Each case lists where its expected numbers come from. Code examples, such as the dwelling calculations in NEC Informative Annex D, are used as numbers with a citation; we do not reproduce the code text. Where no published example exists for a simple formula, the case is a hand calculation with every step written out, so anyone can check it.

Residential Load Calculator

Case and sourceExpectedOur resultStatus
Example D1(a), one-family dwelling, standard method (2023) · NEC 2023NFPA 70 (National Electrical Code), Informative Annex D, Example D1(a), 2023 edition
Inputs
  • Floor area (outside dimensions, habitable): 1,500 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 12 kW
  • Clothes dryer: 5.5 kW
  • Water heater: 0 kW
  • Dishwasher: 0 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 0 kVA
  • Electric heat: None
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Standard (Part III)
18,600 VA · 78 A18,600 VA · 78 A✓ pass
Example D1(a), one-family dwelling, standard method (2026) · NEC 2026NFPA 70 (National Electrical Code), Informative Annex D, Example D1(a), 2026 edition (balloted revision)
Inputs
  • Floor area (outside dimensions, habitable): 1,500 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 12 kW
  • Clothes dryer: 5.5 kW
  • Water heater: 0 kW
  • Dishwasher: 0 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 0 kVA
  • Electric heat: None
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Standard (Part III)
18,075 VA · 75 A18,075 VA · 75 A✓ pass
Example D2(a), optional method, heating larger than A/C (2023) · NEC 2023NFPA 70 (National Electrical Code), Informative Annex D, Example D2(a), 2023 edition
Inputs
  • Floor area (outside dimensions, habitable): 1,500 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 12 kW
  • Clothes dryer: 5 kW
  • Water heater: 2.5 kW
  • Dishwasher: 1.2 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 1.38 kVA
  • Electric heat: Space heat, 4+ units
  • Electric heat: 9 kW
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Optional (one dwelling)
21,480 VA · 90 A21,480 VA · 90 A✓ pass
Example D2(a), optional method, heating larger than A/C (2026) · NEC 2026NFPA 70 (National Electrical Code), Informative Annex D, Example D2(a), 2026 edition (balloted revision)
Inputs
  • Floor area (outside dimensions, habitable): 1,500 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 12 kW
  • Clothes dryer: 5 kW
  • Water heater: 2.5 kW
  • Dishwasher: 1.2 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 1.38 kVA
  • Electric heat: Space heat, 4+ units
  • Electric heat: 9 kW
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Optional (one dwelling)
19,680 VA · 82 A19,680 VA · 82 A✓ pass
Example D2(b), optional method, A/C larger than heating (2023) · NEC 2023NFPA 70 (National Electrical Code), Informative Annex D, Example D2(b), 2023 edition
Inputs
  • Floor area (outside dimensions, habitable): 1,500 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 13.1 kW
  • Clothes dryer: 5 kW
  • Water heater: 4.5 kW
  • Dishwasher: 1.2 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 10.08 kVA
  • Electric heat: Space heat, 1–3 units
  • Electric heat: 1.5 kW
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Optional (one dwelling)
29,200 VA · 122 A29,200 VA · 122 A✓ pass
Example D2(b), optional method, A/C larger than heating (2026) · NEC 2026NFPA 70 (National Electrical Code), Informative Annex D, Example D2(b), 2026 edition (balloted revision)
Inputs
  • Floor area (outside dimensions, habitable): 1,500 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 13.1 kW
  • Clothes dryer: 5 kW
  • Water heater: 4.5 kW
  • Dishwasher: 1.2 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 10.08 kVA
  • Electric heat: Space heat, 1–3 units
  • Electric heat: 1.5 kW
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Optional (one dwelling)
27,400 VA · 114 A27,400 VA · 114 A✓ pass
Example D2(c), optional method, heat pump with supplemental heat (2023) · NEC 2023NFPA 70 (National Electrical Code), Informative Annex D, Example D2(c), 2023 edition
Inputs
  • Floor area (outside dimensions, habitable): 2,000 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 12 kW
  • Clothes dryer: 5 kW
  • Water heater: 4.5 kW
  • Dishwasher: 1.2 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 5.76 kVA
  • Electric heat: Heat-pump supplemental
  • Electric heat: 15 kW
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Optional (one dwelling)
34,790 VA · 145 A34,790 VA · 145 A✓ pass
Example D2(c), optional method, heat pump with supplemental heat (2026) · NEC 2026NFPA 70 (National Electrical Code), Informative Annex D, Example D2(c), 2026 edition (balloted revision)
Inputs
  • Floor area (outside dimensions, habitable): 2,000 ft²
  • Small-appliance circuits: 2
  • Laundry circuits: 1
  • Range or cooking (total): 12 kW
  • Clothes dryer: 5 kW
  • Water heater: 4.5 kW
  • Dishwasher: 1.2 kW
  • Disposal: 0 kW
  • Other fixed appliances (total): 0 kW
  • A/C or heat-pump compressor: 5.76 kVA
  • Electric heat: Heat-pump supplemental
  • Electric heat: 15 kW
  • EV charger (nameplate): 0 kW
  • Service voltage: 120/240 V
  • Method: Optional (one dwelling)
32,790 VA · 137 A32,790 VA · 137 A✓ pass

