Wire Size Calculator (NEC Ampacity + Voltage Drop)
NEC wire size calculator combining ampacity (Table 310.16) + voltage drop. Smart AWG recommendation considering both constraints — residential, commercial, industrial circuits.
Wire Size Calculator
| Design current (with 125% factor if continuous) | — |
| Smallest by ampacity alone | — |
| Smallest by voltage drop alone | — |
| Recommended ampacity (Table 310.16) | — |
| Recommended voltage drop at this size | — |
| Conductor cross-section | — |
How to use the wire size calculator
Enter circuit basics
Phase: single (most residential/commercial 120-240V) or three (industrial 208-480V). Continuous: loads running 3+ hours need 125% factor per NEC. Max % drop: 3% standard for branch circuits, 5% combined feeder+branch.
Enter voltage, current, length
Source voltage at panel. Load current in amps (motor full-load amps, equipment nameplate, calculated load). One-way length from panel to load — calculator handles round-trip multiplier.
Read recommendation
The calculator picks the smallest AWG that satisfies BOTH constraints: ampacity (NEC Table 310.16) AND voltage drop. For short branch circuits, ampacity dominates. For long runs, voltage drop dominates. Both shown separately for transparency.
Check upsizing for continuous loads
Per NEC, continuous loads (≥3 hours running) require conductor + breaker sized for 125% of load. Air conditioning, EV charging, large motors are typical examples. The calculator handles this automatically when you select "yes".
Verify with installer
This is engineering education / preliminary sizing — actual installations must comply with NEC code-of-record + local amendments + verified by licensed electrician + AHJ inspection. Conduit fill, ambient temperature, harmonics may require upsizing beyond this baseline.
Wire sizing — balancing ampacity + voltage drop
Wire size selection in electrical engineering is governed by TWO independent constraints: (1) ampacity — can the conductor carry the current without overheating? (NEC Table 310.16, governed by insulation rating + conductor material + ambient temperature). (2) Voltage drop — will the conductor deliver enough voltage to the load? (NEC 215.2(A) informational, 3% branch / 5% combined). The required wire size is the LARGER of the two constraints — both must be satisfied. For short branch circuits, ampacity dominates: a 20A circuit needs at least 12 AWG regardless of length. For long runs (100+ ft), voltage drop dominates: the same 20A circuit may need 8 AWG or larger at 200 ft to meet 3% drop.
The 125% factor for continuous loads
NEC 210.19 and 215.3 require continuous loads (operating 3+ hours) to be supplied by conductors rated at 125% of the load current. Examples: lighting circuits running all day, EV chargers, large air conditioning. A 32A continuous EV charger needs conductor sized for 40A — so 8 AWG copper (50A ampacity, 25% margin) is appropriate. The factor exists because the conductor + breaker reach thermal equilibrium at full load; the 125% prevents nuisance tripping. Non-continuous loads use rated ampacity directly.
The biggest mistake in wire sizing isn\'t the wrong AWG — it\'s forgetting that ampacity and voltage drop are SEPARATE constraints. A 50-ft run might pass ampacity at 12 AWG but fail voltage drop. A 5-ft run might pass voltage drop at 14 AWG but fail ampacity. Both checks always.
Conductor temperature ratings
NEC Table 310.16 lists ampacity at three insulation temperature ratings: 60°C (oldest, like THHN), 75°C (modern THWN/USE, most common), 90°C (highest, like XHHW-2). At 90°C, conductors carry more current — but only if EVERY component in the circuit (terminals, switches, breakers) is also rated 90°C. Most equipment is 75°C rated, so the 75°C column is the practical limit for most installations. The 60°C column applies for circuits with old fixtures or terminations.
ASEAN wiring practice
ASEAN wiring uses metric mm² cross-section. IEC 60364 (and BS 7671 for Singapore) governs ampacity + voltage drop. Comparable sizes: 2.5 mm² ≈ 14 AWG, 4 mm² ≈ 12 AWG, 6 mm² ≈ 10 AWG, 10 mm² ≈ 8 AWG, 16 mm² ≈ 6 AWG. The fundamental engineering — ampacity + voltage drop dual constraint — is identical across codes. Local codes may have slightly different voltage drop limits (BS 7671: 3% for lighting, 5% other; comparable to NEC).
10 Things to Know About Wire Sizing
Wire size must satisfy BOTH ampacity AND voltage drop. Pick the larger.
NEC Table 310.16 at 75°C is the most-used ampacity column for residential + commercial.
Continuous loads (3+ hr): 125% factor per NEC 210.19 + 215.3.
Common AWG sizes: 14 (15A), 12 (20A), 10 (30A), 8 (50A), 6 (65A).
Long runs (100+ ft) typically need upsizing for voltage drop beyond ampacity.
Conduit fill (multiple conductors): NEC Table 310.15 reduces ampacity 80%, 70%, etc.
Ambient temperature: NEC Table 310.15(B)(1) reduces ampacity in hot environments.
Aluminum needs 2 sizes larger than copper for same ampacity.
EV charging circuits: typically need 6 AWG or 4 AWG for 40A Level 2.
For ASEAN metric: 1 mm² CSA ≈ 5 A rough rule for residential.
Frequently asked questions
Voltage drop. Ampacity says "can this wire carry this current?" but voltage drop says "will the load receive enough voltage at the end of the run?" Long runs require larger conductors for voltage drop even when ampacity would allow smaller. Both must be satisfied.
NEC defines continuous load as one that operates for 3+ hours at full rating. EV charging (4-8 hr sessions), industrial equipment, commercial lighting all qualify. NEC requires conductor + breaker sized for 125% of continuous load current — i.e. a 32A continuous load needs a 40A breaker + 8 AWG copper.
No — this is the single-conductor baseline. NEC Table 310.15(B)(3)(a) requires ampacity reduction when multiple current-carrying conductors share a raceway: 4-6 wires = 80%, 7-9 = 70%, etc. For accurate fill calculation, use the lower derated ampacity.
NEC Table 310.15(B)(1) reduces ampacity in hot environments. At 40°C ambient: 88% of base ampacity. At 50°C: 75%. Important for attic, mechanical room, equatorial outdoor installations (ASEAN especially). This calculator uses 30°C ambient baseline.
Aluminum has ~60% the conductivity of copper, so it needs more cross-section to carry the same current AND has higher voltage drop. Typical rule: bump up 2 AWG sizes (e.g. copper 6 AWG → aluminum 4 AWG). Aluminum is cheaper, lighter — common in service entrance, large feeders.
No. All inputs stay in your browser.
For 14, 12, 10 AWG (the "small conductor" sizes), NEC 240.4(D) restricts breaker size to 15A, 20A, 30A respectively — regardless of higher Table 310.16 ampacity. This calculator uses these practical ampacities (20A for 12, 30A for 10, etc).
Tesla Wall Connector / Level 2 chargers: 40-48A continuous at 240V. Apply 125%: design current 50-60A. Need 50A or 60A breaker. Conductor: 6 AWG copper for 50A breaker (over short distances); 4 AWG for 60A or long runs. ALWAYS consult licensed electrician for actual installation.
Free-air (overhead, single conductor): higher ampacity per NEC Table 310.17. In conduit/raceway: lower (Table 310.16). This calculator uses Table 310.16 (most common). For aerial residential service drops or motor leads in air, higher ampacity applies.
NEC 2023 (NFPA 70) at nfpa.org. NEC Handbook with commentary. Mike Holt videos for practical applications. IEEE Std 141 for industrial systems. ASEAN: BS 7671 (Singapore), MS 1979 (Malaysia), IEC 60364 (international).
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