TryBuildCalc

Wire Size Calculator (AWG Gauge from Load Current, Length & Voltage)

Calculate wire gauge instantly.

Inputs

Load Entered As
A
V

ℹ️Common values: 120V, 208V, 240V, 277V, 480V (US); 230V, 400V (many other countries).

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ℹ️Distance from the panel/breaker to the load, one way — not round trip.

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ℹ️The wire's own insulation rating (check the product's markings, e.g. THHN, THWN-2).

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ℹ️Per NEC 110.14(C) — the breaker/panel/device's own temperature rating. Most equipment rated 100A or less is only terminal-rated for 60°C or 75°C, even with 90°C wire installed. The calculator automatically uses the lower of this and the insulation rating above.

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Continuous Load (3+ hours)?

Advanced (NEC Derating)

Adjust for Conductor Count / Ambient Temperature?

Conduit Sizing

Also Size the Conduit?

Cost

Enable Cost Estimation?

Recommended Wire Size: 10 AWG (Copper)

Sized by voltage drop, not ampacity — recommended breaker/OCPD: 20A

Ampacity Check

Design current: 20.0 A

Minimum size by ampacity: 12 AWG

Derating factor applied: 1.00×

Final wire's derated ampacity: 35.0 A

Voltage Drop Check

Actual load current: 20.0 A

Minimum size by voltage drop: 10 AWG

Allowed drop: 3.0%

Final wire's actual drop: 2.07% (2.49 V)

Circuit Summary

System120 V, single-phase
One-way run length50.0 ft (15.2 m)
Insulation / terminal rating75°C (167°F) — most common (THWN-2, terminals) / 75°C (167°F) — most common (THWN-2, terminals)
Governing rating (NEC 110.14(C))75°C (167°F) — most common (THWN-2, terminals)
Small-conductor OCPD cap30 A (NEC 240.4(D))

Assumptions Used

NEC Table 310.16 ampacity, NEC 110.14(C) governing rating, NEC 240.4(D) small-conductor cap, NEC 310.15(C)(1)/(B)(1)(1) derating, NEC Chapter 9 conduit fill (THHN/THWN-2, uniform AWG). Voltage drop uses K = 12.9 (copper) / 21.2 (aluminum) ohm-cmil/ft. This is a reference estimate — confirm against the code edition adopted in your jurisdiction and a licensed electrician before final installation.

Wire Run VisualizationPanelLoad10 AWG20.0 ALength: 50 ftDiagram simplified for clarity (not to scale)

What Is a Wire Size Calculator?

A wire size calculator recommends the minimum conductor gauge (AWG) for a circuit, based on the load current, the circuit's voltage, and the one-way run length. It checks two independent NEC requirements — ampacity (the conductor's safe current-carrying capacity) and voltage drop (how much voltage is lost over the run) — and recommends whichever one demands the larger conductor.

A wire that passes the ampacity check can still fail on voltage drop over a long run, and vice versa on a very short but heavily-loaded run — this calculator checks both, rather than only the simpler ampacity table lookup most quick references stop at.

Why checking both ampacity and voltage drop matters:

  • Undersized wire on ampacity risks overheating and fire — this is a genuine safety hazard, not just a performance issue
  • Undersized wire on voltage drop causes dim lighting, motor overheating, and voltage-sensitive equipment malfunction, even when ampacity is technically fine
  • Long runs (detached garages, workshops, well pumps) are the most common case where voltage drop, not ampacity, ends up governing
  • Getting this wrong is expensive to fix after conduit/walls are closed up — sizing correctly before installation avoids a costly rework

Wire Size Formula: How Is AWG Gauge Calculated?

Two independent checks are run, and the wire recommended is whichever check requires the larger conductor.

Step 1 — Design Current

Design Current = Load Current × 1.25 (if continuous load, per NEC 210.19/210.20)

Design Current = Load Current (if not continuous)

A continuous load (running 3+ hours) needs a 25% safety margin applied to the wire and breaker sizing, per NEC's continuous-load rule. Voltage drop, further below, uses the actual (non-inflated) current instead.

