Wire Size Calculator

How Wire Sizing Works

Choosing the right wire gauge requires satisfying two constraints simultaneously:

• Ampacity: The wire must carry the current without overheating. NEC Table 310.16 lists the ampacity for each gauge at each insulation temperature column.
• Voltage drop: The wire must not lose too much voltage over distance. NEC 210.19(A) Informational Note 4 recommends a 3% branch-circuit drop target as a performance recommendation, not a code requirement.

At short distances (under 50 feet), ampacity is usually the limiting factor. At longer distances, voltage drop requires upsizing beyond what ampacity alone would demand. This calculator checks both constraints.

NEC Temperature Ratings (60°C / 75°C / 90°C)

NEC Table 310.16 lists three ampacity columns for each wire gauge, corresponding to the insulation's temperature rating. Reading the right column is essential because the ampacity you can legally use is determined by the lowest-rated component in the circuit, not the wire itself.

• 60°C column: Used for TW, UF, and NM-B cable (Romex) in typical residential installations. Per NEC 334.80, NM-B ampacity is limited to the 60°C column even though the underlying conductors are 90°C rated, because the overall cable assembly is derated. This is the row most homeowners and residential electricians actually use.
• 75°C column: Used for THWN, THW, XHHW, and feeder-style installations where the terminal lug is rated 75°C and the conductors are not inside NM-B cable. Common in commercial conduit work.
• 90°C column: Used for THHN, XHHW-2, and similar premium insulations, but only when every terminal and splice is rated for 90°C. Most residential and commercial terminals are 75°C, so the 90°C column is rarely usable at its full value; it is mainly used as a starting point for derating calculations.

This is why a 12 AWG THHN conductor shows 30A in the 90°C column but is almost always limited to 25A in practice (75°C column) and further limited to 20A under the small conductor rule (NEC 240.4(D)). The calculator above uses the NEC branch-circuit OCP ceiling, which incorporates 240.4(D), so 14 AWG caps at 15A, 12 AWG at 20A, and 10 AWG at 30A regardless of temperature column.

Continuous Load Sizing (NEC 210.19(A))

NEC 210.19(A) (branch circuits) and 215.3 (feeders) size the conductor and overcurrent device at not less than 125% of any continuous load plus 100% of the non-continuous load. A continuous load is one that operates for three hours or more. Equivalently, the load on a given breaker cannot exceed 80% of the breaker rating if it is continuous: a 20A branch-circuit breaker supports 16A of continuous draw under this rule.

Common continuous loads include:

• Lighting circuits in offices, retail, and warehouses (runs all day)
• EV chargers on Level 1 and Level 2 (hours per charge session)
• Baseboard and space heaters on thermostat cycles
• Server rack PDUs and 24/7 equipment

For non-continuous loads the 125% sizing rule does not apply; the conductor and OCP are sized at 100% of the load. Many real circuits carry a mix of continuous and non-continuous loads and the 125%/100% rule applies to each part of that total. When in doubt, size for continuous duty.

Copper vs Aluminum

Copper is the standard for branch circuits. It has lower resistance, higher ampacity per gauge, and is easier to work with. Aluminum (AA-8000 series) is less expensive per foot and lighter, making it the industry standard for service entrance cables, sub-panel feeders, and long runs where the cost savings outweigh the gauge upsize.

For the same duty, aluminum is typically one to two gauges larger than the copper pick, with the exact gap depending on whether ampacity or voltage drop is binding on the specific run. Where 10 AWG copper is enough for an ampacity-driven branch, 8 AWG aluminum typically covers the same duty; where 2 AWG copper feeds a 100A service, 1/0 AWG aluminum typically does the same job. On voltage-drop-driven picks at long run lengths the gap can be smaller, sometimes a single step on the gauge ladder.

Aluminum caveat: old solid-strand aluminum branch wiring from the 1960s-70s is associated with connection failures and fires when mixed with brass terminals. Modern AA-8000 series stranded aluminum (used for feeders) is safe when installed with listed anti-oxidant compound, aluminum-rated lugs, and proper torque. Small aluminum conductors (8 AWG or smaller) are not typical for residential branch circuits.

Common Residential Circuits

Most US residential electrical work follows a small set of standardized circuit types. These pair a specific breaker size with a minimum wire gauge:

These are typical starting points. Always verify against the actual load calculation and local amendments. The small conductor rule (NEC 240.4(D)) caps 14 AWG at 15A, 12 AWG at 20A, and 10 AWG at 30A even when the 75°C ampacity column shows higher numbers.

Reading Wire Insulation Markings

Every code-compliant wire is stamped with a letter code that tells you its rating and environment. Understanding the markings lets you verify the correct insulation is being used:

• T = Thermoplastic (PVC-based insulation)
• H = Heat-resistant (75°C rating); HH = High heat-resistant (90°C rating)
• W = Wet locations (can be used in conduit underground or outdoors)
• N = Nylon jacket (adds abrasion and chemical resistance)
• -2 suffix = Wet-location rated at full 90°C (e.g. XHHW-2)

The most common building wires you will see:

• THHN/THWN-2: Thermoplastic, 90°C dry / 75°C wet (THHN) or 90°C wet (THWN-2). The go-to for commercial conduit work.
• NM-B: Non-metallic sheathed cable ("Romex"). 90°C insulation but ampacity is limited to the 60°C column per NEC 334.80 for most residential runs.
• XHHW-2: Cross-linked polyethylene. Used for service conductors and high-temp environments.
• USE/RHW: Underground service entrance cables.

Wire insulation is tested and listed under UL 83 (thermoplastic) and UL 44 (thermoset). The copper conductor itself meets ASTM B3 for soft-drawn copper. Both standards are cited in the references below.