NEC 310.12: Why 2/0 Copper Legally Feeds a 200-Amp House
Look up 200 amps in Table 310.16 and it points at 3/0 copper. Look at what actually feeds houses across the country and it is 2/0. Both are right, and the gap between them is a rule that 60% of candidates in our data get wrong. Here is the 83% rule, its limits, and the subpanel trap.
Last reviewed September 2026
The idea behind the discount
A dwelling has the most diversified load profile in the code. The range runs at dinner, the dryer runs on the weekend, the air conditioner cycles, and the water heater recovers in bursts. The odds of every large load pulling full current together, for hours, are close to zero, and the Article 220 load calculation already accounts for that. The 83% conductor allowance is the same honesty applied to the service wire itself.
The numbers to memorize
- 100-amp service: 4 AWG copper or 2 AWG aluminum.
- 150-amp service: 1 AWG copper or 2/0 aluminum.
- 200-amp service: 2/0 copper or 4/0 aluminum. The most-tested row in the table.
- Any rating 100 through 400 A: rating x 0.83, then Table 310.16.
The 200-amp row earns its place. 2/0 copper or 4/0 aluminum answers a large share of residential service questions on sight, and both values also show up constantly on the truck.
Worked example: a 400-amp service
- One-family dwelling, 400-amp single-phase service, conductors carry the entire load.
- 310.12 applies: 400 x 0.83 = 332 A required ampacity.
- Met in practice with parallel runs sized from Table 310.16.
- At 450 amps there is no 83% allowance. The rule ends at 400.
Where candidates lose the point
- Applying 310.12 to a garage subpanel feeder. Unless the feeder carries the ENTIRE dwelling load, it sizes from Table 310.16.
- Applying it to a store or an apartment house service. It reads on individual dwelling services only.
- Answering 3/0 copper for a 200-amp house because Table 310.16 says so. The dwelling rule is the controlling one.
- Stretching it past 400 amps. The allowance covers ratings of 100 through 400.
- Forgetting the feeder version. A feeder between an outside disconnect and the inside panel, carrying everything, gets the same 83%.
How it shows up on the exam
Two shapes. The direct one names a one-family dwelling service rating and asks for the minimum conductor, and the answer is the table row or the 0.83 arithmetic. The trap one moves the wire downstream, to a subpanel feeding half the house, and offers the 83% answer as a choice. The question inside every 310.12 problem is the same: does this conductor carry the whole dwelling load? Yes means 83%. No means Table 310.16, full value.
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