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Cable Sizing Calculator — NEC/NFPA 70 🇺🇸

United StatesEdition NEC/NFPA 70:2023Free Online Tool

The National Electrical Code (NEC), published as NFPA 70 by the National Fire Protection Association, is the standard for electrical installations throughout the United States and much of the Americas. The 2023 edition reorganised Article 310 on conductors, but the core methodology remains: determine the ampacity from Table 310.16 (the most widely referenced table in the entire code), apply ambient temperature correction factors per Section 310.15(B)(1), apply conductor count adjustment factors per Section 310.15(C)(1), and verify that termination temperature limits per Section 110.14(C) are respected.

Unlike IEC and BS standards that use metric mm² conductor sizes, NEC uses AWG (American Wire Gauge) for conductors up to 4/0 and kcmil (thousand circular mils) for larger conductors. The NEC also introduces the concept of the continuous load 125% factor (Article 210.20), which has no direct equivalent in IEC standards and significantly affects conductor selection for commercial and industrial circuits.

This calculator implements the complete NEC 2023 conductor sizing procedure including the critical 110.14(C) termination temperature check that is the single most common source of NEC sizing errors. Every result includes specific Article and Table references.

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How Cable Sizing Works Under NEC/NFPA 70

Step 1 — Determine the Load Current

Calculate the circuit current from the connected load. For single-phase at 120V or 240V: I = P / (V × PF). For three-phase at 208V or 480V: I = P / (√3 × V × PF). NEC Article 220 provides methods for calculating branch circuit, feeder, and service loads. For motor loads, Article 430.6 requires using the Full Load Current (FLC) from NEC Table 430.250 rather than the nameplate value.

Step 2 — Apply the Continuous Load Factor

NEC Section 210.20(A) requires that where a load operates continuously (3 hours or more), the conductor must be sized for 125% of the continuous load plus 100% of the non-continuous load: Idesign = 1.25 × Icontinuous + Inon-continuous. This requirement also applies to the overcurrent device rating per Section 210.20(A). The 125% factor accounts for thermal accumulation during sustained loading and provides a safety margin that IEC standards address differently through their derating methodology.

Step 3 — Select Conductor Ampacity from Table 310.16

NEC Table 310.16 provides ampacities for insulated conductors rated 0–600V in raceway, cable, or direct buried, based on an ambient temperature of 30°C (86°F). The table has three column groups for conductor temperature ratings: 60°C, 75°C, and 90°C. Common insulation types include:

  • 60°C: TW, UF
  • 75°C: THW, THWN, XHHW, USE
  • 90°C: THWN-2, THHN, XHHW-2, USE-2

Select a conductor size whose ampacity in the appropriate column is at least equal to Idesign. For example, #6 AWG copper THWN-2 at 75°C is rated 65A; at 90°C it is rated 75A.

Step 4 — Apply Ambient Temperature Correction

If the ambient temperature differs from 30°C (86°F), NEC Table 310.15(B)(1) provides correction factors. At 40°C (104°F), the factor is 0.88 for 75°C conductors. The corrected ampacity is: Icorrected = Itable × Correction Factor. For higher temperatures (found in boiler rooms, rooftops, attics), the factors decrease further: 50°C gives 0.75 for 75°C conductors.

Step 5 — Apply Conductor Count Adjustment

NEC Section 310.15(C)(1) requires adjustment when more than 3 current-carrying conductors are in a raceway or cable. The adjustment factors are: 4–6 conductors = 80%, 7–9 = 70%, 10–20 = 50%, 21–30 = 45%, 31–40 = 40%, 41+ = 35%. The adjusted ampacity is: Iadjusted = Icorrected × Adjustment Factor. Note that the neutral conductor counts as current-carrying if it carries unbalanced current, and the equipment grounding conductor never counts.

Step 6 — Check Termination Temperature per Section 110.14(C)

This is the step most commonly missed or misunderstood. NEC Section 110.14(C) requires that the conductor ampacity be determined based on the lowest temperature rating of any connected termination, conductor, or device. For circuits rated:

  • 100A or less: Use the 60°C column unless all terminations, conductors, and devices are rated for 75°C (Section 110.14(C)(1)(a)).
  • Over 100A: Use the 75°C column unless all components are rated for higher temperatures (Section 110.14(C)(1)(b)).

