Breakers
Circuit Breaker Sizing Calculator
Translate loads into standard breaker sizes using continuous-load rules and common NEC sizing ladders.
Open Breaker Sizing ->TestTalkHQ • Electrical Hub
Electrical Calculators and NEC Sizing Tools
Use these calculators to size breakers, wires, conduit, boxes, voltage drop, EV chargers, panels, feeders, motors, and residential loads. Built for practical electrical planning, estimates, and field checks — not textbook fluff.
27Live Calculators
WireAmpacity
ConduitFill limits
LoadService demand
VDVoltage drop
Start Here
For most installs, work in this order: establish realistic loads and continuous/non-continuous factors, pick protective devices that fit NEC sizing rules, size conductors with ampacity and temperature corrections, check voltage drop on longer runs, then verify box fill and conduit fill before rough-in is sealed up.
Main Electrical Calculator Tools
Wire Ampacity
Wire Ampacity Derating Calculator
Apply temperature, bundling, and conductor material corrections before locking in a wire size.
Open Wire Ampacity ->Extension Cords
Extension Cord Voltage Drop Calculator
Check cord AWG and voltage drop for tools, welders, and jobsite runs — separate from branch-circuit VD.
Open Cord VD ->Power Conversion
kW to kVA to Amps Calculator
Convert between kilowatts, kVA, and amps for single-phase and three-phase loads.
Open kW/kVA/Amps ->Generator Fuel
Generator Fuel Runtime Calculator
Estimate fuel burn and tank runtime at a given load — pairs with generator sizing for capacity.
Open Fuel Runtime ->Voltage Drop
Voltage Drop Calculator
Quick-run VD checks for branch circuits with practical assumptions for field verification.
Open Voltage Drop ->Advanced Voltage Drop
Advanced Voltage Drop Calculator
Deeper VD modeling when runs, materials, or parallel paths need more detail than a basic estimate.
Open Advanced VD ->12V DC Automotive
12V DC Wire Gauge Calculator
Accessory copper by voltage drop (amps, one-way feet, 12/24V, 3% default). Not a substitute for the NEC voltage-drop tools above.
Open 12V Wire Gauge ->NEC Box Fill
Box Fill Calculator
Count conductors, devices, and clamps to confirm junction-box volume before drywall and finishes.
Open Box Fill ->NEC Conduit Fill
Conduit Fill Calculator
Cross-check raceway trade sizes against conductor fill rules and practical pulling limits.
Open Conduit Fill ->EV Charging
EV Charger Circuit Sizing Calculator
Size EV supply circuits with breaker, conductor, and realistic voltage-drop targets.
Open EV Sizing ->Feeders
Feeder / Service Load Calculator
Roll feeder loads into demand amps for conductors and protection on larger branch feeder runs.
Open Feeder Load ->Residential Load
Residential Load Calculation Calculator
Optional-method style residential demand math for service sizing sanity checks.
Open Residential Load ->NEC Load Calc
Load Calculation Calculator
General NEC-style load aggregation for planning minimum service and feeder capacity.
Open Load Calc ->Generator Sizing
Generator Sizing Calculator
Estimate standby generator requirements from appliance lists and surge assumptions.
Open Generator Sizing ->Motors
Motor Full Load Amps (FLA) Calculator
FLA, breaker, conductor, and disconnect sizing references from motor nameplate inputs.
Open Motor FLA ->Ohms Law
Ohms Law Power Calculator
Relate volts, amps, ohms, watts, and power factor across DC and AC contexts.
Open Ohms Law ->GEC
Grounding Electrode Conductor Calculator
Size the grounding electrode conductor per NEC Table 250.66 from the largest ungrounded service conductor.
Open GEC Calculator ->Cable Tray
Cable Tray Fill Calculator
Check NEC 392 tray fill for ladder, ventilated trough, and solid-bottom trays.
Open Cable Tray Fill ->Bends
Conduit Bending Calculator
EMT deducts, multipliers, and offsets for common bends before you mark the pipe.
Open Conduit Bending ->EGC
Equipment Grounding Conductor Calculator
Size equipment grounding conductors from OCPD rating using NEC Table 250.122.
Open EGC Calculator ->Motors
Motor Circuit Sizing Calculator
Apply NEC 430 FLA, conductor, and OCPD multipliers for branch-circuit motor sizing.
Open Motor Circuit Sizing ->Motors · Group
Grouped Motor Circuit Sizing Calculator
NEC 430.24 feeder ampacity and 430.53(C) group OCPD for multiple motors on one circuit.
Open Grouped Motor Sizing ->Circuits
Series & Parallel Circuit Calculator
Total resistance, branch current, and voltage drop for series and parallel networks.
Open Series/Parallel ->Fault Current
Short Circuit Current Calculator
Estimate available fault current for equipment AIC and coordination checks.
Open Fault Current ->Solar
Solar Panel Circuit Sizing Calculator
NEC 690 Isc multipliers for PV source and output circuit conductor/OCPD sizing.
Open Solar Circuit Sizing ->Directory · PDF + CSV + JSON
Panel Schedule Builder Calculator
Build a full NEMA odd/even circuit directory with multi-pole spanning and connected-load tally. Export printable PDF, CSV, or JSON — not an NEC demand calculation.
