Voltage Drop: Why Long Wire Runs Need Bigger Wire

By IES team, Innovative Electrical Services (IES), Auburn, CA · Reviewed November 2026 · 3 min read

Key takeaways

Wire resistance turns some voltage into heat before it reaches the load.
Single-phase estimate: VD = 2 × K × I × L ÷ CM (K ≈ 12.9 for copper, 21.2 for aluminum).
The NEC’s 3% and 5% figures are recommendations in informational notes, not hard rules, with some exceptions for special equipment.
Long runs to outbuildings, pumps and chargers are where we most often upsize wire.

Wire isn’t a perfect conductor. Each foot has a small resistance, and when current flows through it some voltage is used up as heat in the wire itself. On a short kitchen circuit that loss is tiny. On a 200-foot run to a barn, a well pump or a detached garage with an EV charger, it can be enough to make motors run hot and lights dim.

Current flows out and back, so a single-phase circuit has two lengths of wire. Each foot has a little resistance, and that is where the lost volts go.

The formula

For a single-phase circuit, a widely used estimate is:

VD = 2 × K × I × L ÷ CM

VD is the voltage drop in volts.
K is the resistance of a circular-mil-foot of the metal: about 12.9 Ω for copper and 21.2 Ω for aluminum near operating temperature.
I is the load in amps.
L is the one-way length in feet. The 2 accounts for the trip out and back.
CM is the wire’s area in circular mils, listed in NEC Chapter 9, Table 8 (for example, 10,380 for 10 AWG).

For three-phase circuits, the 2 is replaced with √3 (about 1.732).

Try it

A worked example

A 30 A, 240 V load at the end of 150 feet of 10 AWG copper: 2 × 12.9 × 30 × 150 ÷ 10,380 ≈ 11.2 V, or about 4.7%. Moving up to 8 AWG (16,510 cmil) brings it down to about 7 V, roughly 2.9%.

What the code says (and doesn’t)

The NEC mentions voltage drop in informational notes, for example near the branch-circuit and feeder conductor sizing rules. Those notes suggest that about 3% on a branch circuit, and about 5% total for feeder plus branch circuit, gives reasonable efficiency. Informational notes explain; they aren’t enforceable requirements on their own. Some specific equipment, such as fire pumps and certain sensitive electronic equipment, does have mandatory limits in its own articles, and the energy code or the equipment maker’s instructions may also apply.

How we use it

Wire is first sized for ampacity (how much current it can carry safely). Voltage drop is a second check on top of that.
Prefer to skip the math? Our homeowner voltage drop calculator does the same estimate.
For long runs we often upsize one or two sizes, especially for motors, which draw more current and run hotter on low voltage.
When we upsize the wire for voltage drop, the equipment grounding conductor usually has to be increased in proportion too (NEC 250.122(B)).

Sources and further reading

Local rules decide. These articles explain general ideas and point to the code sections we use. They are not legal or engineering advice and don’t replace the code itself. California adopts the NEC with its own amendments, cities and counties can add more, and the local inspector (the AHJ, or authority having jurisdiction) has the final say on any job. Read the code free at NFPA’s free access page.