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Wire Size Calculator

IRC Chapter 37 ampacity tables, the NEC's Table 310.16 reproduced for dwellings

The NEC's ampacity table is behind a login, but the residential code reprints it as Table E3705.1, and its widest column is the one you cannot size a breaker from. 12 AWG copper is listed at 30 amps at 90°C and protected at 20. The same 2/0 copper that serves a 200 amp house legally is one size too small for a 200 amp subpanel feeder. This calculator applies every limit the code stacks on top of the table.

Conductor size for a branch circuit, feeder or service

A
A
°F
no.

Continuous means a load expected to run three hours or more. Count current-carrying conductors in the raceway or bundle; a neutral carrying only unbalanced current and equipment grounds do not count. Ambient is the hottest air the run passes through.

Specification summary

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Entered

Result

These bars are drawn from the circuit you describe above.
What limits the conductor

Worked example

A 16 amp kitchen load on a 20 amp circuit, then a 24 amp continuous load, the same wire bundled in a hot attic, and a 200 amp service against a 200 amp subpanel.

  1. 16 A, noncontinuous. Breaker 20 A. Terminations 60°C. 12 AWG copper: 30 A at 90°C, limited to 20 A at the terminals and by the small conductor rule.
  2. 24 A continuous. 125% is 30 A, so a 30 A breaker and 10 AWG.
  3. 18 A in an attic at 100°F with 8 conductors. 12 AWG derates to 30 × 0.91 × 0.70 = 19.1 A, below the 20 A breaker, so 10 AWG (25.5 A).
  4. 200 A service to the whole house. 83% is 166 A: 2/0 copper at 175 A.
  5. 200 A feeder to a subpanel. 100%: 3/0 copper at 200 A.

Four of those five answers are one size larger than the 90°C column alone would suggest, and the fifth is the house service, where the code deliberately allows less.

The formula

The conductor has to pass every one of these at once:

required = noncontinuous + 1.25 × continuous    usable = min(table at insulation rating × correction × adjustment, table at termination rating)    usable ≥ breaker ≤ small conductor cap
Table E3705.1
allowable ampacity by size, material and temperature rating
correction
Table E3705.2, for ambient air other than 86°F
adjustment
Table E3705.3, 80% for 4 to 6 current-carrying conductors down to 35% for 41 or more
termination rating
60°C for circuits of 100 A or less unless marked, 75°C above
small conductor cap
14 AWG copper 15 A, 12 AWG 20 A, 10 AWG 30 A; aluminum 12 AWG 15 A, 10 AWG 25 A, 8 AWG 30 A

Service conductors that carry the whole dwelling skip all of this and use Table E3603.1.1, sized at 83 per cent of the rating.

Why this page reads the residential code

Wire size charts print one number per gauge and rarely say which column it came from. The code prints three and stacks four rules on top of them.

The NEC's Table 310.16 is behind NFPA's login. The International Residential Code reprints it for one and two family dwellings, with the NEC section in brackets, and two states' adoptions were compared to confirm the tables are the model text.

The voltage drop calculator on this site had said ampacity could not be sourced. It now points here.

Small copper conductors: ampacity columns against the breaker limit
Copper sizeTable E3705.1 at 90°CAt 75°CAt 60°CLargest breaker, E3705.5.3What governs
14 AWG25 A20 A15 A15 A60°C terminals and the cap agree
12 AWG30 A25 A20 A20 A60°C terminals and the cap agree
10 AWG40 A35 A30 A30 A60°C terminals and the cap agree

Read 2026-09-13. IRC 2021 Tables E3705.1 and E3705.5.3 as adopted in Utah.

Download this table as CSV. Same cells as printed above, nothing added.

The ampacity table's widest column is the one you cannot size from

Table E3705.1 prints three temperature columns for copper and three for aluminum. The 90°C column has the biggest numbers, and it is the one wire charts get quoted from. The code lets it do one job:

Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

At the breaker and the device, a lower column applies. For the circuits in a house:

Except where the equipment is marked otherwise, termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following: Conductors rated 60°C (140°F); Conductors with higher temperature ratings, provided that the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used;

And for the three smallest sizes a separate cap overrides the 75°C and 90°C columns, even where the equipment is marked for 75°C.

Small copper conductors: three ampacity columns against the overcurrent limit, IRC 2021 as adopted in Utah
Copper sizeTable E3705.1 at 90°CAt 75°CAt 60°CLargest breaker, E3705.5.3What governs
14 AWG25 A20 A15 A15 A60°C terminals and the cap agree
12 AWG30 A25 A20 A20 A60°C terminals and the cap agree
10 AWG40 A35 A30 A30 A60°C terminals and the cap agree

12 AWG is listed at 30 amps and protected at 20. The 30 is real, and it is what a hot attic or a full conduit derates from: 12 AWG THHN in 7 to 9 current-carrying conductors at 70 per cent is 21 A, which still clears a 20 A breaker. Start from the 60°C column instead and the same bundle is 14 A, too little. Whichever way it is derated, the result also has to stay within the 60°C figure at the terminals, 20 A, which it does. That difference is the whole reason the 90°C column exists.

The same wire, 200 amps for the house and not for the subpanel

Table E3603.1.1 sizes the conductors for a whole-dwelling service or feeder, and its rule is explicit:

the service conductors supplying the entire load associated with a one-family dwelling ... shall have an ampacity of not less than 83 percent of the service rating.

