Wire Size Calculator
Wire size selection is a two-constraint optimization problem: the conductor must satisfy BOTH (1) ampacity — the ability to carry the load current continuously without the...
Formula
Source: NEC Article 310, NFPA 70 | Last reviewed: July 26, 2026
Examples
50 Amps
= 0.00653 AWG
- voltage = 240
- length = 100
- material = 1
- insulation = 75
50 A continuous load → 62.5 A minimum → #6 AWG Cu (65 A) then check VD
100 Amps
= 0.01 AWG
- voltage = 480
- length = 200
- material = 1
- insulation = 75
100 A → #3 AWG Cu (100 A x 1.25 = 125 → #1 AWG) then check VD
200 Amps
= 0.016 AWG
- voltage = 208
- length = 350
- material = 1
- insulation = 75
200 A 208V → 250 kcmil. VD governs over ampacity at 350 ft
Quick Reference Table
| AWG / kcmil | 60°C (TW/UF) | 75°C (THWN/THHN*) | 90°C (THHN/XHHW-2) | Area (CM) |
|---|---|---|---|---|
| #14 | 15 | 20 | 25 | 4110 |
| #12 | 20 | 25 | 30 | 6530 |
| #10 | 30 | 35 | 40 | 10380 |
| #8 | 40 | 50 | 55 | 16510 |
| #6 | 55 | 65 | 75 | 26240 |
| #4 | 70 | 85 | 95 | 41740 |
| #3 | 85 | 100 | 110 | 52630 |
| #2 | 95 | 115 | 130 | 66360 |
| #1 | 110 | 130 | 150 | 83690 |
| #1/0 | 125 | 150 | 170 | 105600 |
| #2/0 | 145 | 175 | 195 | 133100 |
| #3/0 | 165 | 200 | 225 | 167800 |
| #4/0 | 195 | 230 | 260 | 211600 |
| 250 kcmil | 215 | 255 | 290 | 250000 |
| 300 kcmil | 240 | 285 | 320 | 300000 |
| 350 kcmil | 260 | 310 | 350 | 350000 |
| 400 kcmil | 280 | 335 | 380 | 400000 |
| 500 kcmil | 320 | 380 | 430 | 500000 |
| 600 kcmil | 350 | 420 | 475 | 600000 |
| 750 kcmil | 400 | 475 | 535 | 750000 |
| Condition | NEC Reference | Factor Range | Example at Worst Case |
|---|---|---|---|
| Ambient Temp 30-40°C | 310.15(B)(1) | 0.91-0.71 | 75°C THWN at 40°C: 0.88 |
| Ambient Temp 40-50°C | 310.15(B)(1) | 0.71-0.50 | 90°C THHN at 50°C: 0.82 |
| Rooftop Adder (≥3.5" height) | 310.15(B)(3)(c) | +17°C to ambient | Adds 17°C → re-enter ambient table |
| Rooftop Adder (0.5-3.5" height) | 310.15(B)(3)(c) | +22°C to ambient | Phoenix 45°C + 22°C = 67°C → 0.39 |
| Rooftop Adder (<0.5" height) | 310.15(B)(3)(c) | +33°C to ambient | Phoenix 45°C + 33°C = 78°C → 0.26 |
| 4-6 CCCs in Raceway | 310.15(C)(1) | 0.80 | 80% of bare-table ampacity |
| 7-9 CCCs in Raceway | 310.15(C)(1) | 0.70 | 70% — typical for 3× 3-phase circuits |
| 10-20 CCCs in Raceway | 310.15(C)(1) | 0.50 | 50% — severe: consider larger conduit or sub-feed panels |
| Burial Depth (direct) | 310.15(B)(2) | Varies by ampacity | Additional load factor reduction for deep burial |
Where is this used?
(1) Residential branch circuits — NEC Table 210.24 summary: 15 A circuits = #14 AWG Cu minimum, 20 A = #12 AWG Cu, 30 A = #10 AWG Cu, 40 A = #8 AWG Cu, 50 A = #6 AWG Cu.
These are the standard sizes electricians carry on their trucks.
(2) Motor circuits — per NEC 430.22, conductors for a single motor must have ampacity ≥ 125% of the motor's full-load current (FLA) from NEC Table 430.250.
A 50 HP, 460V three-phase motor with FLA = 65 A requires conductor ampacity ≥ 65 × 1.25 = 81.25 A → #4 AWG Cu (85 A at 75°C).
