NEC calculators

Wire size calculator (NEC)

The conductor, breaker and equipment ground for a load, with the 125 % rule, derating, terminal temperatures and voltage drop worked out step by step for each NEC edition.

Calculated with electrician.wiki · https://electrician.wiki/calculators/wire-size/

Code / standard

Circuit

Continuous load

Runs 3 hours or more: EV chargers, heaters, lighting.

Noncontinuous load

Conductor

Wire type

Terminal rating

The lowest-rated termination at either end governs (110.14(C)).

Ambient temperature

°F

Unit

Current-carrying conductors

In the raceway or cable, spares included.

Sets in parallel

1/0 AWG and larger only.

Rooftop, in direct sunlightCheck voltage drop and upsize

System

Voltage

Line to line for three-phase.

One-way length

Length unit

Voltage-drop target

The NEC notes suggest 3 % for a branch circuit or feeder, 5 % in total.

Method

Result · NEC 2023

6 AWG Cu · 60 A · 10 AWG EGC

For 48 A continuous on a branch circuit under NEC 2023, use 6 AWG copper on a 60 A breaker or fuse with a 10 AWG copper equipment grounding conductor; it drops 1.97 % over 100 ft.

Conductor: 6 AWG Cu
Breaker or fuse: 60 A
Equipment grounding conductor: 10 AWG Cu
Required at the terminals (125 %): 60 A
Ampacity of the conductor as installed: 65 A
Voltage drop (target 3 %): 1.97 % · 4.72 V

This assumes 75 °C terminations. On a circuit rated 100 A or less, equipment not marked for 75 °C must be treated as 60 °C (110.14(C)(1)(a)); then the minimum would be 4 AWG Cu.

Show the work

1. Minimum ampacity at the terminals: 125 % of the continuous load plus the noncontinuous load (210.19(A)(1))
$I_{\min} = 1.25 \cdot I_{cont} = 1.25 \cdot 48 A = 60 A$
2. Overcurrent device: the next standard rating at or above 60 A (210.20(A), Table 240.6(A))
$I_{OCPD} = 60 A$
3. Terminal temperature (110.14(C)(1))
75 °C terminations
As entered for the equipment’s listing and marking.
4. Correction and adjustment: 86 °F, 3 current-carrying conductors (Table 310.15(B)(1)(1), Table 310.15(C)(1))
$k = k_{amb} \cdot k_{adj} = 1.00 \cdot 1.00 = 1.00$
Factors at the conductor’s 90 °C rating; ambient band 78–86 °F.
5. Smallest conductor that passes: 6 AWG Cu
75 °C column 65 A ≥ 60 A ✓ · 90 °C: 75 A, held to 65 A by the 75 °C terminals ≥ 48 A ✓ · protects up to 70 A (next size up) ≥ 60 A ✓
8 AWG fails: 75 °C column 50 A < 60 A ✗ · 90 °C: 55 A, held to 50 A by the 75 °C terminals ≥ 48 A ✓ · protects up to 50 A < 60 A ✗
6. Voltage drop with 6 AWG at 48 A over 100 ft (K-factor)
$V_{D} = \frac{2 \cdot K \cdot I \cdot L}{CM} = \frac{2 \cdot 12.9 \cdot 48 A \cdot 100 ft}{26,240 cmil} = 4.72 V$
7. Percent of the supply voltage
$%VD = \frac{4.72 V}{240 V} = 1.97 %$
Within the 3 % limit.
8. Equipment grounding conductor for a 60 A device (Table 250.122)
$A_{EGC} = 10 AWG Cu$

References: NEC 210.19(A)(1) (2023): conductor size · NEC 210.20(A) (2023): overcurrent device · NEC Table 310.16 (2023) · NEC 110.14(C)(1) (2023): terminal temperature · NEC Table 250.122 (2023): EGC · NEC 210.19 Informational Note (2023): voltage-drop recommendation

90 °C (THHN, THWN-2) conductors in one raceway or cable; the 75 °C rating applies at both ends.
Continuous loads are those expected to run 3 hours or more. The overcurrent device is a standard one, not listed for 100 % continuous operation, and the circuit is not a multioutlet receptacle circuit (240.4(B) next size up is used).
Motor, air-conditioning, welder and other circuits with their own article (430, 440, 630) are sized by those rules instead.

