The one place a conductor may be smaller than its breaker

When to use this: sizing a new service, upgrading an existing one, or checking what an installer has quoted.

Everywhere else in the code a conductor must carry at least what the device ahead of it is rated for. NEC 310.12 is the exception: service and feeder conductors serving the entire load of a dwelling may be sized at 83% of the overcurrent device rating.

A 200 A house service therefore needs conductors rated 166 A, not 200 A — which is 4/0 AWG aluminium rather than 250 kcmil. One full size, on the most expensive conductor in the job.

The justification is load diversity. A house never draws its calculated load continuously, and the Article 220 calculation that produced the service size is already conservative. Nothing about the conductor changed; the code simply stopped double-counting.

Four conditions, all of them

1. A dwelling unit. Single-family, or an individual unit of a two-family or multifamily dwelling. Never commercial.
2. The entire load of that unit. This is the one people miss.
3. Service or feeder conductors. Never branch circuits.
4. 100 A to 400 A. Outside that range the rule does not exist.

Condition two is where installations go wrong, because the arithmetic looks identical either way. A 200 A feeder to a detached garage, a workshop or an ADU carries part of the load, so it takes full-size conductors from Table 310.16 however large it is. Only the conductors carrying everything qualify.

Table 310.12(A)

The published table, which is simply 83% of each rating resolved against the 75 °C column of Table 310.16 — this calculator derives it rather than transcribing it, which is why a transcription error would show up as a test failure rather than a wrong answer.

Why the 75 °C column

Service equipment terminals are listed for 75 °C, and 110.14(C) holds the circuit to the lowest-rated component. Using the 90 °C column would give 2/0 AWG → one size smaller for a 200 A service, and it is not permitted. This is the same rule that stops THHN buying 90 °C ampacity anywhere else — our wire size calculator covers it in detail.

The other two conductors

A service has three conductor calculations and they use three different inputs. Getting one right does not get the others right.

The neutral is usually smaller than the hots, because 220.61 sizes it on the load between neutral and any one ungrounded conductor rather than on the total. Two limits apply: it may not be smaller than the grounding electrode conductor, and a further demand factor applies to the portion above 200 A.

The grounding electrode conductor comes from Table 250.66 and is sized on the service conductors, not on the device. Two caps override the table: 250.66(A) limits the conductor to a ground rod to 6 AWG, and 250.66(B) limits the conductor to a concrete-encased electrode to 4 AWG. A larger conductor into soil achieves nothing — earth is not a fault-current path. Our ground wire size calculator covers the distinction between this and the equipment grounding conductor, which is the most common confusion in Article 250.

Above 400 amps

The allowance stops. Large dwelling services are usually parallel sets under 310.10(G): two or more conductors per phase, each carrying its share. Every set must be identical in length, material, size, insulation and termination method — because current divides by impedance, and a set that is even slightly shorter carries more than its share and overheats while the others loaf.

Two section numbers, one rule

Searching for this turns up both 310.12 and 310.15(B)(7), which causes a lot of confusion. They are the same allowance: it lived at 310.15(B)(7) in older editions and moved to 310.12 in the 2020 code. Older editions also expressed it as a percentage table rather than a flat 83%, so a document citing 310.15(B)(7) may give slightly different sizes. Use the edition your jurisdiction has adopted.

The service size itself comes from an Article 220 load calculation — our load calculator does that arithmetic, and panel capacity tells you whether an existing service has room for what you are adding.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 2 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 17 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.