The maximum 8 AWG current rating (ampacity) for copper wire is 40 amps when used in standard NM-B (Romex) or UF-B cable, and 55 amps when pulled as individual THHN/THWN-2 conductors in conduit. For aluminum 8 AWG, the rating is 30 amps (NM-B/UF-B) or 45 amps (THHN). These values assume an ambient temperature of 86°F (30°C) and standard residential termination rules.

Safety & Code Caveat: Always de-energize the panel and verify dead with a tested multimeter before working on branch circuits. The ampacity values below are based on NEC-style guidance (NFPA 70). Your local Authority Having Jurisdiction (AHJ) or electrical inspector always has final authority on code compliance and permitted wire types.

The 8 AWG Current Rating Table (NEC 310.16)

To use this table correctly, you must look at both the insulation type printed on the wire jacket and the temperature column (60°C, 75°C, or 90°C). A common mistake is using the 90°C column just because the wire insulation is rated for it. In reality, NEC 110.14(C) forces you to use the lower 60°C or 75°C columns based on the temperature rating of the breaker lugs and receptacles you are terminating the wire into. Bookmark the quick-lookup rows below for your most common residential and commercial pulls.

Table 1: 8 AWG Ampacity by Insulation and Material (Source: NEC Table 310.16)
Insulation / Cable Type Temp Rating Column Copper Ampacity Aluminum Ampacity Typical Application
NM-B / UF-B (Romex) 60°C 40A 30A Indoor/outdoor residential branch circuits (Ranges, EV chargers, subpanels)
THHN / THWN-2 90°C (for derating) 55A 45A Individual conductors pulled in EMT, PVC, or flexible conduit
XHHW-2 90°C (wet/dry) 55A 45A Underground conduit, wet locations, commercial feeders
TW / Older UF 60°C 40A 30A Legacy installations (do not use for new construction)
Quick Lookup for DIYers: If you are wiring a 40-amp EV charger or a small subpanel using standard yellow-jacket Romex (NM-B), your absolute maximum breaker size is 40A. If you are pulling individual black/red/white THHN wires through metal conduit, you can safely use a 50A breaker.

Which Ampacity Column Applies to Your Installation?

Determining the correct column requires understanding the NEC 110.14(C) termination temperature rule. Most modern residential circuit breakers and receptacles are tested and rated for 75°C terminations. However, NEC 110.14(C)(1)(a) states that for circuits rated 100 amps or less, you must use the 60°C column to determine ampacity unless the equipment is explicitly marked and listed for 75°C or higher.

Because most standard residential breakers and disconnects are not explicitly marked for 75°C on the smaller frame sizes, the 60°C column governs NM-B and UF-B cables. This is why 8 AWG NM-B is capped at 40 amps, even though the copper inside could thermally handle more.

How Derating Modifies the Base Value

The 90°C column in the table above is rarely used to determine your final breaker size. Instead, it is used as the starting point for ampacity derating (adjustment factors) under NEC 310.15(C)(1). When you bundle multiple current-carrying conductors in a single raceway, or when ambient temperatures exceed 86°F (30°C), the wire cannot dissipate heat as efficiently.

Worked Derating Example:
Imagine you are pulling four current-carrying 8 AWG THHN copper conductors through a conduit in a garage.

  1. Base Value: The 90°C column gives us 55A.
  2. Adjustment Factor: NEC Table 310.15(C)(1) dictates an 80% adjustment factor for 4-6 current-carrying conductors.
  3. Derated Ampacity: 55A × 0.80 = 44 amps.
  4. Termination Check: Your breaker lugs are rated for 75°C (50A limit for 8 AWG). Since your derated value (44A) is less than the 75°C termination limit (50A), the 44A value governs.

Because 44A is not a standard breaker size, and NEC 240.4(B) allows rounding up to the next standard size (50A) only if the calculated load does not exceed the wire ampacity, you would typically protect this circuit with a 40A or 45A breaker to remain strictly within the derated thermal limits of the bundled wire.

What the Ampacity Table Cannot Tell You

While NFPA 70 (NEC) tables tell you when a wire will melt or degrade its insulation, they completely ignore voltage drop and physical fitment. Relying solely on the ampacity chart will lead to underperforming circuits on longer runs.

Voltage Drop on Long Runs

Ampacity assumes the wire is infinitely short. In reality, wire has resistance. The NEC recommends (via Informational Note to 310.15) that branch circuit voltage drop be limited to 3% for reasonable efficiency.

Worked Voltage Drop Example:
You are wiring a 120V, 40A workshop outlet located 100 feet from the main panel using 8 AWG copper.

  • Resistance of 8 AWG copper at 75°C is approximately 0.778 ohms per 1,000 feet.
  • Total round-trip distance = 200 feet.
  • Total Resistance = 0.778 × (200 / 1000) = 0.1556 ohms.
  • Voltage Drop = Current × Resistance = 40A × 0.1556Ω = 6.22 volts.
  • Percentage Drop = (6.22V / 120V) × 100 = 5.18%.

At 5.18%, this exceeds the 3% recommendation. Your tools and equipment will run hot and inefficiently. To fix this, you must step up to 6 AWG copper for a 100-foot, 40A run, even though 8 AWG is thermally rated to handle the 40A current without catching fire. Use a dedicated voltage drop calculator for runs exceeding 50 feet.

Terminal Lug Sizing and Torque

The table assumes your wire will physically fit into the breaker or disconnect lugs. Many 40A and 50A breakers are designed to accept a range of wire sizes (e.g., #14 to #4 AWG), but some older or specific manufacturer lugs have strict minimums or maximums. Furthermore, NEC 110.14(D) requires that terminations be torqued to the manufacturer's specified values using a calibrated torque screwdriver. An 8 AWG wire torqued to 35 in-lbs on a breaker rated for 45 in-lbs will create a high-resistance connection, leading to localized heating that has nothing to do with the wire's overall ampacity rating.

Pro-Tip for Panel Work: When terminating 8 AWG stranded THHN into a breaker lug designed primarily for solid wire, use a pin terminal or ferrule crimped with a proper ratcheting crimper. Never tin the stranded wire with solder before inserting it into a screw-terminal lug; solder cold-flows under pressure, leading to a loose connection and eventual arcing.