Size the conductor and circuit breaker for a load, or find a conductor's ampacity, using NEC 310.16 ampacity, termination, and derating rules.Learn more ▾Show less ▴
Choosing a conductor is more than reading an ampacity chart. The National Electrical Code ties together three limits: the overcurrent device sized for the load, the termination temperature of the equipment, and the conductor's ampacity after correction for ambient heat and bundling.
This calculator applies all three so the wire size and breaker it reports are consistent with one another — the same procedure an electrician follows by hand. Switch modes to work forward from a load, or backward from a conductor you already have.
A
°C
Recommended conductor
--
How this was determined
Advertisement
About This Tool
The Wire Size & Ampacity Calculator selects conductors and overcurrent devices using the U.S. National Electrical Code. It works in two directions: forward from a load to a recommended conductor and breaker, or backward from a conductor to its usable ampacity.
It is built for electricians, estimators, engineers, and students who need a result that respects every NEC limit at once — ampacity, termination temperature, derating, and overcurrent protection — rather than a single ampacity-chart lookup.
Choose a mode: size the wire from a load, or find a conductor's ampacity.
Enter the load (or conductor) and the installation conditions — material, insulation, terminals, ambient, and conductor count.
Read the recommended conductor and breaker, with the full ampacity and derating breakdown.
How to Use
Choose a mode: size the wire from a load, or find a conductor's ampacity.
Enter the load (or conductor) and the installation conditions — material, insulation, terminals, ambient, and conductor count.
Read the recommended conductor and breaker, with the full ampacity and derating breakdown.
Methodology
Conductor selection follows the standard NEC sequence. First, the overcurrent device is sized at 100% of non-continuous load plus 125% of continuous load and rounded up to the next standard rating (NEC 240.6(A)).
Next, the minimum conductor is chosen from the termination-temperature column — 60 °C or 75 °C — so it meets that same load (NEC 110.14(C)). The choice is then verified for the condition of use: the 90 °C ampacity is reduced by the ambient-temperature factor (Table 310.15(B)(1)) and the bundling factor (Table 310.15(C)(1)), and the result must permit the overcurrent device, allowing the next-size-up rule (240.4(B)) but never exceeding the small-conductor caps (240.4(D)).
Ampacity values come from NEC Table 310.16 for copper and aluminum.
The recommended conductor is the smallest that satisfies every limit at once; the breaker is the standard device that protects it. The "controlling factor" tells you which limit set the size — termination temperature or overcurrent protection — so you know what to change if you want a smaller wire.
The breakdown shows the insulation-column ampacity, the termination-column ampacity, the correction factors, and the final derated ampacity. If voltage drop matters on a long run, size separately for voltage drop and use the larger conductor.
Example 1 — Sizing a wire: a 50 A continuous load on copper with 75 °C terminals. The breaker is sized at 125% × 50 = 62.5 A → 70 A. The conductor must carry 62.5 A at 75 °C: 6 AWG (65 A) works, so the result is 6 AWG copper on a 70 A breaker.
Example 2 — Finding ampacity: 6 AWG copper THHN with six conductors in a 40 °C raceway. Start at the 90 °C value of 75 A, apply 0.91 (40 °C) × 0.80 (4–6 conductors) = 54.6 A, limited by the 75 °C column (65 A). The usable ampacity is about 55 A, protected by a 60 A device.
Tips for accurate wire sizing
• Check the equipment label for its termination rating. Most breakers are 75 °C, but using the 60 °C column when required keeps you compliant.
• Mark continuous loads. Lighting, EV chargers, and HVAC usually run 3+ hours and need the 125% factor.
• Count only current-carrying conductors for bundling — grounding conductors and balanced neutrals do not count.
• For long runs, also check voltage drop and use the larger conductor.
• Aluminum needs terminations and connectors listed for aluminum (AL or CU-AL).
All calculations are performed locally in your browser. No data is sent to any server.
Was this tool helpful?
Want to tell us more?
0/500
Want us to follow up?
Thanks for your feedback!
Frequently Asked Questions
How does the wire size calculator work?
