Wire Size Calculator
This free wire size calculator answers the most-asked question in residential wiring — what gauge wire for how many amps — the way the code answers it, not the way a chart on a forum does. It starts from NEC Table 310.16, applies the ambient-temperature and bundling corrections, caps the result by the termination rules that catch most DIYers (NM-B is limited to the 60°C column no matter what its insulation says), enforces the small-conductor breaker limits of 240.4(D), and sizes continuous loads like EV chargers at 125%.
Appliance presets cover the circuits homeowners actually search for — dryer, range, water heater, hot tub, well pump, EV charger, detached-garage subpanel — with the typical circuit values and the gotchas each one carries. High-variance equipment (mini-splits, welders, saunas) deliberately requires your nameplate value instead of guessing, because on those circuits the nameplate is the code.
Enter your one-way run length and the calculator also checks voltage drop — the thing that actually governs wire size on long runs to wells, hot tubs, and outbuildings — flagged honestly as the NEC Informational Note recommendation it is. Sizing reference only: motor, HVAC, and welder circuits carry additional rules, and every result should be verified with a licensed electrician and your local AHJ. Free, no signup.
Wire Size Calculator
What gauge wire do you need? Pick your appliance or enter the load, and get the minimum AWG for NM-B cable or THHN in conduit — with NEC derating, the 125% continuous rule, and an optional voltage-drop check for long runs. Free, no signup.
What are you wiring?
Conductor & installation
Long runs to wells, detached garages, and hot tubs are usually governed by voltage drop, not ampacity — enter the run length to check both at once.
The long-run effect the ampacity table can’t see
Ampacity says what the wire can carry safely; distance decides what actually arrives. On runs to wells, sheds, and hot tubs, the drop is usually the governing constraint — enter your run length in the calculator to check both at once.
Romex’s 20-amp secret
The most common wire-sizing surprise: 12 AWG NM-B is a 20 A conductor no matter what the ampacity chart says, while the same copper as THHN in conduit reads 25 A — and the breaker is capped at 20 A either way.
Calculation Formulas
Loads expected to run 3+ hours (EV charging per 625.41, some heating) are sized at 125%. Typical appliance circuits use the load directly.
Example:
A 48 A EV charger requires 48 × 1.25 = 60 A of circuit capacity.
Derating starts from the conductor insulation rating column (90°C for NM-B and THHN/THWN-2), corrected for ambient temperature (Table 310.15(B)(1), 30°C base) and for more than three bundled current-carrying conductors (Table 310.15(C)(1)).
Example:
12 AWG THHN in a 105–113°F attic with 7–9 bundled conductors: 30 × 0.87 × 0.70 = 18.3 A.
NEC 110.14(C)(1): circuits ≤100 A use the 60°C column unless equipment is marked 75°C; NM-B is always capped at the 60°C column (334.80) regardless of its 90°C insulation.
Example:
12 AWG copper NM-B: 90°C column is 30 A, but the 60°C cap makes the usable ampacity 20 A.
Regardless of column ampacity, small conductors are limited to these overcurrent-device ratings (with motor/HVAC/welder exceptions under Articles 430/440/630).
Example:
The classic gotcha: 12 AWG copper shows 25 A in the 75°C column but may not be protected above a 20 A breaker.
Sizes are walked smallest to largest; the first conductor that satisfies both the capacity requirement and the small-conductor cap is the minimum size.
Example:
48 A EVSE (60 A required): THHN in conduit → 6 AWG (65 A at 75°C); NM-B → 4 AWG (70 A at 60°C), because 6 AWG NM-B is only 55 A.
When a run length is entered, the chosen conductor is checked with the K-factor method (shared with the voltage drop calculator). The 3% figure is a recommendation (NEC 210.19 IN), not enforceable code (90.5(C)).
Example:
20 A at 120 V over 150 ft on 12 AWG drops 9.9% — 6 AWG brings the run within 3%.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| 12 AWG Cu (60/75/90°C) | 20 / 25 / 30 A | Table 310.16 — the most common 20 A branch-circuit conductor. |
| 6 AWG Cu (60/75/90°C) | 55 / 65 / 75 A | Table 310.16 — the workhorse for 50–60 A circuits (hot tubs, EV chargers). |
| Continuous-load factor | 125% | NEC 210.19/625.41 — loads running 3+ hours are sized at 125%. |
| Bundling 4–6 / 7–9 / 10–20 | × 0.80 / 0.70 / 0.50 | Table 310.15(C)(1) adjustment for more than three current-carrying conductors (2023 values; the claimed 2026 change to 0.65 for 7–9 is unconfirmed and not used). |
| Hot attic (105–113°F) | × 0.87 (90°C col) | Table 310.15(B)(1) ambient correction — attics are the common residential derating case. |
| Standard breakers (residential) | 15–200 A per 240.6(A) | The typical-breaker hint rounds the required amps up to the next standard rating. |
| Aluminum 14 AWG | not listed | Table 310.16 lists aluminum from 12 AWG — 14 AWG aluminum branch wiring is not sized by this table. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
NEC Table 310.16 — Conductor ampacities(NFPA 70 (2023) Table 310.16)
View StandardAllowable ampacities for insulated conductors at 60/75/90°C, copper and aluminum. (Numbered 310.15(B)(16) in the 2017 and earlier editions — values unchanged.)
