A

Result: ADEQUATE / OPTIMAL. Utilization 86.0% — within limit with practical margin. #12 AWG is the smallest compliant size (#14 AWG Cu at 75°C = 20 A < 21.5 A required). NEC 240.4(D): #12 AWG Cu max OCPD 20 A — select OCPD ≤ 20 A.

Example 2: Termination Limit Controls — UNDERSIZED

Inputs: 70 A continuous. Copper, 90°C THHN, 75°C terminals (standard breaker), 30°C ambient, 3 CCC. Manual size: #6 AWG.

I_required = 1.25 × 70 = 87.5 A
I_base (#6 AWG Cu, 90°C) = 75 A
Ambient correction = 1.00, Bundling = 1.00
I_adjusted = 75 A
I_terminal_limit (75°C, #6 AWG Cu) = 65 A
I_final = MIN(75, 65) = 65 A
I_deficit = 87.5 − 65 = 22.5 A

Result: UNDERSIZED. The 90°C insulation supports 75 A but the 75°C terminal limit caps final ampacity at 65 A. Recommended: #3 AWG Cu (90°C base = 110 A; terminal limit 75°C = 100 A; I_final = 100 A; utilization 87.5%). This is the classic 110.14(C) trap — upgrading insulation alone cannot resolve UNDERSIZED when termination controls.

Example 3: Bundling Dominant — AT-LIMIT / OPTIMAL

Inputs: 35 A continuous. Copper, 75°C THWN, 75°C terminals, 30°C ambient, 8 current-carrying conductors. Manual size: #6 AWG.

I_required = 1.25 × 35 = 43.75 A
I_base (#6 AWG Cu, 75°C) = 65 A
Ambient correction (30°C) = 1.00
Bundling (7–9 CCC) = 0.70
I_adjusted = 65 × 0.70 = 45.5 A
I_terminal_limit (75°C, #6 AWG) = 65 A
I_final = MIN(45.5, 65) = 45.5 A
utilization = 43.75 / 45.5 = 96.2%

Result: AT-LIMIT / OPTIMAL. Utilization 96.2% — bundling adjustment of 0.70 is the binding constraint (terminal limit at 65 A does not control). #8 AWG Cu at 75°C with same bundling: 50 × 0.70 = 35 A < 43.75 A required — does not fit. #6 AWG is the smallest compliant size.

Standards & References

  • NFPA 70 (NEC) Article 310 — Conductors for General Wiring. The primary code reference for conductor ratings, ampacity tables, and derating procedures. Free read-only access via NFPA.
  • NFPA 70 — Standard for Electrical Installations — NFPA official page with current edition, supplements, and errata.
  • NEC Table 310.16 — Allowable Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth, Based on Ambient Temperature of 30°C (86°F).
  • NEC Table 310.15(B)(1) — Ambient Temperature Correction Factors Based on 30°C (86°F).
  • NEC 310.15(B)(2) — Adjustment for Ambient Temperature, Special Cases. Rooftop conduit temperature adders.
  • NEC Table 310.15(C)(1) — Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable.
  • NEC 110.14(C) — Temperature Limitations of Conductors and Equipment. Terminal temperature limits final ampacity regardless of conductor insulation rating.
  • NEC 210.20(A) — Continuous and Noncontinuous Loads. Branch circuit conductor minimum size at 125% of continuous current.
  • NEC 215.3 — Feeder Ampacity. Continuous load 125% requirement for feeders.
  • NEC 240.4(D) — Small Conductors. OCPD limits on #14 AWG through #10 AWG copper and #12–#10 AWG aluminum.
  • NEC 310.10(G) — Conductors in Parallel. Requirements for parallel conductor installations.
  • NEC 250.122 — Size of Equipment Grounding Conductors (out of scope — referenced for related sizing).
  • OSHA 1910.305 — Wiring methods, components, and equipment for general use. Federal regulation that adopts NEC installation rules.
  • NFPA 70 free-access portal (read-only) — Browse NEC 2023 online at no cost.
  • OSHA 1910.305 (free federal regulation) — Full regulatory text available online.
  • National Electrical Code overview (Wikipedia) — Background, history, and state adoption map.

