Electrical: Calculators & Wiring Guides
11 calculators · 10 diagrams · 2 guides
Electrical is the trade where a remodel project most often crosses from "I can measure this myself" into "I need to understand this before I talk to anyone." Finishing a basement means new circuits. Converting a garage means a subpanel feed. Adding a hot tub, a well pump, or a workshop at the far end of the yard means a long wire run — and long runs are where wiring plans quietly go wrong, because the wire that is perfectly rated for the load can still starve it of voltage over distance. The calculators in this hub exist for exactly that planning moment: they put real numbers on your circuit before you buy wire or call an electrician, so you can sanity-check a plan or a quote instead of nodding along.
The first tool in the hub is the voltage drop calculator — the long-run workhorse. Enter your circuit, wire gauge, load, and one-way distance and it returns the drop in volts and percent, the voltage your equipment actually receives, and the longest run each gauge supports within the commonly cited 3% recommendation. It is also honest about what that 3% figure is: an Informational Note recommendation in the NEC, not enforceable code — a distinction this hub takes seriously everywhere.
Every electrical tool here follows the same rules: the math comes from published NEC tables with the edition named, results are sizing references only, and every output carries the same advice — electrical work almost always needs a permit and inspection, many states limit what homeowners may legally do themselves, and a licensed electrician and your local Authority Having Jurisdiction have the final word. Wire size, breaker sizing, and project-specific tools for hot tubs, mini-splits, and subpanel feeds are on the roadmap for this hub. Free, no signup.
Electrical calculators
- Voltage Drop CalculatorHow much voltage do you lose over a wire run? Free NEC K-factor calculator for volts and % drop by wire size, length, and load — with the 3% check.
- Wire Size CalculatorWhat gauge wire do you need? Free NEC wire size calculator by amps, wiring method, and run length — with derating, 240.4(D), and appliance presets.
- Breaker Size CalculatorWhat size breaker do you need? Free NEC calculator from amps or watts — 125% continuous rule, standard 240.6(A) ratings, and the wire it must pair with.
- EV Charger Circuit CalculatorWhat wire and breaker does your EV charger need? Free NEC Article 625 calculator by EVSE amps — 125% rule, hardwire vs plug, GFCI, and run length.
- Generator Size CalculatorWhat size generator do you need? Free calculator from the loads you want to back up — running watts, motor-starting surges, and sizing headroom.
- Recessed Lighting CalculatorHow many recessed lights do you need? Free calculator for fixture count, grid spacing, and wall offsets — with a brightness check for your room type.
- Hot Tub Wiring CalculatorWhat size wire and breaker for a hot tub? Free NEC Article 680 calculator — GFCI circuit, wire gauge, the 5-ft disconnect rule, bonding, and burial depth.
- Mini Split Breaker CalculatorWhat size breaker and wire for a mini split or central AC? Free NEC Article 440 calculator from the nameplate MCA and MOP — with real spec examples.
- Tankless Water Heater Breaker CalculatorHow many breakers does an electric tankless water heater need? Free calculator by model — circuits, wire per circuit, total amps, and a service-size check.
- Welder Circuit CalculatorWhat breaker and wire for your welder? Free NEC Article 630 calculator from the nameplate primary amps and duty cycle — the rule most pages skip.
- Garage Subpanel CalculatorWhat wire to a detached garage or shed? Free NEC calculator for the 4-wire feeder — size by amps and distance, plus ground rods, disconnect, and burial.
Guides & references
Electrical · 10 diagrams
- Electrical
Hot tub electrical clearances — the 5-ft disconnect rule and the 3-ft bonding zone
Two distances an inspector checks first: the disconnect at least 5 ft from the water (within sight), and the bonding zone extending 3 ft beyond the spa walls.
- Electrical
Underground feeder burial depths — UF cable 24″, PVC conduit 18″, rigid metal 6″
The wiring method sets the trench depth: 24″ for direct-burial UF, 18″ for PVC conduit, 6″ for rigid metal conduit (NEC Table 300.5).
- Electrical
A 240 V, 4-wire circuit — two hots, a neutral, and the equipment ground
Four wires, three jobs: L1+L2 carry the 240 V load, the neutral serves any 120 V accessories, and the ground clears faults. Spas and EV chargers use dedicated 4-wire (or 3-wire, no-neutral) circuits.
