Wire Size Guide AWG Chart: NEC Ampacity & Breaker Sizing

For standard 120V/240V residential branch circuits, the baseline wire size guide AWG rule is straightforward: a 15A circuit requires 14 AWG copper, 20A requires 12 AWG, 30A requires 10 AWG, 40A requires 8 AWG, and 50A requires 6 AWG. However, ampacity is not just about the wire's insulation; it is governed by the weakest link in the circuit. Per NEC 110.14(C), you must use the 60°C ampacity column for wires 14 through 10 AWG regardless of the insulation's higher 90°C rating, unless the termination hardware is explicitly listed for 75°C. Sizing up for voltage drop or continuous loads is mandatory, not optional.

How to Read the NEC 310.16 Ampacity Table

The National Electrical Code (NEC) Table 310.16 is the master reference for conductor ampacity. To use it correctly, you must understand the temperature columns. Wire insulation like THHN is rated for 90°C, but standard residential breakers, receptacles, and switches are typically rated for 60°C or 75°C terminations. The heat generated at the termination point dictates the safe current limit, not the middle of the wire run.

Complete AWG Wire Size and Breaker Chart

The following spec-sheet-table provides the allowable ampacities for insulated copper conductors rated up to 2000 volts. This data is sourced directly from the NFPA 70 National Electrical Code, specifically Table 310.16 (formerly 310.15(B)(16) in older code cycles). Values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.

AWG / kcmil	60°C (140°F) Copper	75°C (167°F) Copper	90°C (194°F) Copper	Max Standard OCPD (Breaker)
14	15A	20A	25A	15A
12	20A	25A	30A	20A
10	30A	35A	40A	30A
8	40A	50A	55A	40A
6	55A	65A	75A	60A
4	70A	85A	95A	80A
3	85A	100A	115A	100A
2	95A	115A	130A	125A
1	110A	130A	145A	150A
1/0	125A	150A	170A	150A
2/0	145A	175A	195A	175A
3/0	165A	200A	225A	200A
4/0	195A	230A	260A	250A

Note: The Max Standard OCPD column reflects standard breaker sizes per NEC 240.6(A). If your calculated derated ampacity falls between standard sizes, you are permitted to round up to the next standard breaker size, provided the load does not exceed the conductor's actual derated ampacity.

Derating: When Your Base Ampacity Drops

The table above assumes ideal conditions: 30°C ambient temperature and no more than three current-carrying conductors bundled together. When you stuff more wires into a conduit or run them through a hot attic, the wire cannot dissipate heat efficiently. You must apply derating factors to the 90°C column (for THHN/THWN-2), and then verify that the final derated ampacity is sufficient for your breaker and load.

Worked Numeric Example:
You are pulling four current-carrying 12 AWG THHN conductors through a conduit in an attic that reaches 104°F (40°C) in the summer. You want to protect this circuit with a 20A breaker.

1. Base Ampacity: 12 AWG in the 90°C column is 30A.
2. Temperature Correction: At 40°C ambient, the 90°C column correction factor is 0.91. (30A × 0.91 = 27.3A).
3. Bundling Adjustment: Four current-carrying conductors require an 80% adjustment factor. (27.3A × 0.80 = 21.84A).
4. Final Result: Your derated wire ampacity is 21.84A. Because 21.84A is greater than the 20A breaker rating, 12 AWG THHN is legally and safely compliant for this installation.

Decision Tree: Picking Your Exact Wire and Breaker

Use this decision-tree-table to lock in your materials before heading to the supply house. This path terminates in a concrete pick, eliminating guesswork.

Step	Condition / Question	Action / Concrete Pick
1. Calculate Load	Is the load continuous (on for 3+ hours)?	If YES: Multiply load by 1.25. If NO: Use exact load amperage.
2. Size Breaker	What is the calculated load from Step 1?	Pick the next standard breaker size UP from your calculated load (e.g., 16A load -> 20A breaker).
3. Check Terminations	Are the breaker and receptacles rated 75°C?	If YES and wire is 8 AWG or larger: Use 75°C column. If NO or wire is 14-10 AWG: Use 60°C column.
4. Select Wire	Does the wire's column ampacity meet or exceed the BREAKER size?	If YES: Lock in that AWG. If NO: Step up one AWG size and repeat.
5. Verify Derating	Are there 4+ wires in the conduit or high ambient heat?	If YES: Apply derating math to the 90°C column. If derated ampacity drops below the LOAD, step up AWG.

What This Table Cannot Tell You

Ampacity charts dictate thermal limits, but they do not account for the physics of long-distance power transmission or physical installation constraints. You must cross-reference three external factors before finalizing your wire size guide AWG selection:

• Voltage Drop: The NEC recommends a maximum 3% voltage drop for branch circuits and 5% overall. If you are running a 12 AWG wire 150 feet to a 15A receptacle, the voltage drop will exceed 5%, causing motors to overheat and lights to dim. Use a dedicated voltage drop calculator and step up to 10 AWG or 8 AWG for long runs, regardless of what the ampacity table permits.
• Conduit Fill Capacity: NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. You cannot physically jam six 6 AWG THHN wires into a 1/2-inch EMT conduit, even if the derating math technically allows it. Always check the fill tables before buying conduit.
• Physical Termination Limits: A 50A breaker lug might physically reject a 4 AWG wire, even though 4 AWG is electrically overkill and safe. Always check the manufacturer's datasheet for the maximum and minimum wire sizes the specific lug or breaker terminal will accept.