Voltage Drop Calculator per NEC (NFPA 70) for Residential Installations
The NEC 2026 Edition recommends a maximum 3% voltage drop on branch circuits per Article 210.19(A) Informational Note No. 4 and 5% total (feeder plus branch circuit) per Article 215.2(A) Informational Note No. 2. These are recommendations, not mandatory requirements, but are widely adopted as design practice. Informative Annex D provides worked voltage drop calculation examples.
Quick Reference Table
| NEC 2026 Voltage Drop Guidance — Residential — NEC (NFPA 70) (2026 Edition) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Branch circuit recommendation | 3% maximum (3.6 V at 120 V) | Article 210.19(A), Informational Note No. 4 |
| Feeder recommendation | 3% maximum for feeder portion | Article 215.2(A), Informational Note No. 2 |
| Total combined recommendation | 5% maximum feeder + branch circuit (6 V at 120 V) | Article 215.2(A), Informational Note No. 2 |
| Conductor sizing for voltage drop | May require upsizing beyond ampacity minimums | Article 210.19(A) |
| Worked examples | Sample voltage drop calculations for various circuits | Informative Annex D |
| Sensitive electronic loads | Reduced voltage drop limits for technical equipment | Article 647.4 |
How to Calculate Voltage Drop for Residential Installations
- 1
Identify circuit voltage and type
Determine whether the residential circuit is 120 V single-phase (standard receptacles and lighting), 240 V single-phase (dryer, range, water heater), or 120/240 V split-phase. The voltage level affects both the absolute drop in volts and the percentage calculation.
- 2
Establish load current and wire gauge
Calculate the circuit load current. For NEC residential branch circuits, the conductor is typically 14 AWG (15 A circuit), 12 AWG (20 A circuit), or 10 AWG (30 A circuit). Use the conductor resistance from NEC Chapter 9, Table 8 for the selected gauge.
- 3
Measure the circuit run length
Measure the one-way distance from the panel to the furthest outlet or load. In US residential construction, runs of 50-100 feet are common. Include the distance through the attic, basement, and wall cavities.
- 4
Calculate voltage drop
For single-phase: VD = 2 × I × R × L, where R is conductor resistance in ohms per foot from Chapter 9 Table 8, and L is the one-way length in feet. For 12 AWG copper at 75°C, R = 0.00193 Ω/ft. A 20 A load at 80 feet gives VD = 2 × 20 × 0.00193 × 80 = 6.18 V (5.1% at 120 V).
- 5
Compare against NEC recommendations
Check whether the calculated drop exceeds the 3% branch circuit recommendation (3.6 V at 120 V) or the 5% total recommendation (6 V at 120 V). If exceeded, increase the conductor size — for example, from 12 AWG to 10 AWG — and recalculate.
- 6
Document for permit submission
While the NEC voltage drop values are informational notes (not mandatory code), many AHJs (Authorities Having Jurisdiction) require voltage drop calculations as part of the electrical permit submission for new residential construction.
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Run compliant NEC (NFPA 70) calculations for residential installations — free, instant results with full clause references.
Calculate Voltage Drop NowNEC vs IEC 60364 Cable Sizing Comparison
| Parameter | NEC | IEC 60364 |
|---|---|---|
| Conductor sizing unit | AWG/kcmil | mm² |
| Voltage drop recommendation | 3% branch / 5% total | 4% lighting / 5% other |
| Reference ambient temp | 30°C | 30°C (air), 20°C (ground) |
| Continuous load multiplier | 1.25x required | Not explicitly required |
| Ampacity table | Table 310.16 (60/75/90°C) | Tables B.52.2–B.52.13 |
| Conduit fill limit | 40% for 3+ conductors | Not specified (derating instead) |