Wire Size Calculator

Case and sourceExpectedOur resultStatus
Example D7: 175 A dwelling service conductors by the 83 % rule (2023) · NEC 2023NFPA 70 (National Electrical Code), Informative Annex D, Example D7: 175 A × 0.83 = 145.25 A → 1/0 AWG copper
Inputs
  • Circuit: Dwelling service (83 %)
  • Service or main feeder rating: 175 A
  • Wire type: 90 °C: XHHW-2
  • Check voltage drop and upsize: no
1/01/0 AWG Cu · 175 A · 6 AWG EGC✓ pass
Example D7: same service at 49 °C (120 °F) ambient (2026) · NEC 2026NFPA 70 (National Electrical Code), Informative Annex D, Example D7, 2026 edition (balloted revision): 145.25 A ÷ 0.82 → 2/0 AWG copper
Inputs
  • Circuit: Dwelling service (83 %)
  • Service or main feeder rating: 175 A
  • Wire type: 90 °C: XHHW-2
  • Check voltage drop and upsize: no
  • Ambient temperature: 49 °C
  • Unit: °C
2/02/0 AWG Cu · 175 A · 6 AWG EGC✓ pass

Voltage Drop Calculator

Case and sourceExpectedOur resultStatus
40 A over 150 ft of 6 AWG copper at 240 V, single-phase (K method) · NEC 2023Hand calculation: VD = 2 × 12.9 × 40 × 150 / 26,240 cmil = 5.90 V = 2.46 % of 240 V
Inputs
  • System: Single-phase
  • Load current: 40 A
  • Supply voltage: 240 V
  • One-way length: 150 ft
  • Length unit: ft
  • Wire size: 6 AWG
  • Conductor: Copper
5.90 V · 2.46 %5.90 V · 2.46 %✓ pass

Ground Wire Size Calculator

Case and sourceExpectedOur resultStatus
60 A circuit, phase conductors upsized from 6 to 4 AWG copper · NEC 2023Hand calculation, NEC 250.122(B) (2023): 10,380 cmil × 41,740/26,240 = 16,511 cmil, just over 8 AWG (16,510 cmil), so 6 AWG
Inputs
  • Conductor: Equipment ground (EGC)
  • Breaker or fuse rating: 60 A
  • Phase conductors upsized (for voltage drop or future load): yes
  • Minimum size the ampacity required: 6 AWG
  • Phase conductors installed: 4 AWG
6 AWG6 AWG Cu EGC✓ pass

Conduit Fill Calculator

Case and sourceExpectedOur resultStatus
Maximum 12 AWG THHN in 3/4 trade size EMT · NEC 2023Hand calculation from NEC Chapter 9 Tables 1, 4 and 5: 40 % of 0.533 in² = 0.213 in²; ÷ 0.0133 in² = 16
Inputs
  • Raceway: EMT (electrical metallic tubing)
  • Trade size (metric designator): 3/4 (21)
  • Type: THHN / THWN / THWN-2
  • Size: 12 AWG
  • Calculate: Max conductors
16 × 12 AWG3/4 EMT: 16 × 12 AWG THHN✓ pass

Box Fill Calculator

Case and sourceExpectedOur resultStatus
4 × 2-1/8 in square box, six 12 AWG and two devices · NEC 2023Hand calculation from NEC 314.16: 6 × 2.25 + 2 devices × 2 × 2.25 = 22.5 in³ needed; the box gives 30.3 in³
Inputs
  • Box (Table 314.16(A) metal boxes): 4 × 2-1/8 in square (30.3 in³)
  • Plaster ring, extension ring or domed cover: 0 in³
  • 18 AWG conductors: 0
  • 16 AWG conductors: 0
  • 14 AWG conductors: 0
  • 12 AWG conductors: 6
  • 10 AWG conductors: 0
  • 8 AWG conductors: 0
  • 6 AWG conductors: 0
  • Device yokes (gangs): 2
  • Equipment grounding conductors entering: 0
  • How many more of this size fit?: 12 AWG
22.5 in³ · 30.3 in³22.5 in³ needed · 30.3 in³ available✓ pass

