Calculator 08 / Electrical
Electrical Calculator
Voltage drop rises with current and with distance, and falls as conductor size grows — and copper and aluminum do not use the same constant. Check voltage drop on a circuit run, or get a starting-point wire gauge and breaker size for a residential branch circuit.
How this calculator works
Voltage drop matters most on long runs or heavy loads — the NEC's informational notes (2026 edition: 210.19 and 215.4(A)(2); older editions place them at 210.19(A) and 215.2(A)) suggest about 3% or less on a branch circuit and about 5% for feeder and branch combined, for reasonable equipment performance. Those figures are informational notes, not requirements. The K constants are resistance values at 75°C. Wire sizing here uses the 60°C copper ampacity column, the column NM-B (Romex) cable is limited to; larger circuits are often wired with other cable types, so treat results for big loads with extra caution. The continuous-load adjustment follows the NEC's continuous-load rule (2026 NEC 210.19(A)(1) for conductors and 210.20(A) for overcurrent devices; numbering differs in other editions, and the edition your jurisdiction adopted governs) — the circuit is sized for the noncontinuous load plus 125% of the continuous load (a load expected to run 3 hours or more), or 100% only where the assembly is listed for 100% operation. NEC section numbers change between editions, and your local building department decides which edition applies. Confirm any wire or breaker size with a licensed electrician before you buy or install.
What this does not account for
Wire sizing here is a starting point, not a design. It reads a single ampacity column and applies the continuous-load factor. A real installation has several other adjustments that can all push the required size up, and this tool applies none of them.
| Not applied here | Why it matters |
|---|---|
| Ambient temperature correction | An attic or a boiler room derates the conductor |
| Conductor bundling / conduit fill | More than three current-carrying conductors together derates them all |
| Termination temperature rating | The 60°C column is used throughout; equipment listings can allow otherwise |
| Aluminum conductors | The ampacity table is copper only, even though voltage drop offers aluminum |
| Motor, welder and A/C circuits | These have their own NEC articles and different sizing rules |
| Local amendments | Jurisdictions amend the NEC, and some adopt older editions |
Note the mismatch between the two modes deliberately: voltage drop lets you choose aluminum because the resistance constant is all it needs, but the wire-size mode is copper only and says so in its result label. Aluminum generally needs upsizing and listed terminations, so do not carry a copper answer across to an aluminum run.
Two worked examples
Voltage drop: 12 AWG copper, 100 ft run, 16 A load, 120 V single-phase.
- Circular mils for 12 AWG = 6,530
- VD = (12.9 × 2 × 16 × 100) ÷ 6,530 = 6.32 V
- As a percentage = 6.32 ÷ 120 = 5.3%
That is well past the roughly 3% suggested by an NEC informational note, on an entirely ordinary 20 A circuit. It is why long runs get upsized: the same circuit in 10 AWG drops to about 3.3%, and in 8 AWG to about 2.1%.
Wire size: a 2,400 W continuous load at 240 V.
- Current = 2,400 ÷ 240 = 10 A
- Continuous, so × 1.25 = 12.5 A required
- Smallest 60°C copper conductor at or above that = 14 AWG (15 A)
- Smallest standard breaker at or above that = 15 A
Common residential circuits
For orientation only — the actual requirement comes from the load, the run length and your local code. Conductor sizes below are copper.
| Breaker | Copper conductor | Typically serves |
|---|---|---|
| 15 A | 14 AWG | Lighting, general receptacles |
| 20 A | 12 AWG | Kitchen, bathroom and laundry receptacles |
| 30 A | 10 AWG | Dryer, water heater |
| 40 A | 8 AWG | Range, larger appliances |
| 50 A | 6 AWG | Range, EV charger, small subpanel |
Where circuit sizing goes wrong
- Measuring the run as the crow flies. Voltage drop uses the actual conductor length through the building, up and over every obstruction — commonly half again the straight-line distance.
- Forgetting the return path. The multiplier of 2 in the formula is there because current goes out and comes back. Halving it halves your answer and doubles your problem.
- Sizing the breaker to the wire instead of the load. The breaker protects the conductor. It generally should not exceed the conductor's ampacity; the NEC allows rounding up to the next standard size only in limited cases, so confirm with a licensed electrician.
- Treating the 3% figure as a rule. It appears only in an NEC informational note, not as a requirement — but ignoring it still costs you performance in motors, heaters and long lighting runs.
- Doing this instead of hiring an electrician. Branch-circuit work is licensed and inspected in most places for good reason.
Wire size and voltage drop guide
Not sure whether ampacity or voltage drop should decide your wire size? Our guide to choosing wire size for voltage drop works through a 30 A, 240 V, 100 ft circuit in this calculator and tabulates common circuits at 50, 100 and 150 ft.
Where these numbers come from
Every figure here is an estimate against published copper values. The code below is what an inspector applies in the edition your jurisdiction has adopted, and it shifts with ambient temperature, conduit fill and local amendment.
- NFPA 70, the National Electrical Code — NFPA offers free read-only online access to the NEC (a free NFPA account is required), including the ampacity and derating tables.
- Copper Development Association — Conductor properties and the resistance figures behind any voltage-drop calculation.
- EC&M, "Watch Out for that Voltage Drop" — Trade commentary on the 3% and 5% informational-note figures, and why they are recommendations rather than NEC requirements. Its section numbers follow an older edition of the code.
What counts as a "continuous load"?
Anything expected to run at full load for 3 hours or more without interruption — for example some lighting (more common in commercial buildings than in homes), some HVAC equipment, and loads such as storage water heaters or electric-vehicle chargers, which the NEC treats as continuous under its own sections. Standard receptacle and general-use circuits are usually treated as non-continuous. Check your own circuit with a licensed electrician.
Why does this only cover copper?
Aluminum wiring uses different ampacity tables and requires different terminations and anti-oxidant compound — sizing it correctly needs a dedicated table this tool doesn't include. If you're running aluminum conductors, consult a licensed electrician.
Does the breaker size account for wire ampacity limits?
This tool estimates a breaker size from the load, and the breaker it shows can be larger than the wire ampacity shown beside it, because standard sizes step up in increments. The NEC allows rounding up to the next standard size only in limited cases, so compare the two values and confirm with a licensed electrician or your local building department.
Can I save, share, or print a calculation?
Yes — use the Save, Copy link, and Print buttons under your results. Saved calculations can be grouped into a named project on the My Projects page for a combined job estimate with cost totals. Everything is stored only in your browser, never uploaded — see the Privacy Policy for details.
Estimates only, for planning purposes. Always verify against the current NEC, your local code amendments, and a licensed electrician before any installation.