You CAN use the 90°C column for derating/adjustment calculations only, then verify the final derated ampacity against the 60°C or 75°C column value. This is the critical 110.14(C) interaction that often determines the final conductor size.

Step 7 — Verify Voltage Drop

Although the NEC does not set a mandatory voltage drop limit, NEC Sections 210.19(A) Informational Note No. 4 and 215.2(A)(4) Informational Note No. 2 recommend a maximum of 3% for branch circuits and 5% total for feeder plus branch circuit combined. Chapter 9 Table 8 provides DC resistance values for conductors, and Table 9 provides AC resistance and reactance for conductors in various raceways. Calculate: Vdrop = 2 × I × L × R / 1000 for single-phase, or Vdrop = √3 × I × L × Z / 1000 for three-phase, where R is from Table 8 and Z from Table 9.

Key Reference Tables

Table 310.16 — Ampacity of Insulated Conductors (0–600V)

The single most referenced table in the NEC. Provides ampacities for up to 3 current-carrying copper or aluminium conductors in raceway, cable, or direct buried, at 30°C ambient, for 60°C, 75°C, and 90°C insulation ratings. Conductor sizes from 14 AWG to 2000 kcmil.

Select the conductor size whose ampacity in the applicable temperature column meets or exceeds the design current (including continuous load factor). For a 100A circuit with 75°C terminations, #3 AWG copper at 100A (75°C column) would be selected.

Table 310.15(B)(1) — Ambient Temperature Correction Factors

Correction factors for ambient temperatures other than 30°C (86°F). Provides factors for 60°C, 75°C, and 90°C rated conductors at ambient temperatures from 21–80°C.

Multiply the Table 310.16 ampacity by the correction factor. At 36–40°C ambient, the factor for 75°C conductors is 0.88. For outdoor installations in Phoenix, Arizona (50°C+), factors can drop below 0.75.

Section 310.15(C)(1) — Conductor Count Adjustment Factors

Adjustment (formerly derating) factors for more than 3 current-carrying conductors in a raceway or cable assembly. Factors range from 80% (4–6 conductors) down to 35% (41+ conductors).

Count all current-carrying conductors in the raceway (excluding equipment grounds, and excluding neutrals that carry only unbalanced current). For a conduit with 12 current-carrying conductors, apply a 50% adjustment to the ampacity.

Chapter 9 Table 8 — Conductor Properties

DC resistance per 1000 feet for copper and aluminium conductors, both stranded and solid, at 75°C. Covers sizes from 18 AWG to 2000 kcmil. Essential for voltage drop calculations.

Use the resistance value to calculate voltage drop: V_drop = 2 × I × L × R / 1000 (single-phase). For #6 AWG copper stranded: R = 0.510 Ω/1000ft.

Chapter 9 Table 9 — AC Resistance and Reactance

Effective AC impedance (Z), resistance (R), and reactance (X_L) per 1000 feet for copper and aluminium conductors in steel, aluminium, or PVC conduit. Accounts for skin effect and proximity effect in AC circuits.

For accurate three-phase voltage drop: V_drop = √3 × I × L × (R cosφ + X_L sinφ) / 1000. Table 9 values differ from Table 8 because AC impedance includes reactance and conduit material effects.

Section 110.14(C) — Termination Temperature Limitations

Not a table per se, but the governing rule for conductor sizing. Specifies that conductors must be sized based on the lowest temperature rating of any connected component: 60°C for circuits ≤100A (unless all components are 75°C rated), 75°C for circuits >100A.

After selecting a conductor based on the 90°C column (for derating calculations), verify that the derated ampacity does not exceed the 60°C or 75°C column value for that conductor size. This check often governs the final size selection.

