Wire Size Calculator – Wire Gauge (AWG) & Conductor Size (IS / NEC)

Selecting the correct wire size is fundamental to electrical safety and efficiency. An undersized wire overheats and poses a fire risk; an oversized wire wastes material. This calculator determines the minimum conductor cross-section based on load current, circuit length, allowable voltage drop, and installation method derating per NEC.

NEC (Code of Practice for Electrical Wiring Installations) is the primary US standard for sizing wires and cables in building electrical systems. Conductor current ratings are tabulated for different installation methods (clipped to surface, in conduit, underground). Derating factors k1 (ambient temperature), k2 (grouping), k3 (soil thermal resistivity for underground) must be applied. NEC cross-references NEC for all wiring design in buildings.

A single-phase submersible pump motor rated 5.5 kW (230 V, 0.85 power factor, 0.85 efficiency) is fed through a 40 m one-way cable run. Design current: I = 5500 / (230 × 0.85 × 0.85) ≈ 33.1 A. A 6 mm² copper conductor can carry this current, but the voltage drop check governs: ΔV% = (2 × 33.1 × 40 × 0.0179) / (6 × 230) × 100 ≈ 3.4%, exceeding the 3% limit for final circuits.

Upsizing to 10 mm² copper reduces the drop to about 2.1%, which meets NEC requirements - showing that on long runs, voltage drop rather than current-carrying capacity often decides the final wire size.

How the Wire Size Calculator Works

"What size wire do I need?" is the single most common question in electrical work, and the answer always comes from two limits: the wire must carry the current without overheating (ampacity) and it must not lose too much voltage over the run (voltage drop). This calculator applies both, using the National Electrical Code (NEC) ampacity tables and the small-conductor rule of NEC 240.4(D), then cross-checks voltage drop against the 3% target. Give it the circuit amps (or the appliance and voltage), the run length and the wire material, and it returns the correct AWG or kcmil size along with the matching breaker and ground-wire size.

The Wire Sizing Method

For a known load, first find the current, then apply the continuous factor, then read the wire size:

• Current: I = P ÷ V (single-phase) or I = P ÷ (√3 × V) (three-phase).
• Continuous loads (≥ 3 h): size the wire and breaker at 1.25 × I.
• Ampacity check: chosen wire's rated ampacity ≥ design current, capped by NEC 240.4(D) for 14–10 AWG.
• Voltage-drop check: VD = (2 × K × I × L) ÷ CM ≤ 3% of supply (√3 for three-phase).

The NEC 240.4(D) small-conductor rule overrides the ampacity table for the three most common household sizes: 14 AWG is limited to a 15 A breaker, 12 AWG to 20 A and 10 AWG to 30 A — regardless of the higher figures in the 75°C or 90°C columns. This is why a 20 A kitchen circuit is always 12 AWG, never 14.

Variable & Unit Reference

Symbol	Quantity	US Unit	SI Unit
P	Appliance/load power	W / kW / HP	W / kW
V	Circuit voltage	V (120/240)	V (230)
I	Circuit current	A	A
L	One-way run length	ft	m
Wire size	Conductor gauge	AWG / kcmil	mm²

Unit handling: in the US, branch-circuit wire is specified in AWG and run lengths in feet, which is exactly what this calculator uses in imperial mode. The rough metric equivalents are 14 AWG ≈ 2.5 mm², 12 AWG ≈ 4 mm², 10 AWG ≈ 6 mm², 8 AWG ≈ 10 mm² and 6 AWG ≈ 16 mm². Note that NM-B ("Romex") cable is rated at the 60°C column, so its ampacity is lower than the same-size THHN in conduit.

Worked Example 1 — 20 A Kitchen Circuit (120 V)

A residential kitchen small-appliance circuit must supply 20 A at 120 V over a 40 ft run of NM-B cable.