New to this? AWG wire size and ampacity chart

Report a problem with this result

How to use the wire size calculator

Choose the NEC edition your inspector enforces, then the kind of circuit: a branch circuit, a feeder, or a dwelling service or main feeder sized by the 83 % rule. Enter the load in amperes, splitting it into the continuous part (anything expected to run for three hours or more, such as an EV charger, water heater, heat or store lighting) and the noncontinuous part. Pick copper or aluminum and the wire type printed on the jacket, which fixes its temperature rating, and the temperature rating of the terminals at both ends.

Then describe where the wire runs: the hottest ambient it will see, how many current-carrying conductors share the raceway or cable, whether it crosses a sunny roof, and whether you need parallel sets. Leave the voltage-drop check on to have the calculator upsize the conductors when the run is long. The answer is the conductor, the breaker or fuse, and the equipment grounding conductor, with every check written out underneath.

The sizing sequence

Every result follows the same order, and the steps are numbered the same way in the worked solution.

Minimum ampacity at the terminals. 125 % of the continuous load plus 100 % of the noncontinuous load.
Overcurrent device. The next standard rating at or above that figure, from the edition’s list of standard ratings. 10 A joined the list in 2023.
Terminal temperature. 60 °C or 75 °C, from the equipment listing or from the default rule of 110.14(C)(1).
Conditions of use. The ambient correction factor and the adjustment for more than three current-carrying conductors, applied at the wire’s own temperature rating.
Smallest conductor that passes three checks: enough ampacity at the terminal column for step 1, enough corrected ampacity for the actual load, and protection by the breaker under 240.4.
Voltage drop. Upsize until the drop meets your target, 3 % by default.
Equipment grounding conductor. From Table 250.122 for the breaker, increased in proportion when step 6 made the phase conductors larger.

Continuous loads and the two ampacity checks

The 125 % rule is about heat at the terminations. A breaker and its lugs are tested at 80 % of their rating for continuous duty, so the conductor attached to them is sized for 125 % of a continuous load:

I_{\min} = 1.25 \cdot I_{cont} + I_{noncont}

That check uses the ampacity in the terminal’s temperature column, before any correction. A separate check applies the correction and adjustment factors to the conductor’s own rating and compares the result with the actual load, without the 125 %. Doing both at once, multiplying the load by 1.25 and then dividing by the derating factors, double-counts and makes the wire bigger than the Code requires. For the default 48 A charger the first check needs 60 A at 75 °C and the second 48 A after correction; 6 AWG copper passes both with 65 A.

The sizing rules this calculator applies, by edition
Rule	NEC 2017	NEC 2020	NEC 2023	NEC 2026
Branch circuit: 125 % of continuous load	210.19(A)(1)	210.19(A)(1)	210.19(A)(1)	210.19(A)(1)
Feeder: 125 % of continuous load	215.2(A)(1)	215.2(A)(1)	215.2(A)(1)	215.4(A)(1)
Feeder overcurrent device	215.3	215.3	215.3	215.5
Ampacity table	Table 310.15(B)(16)	Table 310.16	Table 310.16	Table 310.16
Ambient correction	Table 310.15(B)(2)(a)	Table 310.15(B)(1)	Table 310.15(B)(1)(1)	Table 310.15(B)(1)(1)
More than three conductors	Table 310.15(B)(3)(a)	Table 310.15(C)(1)	Table 310.15(C)(1)	Table 310.15(C)(1)
Dwelling services (83 %)	310.15(B)(7)	310.12	310.12	310.12
EGC increased with the phase conductors	250.122(B)	250.122(B)	250.122(B)	250.122(D)

Why the terminal rating changes the answer

A 90 °C conductor only runs at 90 °C if both ends can take the heat. Under 110.14(C)(1), equipment for circuits of 100 A or less, or with lugs for 14 to 1 AWG, is treated as 60 °C unless it is listed and marked for 75 °C; above 100 A the default is 75 °C. Most breakers and panelboards sold today are marked 60/75 °C, which is why the calculator starts at 75 °C, but a single 60 °C device in the circuit pulls the whole circuit down. The default charger circuit needs 6 AWG copper with 75 °C terminals and 4 AWG with 60 °C terminals.