If the motor is served by a VFD, additional derating for harmonic heating may apply per the drive manufacturer's instructions.
(3) Service entrance conductors — residential: NEC 310.12 allows the 83% rule for dwellings, so a 200 A service requires conductors rated for 200 × 0.83 = 166 A → #2/0 AWG Cu (175 A) or #4/0 AWG Al (180 A).
Commercial services sized per NEC 310.16 without the 83% reduction.
(4) Feeder tap rules — NEC 240.21(B) permits reduced-size tap conductors under specific conditions (≤ 10 ft, ≤ 25 ft, or unlimited length outside the building) with rules about ampacity ratios and termination at a single overcurrent device.
A 400 A feeder can have 100 A tap conductors feeding a 100 A fused disconnect if the tap length ≤ 10 ft — a common arrangement in industrial switchgear lineups.
(5) Transformer secondary conductors — NEC 240.21(C) rules for transformer secondary protection: conductors ≤ 10 ft with ampacity ≥ primary OCPD × (V_primary/V_secondary) × 1/3, or ≤ 25 ft with ampacity ≥ secondary OCPD rating.
A 150 kVA, 480-208/120V transformer (primary FLA = 180 A, secondary FLA = 416 A) with a 25 ft secondary tap requires conductor ampacity ≥ 416 A → 600 kcmil Cu (420 A) minimum.
(6) Equipment grounding conductors (EGCs) — NEC Table 250.122 sizes the EGC based on the rating of the overcurrent device ahead of the circuit, NOT on the phase conductor size.
A 200 A breaker requires a #6 AWG Cu EGC regardless of whether the phase conductors are #3/0 AWG (minimum ampacity) or 500 kcmil (upsized for voltage drop).
However, NEC 250.122(B) requires that when phase conductors are upsized for voltage drop, the EGC must be upsized proportionally — a frequently missed requirement.
(7) Grounding electrode conductors (GECs) — NEC Table 250.66 sizes the GEC (the conductor from the service neutral to the grounding electrode) based on the largest ungrounded service-entrance conductor.
A 3/0 AWG Cu service uses a #4 AWG Cu GEC.
(8) Data center and mission-critical — 480V to 208/120V PDUs feeding server racks use conductors sized for the maximum continuous load (server nameplate × 0.80 for the 80% rated breaker limitation), with voltage drop checked at the 208V utilization level where the lower voltage makes VD more sensitive.
Real-World Usage Scenarios
Rooftop RTU feeder fails inspection due to missing temperature derating
A mechanical contractor installed a 60 A, 480V rooftop air conditioning unit with #6 AWG THHN copper (65 A at 75°C) in EMT conduit strapped directly to the roof surface. The electrical inspector rejected the installation, citing NEC 310.15(B)(3)(c): conduits exposed to sunlight on rooftops require an ambient temperature adder. The local jurisdiction used the ASHRAE 2% design temperature of 38°C. With the conduit less than ½ inch above the roof, the adder is 33°C (Table 310.15(B)(3)(c)), giving effective ambient = 38 + 33 = 71°C. The 75°C column correction factor for 71-75°C is 0.33. #6 AWG Cu effective ampacity = 65 × 0.33 = 21.5 A — far below the 60 A load (which required 60 × 1.25 = 75 A before derating for a continuous HVAC load). The required pre-derating ampacity was 75 ÷ 0.33 = 227 A → #4/0 AWG Cu (230 A). The contractor's options: (1) re-pull with #4/0 AWG (expensive, and the terminations might not accept it), (2) raise the conduit to ≥ 7/8 inch above the roof (adder drops to 22°C, effective ambient = 60°C, correction = 0.45, required ampacity = 75/0.45 = 167 A → #2/0 AWG), or (3) route the conduit inside the building and penetrate the roof with a sealed conduit body at the unit, eliminating the rooftop adder entirely. Option 3 was chosen.
