United States NEC workflow
NEC Electrical Calculators for Field Sizing
Wire size, conduit fill, voltage drop, dwelling load and breaker compatibility — each tool shows the governing constraint and hands off to charts, guides and direct answers built from the same engines.
Start with a Calculator
Five field tools that show the governing constraint, not just a wire size.
310.16Wire Size & Ampacity CalculatorSize a conductor from load, distance and voltage, with ambient temperature and conductor-bundling corrections applied and shown.Chapter 9 Table 1Conduit Fill Calculator for Multiple Wire SizesMix conductor sizes and insulation types in one raceway, get a pass or fail against the 40 percent limit, and see the cross-section drawn to scale.210.19(A) INVoltage Drop CalculatorSingle-phase and three-phase drop over distance, checked against the three and five percent recommendations.220.82Residential Load Calculation (NEC 220.82)Optional-method service sizing from floor area, appliances, heating and cooling and EV charging, itemised line by line with every demand factor shown.240.4Breaker & Wire Size Compatibility CheckerA pass or fail check on a breaker, conductor and load combination, with the specific rule that decided it.
Reference Tables
Ampacity, fill and sizing charts derived from the same engines as the calculators.
- NEC 310.16 Ampacity Chart with Derating AppliedWhat ampacity does this conductor actually have?310.16
- Wire Size Amp ChartWhat is the quick ampacity starting point for common wire sizes?310.16
- Ambient Temperature Correction FactorsHow does ambient temperature change allowable ampacity?310.15(B)(1)
- Conduit Fill Limits by Conductor CountWhich fill percentage applies to this raceway?Chapter 9 Table 1
- EMT Conduit Fill ChartHow much area is available in EMT conduit?Chapter 9 Table 4
- PVC Schedule 40 Conduit Fill ChartHow much area is available in PVC Schedule 40 conduit?Chapter 9 Table 4
Direct Answers
Appliance breakers, amp and distance wire sizes, and conduit fill counts — each with worked assumptions and calculator handoff.
- What Size Breaker for a Dryer?Electric dryer breakers follow the nameplate current and the conductor that can carry it — not a universal 30 A rule of thumb.breaker appliance
- What Size Breaker for a Water Heater?Fixed electric water heaters are commonly treated as continuous loads, so the breaker and conductor are sized above the nameplate current.breaker appliance
- What Size Breaker for a Electric Stove?Range and cooktop breakers start from the nameplate or the dwelling demand method — a single plug-in amp number is often the wrong path.breaker appliance
- How Many Wires Fit in 1/2 in EMT?Largest same-size THHN counts for 1/2 in EMT under the Chapter 9 area method, plus the mixed-pull and derating caveats that change a real job.conduit fill
- How Many Wires Fit in 3/4 in EMT?Largest same-size THHN counts for 3/4 in EMT under the Chapter 9 area method, plus the mixed-pull and derating caveats that change a real job.conduit fill
- How Many Wires Fit in 1 in EMT?Largest same-size THHN counts for 1 in EMT under the Chapter 9 area method, plus the mixed-pull and derating caveats that change a real job.conduit fill
- What Size Wire for 15 Amps?For a non-continuous 15 amp load under the default short-run assumptions, start with 14 AWG copper. The calculator also shows 12 AWG aluminum when listed aluminum terminations are available, but aluminum is uncommon on small branch circuits.wire amp
- What Size Wire for 20 Amps?For a non-continuous 20 amp load under the default short-run assumptions, start with 12 AWG copper or 10 AWG aluminum. This is the common branch-circuit page where the 12 AWG copper small-conductor limit is the key constraint.wire amp
- What Size Wire for 25 Amps?For a non-continuous 25 amp load under the default short-run assumptions, start with 10 AWG copper. The calculator also shows 10 AWG aluminum only where listed aluminum terminations are available.wire amp
Guides
Procedures in the order the field check needs them.
- How to Size a Wire to NEC RequirementsHow do I size a conductor for this circuit?7 steps
- Conductor Derating ExplainedWhy did derating increase my conductor size?5 steps
- How to Calculate Conduit FillHow do I calculate fill when the pull has mixed wire sizes?5 steps
- How to Check Voltage DropWhen does distance force a larger conductor?5 steps
- Residential Load Calculation 220.82How does the optional dwelling load calculation work?5 steps
Glossary
Terms that directly change calculator results.
- AmpacityThe current a conductor can carry continuously in its installed conditions, and why the tabulated figure is rarely the usable one.310.16
- DeratingDerating is the bridge between a clean ampacity table and the raceway, attic, cable tray or junction box the conductor actually lives in.310.15(B)(1)
- Continuous loadContinuous load changes sizing current for conductors and breakers, while voltage-drop math still follows the current that actually flows.210.19(A)(1)
- Terminal temperature ratingTerminal rating is the reason a 90 C insulated conductor often cannot deliver a 90 C usable ampacity on ordinary equipment lugs.110.14(C)
- Demand factorDemand factor explains why a service or feeder calculation can be smaller than the sum of every nameplate without ignoring those nameplates.Article 220