Somewhere behind your electric meter is a wire that, by the numbers printed in the ampacity table, is too small for the job. If you look up 2/0 copper in NEC Table 310.16, the 75°C column says 175 amps. The main breaker it feeds says 200. Any apprentice who sized a branch circuit that way would fail inspection before lunch. And yet the inspector looked at that service, nodded, and signed the card — because the Code itself says it's fine. Not as an oversight. As a rule, written down, with a section number. Nearly every house on your street is fed this way, and understanding why is one of those moments where the NEC stops looking like a book of arbitrary restrictions and starts looking like what it actually is: eighty years of measured data about how buildings really behave.

The rule with a number attached

The provision is NEC 310.12, and electricians of a certain vintage still call it by its old address, Table 310.15(B)(7). It says that for a one-family dwelling — or an individual unit of a two-family or multifamily dwelling — the service conductors of a 120/240-volt, single-phase service rated 100 through 400 amps may be sized at 83 percent of the service rating. The same allowance applies to a feeder, but only if that feeder carries the entire load of the dwelling.

Run the arithmetic for a 200-amp service: 200 × 0.83 = 166 amps. Now the ampacity table works in your favor. 2/0 copper at 175 amps clears the bar. So does 4/0 aluminum at 180. For a 100-amp service, 83 amps is the target, and 4 AWG copper (85 amps at 75°C) or 2 AWG aluminum makes it. The 2020 Code even prints a table of pre-calculated sizes so nobody has to do the multiplication on a tailgate.

The question worth sitting with isn't what the rule says. It's why the Code — a document that will make you upsize a wire because it shares a conduit with three others — hands out a 17 percent discount on the most important conductors in the building.

Your house never asks for everything at once

The answer is a concept called load diversity, and it's the quiet engine behind most of Article 220.

A 200-amp service exists because, on paper, the house could demand something like that: range, dryer, air conditioner, water heater, every light and receptacle. But decades of utility demand studies show that dwellings never actually do. The range isn't on all four burners while the dryer runs while the AC compressor starts while someone vacuums. Real measured demand at residential services peaks far below the nameplate sum, and the peaks that do occur are brief. The load calculation in Article 220 already acknowledges this — it's why you apply demand factors instead of adding up every appliance at face value — and 310.12 is the same insight applied one more time, to the conductor itself.

Here's where the physics earns its keep. A conductor's ampacity isn't a cliff; it's a thermal budget. Current heats copper at a rate of I²R — current squared times resistance — and the insulation fails not at some magic amperage but at a sustained temperature, 75°C for the insulation systems the table column assumes. Heavy conductors have enormous thermal mass. A 2/0 copper service conductor takes a long time to heat up, and a load spike that lasts ninety seconds while the range preheats barely moves its temperature at all. Ampacity tables are built around continuous, steady-state loading — current held long enough for the conductor to reach thermal equilibrium. A dwelling's load profile is the opposite of that: spiky, diverse, and mostly idle at 3 a.m. The 83 percent factor is the Code translating "this load will never be continuous at full rating" into a number you can buy at the supply house.

That's also why the discount exists here and almost nowhere else. A branch circuit feeding a single big appliance has no diversity — when the water heater runs, it draws its full current for as long as it runs. The service conductor, sitting upstream of forty circuits that take turns, statistically cannot see them all at once.

Where the 83% ends

Every misapplication of this rule comes from missing one of its boundaries, and inspectors see the same three mistakes on repeat.

It's for dwellings only. The demand data behind the rule was measured in homes. A small commercial space with a 200-amp, 120/240 single-phase service gets no discount, because a shop full of equipment can genuinely run flat-out for hours.

The conductor must carry the whole dwelling load. A feeder from the meter-main to an interior panel that serves the entire house qualifies. A feeder to a garage subpanel, or one leg of a house split between two panels, does not — that conductor's load isn't diverse in the same way, and it gets sized the ordinary way, from the ampacity table against its calculated load.

It's single-phase 120/240 only, 100 through 400 amps. No three-phase services, no 50-amp services, no 600-amp estate houses. Outside the studied range, the assumption of diversity hasn't been validated, so the Code doesn't extend it.

And two things the rule never waives: the Article 220 load calculation still has to come in at or under the service rating — 310.12 shrinks the wire, not the math — and voltage drop still exists. On a long feeder to a dwelling unit, the 83 percent conductor may be legal and still deliver mushy voltage at the far end. Legal minimum and good design are different questions, and the second one is yours.

Your next moves

  • Memorize the two anchor pairs. 200-amp service: 2/0 copper or 4/0 aluminum. 100-amp service: 4 AWG copper or 2 AWG aluminum. These four sizes cover the vast majority of residential services you'll ever touch — then verify against Table 310.12 in your Code cycle, because the table is the authority, not your memory.
  • Run the three-question scope test before applying 83 percent: Is it a dwelling unit? Is it 120/240-volt single-phase, 100–400 amps? Does this exact conductor carry the entire load of that dwelling? Any "no" sends you back to Table 310.16 and the standard rules.
  • Do the Article 220 load calculation first, not after. The 83 percent rule assumes the service rating already covers the calculated load. On any panel upgrade — especially ones adding an EV charger or heat pump — run the calc before you order wire.
  • Check voltage drop on anything long. A short mast-to-panel run doesn't care, but a 150-foot feeder to an in-law unit might be Code-legal at 83 percent and still a callback. Size for the run, not just the rule.
  • Read your local amendments. Some jurisdictions modify or restrict 310.12. Five minutes on your AHJ's website beats an afternoon re-pulling service entrance cable.

The rule is simple; the boundaries aren't

The 83 percent rule is a one-line multiplication wrapped in a fence of conditions — dwelling, single-phase, whole load, 100 to 400 amps, load calc satisfied, voltage drop acceptable. The multiplication you can do in your head. The fence is where jobs go sideways, usually at the moment you're standing in a crawlspace deciding whether a feeder "counts." That's the moment Voltly was built for: an offline load calculator, voltage drop, ampacity, and conductor sizing in your pocket, working exactly the same in a basement with no signal as it does in the truck. Run the calc, check the drop, size the wire, and walk into the inspection already knowing the answer — voltly.lumenlabs.works.