In "Size the wire" mode, enter your load in amps and the installation conditions. The calculator sizes the overcurrent device (breaker or fuse) at 100% of non-continuous load plus 125% of continuous load, rounded up to the next standard size. It then picks the smallest conductor whose ampacity in the termination-temperature column meets that load, and verifies the choice against ambient and bundling derating before reporting the wire size and breaker.
In "Find ampacity" mode, enter a conductor and conditions to get its usable ampacity and the largest device that can protect it.
Why doesn't 90 °C wire give me 90 °C ampacity?
Because of the termination-temperature rule (NEC 110.14(C)). Most breakers, lugs, and devices are rated for 60 °C or 75 °C terminations, so a conductor's usable ampacity is limited to the value in that column — even if its insulation is rated 90 °C. The 90 °C rating is still valuable: it is the starting point for ambient and bundling derating. The final ampacity is the lower of the derated 90 °C value and the termination-column value.
What is a continuous load and why does it add 25%?
A continuous load is one expected to run at its maximum for three hours or more (lighting, EV charging, HVAC). The NEC requires the overcurrent device and conductor to be sized at 125% of continuous load so that breakers and terminals are not operated at their limit for long periods. Tick the "continuous load" option and the calculator applies the 125% factor automatically.
How do ambient temperature and conductor count change the result?
Heat reduces how much current a conductor can carry safely. When the ambient temperature is above 30 °C (86 °F), an ambient correction factor lowers the ampacity (NEC Table 310.15(B)(1)). When more than three current-carrying conductors share a raceway or cable, a bundling adjustment factor applies (NEC Table 310.15(C)(1)) — for example 80% for 4–6 conductors. Both factors multiply together and can require a larger conductor.
Why are 14, 12, and 10 AWG limited to 15, 20, and 30 amps?
NEC 240.4(D) sets absolute overcurrent limits for small copper conductors — 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG (15 A and 25 A for 12 and 10 AWG aluminum) — applied after any derating. These caps override the next-standard-size-up allowance, which is why a 35 A load needs 8 AWG even though 10 AWG shows 40 A in the 90 °C column.
When can the breaker be the next size up from the conductor ampacity?
NEC 240.4(B) lets you use the next standard overcurrent size when the conductor's ampacity does not match a standard rating, the device is 800 A or less, and the circuit is not a multi-outlet receptacle branch for cord-and-plug loads. For example, a conductor rated 130 A may be protected by a 150 A device. The small-conductor limits of 240.4(D) still take priority for 14, 12, and 10 AWG.
Should I use copper or aluminum?
Both are listed in NEC Table 310.16. Aluminum is lighter and less expensive but has lower ampacity, so it needs a larger size for the same load — for example a 100 A feeder is typically 3 AWG copper or 1 AWG aluminum. Aluminum requires terminations and connectors listed for aluminum (marked AL or CU-AL) and proper anti-oxidant practices. The calculator covers both; switch the conductor material to compare.
Does the wire size also depend on voltage drop and conduit fill?
Yes. This tool sizes for ampacity, termination, and overcurrent protection. On long runs, voltage drop can require a larger conductor than ampacity alone — use the Voltage Drop Calculator to check, and take the larger of the two results. The number of conductors also affects how many fit in a raceway — use the Conduit Fill Calculator for that. The final conductor must satisfy all three.
Which NEC edition and conditions does the calculator use?
It uses Table 310.16 (not more than three current-carrying conductors in a raceway, cable, or earth, based on a 30 °C / 86 °F ambient), consistent across the 2017, 2020, and 2023 editions, with correction factors from Tables 310.15(B)(1) and 310.15(C)(1). Free-air conductors (Table 310.17) and special applications are outside its scope. Always confirm against the edition adopted in your jurisdiction.
Is my data private?
Yes. Every calculation runs entirely in your browser. Nothing you enter is stored or sent to any server, so you can use the tool offline once the page has loaded.
My Favorites
Drag to reorder
No favorites yet
Tap the ☆ on any tool page to bookmark it for quick access.