Key Requirements:
- •Ampacity by size, material, and temperature rating
- •Basis for all conductor selection in this calculator
NEC 310.15(B)(1) & 310.15(C)(1) — Corrections(NFPA 70 (2023) 310.15)
View StandardAmbient-temperature correction factors (30°C base) and adjustment factors for more than three current-carrying conductors.
Key Requirements:
- •Ambient correction by temperature-rating column
- •80% (4–6), 70% (7–9), 50% (10–20) bundling factors
NEC 110.14(C)(1) — Termination temperature(NFPA 70 (2023) 110.14(C)(1))
View StandardCircuits ≤100 A (or 14–1 AWG) use the 60°C column unless equipment is listed and marked for 75°C; larger circuits use the 75°C column.
Key Requirements:
- •Termination rating caps usable ampacity
- •90°C column usable only when every termination is 90°C-listed
NEC 334.80 — NM-B ampacity(NFPA 70 (2023) 334.80)
View StandardNonmetallic-sheathed cable (Romex) ampacity must be taken from the 60°C column, though its 90°C insulation may be used as the starting point for derating.
Key Requirements:
- •12 AWG NM-B = 20 A usable, not 25 A
- •Derating may start from the 90°C value
NEC 240.4(D) — Small conductors(NFPA 70 (2023) 240.4(D))
View StandardCaps overcurrent protection at 15 A (14 Cu), 20 A (12 Cu), 30 A (10 Cu); 15 A (12 Al), 25 A (10 Al) — regardless of column ampacity.
Key Requirements:
- •Applies after ambient/bundling corrections
- •Motor, HVAC, and welder circuits excepted under their own articles
NEC 240.6(A) — Standard breaker ratings(NFPA 70 (2023) 240.6(A))
View StandardThe standard ampere ratings for fuses and inverse-time breakers, used for the typical-breaker hint (15, 20, 25, 30, 35, 40, 45, 50, 60 A … in the residential range).
Key Requirements:
- •Round up only per the 240.4(B) conditions
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Homeowner electrical permits
Owner-occupied single-family homes
Many states allow owner-occupants to pull their own permit with required inspection; rentals and multifamily are excluded nearly everywhere.
Regional Examples:
Adopted NEC edition
2020 / 2023 / 2026
The ampacity values in this calculator are identical across the 2020, 2023, and 2026 editions; requirements around them (GFCI expansions, EV provisions) differ by edition.
Regional Examples:
Climate-driven derating
Attics and hot regions
Ambient-temperature correction is the practical regional variable: a Phoenix attic run can lose 18–25% of table ampacity, while the same cable in a Minnesota basement loses nothing.
Regional Examples:
Special-rule circuits this tool does not size
Motors, HVAC, welders
Articles 430 (motors), 440 (HVAC), and 630 (welders) allow breakers that legitimately exceed conductor ampacity to handle starting current and duty cycles.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Want to Learn More?
Can you do your own electrical work? State-by-state homeowner rules for all 50 states — license exemptions, permit requirements, and the states that say no.
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Related Calculators
Breaker Size Calculator
What size breaker do you need? Free NEC calculator from amps or watts — 125% continuous rule, standard 240.6(A) ratings, and the wire it must pair with.
Voltage Drop Calculator
How much voltage do you lose over a wire run? Free NEC K-factor calculator for volts and % drop by wire size, length, and load — with the 3% check.
EV Charger Circuit Calculator
What wire and breaker does your EV charger need? Free NEC Article 625 calculator by EVSE amps — 125% rule, hardwire vs plug, GFCI, and run length.
How to Use This Calculator
- Pick your appliance or project from the preset list — it fills the typical circuit amps and voltage, and flags the gotchas (GFCI, bonding, nameplate rules) for that circuit. Or choose Custom and enter your own load.