Worked Example — NEC/NFPA 70 Cable Sizing

Scenario

A 100A continuous feeder in a commercial building in Texas. The feeder runs 200 feet (61 m) from the main distribution panel to a sub-panel through EMT conduit. Ambient temperature is 86°F (30°C). No additional conductors in the conduit beyond the 3-phase set plus neutral and ground. Supply is 208Y/120V three-phase. Conductor type: THWN-2 copper. All terminations are rated 75°C.

1

Determine the load current

The sub-panel serves a continuous commercial lighting and receptacle load with a calculated maximum demand of 95A at 208V three-phase.

I_load = 95A (continuous)

2

Apply continuous load factor

Per NEC Section 215.3 and 210.20(A), the feeder conductor and overcurrent device must be sized for 125% of the continuous load.

I_design = 1.25 × I_continuous = 1.25 × 95 = 118.75A

I_design = 118.75A

3

Select overcurrent protective device

The next standard OCPD size above 118.75A per NEC Section 240.6(A) is 125A.

OCPD = 125A circuit breaker

4

Select conductor from Table 310.16

Since all terminations are rated 75°C and the circuit exceeds 100A, Section 110.14(C)(1)(b) permits using the 75°C column. From Table 310.16, copper at 75°C: #1 AWG = 130A (satisfactory, 130A ≥ 118.75A). #2 AWG = 115A would be insufficient.

Selected: #1 AWG THWN-2 copper (130A at 75°C)

5

Check ambient temperature correction

Ambient is 30°C (86°F), which is the reference temperature for Table 310.16. No correction required.

Correction Factor = 1.00

No ambient correction needed.

6

Check conductor count adjustment

The conduit contains 3 phase conductors + 1 neutral + 1 equipment ground. Per Section 310.15(C)(1), the neutral counts as current-carrying only if it carries unbalanced current. For a balanced three-phase four-wire system with linear loads, the neutral does not count. The equipment ground never counts. Total current-carrying conductors = 3. Since this is 3 or fewer, no adjustment factor applies.

No conductor count adjustment required (3 current-carrying conductors).

7

Verify voltage drop

From NEC Chapter 9 Table 9, the effective Z for #1 AWG copper in steel EMT at 0.85 PF is approximately 0.16 Ω/1000ft (using R = 0.154 and X_L = 0.0682, combined as R cosφ + X_L sinφ = 0.154 × 0.85 + 0.0682 × 0.527 = 0.167 Ω/1000ft).

V_drop = √3 × I × L × Z / 1000 = 1.732 × 95 × 200 × 0.167 / 1000 = 5.50V

V_drop = 5.50V (2.64% of 208V) — within the NEC recommended 3% limit for feeders. PASS.

#1 AWG THWN-2 copper conductors are selected for this 125A-protected continuous feeder. The 125% continuous load factor was the critical sizing driver, increasing the required ampacity from 95A to 118.75A. The 75°C column was used per Section 110.14(C)(1)(b) since all terminations are 75°C rated and the circuit exceeds 100A. The voltage drop of 2.64% at 200 feet is within the NEC recommended 3% for feeders, leaving 0.36% headroom. For the neutral, a #1 AWG conductor is also required per Section 215.2(A)(1), and an equipment grounding conductor is sized per Table 250.122 (for a 125A OCPD: #6 AWG copper minimum).

Common Mistakes When Using NEC/NFPA 70

  1. 1

    Using the 90°C column of Table 310.16 for final conductor selection without checking Section 110.14(C) termination limits. THWN-2 is rated 90°C, but if the circuit breaker or panel lugs are rated 60°C or 75°C, the conductor must be sized using the lower column. The 90°C column may only be used for derating/adjustment calculations—the final ampacity still cannot exceed the value in the termination-limited column. This is the single most common NEC sizing error.

  2. 2

    Forgetting the 125% continuous load factor from Section 210.20(A). For loads operating 3 hours or more (most commercial lighting, HVAC, industrial processes), the conductor and OCPD must be sized at 125% of the continuous current. Sizing at 100% for a continuous load violates the NEC and creates a thermal risk. The only exception is equipment specifically listed for 100% continuous operation.