Units

  • All ampacity values in amperes (A).
  • Temperature inputs and correction factors in degrees Celsius (°C).
  • Wire sizes in AWG (American Wire Gauge) or kcmil (thousands of circular mils).
  • Calculations are performed in SI units.
  • Both Metric and Imperial unit systems display identical results for this calculator since NEC Table 310.16 values are dimensionless within the defined installation conditions.

Limitations

  • Implements NEC Table 310.16 (raceway, cable, or directly buried, ≤3 CCC, 30°C ambient base). Free-air ampacity from Table 310.17 is not covered.
  • Motor branch circuit conductor sizing per NEC 430.22 (125% of FLA from Tables 430.247–430.250) is not covered — motors require specialized handling.
  • Voltage drop per NEC 210.19 informational note (3% branch / 5% combined target) is not computed — verify separately.
  • Conduit fill per NEC Chapter 9 is not computed — verify separately using the Conduit Fill Calculator.
  • Service load calculation per NEC Article 220 is not covered — feeder and service sizing requires demand factors.
  • Short-circuit and ground-fault current calculations are out of scope.
  • Arc-flash incident energy per IEEE 1584 is out of scope.
  • Aluminum conductors smaller than #12 AWG are not in NEC Table 310.16 for general branch/feeder wiring.
  • Parallel conductors per NEC 310.10(G) require each conductor to be #1/0 AWG or larger, equal length, same insulation, and common terminations — the calculator performs single-conductor analysis when the parallel flag is enabled; the user must verify all 310.10(G) requirements separately.
  • NEC 2023 (NFPA 70-2023) Article 310 is used. Earlier editions renumbered 310.15(B)(16) to 310.16 but ampacity values are nearly identical for major conductor types. Verify code edition adopted by AHJ.

Common Mistakes to Avoid

  • Comparing raw load amperes to conductor ampacity without the NEC 210.20 125% continuous load multiplier — I_required = 1.25 × I_continuous + I_non_continuous, not just the load current.
  • Using the 90°C insulation column for base ampacity when equipment terminals are rated 75°C — the NEC 110.14(C) terminal limit caps final ampacity at the 75°C column regardless.
  • Counting EGC and unbalanced-only neutrals in the current-carrying conductor count for bundling adjustment — these conductors are excluded from the count per NEC 310.15(C)(1).
  • Treating INFEASIBLE as 'the load is impossible to serve' — it means no single conductor of the selected material can carry this load; switching material or using parallel conductors per NEC 310.10(G) typically resolves it.
  • Applying the Table 310.16 base ampacity directly without ambient correction — at 40°C ambient, a 75°C conductor has 0.88 times its base ampacity; at 50°C, 0.75 times.
  • Selecting conductor insulation based on availability without confirming terminal temperature compatibility — the conductor insulation must be rated ≥ the termination temperature to use the higher column.