1. Breaker and rule: a 20 A circuit under NEC 240.4(D) requires a minimum of 12 AWG copper.
2. Ampacity: 12 AWG NM-B is rated 20 A at 60°C — exactly matches ✓.
3. Voltage drop: VD = (2 × 12.9 × 20 × 40) ÷ 6530 = 3.16 V = 2.6% ✓.
4. Ground: NM-B cable includes a 12 AWG bare copper equipment ground, satisfying Table 250.122 for a 20 A breaker.

Answer: 12 AWG copper NM-B on a 20 A breaker — the standard kitchen small-appliance circuit.

Worked Example 2 — 48 A EV Charger (240 V)

A 240 V Level 2 EV charger rated 48 A continuous is installed 75 ft from the panel using THWN-2 copper in conduit.

1. Continuous multiplier: EV charging is continuous, so the circuit is sized at 1.25 × 48 = 60 A.
2. Ampacity: from NEC 310.16, 75°C copper, 6 AWG = 65 A ≥ 60 A ✓; the breaker is 60 A.
3. Voltage drop: VD = (2 × 12.9 × 48 × 75) ÷ 26240 = 3.54 V = 1.5% ✓.
4. Ground: Table 250.122 for a 60 A breaker → 10 AWG copper.

Answer: 6 AWG copper conductors, 10 AWG ground, 60 A breaker. Note how the 125% continuous rule pushed a 48 A load up to a 60 A circuit and a 6 AWG wire — sizing an EV charger as if it were a 48 A non-continuous load would violate the code.

Standards & Code References

• NEC 240.4(D) — small-conductor overcurrent limits (14 AWG/15 A, 12 AWG/20 A, 10 AWG/30 A).
• NEC Table 310.16 — ampacity of insulated conductors for THHN/THWN in conduit.
• NEC 334.80 — NM-B cable is limited to the 60°C ampacity column.
• NEC 210.19(A) — 125% continuous-load sizing and the 3% branch voltage-drop recommendation.
• NEC 210.23 / 210.24 — permitted loads and the standard branch-circuit ratings (15, 20, 30, 40, 50 A).
• NEC Table 250.122 — equipment grounding conductor size by breaker rating.
• NEC 625.41 — EV supply equipment is a continuous load; 210.8 covers required GFCI protection.

Key Facts to Remember

• The three household sizes are fixed by rule: 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A.
• NM-B (Romex) uses the 60°C column, so it carries less than THHN of the same gauge in conduit.
• Continuous loads (EV chargers, heaters, lighting) are sized at 125% — this often bumps the wire up a size.
• The breaker protects the wire, not the appliance — never install a breaker larger than the wire can safely carry.
• Aluminum branch wiring needs one to two sizes larger and listed AL terminations; it is rare in modern branch circuits but common on large feeders.
• Long runs may need a larger wire for voltage drop even when ampacity is satisfied.
• A dedicated equipment grounding conductor is required and sized from Table 250.122, not guessed.
• Multi-wire and bundled circuits may require ampacity derating for more than three current-carrying conductors.

Wire Size by Circuit Rating (the "money table")

Breaker / Circuit	Copper (THHN 75°C)	Copper (NM-B 60°C)	Ground (Cu)	Typical Use
15 A	14 AWG	14 AWG	14 AWG	Lighting, general receptacles
20 A	12 AWG	12 AWG	12 AWG	Kitchen, bath, garage receptacles
30 A	10 AWG	10 AWG	10 AWG	Dryer, water heater, A/C
40 A	8 AWG	8 AWG	10 AWG	Electric range, large A/C
50 A	8 AWG	6 AWG	10 AWG	Range, welder, sub-feed
60 A	6 AWG	4 AWG	10 AWG	EV charger, small sub-panel
100 A	3 AWG	—	8 AWG	Sub-panel feeder
200 A	2/0 AWG	—	6 AWG	Residential service

These are general values at 30°C ambient and ≤ 3 current-carrying conductors; verify voltage drop on long runs and apply correction factors in hot locations.