Smallest conductor for a load equal to the breaker rating, 90 °C insulation, 86 °F, three current-carrying conductors (NEC 2023), by the terminal rating at the ends.
Breaker	Copper, 60 °C terminals	Copper, 75 °C terminals	Aluminum, 60 °C	Aluminum, 75 °C
15 A	14 AWG	14 AWG	12 AWG	12 AWG
20 A	12 AWG	12 AWG	10 AWG	10 AWG
25 A	10 AWG	10 AWG	10 AWG	10 AWG
30 A	10 AWG	10 AWG	8 AWG	8 AWG
40 A	8 AWG	8 AWG	6 AWG	8 AWG
50 A	6 AWG	8 AWG	4 AWG	6 AWG
60 A	4 AWG	6 AWG	3 AWG	4 AWG
70 A	4 AWG	4 AWG	2 AWG	3 AWG
80 A	3 AWG	4 AWG	1 AWG	2 AWG
90 A	2 AWG	3 AWG	1/0 AWG	2 AWG
100 A	1 AWG	3 AWG	1/0 AWG	1 AWG

The 90 °C rating still earns its keep: it is the starting point for correction and adjustment. A hot attic or a full conduit derates a 90 °C conductor from a higher number, and the result often stays above the 75 °C limit.

Hot locations and full raceways

Ambient correction and the adjustment for bundled conductors both multiply the table ampacity, and they compound. Nine current-carrying 90 °C conductors at 104 °F keep 0.91 × 0.70 = 0.637 of their rating: a 40 A noncontinuous load then needs 6 AWG copper (47.78 A), where 8 AWG would manage only 35.04 A. The table shows one load across a range of conditions:

Copper THHN for 32 A continuous (40 A breaker), 75 °C terminals, NEC 2023: the minimum size by ambient temperature and number of current-carrying conductors.
Ambient	3 conductors	6 conductors	9 conductors	12 conductors
86 °F	8 AWG (50 A)	8 AWG (44 A)	8 AWG (38.5 A)	6 AWG (37.5 A)
104 °F	8 AWG (50 A)	8 AWG (40.04 A)	8 AWG (35.04 A)	4 AWG (43.23 A)
122 °F	8 AWG (45.1 A)	8 AWG (36.08 A)	6 AWG (43.05 A)	4 AWG (38.95 A)

On a roof, raceways and cables in direct sunlight within 0.75 in. of the surface (7/8 in. before 2023) add 33 °C (60 °F) to the outdoor temperature before you look up the factor (310.15(B)(2)); XHHW-2 is exempt. Raising the run on supports is usually cheaper than the larger conductor.

Upsizing for voltage drop, and the grounding conductor that follows

The NEC recommends keeping a branch circuit or feeder within 3 % voltage drop, and 5 % for both together. When the run is long, the calculator walks up the sizes until the drop fits. Making the phase conductors bigger has a consequence people miss: the wire-type equipment grounding conductor must grow in proportion to the increase in circular mils (250.122(B); 250.122(D) in 2026). Upsizing done only for ambient correction or adjustment does not count from 2020 on.

Take the 48 A charger 175 ft from the panel. 6 AWG copper drops 3.44 %, so the calculator moves to 4 AWG at 2.16 %. The 60 A breaker calls for a 10 AWG EGC, scaled by 41,740 ÷ 26,240: 16,511 cmil. 8 AWG is 16,510 cmil, just 1.48 cmil short. The rule sets a minimum area, so the strict answer is 6 AWG, and the calculator gives it with a warning that explains the edge rather than hiding it. Some inspectors accept the smaller size for a shortfall this small; ask before you rely on that.

Worked examples

A 48 A EV charger, 100 ft from the panel

A Level 2 charger is a continuous load. Copper THHN in conduit, 75 °C terminals, 240 V.