Step 2 — Ampacity Check

Governing Rating = the LOWER of Insulation Rating and Terminal/Equipment Rating (NEC 110.14(C))

Derated Ampacity = Table Ampacity (at Governing Rating) × Conductor-Count Factor × Ambient-Temp Factor

Pick the smallest AWG where Derated Ampacity ≥ Design Current

Table ampacity comes from NEC Table 310.16, by material (copper/aluminum) and the governing 60/75/90°C column — not just the wire's own insulation rating, since most equipment rated 100A or less is only terminal-rated for 60°C or 75°C even when 90°C wire is installed. 14/12/10 AWG are additionally capped at 15A/20A/30A (copper) regardless of table ampacity, per NEC 240.4(D).

Step 3 — Voltage Drop Check

Voltage Drop (V) = (2 × K × I × D) / CM [single-phase]

Voltage Drop (V) = (1.732 × K × I × D) / CM [three-phase]

Pick the smallest AWG where (Voltage Drop ÷ System Voltage) × 100 ≤ Max Allowed %

K is a resistivity constant (12.9 for copper, 21.2 for aluminum, ohm-circular-mil/ft), I is the actual load current, D is the one-way run length in feet, and CM is the conductor's circular-mil area.

Step 4 — Final Recommendation

Recommended AWG = the larger of the ampacity-check size and the voltage-drop-check size

Both requirements must be satisfied at once, so the calculator always recommends whichever check needs the bigger conductor — never just the smaller of the two.

Worked Example

This example walks through your current inputs above, using the same steps as the Formula section.

Input Values Used

InputValueWhy it is used
Load current / voltage20.0 A, 120 V, single-phaseSets design current and the voltage-drop base
Material / rating / lengthCopper, governed by 75°C (167°F) — most common (THWN-2, terminals), 50.0 ftSets ampacity table column (lower of insulation/terminal rating) and voltage-drop distance
Derating / max drop1.00× derate, 3.0% max dropSets the derated ampacity and voltage-drop threshold

Step 1 — Ampacity Check

CalculationResult
Design current20.0 A
Minimum size by ampacity12 AWG

Step 2 — Voltage Drop Check

CalculationResult
Minimum size by voltage drop10 AWG
Final recommended size10 AWG (governed by voltage drop)

Therefore, for a 20.0A load over 50.0 ft at 120V, you need approximately 10 AWG copper wire on a 20A breaker.

Essential Checklist+

Complete these critical checks before approving the work or proceeding to the next construction stage.

✓15 Inspection Points
✓5 Verification Categories
✓Load & Circuit Inputs+
  • Load current confirmed from equipment nameplate, not estimated
  • Continuous vs. non-continuous load correctly classified
  • System voltage and phase confirmed against the actual panel/service
  • Run length measured as actual one-way routed distance, not straight-line or round-trip
✓Ampacity & Derating+
  • Insulation temperature rating and terminal/equipment rating entered separately, not assumed equal
  • 14/12/10 AWG copper conductors capped at 15A/20A/30A breaker regardless of table ampacity
  • Conductor-count derating applied when more than 3 current-carrying conductors share a raceway
  • Aluminum conductor terminations confirmed AL-rated (not CU-only)
✓Voltage Drop+
  • Voltage drop checked independently of the ampacity result, not assumed to be fine
✓Installation & Code Compliance+
  • Breaker/OCPD size does not exceed the selected conductor's allowable ampacity or small-conductor cap
  • Local code edition and any local amendments confirmed before finalizing
  • Permit and inspection requirements confirmed for the scope of work
  • Work beyond a simple, single branch circuit reviewed or performed by a licensed electrician
✓Conduit Sizing+
  • Conduit fill checked as a separate NEC Chapter 9 requirement, not assumed covered by the ampacity result
  • Total conductor count for fill purposes includes every physical conductor, including the equipment grounding conductor
Full QC Checklist+

Verification checklist for wire size selection and circuit installation — covering load classification, ampacity and derating, voltage drop, and code compliance. Use the Essential Checklist for critical checks before wiring; expand to Full QC Checklist for complete electrical quality control.

