Sizing service wires for a house: the 83% rule

What the code requires

This rule applies to one-family homes and individual dwelling units on 120/240 V single-phase power. A service of 100 to 400 A that carries the entire dwelling load may use conductors with an ampacity of at least 83% of the service rating. A feeder of 100 to 400 A that carries a unit's entire load follows the same rule. Single-phase 208Y/120 V feeders with two hots and a neutral may use parts (A) through (C).

Table 310.12(A), copper with aluminum in parentheses: 100 A, 4 AWG (2); 110 A, 3 (1); 125 A, 2 (1/0); 150 A, 1 (2/0); 175 A, 1/0 (3/0); 200 A, 2/0 (4/0); 225 A, 3/0 (250 kcmil); 250 A, 4/0 (300); 300 A, 250 (350); 350 A, 350 (500); 400 A, 400 (600). The table may be used only when no temperature correction or adjustment factor applies.

The neutral may be smaller than the hot conductors if the rules in 220.61 and 230.42 (services) or 215.2 and 220.61 (feeders) are met.

On the job

• For a 200 A home service, the table gives 2/0 copper or 4/0 aluminum.
• Using the table for a subpanel feeder that does not carry the whole dwelling load is a common mistake.
• Using the table in a high-heat location, such as a service on the west side of a house in a very hot climate, is another.

Exceptions that matter

• If correction or adjustment factors apply, such as a hot ambient or more than 3 current-carrying conductors together, the table may not be used. The factors are applied to the ampacity for the conductor’s temperature rating, and the corrected ampacity must still be at least 83% of the service or feeder rating.

In California

For new one- and two-family homes, the California Electrical Code adds Energy Code readiness rules: panel breaker spaces for a heat pump water heater, heat pump space heater, electric cooktop and dryer (408.2(B)), and energy storage readiness with a minimum panel busbar rating as Energy Code Section 150.0(s) specifies (706.10). These affect panel and service selection, not the 83% conductor sizing.

Related sections

• NEC 220.61
• NEC 230.42: Sizing service-entrance conductors
• NEC 215.2: Feeder wire size and minimum ampacity
• NEC 240.4(H)
• NEC 310.15: Derating wire for heat and crowded conduits
• NEC 310.16: Wire ampacity table and the 60/75 °C terminal rule
• NEC 110.14(C)

Where this comes from

• NFPA: 2026-cycle CMP-6 First Draft ballot (2023 baseline text)
• NFPA: 2023-cycle CMP-6 First Draft ballot (FR 8374, Table 310.12(A))
• Electrical Contractor, Heat Is the Enemy: Using 310.12 to size service and feeder conductors
• Read the official NEC free at NFPA