- Mark the load continuous if it runs 3+ hours at a time (EV charging is the big one) — the calculator applies the 125% rule.
- Choose the wiring method — NM-B (Romex) for typical interior circuits, or THHN/THWN-2 in conduit — and the conductor material.
- Set the ambient temperature band (hot attics derate wire) and how many current-carrying conductors are bundled together.
- Optionally enter the one-way run length to add a voltage-drop check — long runs usually govern the size.
- Click Calculate for the minimum AWG, the ampacity math behind it, the size for both wiring methods, and the typical breaker.
Why the Same Load Needs Different Wire in Different Places
Wire sizing is not one lookup — it is a chain of rules, and each link can change the answer. The same 30 A load needs 10 AWG copper NM-B in a basement, but run that cable through a 110°F attic bundled with eight other conductors and the corrections (0.87 ambient × 0.70 bundling) cut its usable capacity below the load, pushing you to 8 AWG. NM-B itself is a special case: its conductors carry 90°C insulation, but NEC 334.80 caps its usable ampacity at the 60°C column — which is why 12 AWG Romex is a 20 A wire even though the same conductor in conduit reads 25 A in the 75°C column. And 240.4(D) adds the final trap: small conductors are breaker-limited (14 AWG to 15 A, 12 to 20 A, 10 to 30 A) no matter what the table says. This calculator runs the whole chain in order, shows which rule governed your result, and tells you when a rule it does not apply — motor, HVAC, or welder provisions — means the nameplate and an electrician have the final say.
Frequently Asked Questions
What size wire do I need for 50 amps?
For a typical 50 A / 240 V circuit: 6 AWG copper NM-B cable (55 A at the 60°C column per NEC 334.80), or 8 AWG copper THHN in conduit with 75°C terminations (50 A at the 75°C column) — assuming normal ambient temperature and no bundling. Long runs change the answer: past roughly 75–100 feet, voltage drop usually pushes the wire a size larger. This calculator runs the full NEC chain — Table 310.16, derating, termination rules, and an optional voltage-drop check — for your exact conditions.
Why is 12 AWG Romex only good for 20 amps when the chart says 25?
Because two different rules cap it. NEC 334.80 requires NM-B cable ampacity to be taken from the 60°C column — 20 A for 12 AWG — even though its conductors carry 90°C insulation. And even for the same conductor in conduit, NEC 240.4(D) limits 12 AWG copper to a 20 A overcurrent device regardless of the 25 A shown in the 75°C column. The 25 A value is real, but it's only usable as headroom for derating — not for a bigger breaker. This is the single most common wire-sizing misunderstanding, and the calculator applies both rules automatically.
What's the difference between wire size and ampacity?
They're inverse questions. Ampacity asks: how many amps can THIS wire carry safely (gauge in, amps out)? Wire size asks: what gauge do I need for THIS load (amps in, gauge out)? This calculator answers the second question. It works by computing the ampacity of each size in order — Table 310.16 value, corrected for ambient temperature and bundling, capped by the termination rules — and returning the smallest wire whose final ampacity covers your required load.
When does a load count as continuous (the 125% rule)?
A continuous load is one expected to run at maximum current for 3 hours or more — and the circuit must be sized at 125% of it (NEC 210.19; 625.41 for EV charging). EV chargers are the classic residential case: a 48 A EVSE needs 48 × 1.25 = 60 A of circuit capacity, which is why it takes a 60 A breaker with 6 AWG copper in conduit (or 4 AWG NM-B). Most ordinary appliances — dryers, ranges, water heaters — are sized as non-continuous at their circuit rating.
Why does my hot attic change the wire size?
Table 310.16 ampacities assume a 30°C (86°F) ambient. NEC Table 310.15(B)(1) corrects for hotter locations: a 105–113°F attic multiplies the 90°C-column value by 0.87, and bundling more than three current-carrying conductors cuts it again (×0.80 for 4–6, ×0.70 for 7–9 per Table 310.15(C)(1)). Stack both on a marginal circuit and the wire that worked in the basement fails in the attic — the calculator's ambient and bundling inputs exist precisely for this.
Why won't this calculator size my mini-split or welder circuit?
Because on those circuits, the nameplate legally outranks the generic table. HVAC equipment is sized from its nameplate MCA (Minimum Circuit Ampacity) with a breaker up to the nameplate MOP — and under NEC Article 440 that breaker may legitimately exceed the wire's ampacity to handle compressor starting current. Welders get duty-cycle allowances under Article 630, and motors get starting allowances under Article 430. The presets for these loads require your nameplate value, and the result is still the conductor floor — verify the full circuit with a licensed electrician.