Service Wire Size Calculator — The 83% Rule

For a dwelling service or the main feeder carrying the whole load, NEC Table 310.12 allows 2/0 copper or 4/0 aluminum. Both are one size smaller than Table 310.16 would require for 200 A, because Table 310.12 permits dwelling service conductors to be rated at 83% of the service rating. Aluminum 4/0 is the overwhelmingly common choice on residential 200 A services, for cost reasons.

It is the allowance behind NEC Table 310.12. A dwelling's service or main power feeder may use conductors sized for 83% of the service rating rather than 100% — so a 200 A service needs conductors good for 166 A, not 200 A. The justification is diversity: a house's calculated load already includes demand factors, and the actual peak rarely approaches the service rating for long enough to matter thermally. Before the 2020 NEC this appeared as the 310.15(B)(7) allowance; it now has its own table.

For a dwelling service, yes — that is exactly what Table 310.12 permits. On its own, Table 310.16 rates 4/0 aluminum at 180 A at 75 °C, which is why the same conductor is not acceptable as a general-purpose 200 A feeder. The 83% allowance is specific to dwelling services and main power feeders, and applying it anywhere else is a genuine code violation.

Only if that feeder carries the entire load of the dwelling. A feeder to a detached garage, a shop subpanel, or any panel serving part of the load falls back to Table 310.16. This is where most online advice goes wrong: 4/0 aluminum is fine for a 200 A house service but is only good for 180 A as a garage feeder, so a 200 A garage subpanel needs 250 kcmil aluminum or 2/0 copper.

Usually yes. NEC 220.61 permits the neutral to be sized for the maximum unbalanced load, which on a typical dwelling with 240 V appliances is less than the full ungrounded load. The floor is set by 250.24(C): the service neutral may never be smaller than the required grounding electrode conductor, because it carries fault current back to the source. In practice a 200 A service often uses a 2/0 aluminum neutral with 4/0 aluminum hots — which is what 4/0-4/0-2/0 SER and mobile-home feeder cable assemblies are built for.

If drop pushes past 3%, yes — and long rural services from a pole often do. Set the run length above and the calculator upsizes automatically when the voltage-drop check fails, telling you it did so. Remember that upsizing the ungrounded conductors also triggers NEC 250.122(B) for the equipment grounding conductor on feeders; the Ground Wire Size Calculator handles that.

Aluminum, almost always, on residential services — it costs substantially less for the same ampacity even after going up a size or two, and utility service drops are aluminum anyway. The requirements are that terminations be listed for aluminum (AL/CU or AL9CU marking) and that connections be made with the correct anti-oxidant compound and torque specification. Copper is preferred where space is very tight, in corrosive environments, or where a specification demands it.