1. Minimum ampacity at the terminals: 125 % of the continuous load plus the noncontinuous load (210.19(A)(1))
$I_{\min} = 1.25 \cdot I_{cont} = 1.25 \cdot 48 A = 60 A$
2. Overcurrent device: the next standard rating at or above 60 A (210.20(A), Table 240.6(A))
$I_{OCPD} = 60 A$
3. Terminal temperature (110.14(C)(1))
75 °C terminations
4. Correction and adjustment: 86 °F, 3 current-carrying conductors (Table 310.15(B)(1)(1), Table 310.15(C)(1))
$k = k_{amb} \cdot k_{adj} = 1.00 \cdot 1.00 = 1.00$
5. Smallest conductor that passes: 6 AWG Cu
75 °C column 65 A ≥ 60 A ✓ · 90 °C: 75 A, held to 65 A by the 75 °C terminals ≥ 48 A ✓ · protects up to 70 A (next size up) ≥ 60 A ✓
6. Voltage drop with 6 AWG at 48 A over 100 ft (K-factor)
$V_{D} = \frac{2 \cdot K \cdot I \cdot L}{CM} = \frac{2 \cdot 12.9 \cdot 48 A \cdot 100 ft}{26,240 cmil} = 4.72 V$
7. Percent of the supply voltage
$%VD = \frac{4.72 V}{240 V} = 1.97 %$
8. Equipment grounding conductor for a 60 A device (Table 250.122)
$A_{EGC} = 10 AWG Cu$

For 48 A continuous on a branch circuit under NEC 2023, use 6 AWG copper on a 60 A breaker or fuse with a 10 AWG copper equipment grounding conductor; it drops 1.97 % over 100 ft.

The same charger at 175 ft: voltage drop and the borderline EGC

The longer run pushes the phase conductors up a size, and the grounding conductor with them.

1. Minimum ampacity at the terminals: 125 % of the continuous load plus the noncontinuous load (210.19(A)(1))
$I_{\min} = 1.25 \cdot I_{cont} = 1.25 \cdot 48 A = 60 A$
2. Overcurrent device: the next standard rating at or above 60 A (210.20(A), Table 240.6(A))
$I_{OCPD} = 60 A$
3. Terminal temperature (110.14(C)(1))
75 °C terminations
4. Correction and adjustment: 86 °F, 3 current-carrying conductors (Table 310.15(B)(1)(1), Table 310.15(C)(1))
$k = k_{amb} \cdot k_{adj} = 1.00 \cdot 1.00 = 1.00$
5. Smallest conductor that passes: 6 AWG Cu
75 °C column 65 A ≥ 60 A ✓ · 90 °C: 75 A, held to 65 A by the 75 °C terminals ≥ 48 A ✓ · protects up to 70 A (next size up) ≥ 60 A ✓
6. Voltage drop with 6 AWG at 48 A over 175 ft (K-factor)
$V_{D} = \frac{2 \cdot K \cdot I \cdot L}{CM} = \frac{2 \cdot 12.9 \cdot 48 A \cdot 175 ft}{26,240 cmil} = 8.26 V$
7. Percent of the supply voltage
$%VD = \frac{8.26 V}{240 V} = 3.44 %$
8. Upsize for voltage drop: 4 AWG is the smallest size within 3 %
$%VD = \frac{5.19 V}{240 V} = 2.16 %$
9. Equipment grounding conductor for a 60 A device (Table 250.122)
$A_{EGC} = 10 AWG Cu$
10. The phase conductors went up from 6 AWG to 4 AWG, so the EGC goes up in proportion (250.122(B))
$A_{EGC} = A_{table} \cdot \frac{A_{installed}}{A_{minimum}} = \frac{10,380 cmil \cdot 41,740 cmil}{26,240 cmil} = 16,511 cmil$

For 48 A continuous on a branch circuit under NEC 2023, use 4 AWG copper on a 60 A breaker or fuse with a 6 AWG copper equipment grounding conductor; it drops 2.16 % over 175 ft, up from 6 AWG (3.44 %).

A 480 V feeder in two parallel sets

400 A continuous plus 100 A noncontinuous, three-phase, steel conduit, voltage drop from Chapter 9 Table 9 at power factor 0.85.

