Voltage Drop: the Formula, the 3% Rule, and a Worked Example
NEC 210.19 IN · Voltage drop · Updated 2026-07-10
Long runs eat voltage. Motors run hot, LED drivers flicker, and the customer calls back. Voltage drop math tells you when to upsize the conductor before the pipe goes in.
The NEC treats voltage drop as a recommendation, not a mandate - but 3% is the number inspectors, engineers, and warranty departments all speak.
The formula
For single-phase circuits: VD = 2 × K × I × L ÷ CM, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum, at 75°C), I is the load current in amps, L is the one-way run length in feet, and CM is the conductor's circular-mil area. For three-phase, replace the 2 with 1.732 (√3) and measure line-to-line.
| Size | Circular mils |
|---|---|
| #14 | 4,110 |
| #12 | 6,530 |
| #10 | 10,380 |
| #8 | 16,510 |
| #6 | 26,240 |
| #4 | 41,740 |
| #2 | 66,360 |
| 1/0 | 105,600 |
Worked example: 240 V, 25 A, 100 ft on #12
- VD = 2 × 12.9 × 25 × 100 ÷ 6,530 = 9.9 V.
- 9.9 ÷ 240 = 4.1% - over the 3% branch-circuit recommendation.
- Upsize to #10: 2 × 12.9 × 25 × 100 ÷ 10,380 = 6.2 V = 2.6%. Pass.
That exact scenario is the app's Voltage Drop screen: it flags the 4.1% as a WARNING against the 3% branch / 5% feeder thresholds, cites NEC Chapter 9 Note 2, and offers the fix as a one-tap suggestion - "Suggest #10 Copper, brings drop ≤ 3%." Reverse mode answers the other question: what's the biggest load, or longest run, this wire size can carry?
Is voltage drop actually code?
In the base NEC it's an Informational Note (210.19 for branch circuits, 215.2 for feeders): 3% for the branch or feeder, 5% combined. Informational Notes aren't enforceable - but there are real exceptions. Fire pumps (695.7) and sensitive-electronics articles impose hard limits, some jurisdictions amend the recommendation into a requirement, and energy codes (like IECC C405) enforce drops on their own authority. When you upsize ungrounded conductors for voltage drop, NEC 250.122(B) makes you upsize the equipment ground proportionally - see the ground wire size guide.
Drop, percentage, threshold, and the fix - in one screen. Forward and reverse voltage-drop modes with cited thresholds and one-tap wire-size suggestions, for copper and aluminum, single- and three-phase. Free on the App Store. Download Conduit Fill & Bending Calc on the App Store.
Frequently asked questions
Is voltage drop an NEC requirement?
Generally no - the 3% branch / 5% total figures are Informational Notes (210.19, 215.2). But fire pumps (695.7), some state and local amendments, and energy codes make it enforceable in specific cases, and equipment warranties often assume it. Treat 3% as the working spec.
What K value do I use for voltage drop?
12.9 for copper and 21.2 for aluminum - approximate resistivity per circular-mil-foot at 75°C, the standard field constants. For exact work, use the actual resistance per 1,000 ft from NEC Chapter 9, Table 8 (the app calculates with those table values).
How far can I run #12 wire on a 20 A, 120 V circuit?
At full 20 A load and 3% (3.6 V), the one-way distance is CM × VD ÷ (2 × K × I) = 6,530 × 3.6 ÷ (2 × 12.9 × 20) ≈ 45 ft. Real circuits rarely pull full load continuously, which is why the rule of thumb is 'upsize past about 75 ft.'
Does voltage drop change my breaker size?
No. Upsizing a conductor for voltage drop doesn't change the overcurrent device - the breaker still protects per the load and termination ratings. It does force a proportionally larger equipment grounding conductor under 250.122(B).