  3. 3

    Confusing AWG sizes with metric mm². NEC uses AWG (where larger numbers mean smaller conductors—#14 AWG is smaller than #10 AWG) and kcmil for sizes above 4/0 AWG. There is no direct mathematical conversion between AWG and mm²; approximate equivalents must be used (e.g. #12 AWG ≈ 3.31 mm², #10 AWG ≈ 5.26 mm²). Using IEC mm² tables for an NEC-governed installation is non-compliant.

  4. 4

    Not counting the neutral as a current-carrying conductor in circuits with non-linear loads. Per Section 310.15(E), when a three-phase four-wire circuit supplies non-linear loads (computers, LED drivers, VFDs), the neutral carries triplen harmonic currents and must be counted as a current-carrying conductor for the adjustment factor calculation. This can push a 4-conductor circuit (3 phases + neutral) into the 4–6 conductor bracket (80% adjustment).

  5. 5

    Treating NEC voltage drop recommendations as optional suggestions. While Sections 210.19(A) and 215.2(A)(4) informational notes are not mandatory requirements, most AHJs (Authorities Having Jurisdiction) enforce the 3% branch circuit / 5% total recommendation. More importantly, excessive voltage drop causes real operational problems: motor overheating, LED driver malfunction, and sensitive equipment failure. Many project specifications impose stricter limits (2% feeder, 3% branch).

How Does NEC/NFPA 70 Compare?

NEC is fundamentally different from IEC/BS/AS standards in structure and units. Conductor sizes use AWG/kcmil instead of mm²; the term 'ampacity' replaces 'current-carrying capacity'; and the critical Section 110.14(C) termination temperature limitation has no direct equivalent in other standards. The NEC’s 125% continuous load factor is a unique requirement—IEC and BS handle sustained loading through their correction factor methodology. NEC also uses a different conduit fill approach (Chapter 9 Tables 1 and 4) rather than the installation method system used by IEC. The 30°C reference ambient matches IEC/BS but differs from AS/NZS 3008’s 40°C baseline.

Frequently Asked Questions

NEC Section 110.14(C) restricts the ampacity to the temperature rating of the lowest-rated component in the circuit—typically the termination (breaker lug, panel bus, receptacle terminal). For circuits 100A or less, you must use the 60°C column unless every termination, conductor, and device is rated 75°C or higher. For circuits over 100A, the 75°C column applies. However, there is an important exception: you can use the 90°C column solely for applying ambient temperature corrections and conductor count adjustments, then verify that the final derated ampacity does not exceed the lower-column value. This means THWN-2’s 90°C rating is useful for compensating derating penalties, but the base conductor size often cannot be smaller than what the 60°C or 75°C column dictates.
NEC Section 210.20(A) requires that the overcurrent device rating and conductor ampacity be at least 125% of the continuous load current. A continuous load is defined in Article 100 as a load where the maximum current is expected to continue for 3 hours or more. This includes most commercial lighting systems, HVAC equipment, industrial process loads, data centre IT loads, and EV charging stations. The 125% factor ensures that the conductor operates at no more than 80% of its rated ampacity during sustained loading, preventing excessive heat accumulation. The only exception is when both the overcurrent device and its assembly are listed and marked for 100% continuous duty—but these are specialty products (typically 100% rated MCCBs) and not standard residential panel breakers.
The NEC does not impose a mandatory voltage drop limit—the 3% (branch circuit) and 5% (total) figures in Sections 210.19(A) and 215.2(A)(4) are informational notes, not enforceable requirements. In contrast, BS 7671 mandates specific limits (3% lighting, 5% power), and IEC 60364 provides a recommended 4% limit. Despite being advisory in the NEC, most US AHJs treat the 3%/5% recommendation as a practical requirement, and it appears as a mandatory specification in nearly all commercial and industrial project specifications. NEC voltage drop calculations use Chapter 9 Table 8 (DC resistance) and Table 9 (AC impedance) with values in ohms per 1000 feet, while IEC/BS standards use mV/A/m—a different unit system requiring different formulas.

Cable Sizing Calculator for Other Standards

🇦🇺AS/NZS 3008
🇬🇧BS 7671
🌍IEC 60364

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