1. Minimum ampacity at the terminals: 125 % of the continuous load plus the noncontinuous load (215.2(A)(1))
$I_{\min} = 1.25 \cdot I_{cont} + I_{noncont} = 1.25 \cdot 400 A + 100 A = 600 A$
2. Overcurrent device: the next standard rating at or above 600 A (215.3, Table 240.6(A))
$I_{OCPD} = 600 A$
3. Terminal temperature (110.14(C)(1))
75 °C terminations
4. Correction and adjustment: 86 °F, 3 current-carrying conductors (Table 310.15(B)(1)(1), Table 310.15(C)(1))
$k = k_{amb} \cdot k_{adj} = 1.00 \cdot 1.00 = 1.00$
5. Smallest conductor that passes: 2 × 350 kcmil Cu
75 °C column 310 A × 2 sets ≥ 600 A ✓ · 90 °C: 350 A, held to 310 A by the 75 °C terminals × 2 sets ≥ 500 A ✓ · protects up to 700 A (next size up) ≥ 600 A ✓
6. Effective impedance at PF 0.85 (Chapter 9, Table 9, steel conduit, copper)
$Z_{e} = R \cdot PF + X \cdot \sin \cdot (arccos PF) = 0.039 Ω/kft \cdot 0.85 + 0.05 Ω/kft \cdot 0.5268 = 0.05949 Ω/kft$
7. Voltage drop with 350 kcmil at 500 A over 250 ft (Chapter 9 Table 9)
$V_{D} = \frac{\sqrt{3} \cdot I \cdot Z_{e} \cdot L}{1,000 \cdot n} = \frac{\sqrt{3} \cdot 500 A \cdot 0.05949 Ω/kft \cdot 250 ft}{1,000 \cdot 2} = 6.44 V$
8. Percent of the supply voltage
$%VD = \frac{6.44 V}{480 V} = 1.34 %$
9. Equipment grounding conductor for a 600 A device (Table 250.122)
$A_{EGC} = 1 AWG Cu$

For 400 A continuous plus 100 A noncontinuous on a feeder under NEC 2023, use 2 sets of 350 kcmil copper per phase on a 600 A breaker or fuse with a 1 AWG copper equipment grounding conductor in each raceway; it drops 1.34 % over 250 ft.

NEC Annex D, Example D7 (2026): a 175 A dwelling service at 49 °C

XHHW-2 service conductors in a 120 °F (49 °C) location, 75 °C terminals.

1. Required ampacity: 83 % of the service or feeder rating (310.12)
$I_{\min} = 0.83 \cdot 175 A = 145.25 A$
2. Terminal temperature (110.14(C)(1))
75 °C terminations
3. Correction and adjustment: 49 °C, 3 current-carrying conductors (Table 310.15(B)(1)(1), Table 310.15(C)(1))
$k = k_{amb} \cdot k_{adj} = 0.82 \cdot 1.00 = 0.82$
4. Smallest conductor that passes: 2/0 AWG Cu
90 °C: 195 A × 0.82 × 1.00 = 159.9 A ≥ 145.25 A ✓
5. Equipment grounding conductor for a 175 A device (Table 250.122)
$A_{EGC} = 6 AWG Cu$

For a 175 A dwelling service or feeder (83 % = 145.25 A) under NEC 2026, use 2/0 AWG copper. Run as a feeder, it needs a 6 AWG copper equipment grounding conductor.

Dwelling services and the 83 % rule

A service or main feeder that carries the whole load of a one-family dwelling, or of one dwelling unit, may use conductors with an ampacity of only 83 % of its rating (310.12; 310.15(B)(7) in 2017). Household loads never all run at once, which is why the rule exists. The Code’s own worked example, Annex D Example D7, sizes a 175 A service: 83 % is 145.25 A, met by 1/0 AWG copper or 3/0 AWG aluminum at 75 °C. The 2026 edition adds a hot location, 49 °C (120 °F), where XHHW-2 keeps 0.82 of its 90 °C rating: 2/0 AWG copper (159.9 A) and 4/0 AWG aluminum (168.1 A). The calculator reproduces both.

Dwelling service or main feeder by 310.12 (83 % of the rating), XHHW-2 with 75 °C terminals, NEC 2023, at 86 °F (no correction) and in a 104 °F (40 °C) location.
Rating	83 %	Copper, 86 °F	Copper, 104 °F	Aluminum, 86 °F	Aluminum, 104 °F
100 A	83 A	4 AWG	4 AWG	2 AWG	2 AWG
150 A	124.5 A	1 AWG	1 AWG	2/0 AWG	2/0 AWG
200 A	166 A	2/0 AWG	2/0 AWG	4/0 AWG	4/0 AWG

A 104 °F location changes nothing here: XHHW-2 is derated from its 90 °C value, and the result still sits above the 75 °C limit that the terminals impose. With a 75 °C conductor type the same location would push each size up.

