Cable Size Calculator (NEC 310) – Online 3-Phase & LV Cable Sizing

Selecting the correct cable size is one of the most critical tasks in electrical design. An undersized cable overheats and becomes a fire hazard; an oversized cable wastes material cost. NEC (Code of Practice for Electrical Wiring Installations) requires that every cable be sized to satisfy two simultaneous criteria: current-carrying capacity and voltage drop.

NEC current capacity tables (Table 4, 5 etc.) give the rated current for different installation methods - clipped direct to surface, in conduit, or in trunking. Each installation method has a derating factor. For example, a 4mm² copper cable clipped direct carries 32A, but in a conduit (grouped with other cables) this may derate to 20A. Always apply the correct derating factors for ambient temperature, grouping, and soil thermal resistivity for buried cables.

A 3-phase 15kW motor (PF 0.85, 415V) runs through a 40m cable. Full load current: I = 15,000 / (1.732 × 415 × 0.85) ≈ 24.6A.

From NEC tables, a 6mm² copper cable (clipped direct) carries 36A - well above 24.6A. Checking voltage drop: Vd = (1.732 × 40 × 24.6 × 0.0172) / 6 ≈ 2.93V (0.71%), comfortably within the 3% limit. If the run were 100m instead, Vd would rise to 7.3V (1.76%) - still compliant, but a 10mm² cable would bring it down to about 4.4V (1.06%) with margin for future load growth.

How the Cable Size Calculator Works

Sizing a conductor is a four-step engineering process, and this calculator automates every step the way a licensed electrical engineer would work it by hand under the National Electrical Code (NEC). First it converts your load into a full-load current (amperes). Second it applies the 125% continuous-load multiplier required by NEC 210.19(A) and 215.2(A). Third it selects the smallest conductor whose ampacity from NEC Table 310.16 meets that adjusted current — after correcting for ambient temperature and the number of current-carrying conductors bundled together. Fourth it verifies the choice against the 3% branch-circuit voltage-drop recommendation of NEC 210.19(A) Informational Note 4. The result is a conductor size in AWG or kcmil that is simultaneously safe (won't overheat), code-compliant (passes inspection), and efficient (won't waste energy or drop excessive voltage).

The Cable Sizing Formula

Everything starts with the design current. For a balanced system:

• Single-phase: I = P ÷ (V × PF)
• Three-phase: I = P ÷ (√3 × V_LL × PF)
• Continuous loads: I_design = 1.25 × I_load (NEC 210.19 / 215.2)
• Motors: use the table FLC from NEC 430.248 (1-φ) or 430.250 (3-φ), then × 1.25 (NEC 430.22)

The conductor must then satisfy the corrected-ampacity inequality:

I_design ≤ I_table × C_t × C_a

where I_table is the base ampacity from NEC 310.16, C_t is the ambient-temperature correction factor (Table 310.15(B)(1)), and C_a is the conductor-bundling adjustment factor (Table 310.15(C)(1)).

Variable & Unit Reference

Unit handling: the US convention is AWG (American Wire Gauge) for conductors up to 4/0 and thousand-circular-mils (kcmil) above that; run lengths are in feet and resistance in ohms-per-1000-ft. When you toggle the calculator to imperial it works directly in these units, so you never have to convert a metric mm² catalog value into AWG in your head. For reference, 2.5 mm² ≈ 14 AWG, 4 mm² ≈ 12 AWG, 6 mm² ≈ 10 AWG, 10 mm² ≈ 8 AWG, 16 mm² ≈ 6 AWG and 25 mm² ≈ 4 AWG.

Worked Example 1 — Single-Phase Continuous Load (120/240 V)

A commercial kitchen has a 240 V, single-phase, 9.6 kW resistance appliance (PF = 1.0) on a 100 ft run using 75°C-rated THWN-2 copper in EMT, ambient 30°C, no more than three current-carrying conductors.

1. Full-load current: I = 9600 ÷ (240 × 1.0) = 40 A.
2. Continuous multiplier: a fixed appliance running ≥ 3 hours is continuous, so I_design = 1.25 × 40 = 50 A.
3. Ampacity selection: from NEC 310.16, 75°C copper column, 8 AWG = 50 A. Corrections C_t = C_a = 1.0, so 50 A ≥ 50 A ✓.
4. Voltage-drop check: 8 AWG copper ≈ 0.778 Ω/1000 ft. VD = 2 × 0.778 × (100/1000) × 40 = 6.22 V = 2.6% of 240 V — under the 3% target ✓.
5. Equipment grounding conductor: NEC 250.122 for a 50 A breaker → 10 AWG copper.

