What Size Copper Wire for a 200 Amp Residential Service?
A user asked the app what size copper wire a 200 amp service needs for a single family dwelling. The short answer is 2/0 copper. That surprises people who go straight to the ampacity table, because 2/0 copper is listed at 175 amps in the 75 degree C column of NEC 2023 Table 310.16, which looks too small for a 200 amp main. The reason it works is a special allowance that only applies to dwellings.
The 83 percent rule in 310.12
NEC 2023 Section 310.12 covers single phase services and feeders rated 100 through 400 amps that supply an entire dwelling unit. For those conductors, the Code allows an ampacity of not less than 83 percent of the service rating. For a 200 amp service that works out to 200 times 0.83, which is 166 amps. Any conductor rated at least 166 amps in the 75 degree C column qualifies, and 2/0 copper at 175 amps clears that bar with room to spare. Table 310.12(A) spells this out directly and lists 2/0 copper and 4/0 aluminum for a 200 amp service.
The logic behind the allowance is diversity. A whole house never runs every load at once, so the demand on service conductors is lower than the nameplate rating suggests. The allowance only applies when the conductors serve the entire load of the dwelling. A 200 amp feeder to a detached shop or a subpanel that serves part of a building does not qualify and must be sized from Table 310.16 the normal way, which pushes you to 3/0 copper.
Details that still matter
Termination ratings drive the column you use. Modern meter sockets, main breakers, and lugs are rated 75 degrees C, so the 75 degree column governs even when the conductor insulation is THHN rated 90 degrees. If you have very long runs, voltage drop can justify upsizing to 3/0 even though the Code does not require it. And the grounding electrode conductor for a 2/0 copper service is 4 AWG copper per Table 250.66.
One more practical note. Most 200 amp residential services in many regions are actually run in 4/0 aluminum SER or XHHW because of copper cost. Both answers are compliant. If the utility owns the overhead drop, their conductor sizing is their business, and your sizing duty starts at the service point.