Electrical Formula Reference
Every formula used in electrical field work and design — organised by workflow. Each entry gives the formula, what each variable means, typical values, and a link to the free calculator that does the arithmetic. Code citations throughout so you can check any of it against the book.
Required Ampacity (continuous load)
Variables
- continuous load
- — Any load expected to operate for 3 hours or more (210.20(A))
- 1.25
- — The 125% adder — the same rule stated backwards as the '80% rule'
- non-continuous
- — Cycling loads: ranges, dryers, ovens, most receptacle circuits
Normal range / typical values
EV charging is continuous by rule (625.41). A 48 A charger → 60 A of required ampacity
Voltage Drop (circular-mil method)
Variables
- K
- — Resistivity: 12.9 copper, 21.2 aluminum (ohm-cmil per foot)
- I
- — Load in amps actually drawn — not the breaker rating
- L
- — One-way run length in feet; the 2 accounts for the return path
- CM
- — Conductor area in circular mils (Chapter 9, Table 8)
Normal range / typical values
3% branch / 5% total is recommended in Informational Notes to 210.19(A) and 215.2(A) — guidance, not enforceable code
Maximum Run Length at a Drop Target
Variables
- limit%
- — Target drop, normally 3 for a branch circuit
- V
- — Circuit voltage — doubling it doubles the reach
- CM
- — Circular mils; doubling the area doubles the distance
Normal range / typical values
12 AWG copper at a full 20 A: ~45 ft on 120 V, ~91 ft on 240 V
Derated Ampacity
Variables
- base A
- — Table 310.16 at the conductor's insulation rating, normally 90 °C
- ambient factor
- — 310.15(B)(1), 1.00 at 30 °C, 0.91 at 40 °C for 90 °C wire
- bundling factor
- — 310.15(C)(1): 0.80 for 4–6, 0.70 for 7–9, 0.50 for 10–20 conductors
- 75 °C cap
- — 110.14(C) — the termination limit the result is checked against
Normal range / typical values
12 AWG THHN, 40 °C, 6 conductors: 30 × 0.91 × 0.80 = 21.8 A — still covers a 20 A breaker because derating started at 90 °C
Wire Size — the governing test
Variables
- ampacity-governed
- — Smallest conductor whose Table 310.16 value ≥ required ampacity
- drop-governed
- — Smallest conductor whose drop over the run stays within target
Normal range / typical values
Ampacity governs short runs; voltage drop governs long ones. The crossover is often near 100 ft
Dwelling Load — Optional Method (220.82)
Variables
- 3 VA/ft²
- — General lighting unit load; excludes garages, open porches, unfinished space
- 10,000 VA
- — The first tier, taken at 100% under 220.82(B)
- 0.40
- — Demand factor on everything above the first tier
- 220.82(C)
- — A/C and heat pumps at 100%, central electric heat at 65%
Normal range / typical values
2,000 ft² all-electric house: 33,200 VA connected → 24,280 VA calculated → 101.2 A
Existing Load from Measured Demand (220.87)
Variables
- 12-month demand
- — From the utility; 30 days of recorded data where a year is unavailable
- 1.25
- — 220.87's safety factor on measured demand
Normal range / typical values
44 A peak → 55 A existing; add a 32 A charger at 125% (40 A) and a 100 A service still fits
Dwelling Service Conductors — the 83% rule
Variables
- 0.83
- — Table 310.12, for a dwelling service or a feeder carrying the entire dwelling load
- scope
- — Does NOT reach an ordinary subpanel feeder — that uses Table 310.16
Normal range / typical values
200 A × 0.83 = 166 A → 2/0 copper or 4/0 aluminum, one size below straight 310.16 sizing
Generator Sizing
Variables
- margin
- — Typically 25%, so a standby set is not run flat out continuously
- largest motor starting
- — Inrush, commonly 2–4× running watts; 1× with a soft starter
Normal range / typical values
9,200 W of essential loads with a 3× compressor surge → 19,200 W → a 20 kW set
Ohm's Law and the Power Wheel
Variables
- V
- — Volts — electrical potential difference
- I
- — Amperes — current
- R
- — Ohms — resistance
- P
- — Watts — real power
Normal range / typical values
Know any two and the other two follow. P = I²R is why a loose connection heats up
Watts to Amps
Variables
- PF
- — Power factor, 0–1. Use 1.0 for resistive loads, 0.8–0.9 for motors
- 1.732