Answer: 8 AWG copper conductors with a 10 AWG copper ground, protected by a 50 A breaker.

Worked Example 2 — Three-Phase Motor Feeder (480 V)

Size the branch conductors for a 100 HP, 460 V, three-phase induction motor in a plant where the raceway may reach 40°C ambient, THWN-2 copper, three current-carrying conductors.

1. Table FLC: NEC 430.250 lists 100 HP at 460 V as 124 A (always use the table value, never the nameplate, for conductor and OCPD sizing).
2. Motor conductor multiplier: NEC 430.22 requires 125%, so I_design = 1.25 × 124 = 155 A.
3. First pass ampacity: 310.16 75°C column → 2/0 AWG = 175 A (1/0 = 150 A is too small).
4. Ambient correction: at 40°C the 75°C factor is 0.88. 175 × 0.88 = 154 A < 155 A — fails. Upsize to 3/0 AWG = 200 A: 200 × 0.88 = 176 A ≥ 155 A ✓.
5. Overcurrent protection: NEC 430.52 allows an inverse-time breaker up to 250% FLC = 310 A → use a 300 A frame.
6. Equipment grounding conductor: 250.122 for 300 A → 4 AWG copper.

Answer: three 3/0 AWG copper conductors plus a 4 AWG copper ground in 2″ conduit, on a 300 A inverse-time breaker. This example shows why ambient temperature must never be ignored — skipping the 0.88 factor would have left a dangerously undersized 2/0 feeder that passes at 30°C but overheats at 40°C.

Standards & Code References

• NEC Table 310.16 — allowable ampacities of insulated conductors (60/75/90°C) rated 0–2000 V, the primary sizing table in the US.
• NEC 240.4(D) — small-conductor rule: 14 AWG max 15 A, 12 AWG max 20 A, 10 AWG max 30 A regardless of ampacity.
• NEC 210.19(A) / 215.2(A) — 125% continuous-load sizing and the 3%/5% voltage-drop recommendation.
• NEC Table 310.15(B)(1) — ambient temperature correction; Table 310.15(C)(1) — adjustment for > 3 current-carrying conductors.
• NEC 110.14(C) — termination temperature rating (most equipment ≤ 100 A terminates at 60°C, larger at 75°C).
• NEC 250.122 — minimum equipment grounding conductor by OCPD size.
• Secondary / international: IEC 60364-5-52 and IS 732 use the same physics with mm² conductors and Tables 4/5 rating factors — use them for projects specified to those codes.

Key Facts to Remember

• Ampacity is chosen from the termination temperature column (usually 75°C), even if the insulation is rated 90°C — the 90°C column is only for applying correction factors.
• The 125% continuous multiplier and the temperature/bundling corrections are applied at different points; never combine them into one fudge factor.
• Copper 8 AWG (50 A) and aluminum 6 AWG (50 A) carry similar current — aluminum always needs one to two sizes larger.
• Voltage drop is a recommendation, not a mandate, in the NEC — but most engineering specs make 3% branch / 5% total a hard requirement.
• Motor conductors use table FLC (430.250) and a 125% factor; OCPD uses a much higher 250% factor because motors draw locked-rotor inrush.
• Bundling more than three conductors in a raceway can cut ampacity by 20–50% — a very common field oversight.
• Neutrals carrying harmonic current in nonlinear (VFD, LED, IT) loads count as current-carrying conductors for adjustment.
• Always size the equipment grounding conductor from Table 250.122 — it does not automatically match the phase conductors.

NEC Table 310.16 — Copper Ampacity (the "money table")

Values are for not more than three current-carrying copper conductors in a raceway at 30°C (86°F) ambient. Multiply by the correction and adjustment factors for any other condition.

Real-World Applications

• Branch circuits for receptacles, lighting and small appliances in commercial and residential fit-outs.
• Motor feeders for HVAC compressors, pumps, air handlers and industrial machinery.
• Panelboard and switchboard feeders from the service entrance to distribution boards.
• EV charger circuits, which are continuous loads and therefore always sized at 125%.
• Rooftop and outdoor runs, where high ambient temperatures force ampacity correction.
• Data-center and IT power, where harmonic neutral currents demand full-size or upsized neutrals.
• Generator and transfer-switch conductors sized to the standby source rating.
• Solar PV DC and AC circuits, which combine the 125% continuous rule with a second 125% irradiance factor.