Frequently Asked Questions

What is the difference between the 75°C and 90°C ampacity columns in NEC Table 310.16?
The 90°C column lists higher base ampacity values because the conductor insulation can tolerate more heat before degrading. However, the NEC 110.14(C) termination limit caps final ampacity at the terminal temperature column regardless of insulation rating. For most commercial equipment with 75°C-rated terminals, specifying 90°C THHN provides no ampacity benefit over 75°C THWN. The 90°C column is useful when both terminations are listed for 90°C operation, or when high ambient temperature makes the less aggressive 90°C correction factor (from Table 310.15(B)(1)) beneficial.
Why does the calculator apply the 125% multiplier to continuous loads?
NEC 210.20 and 215.3 require conductors serving continuous loads (loads expected to operate at maximum current for three or more hours) to have an ampacity of at least 125% of the continuous current component. This multiplier must be applied to the required ampacity before comparing to conductor ampacity — not added as a separate safety factor afterward. Required ampacity = 1.25 × I_continuous + I_non_continuous. Omitting this multiplier is one of the most common field calculation errors and produces a code violation.
How do I determine how many current-carrying conductors to enter?
Count all conductors in the raceway that carry load current: phase conductors, the neutral conductor when it carries unbalanced current (such as with nonlinear loads), and any other load-carrying conductors. Do not count the equipment grounding conductor (EGC) or a neutral that carries only unbalanced current from a balanced three-phase system. The count drives the bundling adjustment factor from NEC Table 310.15(C)(1). A standard two-wire circuit has 2 CCC; a standard three-wire circuit has 3 CCC (no adjustment); a three-phase four-wire feeder has 3 CCC if the neutral is not significantly loaded.
What does INFEASIBLE mean, and does it mean the load cannot be served?
INFEASIBLE means no single conductor of the selected material can carry the required ampacity under the specified installation conditions. It does not mean the load is impossible to serve. Standard alternatives include switching conductor material (Copper carries approximately 24–30% more current per AWG than Aluminum), using parallel conductors per NEC 310.10(G) (each must be #1/0 AWG or larger, equal length, same insulation, common terminations), reducing the ambient temperature exposure, or splitting current-carrying conductors across multiple raceways to reduce bundling derating.
Can I use this calculator for motor branch circuits?
No. Motor branch circuit conductor sizing is governed by NEC 430.22, which requires conductors sized at 125% of the full-load amperes (FLA) from NEC Tables 430.247–430.250 (not the nameplate current), with additional rules for multiple motors, design letters, and service factor. The NEC 210.20 continuous load rule does not apply to motor conductors in the same way. Use a dedicated motor branch circuit calculator for motor applications. This calculator is appropriate for non-motor loads and for the general branch/feeder sizing context defined in NEC Article 310.
Why is the final ampacity lower than the value in NEC Table 310.16?
Table 310.16 lists base ampacity at standard conditions (≤3 current-carrying conductors in raceway, cable, or earth, at 30°C / 86°F ambient). Real installations frequently differ. NEC requires three additional steps: ambient correction per Table 310.15(B)(1) when ambient exceeds 30°C, bundling adjustment per Table 310.15(C)(1) when more than 3 current-carrying conductors share a raceway, and the termination temperature limit per NEC 110.14(C). The final ampacity is the lesser of the derated value and the terminal-column value. The calculator applies all four steps and identifies which one controls.
Do I always need to apply the 125% multiplier for continuous loads?
Yes, when the load is continuous. NEC 210.20(A) for branch circuits and 215.3 for feeders require continuous loads (operating at maximum current for 3 hours or more) to be sized at 125% of the continuous current. Required ampacity = 1.25 × continuous + non-continuous. Skipping the multiplier on continuous loads is a code violation. If a load is entirely non-continuous, the multiplier does not apply — the calculator includes a checkbox for that case to suppress the classification reminder.
Can I use 90°C THHN on a 75°C breaker?
The conductor itself is allowed; the ampacity calculation just uses the 75°C column for that wire size, not the 90°C column. NEC 110.14(C) caps final ampacity at the terminal rating regardless of insulation rating. A #6 AWG Cu THHN-2 wire physically tolerates 90°C operation, but on a 75°C-rated breaker, the connection point cannot exceed 75°C without damaging the breaker. The 90°C insulation rating is still useful as a starting point for derating calculations under elevated ambient or high bundling — it provides a higher base value before the derating multipliers apply, even though the result caps at the terminal column. Using the full 90°C ampacity requires both ends listed and marked for 90°C.
How do I size wire for a continuous load?
Apply the 125% multiplier per NEC 210.20(A) (branch circuits) or 215.3 (feeders). Required ampacity = 1.25 × continuous current + non-continuous current. Then size the wire so its final ampacity (after the full derating chain — base, ambient, bundling, terminal limit) is at least equal to the required ampacity. Example: a 50 A continuous load requires conductor ampacity of 62.5 A and an OCPD that protects 62.5 A. Pick the smallest AWG whose final ampacity meets or exceeds 62.5 A under the actual installation conditions. The OCPD must also be sized at 125% of continuous load, so a 50 A continuous load needs a 62.5 A OCPD — typically rounded up to the next standard size (70 A) per NEC 240.6(A), unless the OCPD is listed for use at 100% of its rating.
