For AC building wiring, a 20 A non-continuous load on copper with 75 °C insulation and 60 °C terminals (assumed per NEC 110.14(C)(1) for circuits up to 100 A) needs 12 AWG — 20 A in the 60 °C column.
Calculator
Recommended Wire Size
Size found
#12 AWGCU
Ampacity at Size
20A at 60°C
Terminal Rating
60°C (110.14(C)(1))
Per NEC 2023 Table 310.16, NEC 2023 110.14(C)(1), NEC 2023 240.4(D)
How it’s calculated
Ireq = Iload × 1.25 for a continuous load, otherwise Iload
Tcol = the lower of Tins and Tterm
Ader = Ains × Ft × Fn
Ause = min(Ader, Acol)
Size = the first size from 14 AWG up with Ause ≥ Ireq and a 240.4(D) limit, if it has one, of at least Ireq
Iload
load current, amperes
Ireq
required ampacity; a continuous load (one expected to run 3 hours or more) counts at 125 %
Tins
insulation temperature rating, °C
Tterm
terminal temperature rating; Auto takes 60 °C when the required ampacity is 100 A or less and 75 °C above (110.14(C)(1))
Tcol
the governing column: a conductor counts on no more than its lowest-rated termination allows (110.14(C))
Ains
Table 310.16 ampacity in the insulation’s column
Acol
Table 310.16 ampacity in the governing column
Ft
ambient correction, Table 310.15(B)(1)(1), on a 30 °C basis (for example 0.91 at 40 °C for 90 °C insulation)
Fn
the factor for the number of current-carrying conductors in the raceway or cable, Table 310.15(C)(1): 100 % up to 3, 80 % for 4–6, 70 % for 7–9, 50 % for 10–20, 45 % for 21–30, 40 % for 31–40, 35 % for 41 or more
Sizes are tried from 14 AWG copper and 12 AWG aluminum up, the smallest sizes 310.3(A) allows. 240.4(D) limits the device ahead of small conductors (copper 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A; aluminum 12 AWG 15 A, 10 AWG 25 A); a size whose limit is below the required ampacity is skipped even when its ampacity would do.
A conductor that may not be used at the ambient gets no size: the correction table ends at 55 °C for 60 °C, 70 °C for 75 °C and 85 °C for 90 °C insulation.
The voltage-drop step (web only) starts at the ampacity size and tries each larger size until the drop, computed at the load current without the 1.25 factor, is within the limit. It computes the drop the way the voltage drop calculator does: Chapter 9 Table 9 wherever it applies, otherwise the K method.
Assumptions
Ambient temperatures are on the table’s 30 °C basis and read in whole degrees: a fraction is dropped, entries are kept within −100 to 200 °C, and an unreadable entry counts as 30 °C. A Fahrenheit reading converts to a fraction, so round it up before you enter it: the colder band gives the larger factor and can hide a size.
All conductors are the same metal and size, in one raceway or cable; you decide which conductors count as current-carrying.
The required ampacity is also the smallest overcurrent device rating the circuit can use; choosing the breaker or fuse is up to you.
The next-size-up allowance of 240.4(B) is not applied.
The dwelling service and feeder sizing of 310.12 is not applied.
The rooftop temperature adder of 310.15(B)(2) is not applied; where it applies, add it to the ambient yourself.
Equipment grounding and neutral conductors are not sized here.
Motor, air-conditioning and EV charger circuits have their own sizing rules, which this calculator does not apply.
Auto decides from the required ampacity alone. Terminations marked for 14 AWG to 1 AWG conductors take the 60 °C column at any rating (110.14(C)(1)); for those, pick 60 °C yourself.
For NM cable, 334.80 holds the ampacity to the 60 °C value and lets derating start from the 90 °C value: pick 90 °C insulation and 60 °C terminals. Its own bundling rules are not applied.
The voltage-drop step uses the load current as entered (not × 1.25), uses Table 9 wherever it applies (the voltage drop calculator’s Table 9 switch is always on here), and takes a limit of 0.5 to 10 %.
Adjusted ampacities are shown to 0.1 A; the required-ampacity row drops the fraction (62.5 A shows as “62A”).
Worked example
Three 240 V circuits in one conduit, 40 A continuous each
Each circuit feeds a 40 A continuous load at 240 V single-phase, 200 ft away, on two current-carrying conductors, so the conduit holds 6. Copper, 90 °C insulation, terminals on Auto, 40 °C ambient.
The load is continuous, so the conductors are sized for 125 % of it:Ireq = 40 × 1.25= 50 A
Auto: 50 A is 100 A or less, so the terminals count as 60 °C (110.14(C)(1)). The 90 °C insulation cannot lift that, so the 60 °C column governs.
