What is the voltage drop formula?

For a single-phase circuit, VD = 2 × I × R × L ÷ 1000, where I is the current in amperes, R is the conductor resistance in ohms per 1000 feet, and L is the one-way run length in feet. The 2 accounts for current travelling out on the hot and back on the neutral. For three-phase, replace the 2 with the square root of 3, about 1.732.

Why is there a 2 in the voltage drop formula?

Because current flows through two conductors — out along the ungrounded conductor and back along the neutral or the second hot leg. The physical length of wire in the circuit is twice the one-way run, so the resistance is doubled. In a three-phase circuit the return currents partly cancel, which is why the multiplier drops to the square root of 3.

What is an acceptable voltage drop?

The NEC Informational Notes to 210.19(A) and 215.2(A) recommend a maximum of 3% on a branch circuit and 5% total across the feeder plus branch circuit combined. These are recommendations rather than enforceable requirements in most installations, but exceeding them causes dim lights, slow motor starts, and wasted energy as heat in the wire.

How do I calculate the percentage voltage drop?

Divide the voltage drop by the nominal system voltage and multiply by 100. A 4.8 volt drop on a 240 V circuit is 4.8 ÷ 240 × 100 = 2.0%. Always use the nominal system voltage, not the measured voltage at the panel.

Does voltage drop change the wire size I need?

It can. Ampacity sets the minimum wire size for safety; voltage drop sets a second minimum for performance on long runs. You use whichever is larger. A 20 A circuit needs 12 AWG for ampacity, but past about 100 feet at 120 V you need 10 AWG to stay under 3%.

Voltage Drop Formula

Single phase

VD = 2 × I × R × L ÷ 1000

Three phase

VD = 1.732 × I × R × L ÷ 1000

I = load current in amperes · R = conductor resistance in ohms per 1000 feet · L = one-way run length in feet

What each term means

The formula is Ohm's law, V = I × R, with the resistance worked out for the actual length of wire in the circuit. Everything else is bookkeeping.

The 2. Current leaves on the ungrounded conductor and returns on the neutral or the opposite hot leg, so it passes through twice the one-way length of wire. In a balanced three-phase circuit the return currents partly cancel and the multiplier becomes √3 ≈ 1.732 instead.
I, the current. Use the actual load, not the breaker size. A 50 A breaker feeding a 32 A load drops voltage for 32 A. For continuous loads, use the full continuous current.
R, the resistance. Ohms per 1000 feet, from NEC Chapter 9, Table 8. This is why the formula divides by 1000 — it converts the tabulated per-1000-foot figure to the actual run.
L, the length. One-way distance from the panel to the load, measured along the actual cable route, not in a straight line. Vertical drops, stud bays, and slack at both ends all count.

Conductor Resistance — Ohms per 1000 Feet (NEC Chapter 9, Table 8)

Size	Uncoated copper	Aluminum
14	3.14	—
12	1.98	3.25
10	1.24	2.04
8	0.778	1.28
6	0.491	0.808
4	0.308	0.508
3	0.245	0.403
2	0.194	0.319
1	0.154	0.253
1/0	0.122	0.201
2/0	0.0967	0.159
3/0	0.0766	0.126
4/0	0.0608	0.1
250 kcmil	0.0515	0.0847
350 kcmil	0.0367	0.0605
500 kcmil	0.0258	0.0424

Direct-current resistance at 75°C for stranded conductors. Solid conductors and coated (tinned) copper differ slightly. For alternating-current circuits above about 1/0 in a metal raceway, NEC Chapter 9, Table 9 gives effective impedance values that are marginally higher because of skin effect and reactance.

Example 1 — 20 A branch circuit, 12 AWG copper, 100 feet, 120 V

A 20 amp kitchen circuit run 100 feet from the panel, carrying a full 20 A load.