Does the neutral count in ampacity derating?
Situational. A neutral that carries only the unbalanced current of a balanced 3-phase 4-wire circuit is NOT counted as a current-carrying conductor per Table 310.15(C)(1). A neutral on a circuit with significant harmonic content from non-linear loads (computers, LED drivers, VFDs, switching power supplies) IS counted, because triplen harmonics (3rd, 9th, 15th) flow on the neutral even when phase currents are balanced. NEC 310.15(E) specifically requires the neutral to be counted when a major portion of the load is non-linear. A neutral on a single-phase 2-wire circuit (one hot, one neutral) IS counted because it carries the full circuit current as a return.
Does the ground wire count as a current-carrying conductor?
No. Equipment grounding conductors (EGC) are never counted as current-carrying for the bundling adjustment per NEC Table 310.15(C)(1). EGCs carry current only during fault conditions, not under normal operation, so they do not contribute to the heat load that drives the bundling derating. The calculator does not include EGC in the current-carrying conductor count input. Note that EGC must still be included in conduit fill calculations per NEC Chapter 9, Table 1 — that is a separate calculation for raceway sizing, not for ampacity.
Why is my wire ampacity okay but my breaker size still limited?
The small conductor rule per NEC 240.4(D) caps OCPD on conductors #14 through #10 AWG regardless of how high the conductor's derated ampacity computes. #14 AWG Cu can show final ampacity of 25 A under favorable conditions (90°C insulation, 90°C terminations, no derating), but NEC 240.4(D) still caps the OCPD at 15 A for #14 AWG Cu. The conductor is "ampacity-adequate but OCPD-constrained". Resolution: either reduce the OCPD to the small conductor rule limit, or upsize the conductor to #8 AWG or larger, where the rule no longer applies. The calculator surfaces this through a separate soft check when an intended OCPD is provided as input.
Why doesn't 90°C insulation always give 90°C ampacity?
The 90°C column in Table 310.16 represents the conductor's thermal capability, but final usable ampacity depends on the entire circuit, not just the conductor. NEC 110.14(C) limits final ampacity to the column matching the equipment terminal rating. Most circuit breakers, receptacles, panels, and disconnects are rated 75°C. Even 90°C-rated wire on a 75°C-rated panel is capped at the 75°C column for that wire size. The 90°C insulation is still useful: it allows higher base values during derating calculations, so under elevated ambient or high bundling, the derated 90°C result can exceed what 75°C would have produced under the same factors. But the final cap remains at the terminal rating unless both ends are 90°C-listed.
How do I count current-carrying conductors for the bundling adjustment?
Count all phase conductors that carry current under normal operation. Equipment grounding conductors (EGC) are NOT counted. Neutral conductors are situational based on circuit type (see "Does the neutral count in ampacity derating?" above). Signaling and control conductors that carry current are counted. The calculator uses the user-supplied count — verifying which conductors are current-carrying is the user's responsibility, since it depends on the actual circuit topology and load type.
Can I switch from aluminum to copper without changing wire size?
Sometimes, but not always. Copper has 24-30% higher ampacity per AWG than aluminum at the same insulation and termination rating. A switch from aluminum to copper at the same AWG often resolves UNDERSIZED status without upsizing. The calculator's material change soft check evaluates this explicitly when an aluminum conductor is UNDERSIZED. Practical considerations beyond ampacity: aluminum requires listed AL-rated terminals (CO/ALR or AL/CU), antioxidant compound at terminations, and different torque values. Material substitution during construction without specification revision can create installation problems.
Does NEC Table 310.16 ampacity include voltage drop?
No. NEC ampacity is a thermal limit — it ensures the conductor does not exceed its insulation temperature rating under load. Voltage drop is a separate concern addressed in NEC 210.19 informational note (3% on branch circuits, 5% combined feeder + branch as recommended targets) and is governed by conductor resistance, run length, and load current. Long runs frequently require upsizing for voltage drop independent of ampacity. The calculator does not compute voltage drop — verify separately with a voltage drop calculator.
What's the difference between 60°C, 75°C, and 90°C insulation ratings?
The temperature rating defines the maximum operating temperature the conductor's insulation can handle without degradation. Higher ratings allow higher base ampacity at the same wire size and benefit more from elevated ambient (less aggressive derating per Table 310.15(B)(1)). 60°C insulation (TW) is older technology, mostly used in fixtures and older residential work. 75°C insulation (THWN, RHW) is standard for most commercial work and matches typical equipment terminal ratings. 90°C insulation (THHN, THWN-2, XHHW-2) is increasingly standard but typically capped by 75°C terminal limits per 110.14(C). The 90°C column is most useful as a starting point for derating in high-ambient or high-bundling installations, where its more favorable derating produces a higher final value than the 75°C base would have provided.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Conduit Fill Calculator Nec

Voltage Drop Calculator Nec Copper

Breaker Size Calculator

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