- Electrical
Voltage drop grows with run length — the same wire loses more volts the farther it goes
Same 12 AWG wire, same 15 A load — only the distance changes. Past ~75–100 ft, voltage drop (not ampacity) usually decides the wire size.
- Electrical
Recessed lighting layout — spacing equals half the ceiling height, first row at half the spacing
The even-coverage rule: spacing = ceiling height ÷ 2 (4 ft under an 8-ft ceiling), with the first row a half-spacing (2 ft) off the walls.
- Electrical
Generator sizing — motors demand far more to start than to run
Motors draw 2–3× their running watts for the moment they start. Size to running total + the single largest surge, × 1.25 headroom.
- Electrical
Reading an HVAC nameplate — MCA sizes the wire, MOP sizes the breaker
Two numbers do all the work: wire ≥ MCA, breaker ≤ MOP. On HVAC the breaker may legally exceed the wire's ampacity (Art. 440) — that's normal, not an error.
- Electrical
Inside a breaker panel — main breaker, single-pole vs double-pole circuits
One slot = 120 V. Two slots (double-pole) = 240 V — that's why every big appliance breaker is a double. The main breaker at top is the service rating.
- Electrical
NM-B cable vs THHN in conduit — same copper, different usable ampacity
Same 12 AWG copper: 20 A usable in NM-B (334.80), 25 A as THHN at 75°C terminations — but 240.4(D) caps the breaker at 20 A either way. The THHN headroom is for derating, not bigger breakers.
- Electrical
Sizing an EV charger circuit — the 125% chain from charger to wire
EV charging is continuous by definition: EVSE amps × 125% → breaker → wire. Above 40 A output, hardwired only; plug-in installs need GFCI.
Frequently Asked Questions
What is voltage drop and why does it matter?
Voltage drop is the voltage lost as current pushes through the resistance of the wire itself — the longer and thinner the wire, the more you lose. It matters most on long runs to wells, detached garages, sheds, and hot tubs, and it matters most for motors: a pump starved of voltage draws more current, runs hotter, and fails earlier. The voltage drop calculator computes the loss for your exact circuit and run.
Is the 3% voltage drop limit an actual code requirement?
No. The widely quoted 3% branch-circuit and 5% combined figures come from Informational Notes to NEC 210.19 and 215.2(A), and NEC 90.5(C) states Informational Notes are explanatory, not enforceable. They remain good engineering practice — excessive drop wastes energy and shortens motor life — but in most jurisdictions an inspector cannot fail an installation on voltage drop alone. A few local codes do adopt hard limits, so always check yours.
Can homeowners legally do their own electrical work?
It varies by state and local jurisdiction. Many states let an owner-occupant pull a permit and wire their own single-family home with required inspections — Washington, Minnesota, and Michigan have explicit homeowner provisions. Others are restrictive: Massachusetts largely requires licensed electricians, New York City requires a licensed master electrician, and Hawaii generally requires a contractor even for homeowners. Rentals and multifamily buildings are excluded nearly everywhere. Confirm with your local Authority Having Jurisdiction before planning any work.
What size wire do I need for my circuit?
Two separate checks decide it: the conductor must carry the load safely (ampacity, per NEC Table 310.16 with its correction factors and the breaker rules of 240.4(D)), and it must deliver acceptable voltage over the distance. The wire size calculator runs the full ampacity chain — with appliance presets for dryers, hot tubs, EV chargers and more — and the voltage drop calculator handles the distance side. On short runs ampacity governs; past roughly 75–100 feet, voltage drop usually forces the wire a size or two larger. Verify conductor choices with a licensed electrician.
Which edition of the NEC applies to my project?
Whichever edition your state or local jurisdiction has adopted — typically the 2020 or 2023 NEC as of 2026, with adoption lagging publication by two to four years. The core math on this hub (the K-factor voltage-drop method from Chapter 9 Tables 8–9) is unchanged across editions, but table numbering and some requirements differ between editions, so any code citation should name the edition. Your local Authority Having Jurisdiction can tell you which edition is enforced where you live.