Subpanel feeder sized for ampacity but LED lights flicker on motor start
A woodworking shop added a 100 A subpanel 250 ft from the main service to feed a 5 HP table saw (FLA = 28 A at 240V single-phase), dust collector (FLA = 20 A), lighting (15 A LED), and misc. receptacles. The electrician sized the feeder at #1 AWG aluminum (100 A at 75°C) based on ampacity alone. Voltage drop calculation at the 100 A panel rating: VD = (2 × 250 × 21.2 × 100) ÷ 83,690 = 12.66 V = 5.27% — exceeding the 2% feeder recommendation and consuming the entire 5% combined budget before any branch circuits. The actual problem manifested when the table saw started: the inrush current of ~170 A (6 × FLA) caused a feeder VD of (2 × 250 × 21.2 × 170) ÷ 83,690 = 21.5 V (9.0%), dropping the subpanel bus to 218.5V. The LED lights on a branch circuit dimmed noticeably with each saw start, and the dust collector's capacitor-start motor occasionally failed to start, tripping its thermal overload. The fix: re-feed with #3/0 AWG aluminum (167,800 CM for 155 A at 75°C, but selected for VD, not ampacity). Steady-state VD at 100 A: (2 × 250 × 21.2 × 100) ÷ 167,800 = 6.31V = 2.63% feeder. Starting VD at 170 A: 10.7V (4.5%) — the LED flicker disappeared and the dust collector started reliably. The material cost increase was approximately $180 in aluminum wire — far cheaper than diagnosing flickering lights and nuisance trips for two years.
Data center PDU whip overheating due to harmonic-rich server loads
A colocation data center installed 60 A, 208/120V three-phase PDUs feeding racks via 10 ft, #6 AWG THHN copper whips under the raised floor. Each whip served 20 A of nameplate IT load per phase (servers with power-factor-corrected supplies, PF > 0.95). Continuous load = 20 × 1.25 = 25 A, well within #6 AWG's 65 A rating. After six months of operation, thermal imaging during a preventive maintenance walkthrough revealed the #6 AWG whips running at 78°C — above the 75°C termination rating. Investigation revealed the server power supplies, despite high power factor, generated substantial third-harmonic and triplen currents (3rd, 9th, 15th harmonics) that ADD in the neutral rather than cancel. The neutral current measured 28 A (140% of phase current) due to triplen harmonic accumulation. NEC 310.15(E) Note 3 requires the neutral to be counted as a current-carrying conductor when the major portion of the load is nonlinear, bringing the total to 4 CCCs in the conduit — triggering an 80% derating factor. #6 AWG effective ampacity = 65 × 0.80 = 52 A. Still above 25 A per phase — but the neutral was carrying 28 A on a #6 AWG conductor with no dedicated overcurrent protection and the accumulated heating from four conductors raised the conduit interior temperature. The solution: (1) double the neutral (run a second #6 AWG neutral in parallel or upsize to #4 AWG for the neutral), and (2) derate all phase conductors for 4+ CCC. The new whip design used #4 AWG phases and an up-sized #2 AWG neutral to handle triplen harmonics, keeping conductor temperatures at 62°C (within 75°C rating with margin).
Common Mistakes to Avoid
Forgetting the 125% continuous load factor
NEC 210.19(A)(1) requires branch-circuit conductors to have ampacity not less than 125% of the continuous load (defined as maximum current expected to continue for 3 hours or more). A designer who sizes a 100 A continuous HVAC chiller at 100 A (using #3 AWG Cu at 100 A) rather than at 100 × 1.25 = 125 A (#1 AWG Cu at 130 A) is undersized by 25%. The consequences: the conductor operates at a temperature above its insulation rating, accelerating insulation degradation — a THHN conductor rated for 90°C might survive initially, but the 75°C terminations (breakers, lugs) are rated for 75°C and will overheat. Thermal imaging will show hot spots at the breaker terminals. Over time (months to years, depending on load factor), the insulation becomes brittle, the conductor oxidizes at terminations, resistance increases, heating increases further in a positive feedback loop, and the termination eventually fails — either as an open circuit (equipment stops) or as a high-resistance arc fault (fire risk). Always multiply continuous loads by 1.25 before entering ampacity tables.
Selecting the smallest compliant conductor without checking voltage drop
Ampacity governs the minimum safe conductor size; voltage drop governs the minimum functional conductor size. For short runs (< 50 ft at 120V, < 150 ft at 480V), ampacity usually dominates. Beyond these distances, VD almost always dictates a larger conductor. A common error: specifying #12 AWG Cu for a 16 A, 120V garage circuit at 200 ft because 'it's rated for 20 A.' The voltage drop is: VD = (2 × 200 × 12.9 × 16) ÷ 6,530 = 12.64V = 10.5% — the load receives 107.4V. A refrigerator compressor designed for 115V ±10% (103.5-126.5V) would be within tolerance but a microwave or laser printer expecting 120V will underperform or fail to start. The NEC Informational Note recommends 3% max for branch circuits. The fix requires #8 AWG Cu (16,510 CM): VD = (2 × 200 × 12.9 × 16) ÷ 16,510 = 5.0V (4.2%) — improved but still above 3%. Moving to #6 AWG (26,240 CM): VD = 3.14V (2.62%) — compliant. The cost difference from #12 AWG to #6 AWG over 200 ft is substantial (~$80 vs. ~$400 in copper alone), which is why long residential garage/outbuilding circuits are a common place where DIY installers cut corners and create latent performance problems.