Every row of the table is exactly that rule. For all eleven ratings from 100 to 400 amps, copper and aluminum, the conductor listed is the smallest whose 75°C ampacity reaches 83 per cent of the rating. A 200 A service needs 166 A, and 2/0 copper carries 175.

That allowance belongs to the service, not to the wire. A 200 A feeder to a subpanel serving part of the house is sized under Chapter 37 at its full rating. 2/0 copper carries 175 at 75°C, which is short of 200, so that feeder is 3/0. Pulling the same cable to a garage subpanel on the same size breaker as the main is one size too small.

The next-size-up rule in E3705.5.2 allows a breaker one standard rating above a conductor's ampacity under three conditions, and the calculator notes where it could apply without relying on it.

What the common circuits come out as

With copper, 60°C terminations, ordinary room temperature and no more than three current-carrying conductors: a 15 A circuit is 14 AWG, 20 A is 12 AWG and 30 A is 10 AWG, which are the sizes Table E3702.14 lists for general branch circuits.

Heat and bundling change that faster than people expect. Four to six conductors in one conduit take 80 per cent; an attic at 100°F takes another 9 per cent off a 90°C conductor. A 20 A circuit sharing a conduit with seven others in a hot attic needs 10 AWG.

Continuous loads count at 125 per cent. A 24 A electric vehicle charger or water heater running for hours needs a 30 A circuit and 10 AWG, not a 25 A circuit on 12.

The sources, and why the residential code

Read 2026-09-13.

Utah Residential Code 2021, Chapter 37 Branch Circuit and Feeder Requirements, International Residential Code 2021 as adopted with amendments in Utah, read through UpCodes with the jurisdiction and edition stated on the page. Sections E3701.2, E3705.1 to E3705.6 and Tables E3705.1, E3705.2, E3705.3, E3705.5.3 and E3705.6, each carrying its NEC section in brackets.

Utah Residential Code 2021, Chapter 36 Services, International Residential Code 2021 as adopted in Utah, read through UpCodes. Section E3603.1.1 and Table E3603.1.1, single-phase dwelling services and feeders at 83 per cent.

Virginia Residential Code 2021, Chapters 36 and 37, Read through UpCodes to confirm the Utah tables are the model text. Tables E3705.1, E3705.3, E3602.2, E3603.1.1 and E3704.2(1) are character for character identical to Utah's.

The NEC's own text sits behind a login, so this page reads the IRC's electrical chapters, which apply to one and two family dwellings and cite the NEC section for each rule. Two states' adoptions were compared to confirm the tables are the model code rather than a local amendment. Your jurisdiction may have adopted the NEC directly, a different edition, or amendments.

This page sizes for ampacity, which is the safety limit. On a long run the wire may need to be larger again for voltage drop, which the voltage drop calculator works out.

Frequently asked questions

What size wire for a 20 amp breaker?
12 AWG copper for an ordinary circuit. IRC Table E3705.5.3 limits 12 AWG copper to a 20 A overcurrent device and 14 AWG to 15 A. In a hot space or a bundle of more than three current-carrying conductors it may need to be 10 AWG.
What size wire for 30 amps?
10 AWG copper, which Table E3705.5.3 allows to be protected at 30 A. In aluminum, 8 AWG, which is limited to 30 A.
What size wire for a 200 amp service?
2/0 copper or 4/0 aluminum for a service or feeder carrying the entire load of a one-family dwelling, from IRC Table E3603.1.1, which sizes those conductors at 83 per cent of the rating. A 200 A feeder to a subpanel serving part of the house needs 3/0 copper or 250 kcmil aluminum.
What size wire for a 100 amp subpanel?
At 100 A, 60°C terminations apply unless the equipment is marked 75°C. Copper needs 1 AWG at 60°C (110 A), or 3 AWG where the terminations are marked 75°C (100 A). Aluminum needs 1/0 at 60°C.
Why is 12 gauge rated 30 amps in some charts?
That is the 90°C column of the ampacity table. The IRC allows it only as the starting point for temperature and bundling adjustments; the breaker is limited to 20 A by the small conductor rule and the terminations are usually rated 60°C.
How many wires can be in a conduit before derating?
Three current-carrying conductors. From 4 to 6 the ampacity is 80 per cent of the table, 7 to 9 is 70 per cent, 10 to 20 is 50 per cent, per IRC Table E3705.3.
Does wire size depend on distance?
Not for ampacity. The code's ampacity tables have no length. A long run may need a larger wire for voltage drop, which the voltage drop calculator works out.
Is this the NEC?
It is the International Residential Code's electrical chapters, which reproduce the NEC's rules for one and two family dwellings with the NEC section cited in brackets. Where your jurisdiction adopts the NEC directly, confirm against its edition.

Check these numbers yourself

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Sizes follow the 2021 International Residential Code as adopted in Utah, read on 13 September 2026, which reproduces NEC rules for one and two family dwellings. Your jurisdiction may adopt the NEC directly, a different edition or amendments, and cable types can carry their own limits that this page does not apply. Electrical work is commonly permit work that must be done by a licensed electrician where required. This page does not size for voltage drop, grounding conductors or motor circuits.