Real-World Applications

• Residential branch circuits — lighting, receptacles, kitchens and bathrooms.
• Large-appliance circuits — ranges, dryers, water heaters and HVAC.
• EV charger installations, the fastest-growing 240 V continuous load in homes.
• Sub-panel feeders to garages, workshops and additions.
• Commercial receptacle and lighting circuits in tenant fit-outs.
• Well pumps and outbuildings on long outdoor runs where voltage drop dominates.
• Solar and battery interconnections with their double continuous factor.
• Generator and transfer-switch wiring for backup power.

Common Mistakes

• Putting 14 AWG on a 20 A breaker — a fire hazard and an instant code failure.
• Using the THHN ampacity for NM-B cable, which is only rated at 60°C.
• Ignoring the 125% continuous rule for EV chargers and electric heat.
• Oversizing the breaker to stop nuisance trips, defeating the wire's protection.
• Skipping voltage drop on long runs to detached structures.
• Reusing an undersized existing wire when upgrading to a larger appliance.
• Forgetting the correctly sized equipment ground.
• Mixing aluminum and copper at terminals without listed connectors and anti-oxidant.

Step-by-Step: Choosing a Wire Size

1. Identify the load. Read the appliance nameplate in watts or amps, or use the circuit's breaker rating if you are wiring a general-purpose circuit.
2. Determine the voltage. Lighting and standard receptacles are 120 V; ranges, dryers, water heaters and EV chargers are 240 V.
3. Compute the current (amps = watts ÷ volts) and, if the load runs three hours or more, multiply by 1.25.
4. Select the wire and breaker from the ampacity tables, honoring the NEC 240.4(D) caps for 14, 12 and 10 AWG.
5. Pick the right ampacity column for your wiring method — 60°C for NM-B cable, 75°C for THHN/THWN in conduit.
6. Check voltage drop on runs over about 100 ft and upsize if it exceeds 3%.
7. Add the equipment ground from Table 250.122 (NM-B cable already includes it).

The breaker and the wire are a matched pair: the breaker's job is to open before the wire it protects can overheat. That is why you can never fix nuisance tripping by simply installing a larger breaker on the same wire.

Understanding Wiring Methods

The same copper conductor carries different amounts of current depending on how it is installed, because heat dissipation varies with the method. Choosing the right ampacity column is as important as choosing the gauge.

• NM-B cable ("Romex"): the flat, jacketed cable used inside walls of homes. NEC 334.80 requires the 60°C column, so it carries the least current of any method — 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A. It is inexpensive and fast to install but not permitted where exposed to damage, wet locations or high heat.
• THHN/THWN-2 in conduit: individual conductors pulled through EMT, PVC or rigid conduit. Rated at 75°C (or 90°C dry for THHN), it carries more current and is used for feeders, commercial work and any exposed or wet run.
• UF cable: underground feeder cable rated for direct burial and wet locations, used for outdoor circuits to detached structures.
• SE and USE cable: service-entrance conductors sized to the service rating.

Because of the temperature-column difference, a 6 AWG copper conductor carries 55 A as NM-B but 65 A as THWN in conduit — a real distinction when a load falls between two standard sizes.

Aluminum Branch Wiring

Older homes wired in the 1960s and 70s used solid aluminum branch conductors that developed loose, overheating connections at receptacles and switches. Modern aluminum building wire uses the AA-8000 alloy and is safe and code-compliant, but it is used mainly for larger feeders (100 A sub-panels and services) rather than 15/20 A branch circuits, where copper remains standard. If you must terminate aluminum, use only devices and lugs marked CO/ALR or AL-CU, apply anti-oxidant compound, and torque connections to specification. For most homeowners and small commercial circuits, copper is the simpler and more reliable choice, and this calculator defaults to copper while letting you compare aluminum where it makes economic sense.