Applying temperature derating to the wrong insulation column
NEC Table 310.16 lists ampacities for 60°C, 75°C, and 90°C insulation. The correct starting column depends on the termination temperature rating, per NEC 110.14(C): for equipment rated 100 A or less (or marked for #14-#1 AWG), use the 60°C column unless both the equipment and conductor are listed for 75°C. For equipment above 100 A (or marked for #1/0 AWG and larger), use the 75°C column. A common mistake: using the 90°C THHN column directly for ampacity selection. The 90°C column can be used as the STARTING POINT for DERATING calculations (ambient temperature, conduit fill), but the FINAL derated ampacity must not exceed the 75°C (or 60°C) column value for the termination's rating. Example: #10 AWG THHN at 90°C column = 40 A. In a 45°C ambient, derating factor = 0.91. Derated ampacity = 40 × 0.91 = 36.4 A. But the terminations are rated 75°C, where #10 AWG = 35 A. The final ampacity is min(36.4, 35) = 35 A. This 'double check' (derated value vs. termination-limited value) is a frequent exam question and a real-world design pitfall.
Industry Standards Referenced
Frequently Asked Questions
What size wire do I need for 100 amps?
At 75°C copper: NEC 310.16 lists #3 AWG = 100 A, #1 AWG = 130 A. For a non-continuous 100 A load (load does not persist for 3 hours), #3 AWG Cu meets the letter of the code. However, in practice, 100 A circuits are almost always loaded to at least 80 A continuously, and most designers default to #1 AWG Cu (130 A) for 100 A to provide margin. For a continuous load (≥ 3 hours per NEC definition), apply the 125% factor: 100 × 1.25 = 125 A → #1 AWG Cu (130 A) minimum. For 75°C aluminum: non-continuous 100 A → #1/0 AWG Al (120 A); continuous → #2/0 AWG Al (150 A) or equivalent. These are baseline values before any derating for temperature, rooftop exposure, or conduit fill. A 100 A rooftop circuit in Arizona can require #2/0 Cu or larger after derating. Also consider voltage drop: at 480V over 250 ft, #1 AWG Cu VD ≈ 2.0%; at 208V over the same distance, VD ≈ 4.6% — requiring an upsize to #2/0.
How does wire gauge (AWG) work?
American Wire Gauge is a logarithmic scale defined such that 36 steps span a diameter ratio of exactly 1:128 from #36 AWG (5.0 mils, the smallest) to #4/0 AWG (460 mils). Each 3-gauge step approximately doubles (or halves) the cross-sectional area: #12 AWG = 6,530 CM; #9 AWG ≈ 13,100 CM (2× area of #12); #6 AWG = 26,240 CM (2× area of #9, 4× area of #12). Each 6-gauge step approximately doubles the diameter: #10 AWG = 0.102 inch diameter; #4 AWG = 0.204 inch (~2×). The gauge number itself is largely irrelevant beyond memorizing the key reference points: #14 for 15 A lighting, #12 for 20 A receptacles and general purpose, #10 for 30 A (water heaters, dryers smaller than 5 kW, residential AC condensers), #8 for 40-50 A (ranges, larger AC units), #6 for 50-60 A (hot tubs, subpanels, EV chargers up to 48 A continuous), #4-#1 for 70-100 A (subpanels, small commercial feeders), 1/0-4/0 for 125-230 A (residential services, larger feeders), and 250 kcmil+ for 255 A and above (commercial switchgear, large services). Above #4/0 AWG, sizes are given directly in kcmil: 250, 300, 350, 400, 500, 600, 750, 1,000 kcmil, eliminating the AWG increment system for large conductors.
When should I use aluminum wire instead of copper?