Factors for 90 °C conductors: 40 °C ambient (Table 310.15(B)(1)(1)) and 6 current-carrying conductors (Table 310.15(C)(1)).Ft = 0.91; Fn = 0.80
8 AWG: the 90 °C value, derated, then held to the 60 °C column (40 A); 240.4(D) sets no limit for it.Ause = min(55 × 0.91 × 0.80, 40) = min(40.0, 40)= 40.0 A, less than 50 A: too small
6 AWG: the same steps; 240.4(D) sets no limit for it.Ause = min(75 × 0.91 × 0.80, 55) = min(54.6, 55)= 54.6 A, at least 50 A: 6 AWG carries the load
The voltage-drop step works with the load current, 40 A, not the 50 A sizing figure. At a power factor of 1 and for a size below 1/0 AWG, the K method applies (single-phase, 200 ft one way):VD = 2 × 12.9 × 40 × 200 ÷ 26,240= 7.9 V, 3.3 % of 240 V: over the 3 % limit
4 AWG, the first larger size within 3 %, computed the same way:VD = 2 × 12.9 × 40 × 200 ÷ 41,740= 4.9 V, 2.1 %
Answer: 6 AWG copper by ampacity (54.6 A usable for 50 A required); with the 3 % voltage-drop limit, 4 AWG copper (2.1 % instead of 3.3 %).
Section numbers are from NEC 2023 (NFPA 70). This page explains them in its own words and quotes no code text.
NEC 2023 Table 310.16: ampacities for up to three current-carrying conductors at 30 °C ambient, in the 60, 75 and 90 °C columns; aluminum from 12 AWG
NEC 2023 110.14(C)(1): the terminal column when the equipment is not listed and marked otherwise: 60 °C for circuits of 100 A or less and for terminations marked for 14 AWG to 1 AWG, 75 °C for circuits over 100 A and terminations for conductors larger than 1 AWG (what Auto applies, by the circuit amperes)
NEC 2023 110.14(C): a conductor counts on no more than its lowest-rated termination allows; a higher-rated conductor may start correction and adjustment from its own column
NEC 2023 210.19(A)(1): branch-circuit conductors: at least the noncontinuous load plus 125 % of the continuous load (the conductor side of the ×1.25 row)
NEC 2023 215.2(A)(1): the same 125 % rule for feeder conductors
NEC 2023 210.20(A): the branch-circuit overcurrent device follows the same 125 % rule (the device side; the result card cites this section for the ×1.25 row)
NEC 2023 Article 100: a continuous load is one whose maximum current is expected to last 3 hours or more
NEC 2023 Table 310.15(B)(1)(1): ambient correction factors on a 30 °C basis
NEC 2023 Table 310.15(C)(1): how much the ampacity drops as a raceway or cable carries more conductors (100 % up to 3, 80 % for 4–6, 70 % for 7–9, 50 % for 10–20, 45 % for 21–30, 40 % for 31–40, 35 % for 41 or more)
NEC 2023 240.4(D): the largest device ahead of small conductors: copper 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A; aluminum 12 AWG 15 A, 10 AWG 25 A. 240.4(E) and 240.4(G) send some circuits elsewhere, motor and air-conditioning circuits to Articles 430 and 440
NEC 2023 310.3(A): the smallest conductors, 14 AWG copper and 12 AWG aluminum, where the search starts
NEC 2023 240.4(B): the next higher standard device rating, up to 800 A and under conditions; not applied here
NEC 2023 310.12: 83 % sizing for the services and main feeders of one-family dwellings and individual dwelling units (120/240 V single-phase, 100 to 400 A); not applied here
NEC 2023 334.80: NM cable: the 60 °C ampacity, with the 90 °C value as the start for derating, and its own bundling rules; not applied here
NEC 2023 310.15(B)(2): a 33 °C adder to the ambient for raceways and cables in sunlight on or above a roof and less than 3/4 in. from it (XHHW-2 is exempt); not applied here
NEC 2026
Where the 2026 edition differs for this calculator:
310.5(A), Table 310.16: The minimum branch-circuit conductor moves to 310.5(A) and becomes 16 AWG copper for the 10 A branch circuits of 210.23(A) (lighting and exhaust-fan outlets, a NEC 2023 rule), and Table 310.16 rates 16 AWG copper at 10 A in the 60 °C column. This page follows NEC 2023.
Many charts and calculators read the 75 °C column. For circuits of 100 A or less, 110.14(C)(1) sends you to the 60 °C column unless every termination is listed and marked for 75 °C, and Auto follows that: a 50 A load gets 6 AWG copper on Auto against 8 AWG at 75 °C.
Check: which column the other tool reads, and what your breaker, lugs and devices are marked for.
The 125 % continuous-load factor
A load expected to run 3 hours or more needs conductors for 125 % of it (210.19(A)(1) for branch circuits, 215.2(A)(1) for feeders). A tool that skips it sizes the worked example’s 40 A load for 40 A instead of 50 A.
Check: whether the other tool asks if the load is continuous.