Available Fault Current Calculator

Case and sourceExpectedOur resultStatus
75 kVA three-phase transformer, 208 V secondary, 2 %Z, infinite primary · NEC 2023Hand calculation: full-load 75,000/(√3 × 208) = 208.2 A; ÷ 0.02 = 10,409 A
Inputs
  • Source: Transformer
  • Transformer rating: 75 kVA
  • Voltage at the fault: 208 V
  • Nameplate impedance: 2 %Z
  • Conductor length to the fault: 0 ft
10.41 kA10.41 kA available✓ pass

Ohm’s Law Calculator

Case and sourceExpectedOur resultStatus
12 V across 4.7 kΩHand calculation: I = V/R = 12/4700 = 2.553 mA; P = V²/R = 30.6 mW
Inputs
  • I know: Voltage + resistance
  • Voltage: 12 V
  • Resistance: 4.7 kΩ
2.55 mA · 30.6 mWI = 2.55 mA · P = 30.6 mW✓ pass
12 V driving 2 AHand calculation: R = V/I = 6 Ω; P = V·I = 24 W
Inputs
  • I know: Voltage + current
  • Voltage: 12 V
  • Current: 2 A
6 Ω · 24 WR = 6 Ω · P = 24 W✓ pass

Amps to Watts Calculator

Case and sourceExpectedOur resultStatus
15 A at 120 V, single-phase, unity power factorHand calculation: P = V·I·PF = 120 × 15 × 1 = 1,800 W
Inputs
  • System: AC single-phase
  • Current: 15 A
  • Voltage: 120 V
  • Power factor: 1
1,800 WP = 1,800 W (1.8 kW)✓ pass

Watts to Amps Calculator

Case and sourceExpectedOur resultStatus
1,500 W at 120 V, single-phaseHand calculation: I = P/(V·PF) = 1500/120 = 12.5 A
Inputs
  • System: AC single-phase
  • Power: 1,500 W
  • Power unit: W
  • Voltage: 120 V
  • Power factor: 1
12.5 AI = 12.5 A✓ pass

Three-Phase Power Calculator

Case and sourceExpectedOur resultStatus
100 A per line at 480 V, power factor 0.85Hand calculation: P = √3 × 480 × 100 × 0.85 = 70,668 W; S = √3 × 480 × 100 = 83,138 VA
Inputs
  • Calculate: From current
  • Line-to-line voltage: 480 V
  • Line current: 100 A
  • Power factor: 0.85
70.67 kW · 83.14 kVAP = 70.67 kW · S = 83.14 kVA✓ pass

Transformer Calculator

Case and sourceExpectedOur resultStatus
75 kVA three-phase, 480 V to 208 V, full-load currentHand calculation: I = S/(√3·V) = 75,000/(√3 × 480) = 90.21 A and 75,000/(√3 × 208) = 208.2 A
Inputs
  • Calculate: Full-load current
  • Phases: Three-phase
  • Transformer rating: 75 kVA
  • Primary voltage: 480 V
  • Secondary voltage: 208 V
90.21 A · 208.2 A90.21 A primary · 208.2 A secondary✓ pass

Cable Size Calculator

Case and sourceExpectedOur resultStatus
Multicore PVC on tray, 40 °C, 7 circuits bunched, 100 A, three-phase · IEC 60364ABB Electrical installation handbook (6th ed., 2010), pp. 304–305: It ≥ 100/(0.87 × 0.54) = 212.85 A → 95 mm², Iz = 111.81 A
Inputs
  • Supply: Three-phase
  • Design current Ib: 100 A
  • Device rating In: 100 A
  • Installation method (IEC 60364-5-52): E · multicore in free air or on perforated tray
  • Insulation: PVC 70 °C
  • Conductor: Copper
  • Ambient air temperature: 40 °C
  • Circuits grouped together: 7
  • Grouping arrangement: Bunched, or enclosed together
  • Route length: 10 m
95 mm²95 mm² · Iz 111.8 A · 0.10 % drop✓ pass
Multicore PVC on tray, 30 °C, no other circuits, 115 A, three-phase · IEC 60364ABB Electrical installation handbook (6th ed., 2010), p. 306: 35 mm² (126 A)
Inputs
  • Supply: Three-phase
  • Design current Ib: 115 A
  • Device rating In: 115 A
  • Installation method (IEC 60364-5-52): E · multicore in free air or on perforated tray
  • Insulation: PVC 70 °C
  • Conductor: Copper
  • Ambient air temperature: 30 °C
  • Circuits grouped together: 1
  • Route length: 10 m
35 mm²35 mm² · Iz 126 A · 0.32 % drop✓ pass