Common Mistakes

• Sizing from the 90°C column when the terminals are only rated 75°C — a code violation under 110.14(C).
• Forgetting the 125% continuous factor, leaving the conductor and breaker marginally undersized.
• Ignoring ambient temperature on rooftops, in boiler rooms and in direct sun, where derating is severe.
• Not counting bundled conductors — packing 6–9 wires in one conduit without the 70–80% adjustment.
• Using nameplate current instead of table FLC for motor circuits.
• Assuming the ground matches the phase conductor instead of reading Table 250.122.
• Skipping the voltage-drop check on long runs, then wondering why motors stall or lights dim.
• Mixing copper and aluminum ampacities — always confirm which metal the table column represents.

Step-by-Step: Sizing a Conductor in Practice

Working a cable size by hand follows a fixed sequence, and doing the steps in the wrong order is the source of most sizing errors. Use this checklist every time:

1. Establish the load. Convert the connected load to amperes with the single- or three-phase formula, or read the table FLC for a motor. Identify whether the load is continuous (operating 3 hours or more).
2. Apply the demand and continuous factors. Multiply continuous loads by 125%. For feeders serving several loads, apply the appropriate demand factors from NEC Article 220 before sizing — you rarely size a feeder for the arithmetic sum of every branch.
3. Determine the termination temperature. Per NEC 110.14(C), equipment rated 100 A or less generally terminates at 60°C, and larger equipment at 75°C. You may not exceed the lowest-rated component in the circuit, so a 90°C conductor on a 75°C lug is still a 75°C circuit at its terminations.
4. Pick a trial conductor from the correct temperature column of NEC 310.16 whose base ampacity meets the design current.
5. Apply correction and adjustment factors. Multiply the base ampacity by the ambient-temperature factor and the bundling adjustment factor, then confirm the corrected ampacity still meets the design current. If not, upsize and repeat.
6. Check voltage drop over the actual run length and upsize again if the drop exceeds the project limit.
7. Size the overcurrent device and the equipment grounding conductor from the final conductor and load type.

Notice that ampacity correction (step 5) and voltage drop (step 6) can each independently force a larger conductor than the raw load current suggests. The final size is always the largest wire that any single check demands — never the average.

Ambient & Bundling Correction Factors

The NEC ampacity tables assume an ideal condition: 30°C (86°F) ambient and no more than three current-carrying conductors sharing a raceway. Real installations rarely match that, so two multipliers bring the rating down to reality. The ambient factor (NEC Table 310.15(B)(1)) accounts for hotter or cooler surroundings, and the adjustment factor (Table 310.15(C)(1)) accounts for the mutual heating of bundled conductors.

The two factors multiply together. A 90°C conductor carrying 100 A base, in a 40°C rooftop conduit (0.91) with seven conductors (0.70), delivers only 100 × 0.91 × 0.70 = 64 A of usable ampacity — a 36% reduction that routinely surprises designers who size from the table alone. This is exactly why the 90°C column exists: you start from the higher 90°C ampacity, apply the derating, and then compare against the 75°C termination limit.

Copper vs Aluminum: Making the Right Choice

Conductor metal is an economic and engineering decision, not just a habit. Copper has about 61% higher conductivity than aluminum, so it carries more current per size, flexes and terminates more reliably, and resists the cold-flow and oxidation problems that plagued early aluminum branch wiring. It is the default for branch circuits, control wiring and small feeders. Aluminum — specifically modern AA-8000 alloy building wire — is roughly half the weight and a third to a half the cost per ampere, which makes it the economical choice for large feeders, service-entrance conductors and long runs of 1/0 and larger.

The trade-offs are concrete: aluminum needs one to two AWG sizes larger for the same ampacity (compare 3/0 aluminum at 155 A to 2/0 copper at 175 A), requires terminals and lugs listed AL-CU or AL7CU-rated, and benefits from an anti-oxidant compound and correct torque to prevent loose, high-resistance connections. For a large service, aluminum can cut conductor cost by half even after upsizing; for a 20 A receptacle circuit, copper's reliability wins every time. This calculator lets you compare both metals directly so the cost-versus-size trade-off is visible before you specify.

Parallel Conductors & Large Feeders

Above about 400 A, a single conductor becomes physically unwieldy and its ampacity gains diminish, so engineers run conductors in parallel — two or more identical conductors per phase acting as one. NEC 310.10(G) permits paralleling only for sizes 1/0 AWG and larger, and every parallel set must be identical in length, material, size, insulation and termination so the current divides evenly. Two 250 kcmil copper conductors per phase (2 × 255 A = 510 A) commonly replace a single 750 kcmil conductor, saving cost and easing installation. Remember that paralleled conductors in separate raceways each count toward their own raceway's fill and bundling adjustment, and that the equipment grounding conductor in each raceway is sized to the full circuit OCPD, not divided.