Before 2026 the same example used 38 °C and a factor of 0.91 and reached 2/0 copper by dividing 145.25 A by 0.91 and reading the result in the 75 °C column. Applying the factor to the XHHW-2 conductor’s own 90 °C rating and capping the result at the 75 °C value, as 110.14(C) and the 2026 example do, gives 1/0 AWG at 38 °C. The 2026 edition also limits the Table 310.12(A) shortcut to conductors rated 75 °C or higher; the calculation itself is unchanged.

Common mistakes

Reading the 90 °C column for a breaker connection. 10 AWG THHN is 40 A in the table, but 35 A on 75 °C terminals and 30 A at most by 240.4(D).
Applying 125 % and the derating together. They are separate checks against different ampacities.
Counting the equipment ground as current-carrying. It never is. Nor is a neutral that carries only its own circuit’s unbalanced current; the neutral of a two-phase-and-neutral circuit from a wye system, or of a wye circuit feeding mostly nonlinear loads, does count.
Forgetting the EGC after upsizing. A bigger phase conductor for voltage drop means a bigger EGC under 250.122(B).
Using the 83 % rule on a subpanel. It applies only to the service or a feeder that carries the entire dwelling load.
Sizing NM cable from the 75 °C column. NM cable stops at its 60 °C ampacity (334.80), whatever the terminals.

Method and verification

The calculator tries every size from the edition’s minimum (14 AWG copper and 12 AWG aluminum; 16 AWG copper on 10 A circuits in 2026) up to 2000 kcmil and keeps the first that passes every check. Ampacities, correction and adjustment factors, standard ratings and Table 250.122 come from the research data behind this site, checked value by value against at least two independent copies of each table. The 2026 additions for 16 AWG copper and 14 AWG copper-clad aluminum rest on a single NFPA draft, so any result that uses them carries a warning.

The tests reproduce Annex D Example D7 in both editions, hand-worked branch-circuit and feeder cases, the borderline EGC above, and the properties every answer must keep: a larger load, a hotter location, more conductors or a longer run never gives a smaller wire. Check the result against the edition your jurisdiction enforces; the NEC adoption map lists each state’s edition. For the drop alone, use the voltage drop calculator; for a single conductor’s ampacity, the ampacity calculator.

Updated 2026-10-14. Found a problem? Report it; every fix is logged publicly.

Questions people ask

Is this an NEC Article 310 wire size calculator?

Yes. Ampacity comes from NEC Article 310 (Table 310.16 for up to three current-carrying conductors in raceway, cable or earth), then the result is checked against the terminal temperature rating in 110.14(C) and the overcurrent limits in 240.4.

What size wire do I need for a 60 amp breaker?

With 75 °C terminals, 6 AWG copper or 4 AWG aluminum, before any derating or voltage drop. If the equipment is rated only 60 °C, copper goes up to 4 AWG.

What size wire for a 48 amp EV charger?

A 48 A charger is continuous, so the circuit needs 125 %: 60 A and a 60 A breaker. With 75 °C terminals that is 6 AWG copper with a 10 AWG copper EGC; long runs may need a size up for voltage drop.

Do I size wire at 60 °C or 75 °C?

At the lowest temperature rating of any termination on the circuit. Equipment for 100 A or less counts as 60 °C unless it is listed and marked for 75 °C; most breakers are marked 60/75 °C. Above 100 A the default is 75 °C.

Does voltage drop change the ground wire size?

Yes. When you upsize the phase conductors for voltage drop, the wire-type equipment grounding conductor must increase in proportion to the circular-mil increase (250.122(B), or 250.122(D) in the 2026 NEC).

What is the 83 % rule for dwelling services?

For a service or main feeder carrying the whole load of a one-family dwelling or dwelling unit, rated 100 to 400 A, the conductors need an ampacity of only 83 % of the rating (310.12). A 200 A service needs 166 A: 2/0 AWG copper or 4/0 AWG aluminum with no correction.

Did the 2026 NEC change wire sizes?

Not from 14 AWG up: Table 310.16 values are the same as 2023. The 2026 edition adds 16 AWG copper at 10 A for 10 A lighting circuits, renumbers the feeder rules to 215.4 and 215.5, and re-letters the EGC rules in 250.122.