VD = 2 × 20 × 1.98 × 100 ÷ 1000

VD = 2 × 20 × 0.198 = 7.92 volts

Percentage = 7.92 ÷ 120 × 100 = 6.6%

Fails the 3% recommendation badly. Upsize to 10 AWG (4.96 V, 4.1%) or 8 AWG (3.11 V, 2.6%). At 120 V, long runs are punishing — the same circuit at 240 V would be at 3.3%.

Example 2 — 50 A range circuit, 6 AWG copper, 60 feet, 240 V

An electric range on a 50 A double-pole breaker, 60 feet of 6 AWG copper.

VD = 2 × 50 × 0.491 × 60 ÷ 1000

VD = 2 × 50 × 0.02946 = 2.95 volts

Percentage = 2.95 ÷ 240 × 100 = 1.2%

Comfortably within the 3% recommendation. No upsizing needed.

Example 3 — 100 A sub-panel to a detached garage, 3 AWG copper, 150 feet, 240 V

A feeder to a detached garage, 150 feet one way, carrying a calculated 80 A load.

VD = 2 × 80 × 0.245 × 150 ÷ 1000

VD = 2 × 80 × 0.03675 = 5.88 volts

Percentage = 5.88 ÷ 240 × 100 = 2.5%

Within the 5% feeder-plus-branch allowance with 2.5% left for the branch circuits inside the garage. Note that upsizing here would also require upsizing the equipment grounding conductor under NEC 250.122(B).

Example 4 — three-phase 480 V motor feeder, 2 AWG copper, 400 feet

A 65 A three-phase motor load 400 feet from the distribution panel.

VD = 1.732 × 65 × 0.194 × 400 ÷ 1000

VD = 1.732 × 65 × 0.0776 = 8.74 volts

Percentage = 8.74 ÷ 480 × 100 = 1.8%

Within 3%. Higher voltages carry the same power at lower current, which is exactly why long runs are done at 480 V rather than 240 V.

Rearranging the formula

Most real problems are not "what is the drop" but "how big a wire do I need" or "how far can I run this". Solve for the term you want:

R = (VD × 1000) ÷ (2 × I × L)

Maximum allowable resistance per 1000 feet. Look up the first conductor in the table above with a resistance at or below this figure.

L = (VD × 1000) ÷ (2 × I × R)

Maximum one-way run for a given conductor. This is what the distance charts tabulate.

VD_max = V_nominal × 0.03

The 3% branch-circuit target: 3.6 V at 120 V, 7.2 V at 240 V, 14.4 V at 480 V.

Worked backwards: a 30 A circuit at 240 V running 200 feet. Allowable drop at 3% is 7.2 V. R = (7.2 × 1000) ÷ (2 × 30 × 200) = 0.60 ohms per 1000 feet. The first copper conductor at or below 0.60 is 6 AWG at 0.491. So a 30 A circuit that only needs 10 AWG for ampacity needs 6 AWG at 200 feet.

The 3% and 5% recommendations

Voltage drop appears in the NEC as Informational Notes, not as enforceable rules, in 210.19(A) for branch circuits and 215.2(A) for feeders. The guidance is 3% maximum on the branch circuit and 5% maximum for the feeder and branch circuit combined.

Informational Notes are explicitly not enforceable under NEC 90.5(C), so an inspector cannot fail an installation for a 4% drop on that basis alone. Several other articles do make it mandatory in specific cases — 647.4(D) for sensitive electronic equipment, 695.7 for fire pump feeders, and Article 680 for some pool equipment.

What excessive drop actually costs you: incandescent lamps dim noticeably and lose far more light output than the voltage lost; motors draw higher current at lower voltage and run hotter, shortening their life; electronics with switching supplies compensate by drawing more current, which makes the drop worse; and every volt dropped is power turned into heat inside your walls.

Skip the arithmetic

Our calculator applies this formula along with NEC Table 310.16 ampacity and the 240.4(D) small-conductor limits, and returns the larger of the two requirements.

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Wire size & voltage drop calculator → Voltage drop by distance charts → AWG chart with resistance values → NEC Table 310.16 — conductor ampacity → NEC 250.122(B) — upsizing the ground wire →