✓23 Inspection Points
✓5 Verification Categories
✓Load & Circuit Inputs+
  • Load current confirmed from equipment nameplate, not estimated
  • Continuous vs. non-continuous load correctly classified
  • System voltage and phase confirmed against the actual panel/service
  • Run length measured as actual one-way routed distance, not straight-line or round-trip
  • Power factor entered correctly when sizing from a wattage nameplate rating on a motor or reactive load
✓Ampacity & Derating+
  • Insulation temperature rating and terminal/equipment rating entered separately, not assumed equal
  • 14/12/10 AWG copper conductors capped at 15A/20A/30A breaker regardless of table ampacity
  • Conductor-count derating applied when more than 3 current-carrying conductors share a raceway
  • Ambient temperature correction applied for hot locations (attics, rooftops, outdoor conduit in sun)
  • Aluminum conductor terminations confirmed AL-rated (not CU-only)
  • Cable run through or in contact with thermal insulation checked against the cable-type-specific derating rule
✓Voltage Drop+
  • Voltage drop checked independently of the ampacity result, not assumed to be fine
  • Maximum voltage drop percentage matched to the load's actual sensitivity
  • Detached structures, well pumps, and other long runs specifically flagged for voltage-drop review
  • Single-phase vs. three-phase voltage-drop multiplier confirmed correct for the actual circuit
✓Installation & Code Compliance+
  • Breaker/OCPD size does not exceed the selected conductor's allowable ampacity or small-conductor cap
  • Conductor color coding (hot, neutral, ground) follows local code convention, consistently across the circuit
  • Local code edition and any local amendments confirmed before finalizing
  • Permit and inspection requirements confirmed for the scope of work
  • Work beyond a simple, single branch circuit reviewed or performed by a licensed electrician
✓Conduit Sizing+
  • Conduit fill checked as a separate NEC Chapter 9 requirement, not assumed covered by the ampacity result
  • Total conductor count for fill purposes includes every physical conductor, including the equipment grounding conductor
  • Conduit type selected matches what will actually be installed (EMT, PVC Schedule 40, or RMC)

Copper Wire Ampacity Reference (NEC Table 310.16)

Base ampacity (amps) for copper conductors, not more than 3 current-carrying conductors, 30°C ambient — before any derating from the Advanced section above.

AWG60°C75°C90°CSmall-Conductor OCPD Cap
14 AWG15A20A25A15A
12 AWG20A25A30A20A
10 AWG30A35A40A30A
8 AWG40A50A55A—
6 AWG55A65A75A—
4 AWG70A85A95A—
2 AWG95A115A130A—
1/0 AWG125A150A170A—

14/12/10 AWG copper conductors can never be protected by a breaker larger than their small-conductor cap, regardless of a higher table ampacity at the 75°C/90°C columns — this is a fixed NEC 240.4(D) rule, not a derating condition.

EMT Conduit Fill Reference (NEC Chapter 9)

Maximum number of same-size THHN/THWN-2 conductors in EMT conduit, at the NEC 40% fill allowance (3 or more conductors) — from this calculator's own Conduit Sizing section.

Trade Size12 AWG (max count)10 AWG (max count)8 AWG (max count)
1/2"953
3/4"16105
1"25169
1-1/4"442816

Figures assume all conductors are the same AWG and insulation type — a real conduit with mixed sizes needs the calculator's own area-based check (or a manual NEC Chapter 9 calculation) rather than this simplified reference table.

When should you use this wire size calculator?

  • Sizing a branch circuit for a new outlet, appliance, or fixture.
  • Sizing a long run to a detached garage, workshop, well pump, or outbuilding, where voltage drop often governs.
  • Checking whether an existing circuit's wire size is adequate for a planned load increase.
  • Comparing copper vs. aluminum conductor size for the same circuit.
  • Cross-checking an electrician's proposed wire size against an independent estimate.

