What table does this calculator use for ampacity?

NEC Table 310.16, 75 degree C copper column. That is the standard termination temperature for residential and most commercial work and is the column the AHJ will check first.

Why is voltage drop checked separately from ampacity?

NEC 310.16 sets the minimum size for safe heating; voltage drop is an Informational Note in NEC 210.19(A) and 215.2(A) and is not enforced. The widely used targets are 3 percent on branch circuits and 5 percent total feeder plus branch.

Should I use copper or aluminum?

Copper for branch circuits and small feeders; aluminum is common on services and large feeders where the cost difference is material. Aluminum needs roughly 1.6 times the cross-section of copper for the same ampacity, and aluminum-rated terminations.

NEC Wire Size Calculator - Project Ouroboros

NEC Wire Size Calculator

Estimates a copper conductor size from NEC 310.16 (75°C column) and computes one-way voltage drop. For planning only; verify with a licensed electrician.

Load current (amps) System voltage One-way distance (feet) Circuit type

Results

Recommended copper AWG—

Ampacity (75°C)—

Voltage drop—

Within 3% target?—

Enter values above to recalculate.

Last reviewed: April 2026 (NEC 2023)

Tools we used or recommend for this work: Klein Tools wire strippers (Klein 11055), a Klein digital multimeter for verifying voltage at the load end, and Southwire Romex (or THHN if you are pulling in conduit) in the gauge this calculator returned. Links may be affiliate-tagged where the brand has a program.

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