Open Panel Schedule Builder ->Wire Fill
Wire Fill Calculator
Audit a mixed conductor list against a chosen raceway using Chapter 9 Table 1 percentages.
Open Wire Fill ->Companion Guides and Charts
Conduit Bending GuideEMT deducts, multipliers, and saddle constants.
Conduit Bending Layout GuideChoose offset, saddle, rolling, or segmented 90° for the job.
Conduit Bending Layout TroubleshootingDog-legs, short/long bends, wrong roll plane, uneven segments.
Offset vs Rolling OffsetWhen a flat offset is enough — and when you need rolling.
Box Fill Calculations GuideNEC 314.16 volumes and unit counts.Conduit Fill CalculatorEMT Annex C counts and 40% areas.EGC CalculatorEquipment grounding conductor sizes.GEC Chart (250.66)Service electrode conductor sizes.Circuit Breaker Sizing ChartStandard ratings and 125% continuous.Motor Circuit Sizing Chart430.250 FLA and OCPD multipliers.Grouped Motor Circuit Sizing Guide430.24 feeders and 430.53 group protection.Transformer Sizing ChartkVA to full-load amps.
Transformer Sizing TroubleshootingUndersizing, inrush, tap settings, ambient derating.Three-Phase Power ChartkW to amps at common voltages.Short Circuit Current ChartTransformer secondary Isc estimates.Solar Circuit Sizing ChartNEC 690 Isc multipliers.Cable Tray Fill ChartNEC 392 percent fill rules.Feeder & Service Load ChartArticle 220 demand factors.
Feeder Load TroubleshootingDemand errors, continuous 125%, trips, and undersize.
Service Upgrade vs SubpanelGrow the service or add a feeder panel.
Extension Cord Voltage Drop GuideCord AWG, round-trip length, and jobsite VD limits.
Extension Cord TroubleshootingHot cords, bogging tools, trips, and coiled-cord heat.
One Long Cord vs Daisy-ChainingChoose the safer cord setup for long jobsite runs.
Extension Cord Gauge ChartAWG vs amps vs recommended max cord length.
kW / kVA / Amps GuideReal vs apparent power and 1φ/3φ conversion logic.kW / kVA / Amps TroubleshootingPF=1.0 traps, 1φ/3φ mix-ups, generator kVA vs kW.
kW / kVA / Amps ChartConversion tables at common voltages.
Generator Fuel Runtime GuideFuel burn vs load and tank runtime planning.
Generator Sizing TroubleshootingUndersize in practice, surge stall, voltage sag, stacked interlock circuits.
Generator Transfer Switch TroubleshootingBreaker mapping, interlock/backfeed, nuisance trips, L14-30 vs L5-30 plugs.
Generator Fuel Runtime TroubleshootingEarly empty, load-factor mistakes, gauge lies, altitude derate, propane math.
Arc Flash Boundary GuideHow boundary and incident energy results apply on the job.
Arc Flash PPE Category ChartNFPA 70E PPE category overview (educational).
Residential Load Calculation GuideNEC 220.82 optional method — pairs with residential and load-calculation calculators.
Commercial Load Calculation GuideArticle 220 Part III occupancy demand — pairs with the commercial load calculator.
Panel Schedule Builder CalculatorFull NEMA panel directory — PDF, CSV, JSON export.Panel Schedule Builder GuideHeader info, slots, multi-pole, save/load.Panel Schedule Builder TroubleshootingMulti-pole, VA vs demand, JSON errors.Builder vs Calculator — Which to UseSize one circuit, then document the full panel.Panel Schedule GuideNEC 408.4 circuit directories — pairs with the panel schedule calculator.
Sub-Panel Sizing GuideGarage/shop feeders, 4-wire rules, grounding — pairs with the sub-panel calculator.
Wire Fill GuideNEC Chapter 9 Table 1 fill percentages for mixed conductor lists.
EV Charger Circuit GuideNEC 220.57 continuous load, breaker, and wire — pairs with the EVSE calculator.
Common Electrical Workflows
EV charger circuit sizingStart with charger level and continuous-load rules, then verify conductor ampacity and VD on the actual route length.
Detached garage / subpanel planningSize feeder demand first, then feeder conductors and grounding/bonding decisions for the outbuilding run.
Residential service sizingUse load calculators to compare calculated demand against planned service size before upgrading panels or mains.
Long wire run voltage dropAfter ampacity passes, model VD—undersized installs often fail here even when breakers look fine on paper.
Box and conduit fill checksCatch raceway and junction-box violations early while pulls are still accessible and adjustments are cheap.
From the TestTalkHQ shop
Electrical Reference Guide — $12.99
Want NEC ampacity, voltage drop, and conduit fill in one 15-page reference instead of jumping between calculators?
View on Etsy →Practical Notes / FAQ
Do these replace code books?
No. They assist planning and checking calculations—always verify against current NEC, local amendments, and manufacturer instructions.
Why do wire size and breaker size not always match?
Ampacity follows conductor temperature rating and derating factors; breaker sizing follows branch-circuit rules and continuous-load multipliers. They relate but are not interchangeable labels.
Why check voltage drop after ampacity?
Ampacity answers overheating risk first; voltage drop answers performance and equipment tolerance—both must fit the installation.
When should I call a licensed electrician?
For unfamiliar codes, service equipment, inspections, arc-flash exposure, or liability-sensitive work—especially near utility demarcation points.
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