Common Appliance Wire Sizes (the second "money table")

Appliance / Load	Typical Circuit	Copper Wire
General lighting / receptacles	15 A, 120 V	14 AWG
Kitchen / bath / garage receptacles	20 A, 120 V	12 AWG
Microwave, dishwasher, disposal	20 A, 120 V	12 AWG
Electric water heater (4.5 kW)	30 A, 240 V	10 AWG
Clothes dryer	30 A, 240 V	10 AWG
Central A/C (3–4 ton)	30–40 A, 240 V	10–8 AWG
Electric range / oven	40–50 A, 240 V	8–6 AWG
Level 2 EV charger (40–48 A)	50–60 A, 240 V	8–6 AWG
Sub-panel feeder	100 A, 240 V	3 AWG (or 1 AWG AL)

These are typical values; always confirm against the specific nameplate rating and the run length. Continuous loads such as EV chargers and electric heat are sized at 125%, which is already reflected in the circuit ratings shown.

Safety & Best-Practice Tips

• Match the breaker to the wire, never the load. The overcurrent device protects the conductor first.
• Use the 60°C column for NM-B even though the conductors inside are marked 90°C.
• GFCI and AFCI protection is required for many modern circuits (kitchens, baths, outdoors, bedrooms, EV) — verify NEC 210.8 and 210.12.
• Don't back-stab larger conductors; use the screw terminals or listed connectors, torqued correctly.
• Derate for heat and bundling in attics, near recessed lighting and where many cables share a stud bay.
• Pull permits and inspect. Electrical work is life-safety work; when in doubt, hire a licensed electrician.

Circuit Types & the 80% Rule

Standard circuit breakers are rated for continuous operation at only 80% of their marked rating unless they are specifically listed as 100%-rated (most residential and small commercial breakers are not). That is the flip side of the 125% continuous-load rule: sizing a continuous load at 125% is mathematically the same as loading the breaker to no more than 80%. In practical terms, a 20 A circuit should carry no more than 16 A of continuous load, and a 50 A EV circuit no more than 40 A of continuous charging. When you plan how many devices to place on a circuit, apply this 80% ceiling for anything that runs three hours or more — general receptacle circuits are treated as non-continuous, but dedicated appliance, lighting and EV circuits usually are not. Understanding the 80% rule prevents both nuisance tripping and the temptation to overload a circuit up to its nameplate number.

Conductor Identification & Color Codes

Correctly sizing the wire is only useful if it is also correctly identified and terminated. In US 120/240 V single-phase systems, the common convention is black and red for the two ungrounded (hot) conductors, white for the grounded neutral, and green or bare for the equipment ground. In 208/120 V three-phase, hots are black, red and blue with a white neutral; in 480/277 V three-phase, hots are brown, orange and yellow with a gray neutral. These colors are largely convention rather than mandatory NEC requirements above certain sizes, but consistent identification is essential for safety and is often enforced by the local jurisdiction and by facility standards. The grounded (neutral) and grounding (green) conductors, however, do have mandatory color rules. Always confirm the local and project color scheme before pulling wire.

Troubleshooting Undersized Wiring

Undersized wire announces itself in predictable ways: breakers that trip under normal load, receptacles or plugs that feel warm, lights that dim when a large appliance starts, and voltage that sags at the end of long runs. The correct response is never to install a larger breaker on the existing wire — that removes the wire's protection and creates a fire hazard. Instead, either reduce the load on the circuit, add a new dedicated circuit of the correct size, or replace both the wire and the breaker together with a properly sized pair. Warm terminations often indicate a loose connection rather than an undersized conductor, so re-torquing to specification is the first check. When an older home's aluminum branch wiring shows these symptoms, a licensed electrician should evaluate the terminations and remediate with listed connectors. Any time the diagnosis is uncertain, treat it as a life-safety issue and bring in a professional.