Quick Wire Sizing Tips

  • Always check voltage drop separately from ampacity — a wire that's fine on ampacity can still fail on voltage drop over a long run.
  • Measure run length as the actual routed distance (following walls, up/down, around corners), not the straight-line distance.
  • Mark continuous loads (3+ hours of operation) correctly — the 125% factor changes both wire and breaker sizing.
  • Use the Advanced derating section for any raceway with more than 3 current-carrying conductors, or a hot ambient location like an attic.
  • Remember 14/12/10 AWG copper conductors are always capped at 15A/20A/30A breakers, regardless of table ampacity.
  • Enter the terminal/equipment rating separately from the wire's insulation rating — the lower of the two governs, and most breakers/panels rated 100A or less are only 60°C or 75°C-rated even with 90°C wire installed.
  • Turn on Conduit Sizing whenever more than one circuit's conductors share a raceway — it's a genuinely separate NEC Chapter 9 check from ampacity, not something the ampacity result covers.

Common Mistakes

  • Sizing wire only by ampacity and skipping the voltage drop check entirely — this is the single most common wire-sizing error on long runs.
  • Using a 20A-rated breaker on 14 AWG copper wire because the 75°C table ampacity reads 20A — NEC 240.4(D) caps it at 15A regardless.
  • Forgetting the 125% continuous-load factor on circuits that genuinely run 3+ hours, like most lighting and HVAC circuits.
  • Ignoring conductor-count derating when bundling many current-carrying conductors in one conduit or cable.
  • Using round-trip distance instead of one-way distance for run length — this doubles the apparent length and oversizes the wire.
  • Assuming 90°C-rated wire always gets the 90°C ampacity column — the terminal/equipment rating (often only 60°C or 75°C on standard breakers and panels) can override it.
  • Sizing the wire correctly but pulling too many conductors through an undersized conduit — ampacity and conduit fill are two separate NEC checks, and passing one doesn't mean the other is fine.

Limitations

  • NEC-based only (US) — other regions use different tables (IEC/BS 7671 and similar) and this calculator does not model them.
  • Covers standard AWG conductors up to 4/0 (~260A) — larger service/feeder conductors sized in kcmil are outside this calculator's scope.
  • Voltage drop uses a simplified resistance-only formula (no reactance) — standard practice for most branch/feeder sizing, but not exact for very long, high-current three-phase runs.
  • Breaker/OCPD sizing here is a reference estimate — final overcurrent protection selection should be confirmed by a licensed electrician against the exact NEC edition adopted locally.
  • Conduit fill assumes every conductor in the conduit is the same AWG and THHN/THWN-2 insulation as the recommended wire — a raceway with mixed sizes or insulation types needs a manual NEC Chapter 9 check instead.
  • Conduit fill covers EMT, PVC Schedule 40, and RMC only, up to 4" trade size — other conduit types (ENT, FMC, IMC, PVC Schedule 80, larger trade sizes) are outside this calculator's scope.

Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.

FAQ

Two checks are needed, and you use whichever result needs the bigger wire. First, the ampacity check: look up the wire's NEC Table 310.16 ampacity for its material and insulation rating, apply any derating for conductor count or ambient temperature, and confirm it covers your design current (load current, or 125% of it for a continuous load). Second, the voltage drop check: using the formula (2 × K × I × D) / CM for single-phase (1.732 instead of 2 for three-phase), confirm the resulting voltage drop percentage stays within your allowed limit (commonly 3%) over the actual run length. This calculator runs both checks automatically and recommends the larger of the two results.
Ampacity is about heat — how much current a conductor can carry without overheating — and doesn't care how long the wire run is. Voltage drop is about resistance accumulating over distance — the longer the run, the more voltage is lost before it reaches the load, regardless of whether the wire is running hot. A 20A load over 20 feet might be perfectly fine on 12 AWG for ampacity, but the same 20A load over 200 feet (a detached workshop, a well pump) can lose more than 3% of the voltage on that same 12 AWG wire, requiring a larger conductor just to keep the voltage drop acceptable — even though the wire was never at risk of overheating.
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