NEC 310.12 Dwelling Conductor Lookup
NEC 310.12 is the dwelling service and feeder conductor rule that can reduce the ampacity required for qualifying single-phase dwelling services and feeders. Use it only after the dwelling load basis and service or feeder rating are clear.
| Field Item | NEC Reference | Field Meaning |
|---|---|---|
| Single-phase dwelling scope | NEC 310.12 | The dwelling service or feeder must fit the rule scope before the reduced dwelling conductor sizing path is considered. |
| Service conductors | NEC 310.12(A) | Qualifying service conductors supplying the dwelling load can be checked against the 83% ampacity basis instead of treating the full service rating as the required conductor ampacity. |
| Feeder conductors | NEC 310.12(B) | Qualifying feeder conductors can use the dwelling feeder sizing path when they supply the applicable dwelling-unit load and meet the rule conditions. |
| 83% ampacity basis | NEC 310.12(A), 310.12(B) | The required ampacity check can be based on 83% of the service or feeder rating when the installation qualifies. |
| Entire dwelling load | NEC 310.12 scope | The rule is not a blanket reduction for partial-load feeders, detached building feeders, branch circuits, or conductors serving only selected equipment. |
| Conductor insulation suitability | NEC 310.16 Ampacity Table | Before using the table shortcut, confirm the conductor or cable insulation rating and installation conditions are suitable for the selected sizing path. |
| Adjustment and correction | NEC 310.15(B)(1) Temperature Correction, NEC 310.15(C)(1) Ampacity Adjustment | When ambient correction or conductor-count adjustment is required, derating review must stay separate from the 83% dwelling sizing allowance. |
| Downstream checks | NEC 110.14(C) Terminal Temperature Limits, NEC 250.122 Equipment Grounding Conductor Sizing | NEC 310.12 does not finish terminal temperature review, overcurrent protection, equipment grounding conductor sizing, voltage drop, or raceway fill. |
Field Example
Applying the 83% Dwelling Conductor Basis
NEC 310.12 uses the service or feeder rating as the basis when the dwelling installation qualifies. Do not apply the 83% factor to only the calculated load.
Dwelling conductor ampacity basis = service or feeder rating × 83%
Field example: for a qualifying 200A dwelling service, the conductor ampacity basis is 166A before conductor selection and downstream checks. Confirm the dwelling load path with the Residential Load Calculator, then use the Wire Size Calculator or Ampacity Calculator when conductor material, temperature limits, or derating conditions need review.
FIELD WORKFLOW
83% Rule Workflow
Use NEC 310.12 after the Residential Load Calculator or another accepted dwelling load basis establishes the service or feeder rating. The important field step is confirming that the conductor path actually qualifies for the dwelling service or feeder rule.
Establish the load basis. Start with the dwelling calculated load and service or feeder rating before applying the 83% conductor check.
Confirm the scope. Verify whether the service or feeder supplies the dwelling-unit load required for NEC 310.12 use.
Apply the 83% check. Use the reduced dwelling ampacity basis only when the service or feeder rating and conductor path qualify.
Finish the review. Check terminal limits, derating, overcurrent rules, equipment grounding, voltage drop, and conduit fill before treating the conductor as field-ready.
83% RULE SCOPE
83% Rule Scope
NEC 310.12 is a dwelling service and feeder sizing allowance, not a general shortcut for every conductor on a residential job.
Field Translation
Do not apply the 83% rule to every feeder, subpanel, branch circuit, or partial-load conductor without confirming that the service or feeder supplies the qualifying dwelling load.
Use the rule for qualifying dwelling service conductors tied to the applicable service rating.
Use the rule for qualifying dwelling feeder conductors when the feeder supplies the applicable dwelling-unit load and meets the rule conditions.
Do not use it as a blanket reduction for partial-load feeders, detached building feeders, branch circuits, appliance circuits, or selected equipment conductors.
Do not skip downstream checks. The 83% rule does not remove terminal, derating, overcurrent, grounding, voltage-drop, or raceway review.
AMPACITY RELATIONSHIP
310.12 vs 310.16 Ampacity
NEC 310.16 Ampacity Table gives conductor table ampacity based on conductor material, size, insulation temperature rating, and terminal-use context. NEC 310.12 is a dwelling service and feeder sizing allowance that can change the required ampacity basis when the installation qualifies.
The two checks should not be blended into one shortcut. If adjustment or correction is required, NEC 310.15 Ampacity Adjustment and Correction still matters. If the conductor lands on equipment with a lower temperature rating, NEC 110.14(C) Terminal Temperature Limits still need to be checked.
FIELD CHECKS
Dwelling Service and Feeder Checks
Confirm the adopted NEC cycle. Older plans or local amendments may reference a different table layout or Article 310.12 wording.
Check the service or feeder rating first. The 83% comparison is tied to the service or feeder rating, not a random load number.
Confirm the conductor path supplies the qualifying dwelling load. Partial-load conductors need separate review.
Check conductor material, insulation, and terminal limits. NEC 310.12 does not erase conductor suitability or equipment marking requirements.
Keep voltage drop and raceway checks downstream. Long runs and raceway routing still need separate field review.
Calculator Use
TradeHub Calculator Application
TradeHub calculators treat NEC 310.12 as a dwelling service and feeder conductor path only when the service or feeder scope supports the 83% ampacity basis.
Related TradeHub Calculators
RELATED REFERENCES
Related NEC Field References
SOURCE ALIGNMENT
Source Alignment and Use Scope
This field reference is based on NEC 310.12 and related TradeHub source alignment records for dwelling service and feeder conductor sizing, the 83% ampacity basis, qualifying service and feeder scope, and adjustment/correction handoffs. It is for screening and planning only, does not reproduce proprietary NEC table text, and does not approve installations, replace adopted-code review, override local amendments, or replace the Authority Having Jurisdiction. Review the TradeHub Code Citation & Source Log for source alignment records and the TradeHub Methodology page for how field references are scoped.
FAQ
Dwelling Conductor FAQ
What is the NEC 310.12 83% rule?
It is a dwelling service and feeder conductor sizing allowance that permits qualifying conductors to be checked at not less than 83% of the service or feeder rating when the rule scope is met.
Can NEC 310.12 be used for any feeder in a house?
No. The feeder must fit the dwelling feeder scope. Feeders serving only part of the dwelling load, detached structures, appliances, or selected equipment need separate review.
Does NEC 310.12 replace derating or terminal temperature limits?
No. NEC 310.15 adjustment/correction and NEC 110.14(C) terminal temperature limits still need to be checked before the conductor sizing result is trusted.
Does voltage drop change the NEC 310.12 ampacity rule?
Voltage drop does not change the 83% ampacity basis, but long service or feeder runs may still need a separate voltage-drop review. If conductors are upsized for voltage drop, grounding conductor and raceway fill handoffs may also need review.