If you are sizing a branch circuit, the direct answer for standard residential NM-B (Romex) cable is to use the 60°C column of the electrical wiring chart. For individual THHN/THWN-2 conductors in conduit, you calculate ampacity using the 75°C or 90°C column, but your final breaker size is ultimately limited by the temperature rating of the terminations (lugs and breakers), which are typically rated for 75°C.
An electrical wiring chart is not just a suggestion; it is the legal baseline for preventing conductor insulation meltdown and electrical fires. The table below is derived directly from the 2023 National Electrical Code (NEC) Table 310.16. Before you pull wire, you need to understand exactly how to read the columns, how bundling derates your capacity, and what this chart completely ignores.
How to Read the NEC Electrical Wiring Chart
The most common mistake DIYers and junior apprentices make is looking at the highest number on the chart and assuming that is the breaker size they can use. The chart is divided into temperature columns, and the column you are legally allowed to use depends on your insulation type and termination hardware.
- 60°C (140°F) Column: Use this for NM-B (Romex) cable, UF-B cable, and any circuit rated 100A or less where the termination temperature is unknown. This is your default for standard residential interior wiring.
- 75°C (167°F) Column: Use this for THHN/THWN-2 wires in conduit when terminating at standard modern breakers and lugs rated for 75°C. This applies to most residential subpanel feeders and commercial branch circuits.
- 90°C (194°F) Column: You can only use this column for derating calculations (adjusting for heat or bundling). Per NEC 110.14(C), you cannot terminate a wire at a device rated lower than the wire's insulation. Since almost no standard breaker is rated for 90°C, you calculate your derated ampacity here, but your final allowable ampacity cannot exceed the 75°C or 60°C column limits.
The Master Electrical Wiring Chart (NEC Table 310.16)
Source: 2023 NEC Table 310.16 (Copper Conductors, 60°C to 90°C, 30°C Ambient Temperature). Max Breaker sizes assume standard overcurrent protection per NEC 240.4(B).
Bookmark Quick-Jumps: 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG
| AWG Size | 60°C (NM-B/Romex) | 75°C (THHN Term) | 90°C (THHN Conduit) | Max Standard Breaker |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15A* |
| 12 AWG | 20A | 25A | 30A | 20A* |
| 10 AWG | 30A | 35A | 40A | 30A |
| 8 AWG | 40A | 50A | 55A | 40A (or 50A for THHN) |
| 6 AWG | 55A | 65A | 75A | 60A (or 70A for THHN) |
| 4 AWG | 70A | 85A | 95A | 80A (or 90A for THHN) |
| 3 AWG | 85A | 100A | 110A | 100A |
| 2 AWG | 95A | 115A | 130A | 110A (or 125A for THHN) |
| 1 AWG | 110A | 130A | 145A | 125A (or 150A for THHN) |
| 1/0 AWG | 125A | 150A | 170A | 150A |
| 2/0 AWG | 145A | 175A | 195A | 175A |
| 3/0 AWG | 165A | 200A | 225A | 200A |
| 4/0 AWG | 195A | 230A | 260A | 225A (or 250A for THHN) |
How Derating Modifies Your Base Ampacity
The ampacity values in the electrical wiring chart assume two things: an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors bundled together. When you violate either condition, you must apply derating factors per NEC 310.15.
Example: Bundling Derating (NEC 310.15(C)(1))
Imagine you are pulling four current-carrying 12 AWG THHN wires through a single conduit to feed two separate 240V circuits.
- Base Value: Look at the 90°C column for 12 AWG. The base ampacity is 30A.
- Adjustment Factor: Table 310.15(C)(1) states that for 4 to 6 current-carrying conductors, you must multiply the base ampacity by 80%.
- Calculation: 30A × 0.80 = 24A.
- Final Sizing: Your derated ampacity is 24A. Because 24A is not a standard breaker size, NEC 240.4(B) allows you to round up to the next standard size, which is 25A. However, because of the 240.4(D) small conductor rule mentioned above, you are legally capped at a 20A breaker for 12 AWG wire.
If you had used 10 AWG THHN for the same run, your base 90°C ampacity would be 40A. Derated by 80%, that yields 32A. Rounding up to the next standard breaker size gives you a perfectly legal 35A breaker (or 30A to be conservative).
What This Electrical Wiring Chart Cannot Tell You
Relying solely on Table 310.16 will leave you blind to three critical installation factors that cause voltage drop, overheated panels, and failed inspections.
- Voltage Drop: The chart tells you the wire won't melt, but it doesn't guarantee your tools will run. NEC 310.15(B) Informational Note recommends keeping voltage drop under 3% for branch circuits and 5% total. For a 120V circuit, 3% is 3.6 volts. If you are running a 15A load on 14 AWG wire 150 feet away, the wire won't catch fire, but your voltage will drop below 114V, potentially damaging motorized appliances. Always upsize wire for runs over 100 feet.
- Conduit Fill Capacity: The chart ignores physical space. You might legally derate four 6 AWG THHN wires in a 3/4-inch EMT conduit electrically, but Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Four 6 AWG THHN wires require a minimum of 1-inch EMT. Always check conduit fill before pulling.
- Lug and Breaker Physical Limits: A 100A subpanel lug might be rated for 75°C, but the physical hole might only accept up to 1/0 AWG wire. If your voltage drop calculation demands 2/0 AWG, you will need to use a lug adapter or a larger panel.
Electrical Wiring Chart FAQ
What size breaker do I use for 10 AWG wire on this electrical wiring chart?
For standard residential and commercial applications, the maximum breaker size for 10 AWG copper wire is 30 Amps. This applies whether you are using 10 AWG NM-B (Romex) or 10 AWG THHN in conduit. The 30A limit is hard-capped by NEC 240.4(D) for conductors 10 AWG and smaller, meaning you cannot use the 'round up' rule to install a 35A or 40A breaker even if your derating math allows it.
Why does the electrical wiring chart show 12 AWG at 25A but I can only use a 20A breaker?
The 25A value is found in the 75°C column, which represents the thermal limit of the copper and insulation under ideal conditions. However, NEC 240.4(D) specifically overrides this for small conductors to protect against high-fault currents and physical damage. It strictly limits 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A. The only exception is for specific motor circuits where the breaker is sized for motor starting inrush current, not conductor protection.
Can I use the 90°C column for my entire residential circuit?
No. You can only use the 90°C column to calculate derating adjustments for bundling or high ambient temperatures. Per NEC 110.14(C), the final ampacity of the circuit is limited by the lowest temperature rating of any connected device, termination, or conductor. Since standard residential breakers, receptacles, and switches are rated for 60°C or 75°C, your final allowable ampacity cannot exceed the values in the 60°C or 75°C columns.
Does the neutral wire count for derating on the electrical wiring chart?
In a standard single-phase, 120/240V residential circuit, the grounded neutral conductor does not count as a current-carrying conductor for derating purposes under NEC 310.15(B)(5)(b). It only carries unbalanced current. However, if you are wiring a 3-phase, 4-wire wye circuit supplying non-linear loads (like LED drivers or computer power supplies), the neutral does count because harmonic currents can cause the neutral to carry more current than the phase conductors, requiring derating.