- — √3 — the line-to-line factor in balanced three-phase
Normal range / typical values
48 A × 240 V = 11,520 W for a Level 2 charger. 10 kW at 480 V 3φ, PF 0.9 → 13.4 A
Apparent, Real & Reactive Power
Variables
- S
- — Apparent power in kVA — what the conductors and transformer must carry
- P
- — Real power in kW — what does work and what the energy meter bills
- Q
- — Reactive power in kVAR — what capacitor correction cancels
Normal range / typical values
Generators and transformers are rated in kVA precisely because PF is the customer's variable
Power Factor Correction
Variables
- Qc
- — Capacitor bank size in kVAR
- PF₁ / PF₂
- — Existing and target power factor
Normal range / typical values
Correcting 0.75 → 0.95 on 100 kW needs about 55 kVAR and cuts the kVA demand charge
Running Cost
Variables
- hours
- — Real run time — thermostatted loads cycle rather than running continuously
- $/kWh
- — Take it from a bill: total charge ÷ kWh delivered, to capture taxes and fees
Normal range / typical values
US residential averages near $0.17/kWh, ranging from about $0.11 to over $0.30 by state
Conduit Fill
Variables
- conductor area
- — Chapter 9, Table 5 — including insulation
- raceway area
- — Chapter 9, Table 4 — 100% internal area by type and trade size
- fill limit
- — Table 1: 53% for one conductor, 31% for two, 40% for three or more
Normal range / typical values
Everything in the raceway counts toward fill, grounds included. Nipples ≤ 24 in go to 60% (Note 4)
Box Fill
Variables
- V_size
- — 314.16(B): 2.00 in³ for 14 AWG, 2.25 for 12, 2.50 for 10, 3.00 for 8
- device yoke
- — Counts as TWO allowances at the largest conductor connected to it
- counts zero
- — Pigtails wholly inside the box, external clamps, wire connectors
Normal range / typical values
Two 12/2 cables + clamps + one duplex = 18.00 in³, which needs a 20.3 in³ box or larger
Equipment Grounding Conductor
Variables
- keyed to
- — The overcurrent device rating — not the conductor size
- 250.122(A)
- — Never required to be larger than the circuit conductors
- 250.122(B)
- — Upsize proportionally when phase conductors are enlarged
- 250.122(C)
- — One EGC shared by several circuits is sized to the largest OCPD
Normal range / typical values
20 A → 12 AWG Cu · 60 A → 10 AWG · 100 A → 8 AWG · 200 A → 6 AWG. There is no 30 A row
Grounding Electrode Conductor
Variables
- 250.66(A)
- — To a rod, pipe or plate — need not exceed 6 AWG copper
- 250.66(B)
- — To a concrete-encased electrode — capped at 4 AWG copper
- 250.66(C)
- — To a ground ring — no larger than the ring conductor itself
- water pipe
- — No cap; the full table size applies
Normal range / typical values
A 200 A service on 2/0 copper: table says 4 AWG, but rods cap it at 6 AWG copper
Loaded Labor Rate
Variables
- burden
- — Payroll tax, workers' comp, insurance, PTO — typically 30–40%
- billable %
- — The strongest lever in the whole calculation; 75% is a fair assumption
- margin
- — Applied by DIVIDING by (1 − margin), not multiplying
Normal range / typical values
A $32/hr wage supports a $105.62/hr bill rate — a 3.30× multiplier, not 2×
Job Price — Margin, not Markup
Variables
- margin
- — A percentage of the selling price — what shows on your accounts
- markup
- — A percentage of cost. A 25% markup is only a 20% margin
Normal range / typical values
$2,979 break-even at 25%: margin gives $3,971, markup gives $3,723 — $248 of lost profit
Sources & standards: NEC (NFPA 70) 2023 — 110.14(C), 210.19(A), 210.20(A), 215.2(A), 220.12, 220.82, 220.87, 240.4(D), 240.6(A), Table 250.66 and 250.66(A)–(C), Table 250.122 and 250.122(A)–(C), Table 310.12, Table 310.16, 310.15(B)(1), 310.15(C)(1), 314.16 and 314.16(B), Chapter 9 Tables 1, 4, 5 and 8, 625.41, Articles 445 and 702. The 3% and 5% voltage-drop figures come from Informational Notes and are recommendations rather than enforceable requirements. Pricing formulas are standard contracting practice, not code. Local amendments override the model code, and a licensed electrician plus the AHJ have final say on anything installed.
Let the calculator do the arithmetic
Every formula on this page has a free interactive calculator — and they all read the same NEC tables.