Earth Fault Loop Impedance Calculator

Case and sourceExpectedOur resultStatus
B16, 77 m of 1.5/1.0 mm² twin and earth, Ze 0.35 Ω · BS 7671IET Wiring Matters, May 2024: Zs = 0.35 + 0.0302 × 1.2 × 77 = 3.14 Ω, over the 2.73 Ω maximum for a B16
Inputs
  • Ze: Measured or declared
  • Ze, external loop impedance: 0.35 Ω
  • Cable: Twin and earth
  • Line conductor: 1.5 mm² / 1 mm² cpc
  • Cable length: 77 m
  • Protective device: Type B MCB or RCBO
  • Device rating In: 16 A
3.14 Ω · 2.73 ΩZs 3.14 Ω > 2.73 Ω · Fail✓ pass
Maximum Zs for a B32 MCB at 0.4 s · BS 7671BS 7671:2018 Regulation 411.4.4 and Table 41.3: Zs ≤ Cmin × U0/Ia = 0.95 × 230/(5 × 32) = 1.37 Ω
Inputs
  • Protective device: Type B MCB or RCBO
  • Device rating In: 32 A
1.37 ΩZs 1.05 Ω ≤ 1.37 Ω · Pass✓ pass

CPC Size Calculator

Case and sourceExpectedOur resultStatus
6 mm² cpc on a BS 88-3 63 A fuse, 320 A for 5 s · BS 7671NICEIC technical article, Professional Electrician (2018): k²S² = 476,100 A²s < I²t = 512,000 A²s, so 6 mm² fails; S = √(320² × 5)/115 = 6.22 mm² → 10 mm²
Inputs
  • Fault current: I know the fault current
  • Earth fault current: 320 A
  • Disconnection: Enter the time from the device curve
  • Disconnection time: 5 s
  • Line conductor: 16 mm²
  • cpc size to check: 6 mm²
6.22 mm² · 10 mm²6.22 mm² minimum → 10 mm² · 6 mm² too small✓ pass

Resistor Color Code Calculator

Case and sourceExpectedOur resultStatus
Brown, black, orange, goldIEC 60062:2016, Table 1 (digits, multipliers and tolerances)
Inputs
  • I want to: Read the bands
  • Number of bands: 4
  • Band 1: Brown
  • Band 2: Black
  • Band 3: Orange
  • Band 4: Gold
10 kΩ · 5 %10 kΩ ±5 %✓ pass
Yellow, violet, red, goldIEC 60062:2016, Table 1
Inputs
  • I want to: Read the bands
  • Number of bands: 4
  • Band 1: Yellow
  • Band 2: Violet
  • Band 3: Red
  • Band 4: Gold
4.7 kΩ4.7 kΩ ±5 %✓ pass

Voltage Divider Calculator

Case and sourceExpectedOur resultStatus
9 V across 10 kΩ over 4.7 kΩHand calculation: Vout = 9 × 4.7/(10 + 4.7) = 2.878 V
Inputs
  • Find: Find Vout
  • Input voltage Vin: 9 V
  • R1 (top): 10 kΩ
  • R2 (bottom): 4.7 kΩ
  • Load on the output (RL): no
2.878 VVout = 2.878 V✓ pass

LED Resistor Calculator

Case and sourceExpectedOur resultStatus
Red LED at 20 mA from 12 VHand calculation: R = (12 − 2.0)/0.020 = 500 Ω; next E12 value up is 560 Ω
Inputs
  • Supply voltage: 12 V
  • LED colour: Red, typical 2.0 V
  • LED current: 20 mA
  • Arrangement: Single LED
500 Ω · 560 ΩR = 560 Ω · ½ W✓ pass

555 Timer Calculator

Case and sourceExpectedOur resultStatus
Astable, R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µFTexas Instruments NE555 datasheet (SLFS022): f = 1/(ln 2 · (R1 + 2R2) · C)
Inputs
  • Mode: Astable
  • R1: 1 kΩ
  • R2: 10 kΩ
  • C: 10 µF
  • Diode across R2: no
6.87 Hz · 52.4 %f = 6.87 Hz · duty 52.4 %✓ pass
Monostable, R = 100 kΩ, C = 10 µFTexas Instruments NE555 datasheet (SLFS022): t = ln 3 · R · C ≈ 1.1RC
Inputs
  • Mode: Monostable
  • R: 100 kΩ
  • C: 10 µF
1.099 st = 1.099 s✓ pass