Derating from the wrong column
Correction and adjustment may start from the insulation’s own column (90 °C for 90 °C wire), but the result may not exceed the terminal column (110.14(C)). Starting a 90 °C conductor from its 75 °C or 60 °C value instead can give a larger conductor than needed; keeping a derated 90 °C value that is above the terminal column gives one that is too small.
Check: which column the derating starts from, and whether the result is held to the terminal column.
The small-conductor limits of 240.4(D)
14 AWG copper is listed at 20 A in the 75 °C column, but the device ahead of it is limited to 15 A (12 AWG: 20 A, 10 AWG: 30 A). A chart without those limits can put 14 AWG on a 20 A circuit.
Check: whether the other tool applies 240.4(D), including aluminum’s own limits.
Dwelling service tables
310.12 lets the services and main feeders of one-family dwellings and individual dwelling units, 120/240 V single-phase, 100 to 400 A, be sized at 83 % of the rating. Charts built on it give smaller conductors than this calculator, which does not apply 310.12.
Check: whether the other chart is for dwelling services and main feeders only.
Distance ignored
Ampacity says nothing about the length of the run. In the worked example, 6 AWG carries the load but drops 3.3 % over 200 ft; the voltage-drop step moves it to 4 AWG.
Check: whether the other tool checks the voltage drop, and at which current.
DC and automotive gauge charts
Charts for vehicles, boats, low-voltage DC or chassis wiring list amperes for other conditions and limits. They do not follow the NEC ampacity tables and do not fit AC building wiring.
Check: that the chart is meant for AC building wiring under the NEC.
The next size up (240.4(B))
Where a conductor’s ampacity falls between standard device ratings, 240.4(B) allows the next higher standard rating, up to 800 A and under conditions. A tool that takes the breaker rating as its input and applies it can answer 2 AWG copper (115 A at 75 °C) for a 125 A breaker; for a full 125 A load, this calculator answers 1 AWG.
Check: whether the other tool’s input is the load or the breaker, and whether it applies 240.4(B).
Questions
What size wire does a 50 A load need?
With 75 °C insulation, 30 °C ambient and up to 3 current-carrying conductors: 6 AWG copper (55 A in the 60 °C column) with the terminals on Auto, which takes 60 °C for a circuit of 100 A or less, and 8 AWG copper (50 A) when every termination is rated 75 °C. A continuous 50 A load needs 62.5 A: 4 AWG copper on Auto, 6 AWG at 75 °C. Aluminum needs 4 AWG on Auto and 6 AWG at 75 °C.
Why does the calculator use the 60 °C column when the wire is rated 90 °C?
The lowest-rated part of the circuit sets the column. For a circuit of 100 A or less, 110.14(C)(1) takes the 60 °C column unless every termination is listed and marked for 75 °C, whatever the insulation: 12 AWG copper is 30 A in the 90 °C column but 20 A in the 60 °C column. The 90 °C rating still helps, because correction and adjustment may start from the 90 °C value as long as the result stays within the terminal column (110.14(C)).
Why is 14 AWG not enough for a 20 A circuit?
14 AWG copper is listed at 20 A in the 75 °C column, but 240.4(D) limits the overcurrent device ahead of it to 15 A. Even with 75 °C terminals, a 20 A load therefore needs 12 AWG copper, whose limit is 20 A. Aluminum has its own limits (12 AWG: 15 A, 10 AWG: 25 A), so a 20 A load on aluminum needs 10 AWG.
Does this calculator include voltage drop?
Yes, as an optional web-only step: switch on “Also check voltage drop (web only)”, and the size grows until the drop at the load current is within your limit (3 % unless you change it). With the step on, the default 20 A circuit at 120 V over 100 ft moves from 12 AWG (6.6 %) to 8 AWG (2.6 %). Wire Sizing in the Electric Master app sizes by ampacity only; its Voltage Drop calculator covers the run.
The worked example’s 6 AWG at 40 °C with 6 conductors, factor by factor.
About this calculator
Written and maintained by Yusuf Can Samiloglu: MEP/HVAC design background, mechanical-engineering degree, not a licensed electrician.
How it’s checked: the calculation is the method of the Electric Master app, ported to the web and tested case by case against the app’s source code. Every table value was checked against the NEC 2023 values in at least two independent published sources.
The App Store version of Electric Master (2.2.0) predates the NEC corrections this page follows; until its next update, its result can differ from this page for some inputs. Using version 2.2.0? For circuits up to 100 A, set Terminal Temp Rating to 60 °C unless every termination is marked 75 °C.
: First version, NEC 2023. Terminal Temp Rating “Auto” (60 °C up to 100 A, 75 °C above, 110.14(C)(1)); the 240.4(D) limits are checked against the required ampacity, with aluminum’s own limits; sizes start at the 310.3(A) minimum; an optional voltage-drop step (web only).