Cost & When to Call an Electrician

Wire is a small fraction of a project's cost, so there is rarely a good reason to skimp — the labor to pull a conductor is the same whether it is one size up or not, and the larger conductor buys ampacity headroom, lower voltage drop and future flexibility. Where the budget bites is on long feeders and services, where the copper-versus-aluminum decision and the exact size have real dollar impact; that is precisely where a careful calculation pays off. For homeowners, simple like-for-like replacements are often within reach, but new circuits, panel work, service upgrades and any aluminum remediation are permit-required, life-safety tasks best handled by a licensed electrician. This calculator is an excellent planning and learning tool, but it does not replace the code knowledge, permitting and inspection that a qualified professional brings.

Special Circuits: Multi-Wire, Shared Neutrals & Taps

A few common circuit configurations change how you size and protect conductors, and misunderstanding them causes real hazards. A multi-wire branch circuit (MWBC) shares one neutral between two hot conductors on opposite legs (for example a red and a black sharing a white in a 120/240 V panel). When the legs are balanced, the shared neutral carries only the difference of the two currents, so it can be the same size as the hots. But the neutral must be sized for the maximum unbalanced load, and if the two hots are ever placed on the same leg the neutral can be overloaded — which is why NEC 210.4 requires a common disconnect (a handle tie or two-pole breaker) so both hots de-energize together for safety.

Shared neutrals with nonlinear loads deserve caution: computers, LED drivers and other electronic loads inject third-harmonic current that adds rather than cancels in the shared neutral, so the neutral can carry more current than either hot. In those cases the neutral is counted as a current-carrying conductor for derating and is often upsized. Tap conductors — smaller conductors connected to a larger feeder — are permitted under the NEC 240.21 tap rules (the 10-foot and 25-foot tap rules), which allow a conductor smaller than the upstream overcurrent device would normally protect, provided strict length and termination conditions are met. These are the exceptions that prove the rule: in every ordinary case the wire must be protected at its ampacity, but the code carves out carefully bounded situations where engineered judgment applies. When your design involves an MWBC, a shared neutral on electronic loads, or a feeder tap, verify the specific NEC article rather than relying on the basic ampacity table alone.

Quick Answers by Amperage

For fast reference, here are the copper wire sizes that answer the most-searched "what size wire for X amps" questions, assuming a standard 30°C ambient, no more than three current-carrying conductors, and a reasonable run length. A 15 amp circuit uses 14 AWG; a 20 amp circuit uses 12 AWG; a 30 amp circuit (dryer, water heater, small A/C) uses 10 AWG; a 40 amp circuit (range, larger A/C) uses 8 AWG; a 50 amp circuit (range, welder, sub-feed) uses 8 AWG THHN in conduit or 6 AWG NM-B cable; a 60 amp circuit (EV charger, small sub-panel) uses 6 AWG; a 100 amp feeder uses 3 AWG copper or 1 AWG aluminum; and a 200 amp residential service uses 2/0 copper or 4/0 aluminum. Each of these must still be paired with a correctly sized equipment grounding conductor from Table 250.122 and checked for voltage drop on long runs.

Two caveats keep these quick answers safe. First, if the load is continuous — running three hours or more, as EV chargers, electric heat and much commercial lighting do — multiply the load by 1.25 before you read the size, which frequently bumps you up one gauge. Second, confirm the wiring method's temperature column: NM-B cable uses the lower 60°C values, so a size that works as THHN in conduit may need to be one gauge larger as Romex. When these two checks are satisfied and the run is not unusually long, the sizes above are the correct, code-compliant answers that this calculator will confirm for your specific conditions.

Limitations & Disclaimer

This calculator gives a code-based starting point for common branch circuits and feeders; it is not a substitute for a permitted design or a licensed electrician's judgment. It assumes standard insulation types, 30°C ambient and no more than three current-carrying conductors unless you specify otherwise, and it does not evaluate every special condition (rooftop adders, buried cable, parallel sets, harmonic neutrals). Always follow the NEC edition and any local amendments enforced by your Authority Having Jurisdiction, obtain required permits, and have safety-critical work performed or reviewed by a qualified electrician.