Calculator 08 / Electrical
Electrical Calculator
Voltage drop rises with current and with distance, and falls as conductor size grows — and copper and aluminium 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 recommends keeping voltage drop to 3% or less on a branch circuit (5% total including the feeder) for reasonable equipment performance. Wire sizing here uses the standard 60°C copper ampacity column, which matches common residential NM-B (Romex) at typical 15–200A branch circuit sizes. The continuous-load adjustment follows NEC 210.19(A)/210.20(A) — circuits that run 3 hours or more at full load need 125% of the wire and breaker capacity.
What this does not account for
<strong>Wire sizing here is a starting point, not a design.</strong> 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 3% the NEC suggests, 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 must never exceed the conductor's ampacity after all derating.
- Treating the 3% figure as a rule. It is a recommendation in the NEC, not 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.
Where these numbers come from
Every figure here is an estimate against published copper values. The code below is what an inspector actually applies, and it shifts with ambient temperature, conduit fill and local amendment.
- NFPA 70, the National Electrical Code — NFPA provides free read-only access to the current NEC, including the ampacity and derating tables.
- Copper Development Association — Conductor properties and the resistance figures behind any voltage-drop calculation.
What counts as a "continuous load"?
Anything expected to run at full load for 3 hours or more without interruption — most lighting circuits, some HVAC and water heater circuits. Standard receptacle and general-use circuits are usually treated as non-continuous.
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 recommends a breaker based on the load; it's still on you to confirm the recommended breaker doesn't exceed the ampacity of the wire gauge shown — the two are displayed together so you can check.
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.