Voltage Drop Calculator

The Voltage Drop Formula

Voltage drop happens because every length of wire has some resistance, and Ohm's law (V = I × R) tells us that current flowing through that resistance produces a voltage loss. The longer the run, the more the loss. For single-phase circuits the formula is Vd = 2 × L × I × R ÷ 1000, where L is one-way length in feet, I is current in amps, and R is the resistance of the wire in ohms per 1000 ft (from NEC Chapter 9 Table 8). Three-phase circuits substitute √3 ≈ 1.732 for the 2 because current flows through two conductors at a time rather than making a complete round trip.

NEC Voltage Drop Recommendations

NEC 210.19(A) Informational Note 4 and 215.2(A)(4) Note 2 recommend voltage drop not exceed 3% on branch circuits and 5% on the combination of feeder plus branch. These are informational — the code does not technically require them in most installations — but they are enforced by many jurisdictions, utility interconnection agreements, and best-practice standards. Exceed them and motors stall, LED drivers flicker, and breakers trip on inrush.

Common Scenarios

A 20 A 120 V circuit on 12 AWG copper loses about 6.6% over 100 ft one-way — way above the 3% branch limit. The fix: upsize to 10 AWG (about 4.1% drop, still over), to 8 AWG (2.6% drop, OK), or shorten the run. A 50 A 240 V EV charger on 6 AWG copper at 200 ft drops 4.1% — OK for a feeder+branch combined, borderline for a standalone branch. Go to 4 AWG for comfort.

Copper vs Aluminum

Aluminum has roughly 60% higher resistance per AWG than copper, so a 2/0 aluminum feeder is about equivalent to a 1 AWG copper feeder in voltage drop. Aluminum is cheaper per foot but requires anti-oxidant paste on terminations and CO/ALR-rated devices. For runs over 100 ft, larger aluminum often wins on total installed cost. Copper stays the default for branch circuits and service drops to small loads.

Typical Voltage Drop Reference Values

Calculated from NEC Chapter 9 Table 8 DC resistances for copper, one-way distance on 120 V single-phase circuits unless noted. Use these as a quick sanity check on your calculator result — if you get a wildly different number, check your inputs.

• 14 AWG, 15 A, 50 ft: ~3.3% drop — right at the NEC 3% branch limit.
• 12 AWG, 20 A, 50 ft: ~3.3% — at the limit; acceptable for a standard kitchen or bedroom circuit.
• 12 AWG, 20 A, 100 ft: ~6.6% — exceeds 3%. Upsize to 10 AWG for long branches.
• 10 AWG, 30 A, 100 ft: ~6.2% — upsize to 8 AWG.
• 8 AWG, 40 A, 100 ft: ~5.2% — marginal.
• 6 AWG copper, 50 A EV charger, 150 ft @ 240 V: ~3.1% — fine for a garage subpanel.
• 2 AWG copper, 100 A subpanel feeder, 100 ft @ 240 V: ~1.6% — excellent.
• 4/0 aluminum, 200 A service, 100 ft @ 240 V: ~1.3% — standard modern residential service.

Rule of thumb: one AWG size upgrade (e.g., 12 → 10) cuts voltage drop by ~37%. Copper → aluminum at the same AWG adds ~60%. Three-phase circuits drop ~13% less than single-phase at the same line current.

Frequently Asked Questions

Do I use one-way or round-trip distance?

One-way distance from the panel to the load. The formula has a factor of 2 built in for single-phase round-trip.

Does the NEC enforce 3% voltage drop?

No — it is "recommended" in an Informational Note, not a Rule. Most local jurisdictions still check it on long runs.

What AWG is used for 100 A service?

2 AWG copper or 1/0 aluminum USE/SER, per NEC Table 310.12 residential service conductors. Check voltage drop separately for long runs.

How does temperature affect voltage drop?

NEC Table 8 values are at 75 °C. Hotter conductors have higher resistance; this calculator uses the table value as a standard approximation.

Why does my calculation differ from my meter reading?

Real-world wires have AC impedance (resistance + reactance), temperature variation, and loose terminations. This tool gives a design-stage DC resistance estimate.