Dwelling Service & Feeder Conductor Calculator
Pick a 100 A to 400 A rating and get the minimum copper and aluminum service or feeder conductor for a single-phase dwelling under the NEC 310.12 83% allowance, alongside the larger conductor ordinary Table 310.16 sizing would require.
How to use this calculator
- Select the service or feeder ampere rating (100 A through 400 A, the range Table 310.12 covers).
- Choose whether it is a service (NEC 310.12(A)) or a feeder (310.12(B)). This changes the citation, not the conductor size.
- Read the minimum copper and aluminum conductor. The comparison row shows the larger size ordinary Table 310.16 sizing would require without the dwelling allowance.
- Confirm the eligibility conditions in the notes: a 120/240 V single-phase system carrying the entire dwelling load, with no derating factors.
NEC reference
Sizing follows NEC 2020 Table 310.12, Single-Phase Dwelling Services and Feeders, and its 83% allowance in 310.12(A) (services) and 310.12(B) (feeders). The conductor ampacities come from Table 310.16, 75 C column, the standard 110.14(C) termination rating for a 100 A to 400 A service. The tool computes each size directly from Table 310.16 (smallest conductor whose 75 C ampacity is at least 83% of the rating), so it always agrees with the published Table 310.12. The same 83% allowance appeared at 310.15(B)(7) in the 2017 edition and the sizes are unchanged in 2023.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
NEC 2020 Table 310.12 conductor sizes
| Service / feeder rating (A) | Copper | Aluminum / copper-clad |
|---|---|---|
| 100 | 4 | 2 |
| 110 | 3 | 1 |
| 125 | 2 | 1/0 |
| 150 | 1 | 2/0 |
| 175 | 1/0 | 3/0 |
| 200 | 2/0 | 4/0 |
| 225 | 3/0 | 250 kcmil |
| 250 | 4/0 | 300 kcmil |
| 300 | 250 kcmil | 350 kcmil |
| 350 | 350 kcmil | 500 kcmil |
| 400 | 400 kcmil | 600 kcmil |
Values are the minimum ungrounded conductor from NEC 2020 Table 310.12. Sizes are AWG unless marked kcmil. The aluminum column also covers copper-clad aluminum. The same sizes hold in the 2017 (310.15(B)(7)) and 2023 editions.
Why 200 amp dwelling services use 2/0 copper
Ordinary Table 310.16 sizing puts a 200 A load on 3/0 copper (200 A at 75 C) or 250 kcmil aluminum (205 A). A dwelling, though, almost never draws its full calculated demand for long, so NEC 310.12 lets the service conductors carry only 83% of the rating. For a 200 A service that is 0.83 times 200 A, or 166 A. The smallest 75 C conductor that meets 166 A is 2/0 copper (175 A) or 4/0 aluminum (180 A) - one trade size smaller each. That single-step reduction is why 2/0 copper and 4/0 aluminum are the everyday answer for a 200 A house, and it repeats at every rating from 100 A to 400 A.
When the allowance does not apply
- Derating is required. The allowance assumes no ambient-temperature correction (30 C) and no more-than-three-conductor adjustment. Conductors in a hot attic or bundled in conduit are sized by the full 310.15 method instead.
- Three-phase or 208Y/120 V. Table 310.12 is single-phase, 120/240 V only.
- Partial-load feeders. A feeder must carry the entire load of the dwelling or dwelling unit; a subpanel that serves only part of the load does not qualify.
- Below 100 A or above 400 A. Outside that band, size by ordinary Table 310.16 methods.
The grounded (neutral) conductor is sized separately: NEC 310.12(D) permits it smaller than the ungrounded conductors, but not below the maximum unbalanced load required by 220.61. This tool sizes only the ungrounded (hot) conductors.
Frequently asked questions
What wire size for a 100 amp service?
For a 120/240 V single-phase dwelling service, NEC Table 310.12 allows 4 AWG copper or 2 AWG aluminum at 100 A (0.83 times 100 A = 83 A; 4 copper is 85 A and 2 aluminum is 90 A at 75 C). Ordinary sizing without the dwelling allowance would need 3 copper or 1 aluminum.
What wire size for a 400 amp service?
400 kcmil copper or 600 kcmil aluminum under Table 310.12 (0.83 times 400 A = 332 A; 400 kcmil copper is 335 A and 600 kcmil aluminum is 340 A). This is a two-size saving over the ordinary 600 kcmil copper the full method would require, which is a large part of why the allowance matters at higher ratings.
Is this the same as the Wire Size or Service Size calculator?
No. The Service Size Calculator sizes service conductors from a computed load by the full 100% method and lists the 310.12 allowance as out of scope; the Wire Size Calculator sizes a general circuit for ampacity and voltage drop. This tool applies the specific dwelling 83% rule keyed on the service or feeder rating.
Related calculators
- Service Size Calculator: full-method service-entrance sizing from a load, plus disconnect and grounding electrode conductor.
- Electrical Load Calculator: NEC 220.82 optional-method dwelling load to pick the service rating this tool sizes for.
- Wire Ampacity Calculator: derated ampacity per Table 310.16 when correction or adjustment factors apply.
- Equipment Grounding Conductor Size: the ground wire sized from the overcurrent device per Table 250.122.
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