Aluminum is the default choice for utility transmission and distribution (essentially all overhead lines are aluminum conductor steel-reinforced, ACSR), for large service entrance cables (residential 200 A service: 4/0 Al is standard and roughly 1/3 the cost of 2/0 Cu), and for commercial/industrial feeders above 200 A where the cost differential justifies the larger conduit. Aluminum is roughly 30-50% of the cost of copper per ampere-foot of equivalent ampacity. However, aluminum has three significant engineering drawbacks: (1) higher resistivity (K=21.2 vs. 12.9 for Cu) means 1-2 AWG sizes larger for equal ampacity and voltage drop; (2) aluminum oxide forms an insulating layer that must be removed by wire brushing immediately before terminating, with anti-oxidant compound (e.g., Noalox) applied to prevent re-oxidation; (3) higher coefficient of thermal expansion (23.1 vs. 16.5 µm/m·°C for Cu) causes connections to loosen under thermal cycling — terminations must be re-torqued per manufacturer specifications after initial thermal cycling. NEC requires that aluminum conductors be terminated only in lugs and equipment listed for aluminum (marked AL or AL/CU). Modern AA-8000 series aluminum alloy (required by NEC since 1987 for building wire) resolves the creep and cold-flow issues that plagued 1960s-era AA-1350 aluminum in small branch circuits. For residential 15-20 A branch circuits, copper remains the overwhelming standard due to smaller physical size, greater mechanical robustness, and installer familiarity. For commercial projects with 400 A+ feeders, aluminum is frequently the engineer's recommended choice on value-engineering grounds.
What's the difference between THHN, THWN, XHHW, and NM-B wire types?
These are insulation types defined by NEC Table 310.4(A), each with different temperature ratings and environmental suitability. THHN: Thermoplastic High Heat-resistant Nylon-coated, rated 90°C dry locations only, the most common building wire (single conductor, pulled in conduit). THWN: Thermoplastic Heat and Water-resistant Nylon-coated, rated 75°C wet or dry — the 'W' adds wet location suitability. Most modern wire is dual-rated THHN/THWN-2, giving 90°C dry and 90°C wet rating. XHHW-2: Cross-linked High Heat-resistant Water-resistant, rated 90°C wet or dry, with thicker cross-linked polyethylene insulation that is more abrasion-resistant and has better chemical resistance than THHN — standard for industrial and some commercial specifications, and preferred where the conductor may be exposed to oils or solvents. NM-B (Romex): Non-Metallic sheathed cable, rated 60°C per NEC 334.80 (must use 60°C ampacity column despite the individual conductors inside being rated 90°C), used in residential interior wiring where the cable is protected from physical damage. UF (Underground Feeder): similar to NM-B but with moisture-resistant outer jacket rated for direct burial. USE-2: Underground Service Entrance, 90°C rated for direct burial and wet locations, common for solar PV array wiring. For any given circuit, you must use the ampacity column corresponding to the LOWEST temperature rating among the conductor insulation, the terminations, and the equipment — per NEC 110.14(C).
How does continuous vs. non-continuous loading affect wire sizing?
NEC Article 100 defines a continuous load as one 'where the maximum current is expected to continue for 3 hours or more.' Common continuous loads: general lighting in offices and commercial spaces, data center servers, HVAC chiller compressors during peak cooling, industrial process heaters, and EV chargers. Non-continuous loads: residential kitchen appliances (toaster, microwave), intermittent-duty motors (garage door openers, sump pumps), and welders. NEC 210.19(A)(1) requires branch-circuit conductors to have an ampacity not less than the non-continuous load plus 125% of the continuous load. For a circuit with 10 A continuous LED lighting + 8 A non-continuous receptacles: minimum conductor ampacity = 10 × 1.25 + 8 = 20.5 A → #12 AWG Cu at 75°C (25 A) — meets both the calculation and the 20 A overcurrent device limit per 240.4(D). If the entire 18 A were continuous: 18 × 1.25 = 22.5 A → still #12 AWG could work since 25 A > 22.5 A, but the 20 A breaker limit makes the practical maximum continuous load on a 20 A circuit = 16 A (80% rule from NEC 210.20(A) for overcurrent devices). This 80% breaker rule — overcurrent devices protecting continuous loads must be rated at 125% of the continuous load — creates the practical result that a 20 A breaker supports a maximum 16 A continuous load, and a 30 A breaker supports 24 A continuous.
Reviewed for accuracy
Cross-referenced against NEC 2020 Article 310 ampacity tables, Article 210 and 215 voltage drop recommendations, and Article 110 termination temperature requirements · Last reviewed: July 26, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.