Design Tips from the Field

• Leave headroom. Size feeders to about 80% of corrected ampacity where future load growth is likely — repulling a maxed-out feeder is far more expensive than one size up today.
• Coordinate with conduit fill early. A conductor that passes ampacity but won't fit the specified conduit forces a redesign; check Chapter 9 fill in parallel with sizing.
• Watch the rooftop. Conduit exposed to direct sun gains a significant temperature adder — treat rooftop runs as a hot ambient from the start.
• Mind the neutral on nonlinear loads. LED lighting, VFDs and IT power push triplen harmonics into the neutral; size it as a current-carrying conductor and consider a full-size or oversized neutral.
• Torque matters. The majority of conductor failures are termination failures — specify and verify lug torque, especially on aluminum.
• Document your derating. Show ambient and bundling factors on the drawings so the inspector and the next engineer can follow the logic.

Coordinating Conductor Size with Conduit Fill

A conductor that satisfies ampacity is only half the design — it must also physically fit the raceway within the NEC Chapter 9 fill limits, which cap the percentage of the conduit's cross-section that conductors may occupy: 53% for one conductor, 31% for two, and 40% for three or more. Exceeding these limits makes the pull difficult, damages insulation, and traps heat that further reduces ampacity. As a rule of thumb, three 3/0 THWN conductors plus a ground fit comfortably in 2″ EMT, while three 500 kcmil conductors need 3″. When you upsize a conductor for voltage drop or derating, always re-check the conduit — a common late-stage surprise is a conductor that passes every electrical test but no longer fits the raceway shown on the drawings. Upsizing conductors purely for voltage drop while keeping the same overcurrent device also means the equipment grounding conductor must be increased proportionally per NEC 250.122(B).

Short-Circuit Withstand

Ampacity governs a conductor under normal load, but during a fault the conductor must also survive the immense heat of the short-circuit current for the brief time until the protective device clears it. This is the conductor's withstand rating, evaluated with the formula I²t ≤ (K × CM)², where K is a material constant (about 0.0297 for copper) and CM is the conductor area. On systems with high available fault current — near large transformers or utility services — a conductor sized correctly for load can still be destroyed by a fault if it is too small to withstand the let-through energy of the upstream breaker. This is why service and feeder conductors near high-capacity sources are sometimes larger than load alone requires, and why fault-current and protective-device coordination studies accompany the ampacity calculation on larger projects. Use the companion short-circuit calculator to confirm the available fault current at the point of installation.

Life-Cycle Cost & Conductor Economics

The cheapest conductor to buy is rarely the cheapest to own. A smaller conductor costs less up front but dissipates more energy as I²R heat every hour it carries current, and over a 25–30 year service life that wasted energy can exceed the purchase price several times over. For heavily loaded feeders that run continuously, engineers perform an economic conductor sizing analysis: the optimum size is usually one or two steps larger than the minimum code size, because the energy saved by the lower resistance pays back the extra copper within a few years. The IEEE 141 (Red Book) and copper-industry guides publish the method. In practice, the decision balances copper price, load factor, hours of operation and the local cost of electricity — a data center running 24/7 justifies far more copper than a lightly loaded lighting feeder. Sizing generously also leaves headroom for the load growth that almost every facility eventually experiences, avoiding the large expense of repulling feeders later.

Grounding & Bonding Considerations

Every circuit needs a properly sized equipment grounding conductor (EGC) so that a ground fault produces enough current to trip the protective device quickly and safely. The EGC is sized from NEC Table 250.122 by the rating of the overcurrent device — not the phase conductor — so a 400 A feeder takes a 3 AWG copper EGC regardless of whether the phase conductors are 500 kcmil or a paralleled set. When phase conductors are upsized for voltage drop, the EGC must be increased in the same proportion (250.122(B)). Grounding electrode conductors, by contrast, are sized from Table 250.66 by the service-entrance conductor size. Keeping these two tables straight — 250.122 for equipment grounds, 250.66 for grounding electrodes — prevents one of the most frequent plan-review comments on electrical submittals.

Limitations & Disclaimer

This calculator provides a first-pass engineering estimate based on standard NEC tables and typical conductor properties. It does not replace a stamped design by a licensed Professional Engineer, nor does it account for every site-specific factor — parallel conductor sets, unusual raceway fill, buried-duct-bank thermal resistance (NEC 310.60 / Annex B), voltage-drop on unbalanced or non-linear loads, short-circuit withstand, or local amendments to the adopted code cycle. Always verify the final design against the edition of the NEC (or IEC/IS) enforced by your Authority Having Jurisdiction, and have safety-critical circuits reviewed by a qualified engineer before installation.