A 200-amp house service needs 2/0 copper or 4/0 aluminum wire under the 2020 NEC. Dwelling services can be sized at 83% of the rating, and Table 310.12 lists sizes from 100 A to 400 A. It applies only to single-phase dwelling services and feeders that carry the whole home's load.
| Quick facts | 310.12 dwelling service and feeder sizing |
|---|---|
| Who can use it | One-family dwellings, and each dwelling unit in a two-family or multifamily building |
| System | 120/240 V single-phase services and feeders. Also 208Y/120 V single-phase feeders made of two hots and a neutral. |
| Size range | Services or feeders rated 100 A through 400 A |
| The 83% rule | Conductor ampacity must be at least 83% of the service or feeder rating |
| Must carry | The entire load of the dwelling or dwelling unit |
| When the table works | Only when no temperature correction or adjustment factors apply |
| Neutral | May be smaller than the hots if 220.61 and 230.42 (services) or 215.2 and 220.61 (feeders) are met |
Service wire size lookup (Table 310.12)
Pick the service or feeder rating and the wire metal. The lookup gives the Table 310.12 size and shows the 83% math behind it.
Minimum size: 2/0 AWG copper
83% of 200 A is 166 A. 2/0 AWG copper is rated 175 A in the 75°C column of Table 310.16.
Uses NEC Table 310.12. It applies only to a 120/240-volt single-phase dwelling service, or a feeder carrying the whole dwelling unit's load, with no temperature correction or adjustment factors. Anything else: size from Table 310.16.
An informational note says the table's ratings follow the standard fuse and breaker ratings in 240.6(A).
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Worked example: a 200 A dwelling service
A one-family house has a 200 A, 120/240 V single-phase service carrying the whole load, with no correction or adjustment.
- Check the rule fits: one-family, 120/240 V single-phase, 100–400 A, entire load. Yes.
- Find 83%: 200 A × 0.83 = 166 A.
- Copper, Table 310.16 75°C column: 1/0 is 150 A (too small). 2/0 is 175 A. Use 2/0.
- Aluminum: 3/0 is 155 A (too small). 4/0 is 180 A. Use 4/0.
- Check: Table 310.12's 200 A row matches.
Same service in a hot location
Now the 4/0 aluminum is 90°C XHHW-2 in a 38°C (100°F) ambient. Correction applies, so skip Table 310.12. Section 310.12 lets you correct from the conductor's own temperature rating.
- 4/0 aluminum at 90°C, Table 310.16: 205 A.
- Table 310.15(B)(1) factor for 36–40°C, 90°C column: 0.91.
- 205 A × 0.91 = 186.6 A, more than the 166 A needed. Its 75°C ampacity, 180 A, also beats 166 A.
- Answer: 4/0 aluminum XHHW-2 still works.
Table 310.12 and Table 310.16 sit a few pages apart in the 2020 NEC softbound book.
Can you use 310.12 for feeders?
Yes, if the feeder carries the whole dwelling unit's load, under 310.12(B). Section 310.12(C) adds that a dwelling unit feeder never has to be larger than the service or feeder sizes the 83% rule allows.
- Allowed: a 200 A outdoor meter-main feeding the indoor panel for the whole house. The feeder can be 2/0 copper or 4/0 aluminum.
- Allowed: an apartment fed entirely by a 125 A, 208Y/120 V single-phase feeder (two hots and a neutral). 125 × 0.83 = 103.75 A, so 2 AWG copper (115 A) or 1/0 aluminum (120 A), matching the 125 A row.
- Not allowed: a 100 A garage subpanel feeder, which carries only part of the load. Size it from Table 310.16 at 100 A. With 75°C terminals, that's 3 AWG copper, not the table's 4 AWG.
The service rating comes from the load calculation (see JCR's dwelling service load calculation guide).
Common service conductor exam traps
- Commercial or three-phase services. They don't qualify.
- 208Y/120 V services. The 208Y/120 V sentence covers feeders only, not services.
- Taking 83% of the load. Take 83% of the service or feeder rating.
Table 310.12 is new in 2020, so states on the 2017 NEC won't have it. See each state's edition on JCR's state requirements page.
Service wire size questions
What size wire do I need for a 200 amp service?
For a single-phase dwelling service, 2/0 copper or 4/0 aluminum. That's 83% of 200 A, or 166 A. If correction or adjustment applies, recheck with Table 310.16.
What size wire is needed for a 100 amp service?
Table 310.12 lists 4 AWG copper or 2 AWG aluminum for a 100 A dwelling service. That's also the minimum service disconnect rating for a one-family dwelling under 230.79(C).
What size wire is needed for a 150 amp service?
Table 310.12 lists 1 AWG copper or 2/0 aluminum for a 150 A dwelling service. That's 83% of 150 A, or 124.5 A.
Can I use Table 310.12 for a subpanel feeder?
Only if it carries the whole dwelling unit's load, like a feeder from an outdoor meter-main to the main panel. A subpanel serving part of the house is sized at 100% of its rating from Table 310.16.
Does the 83% rule apply to the neutral?
The neutral may be smaller than the hots under 310.12(D). It must meet 220.61 and 230.42 for services, or 215.2 and 220.61 for feeders. Size it from the calculated neutral load.
What replaced 310.15(B)(7) in the 2020 NEC?
Section 310.12. The 83% rule stayed the same. The 2020 edition added Table 310.12, with sizes from 100 A to 400 A.
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Related guides: Dwelling service load calculation · Grounding electrode conductor size (Table 250.66) · Ampacity derating step by step
More worked examples: voltage drop calculation, working space at service equipment (110.26) and all NEC Answers.
References
- NFPA 70, National Electrical Code, 2020 edition
- 2020 NEC 310.12(A) through (D) and Table 310.12
- 2020 NEC Table 310.16, ampacities
- 2020 NEC Table 310.15(B)(1), ambient temperature correction
- 2020 NEC 230.79(C), one-family service disconnect rating
- 2017 NEC 310.15(B)(7), for the 2020 change
JCR checked every rule and table value above against the 2020 and 2017 NEC in NFPA LiNK on October 14, 2026, and re-checked 310.12(A), Table 310.12 and the Table 310.16 values in the lookup on October 21, 2026.