NEC|National Electrical Code

Voltage Drop Calculator

Size your conductors right the first time. Built on NEC Chapter 9, Table 8 values—no guessing, no callbacks.

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Built by a 25-year journeyman

Conductor Size

Material

Phase

Voltage

One-Way Distance

Load Current

5.79V

(4.83%)

0%3%5%7.5%10%+

Between 3% and 5% — Acceptable for feeders, review for branch circuits

Minimum for 3%:

Voltage at Load:114.2 V

How We Calculate Voltage Drop

This calculator uses the resistance values from NEC Chapter 9, Table 8 for uncoated copper and aluminum conductors. We use AC resistance values in steel conduit for a conservative approach that accounts for real-world installation conditions.

Single-Phase Formula:

Vd = (2 × L × I × R) / 1000

Three-Phase Formula:

Vd = (1.732 × L × I × R) / 1000

Where:

Vd = Voltage drop in volts
L = One-way length of conductor in feet
I = Load current in amperes
R = Resistance per 1,000 feet (from NEC Table 8)

We apply the standard 3% maximum for branch circuits and 5% maximum for feeders + branch combined per NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2.

Voltage Drop: What Every Electrician Should Know

What is voltage drop and why does it matter?

Voltage drop is the reduction in voltage as electricity travels through a conductor. Every wire has resistance, and that resistance converts some electrical energy into heat, lowering the voltage available at the load.

This matters because equipment is designed to operate within specific voltage ranges. Too much voltage drop and motors run hot, lights dim, and sensitive electronics malfunction. In extreme cases, it can cause equipment damage or premature failure.

What's the maximum allowable voltage drop per NEC?

The NEC does not mandate specific voltage drop limits, but it provides recommendations in informational notes:

3% maximum for branch circuits
5% maximum for feeder and branch circuit combined

These are recommendations, not requirements—but inspectors and engineers often treat them as de facto standards. For sensitive equipment, you may want to design for even lower voltage drop.

How do I reduce voltage drop on a long run?

You have a few options:

Upsize the conductor — Larger wire has lower resistance. This is the most common solution.
Increase voltage — If you can run 240V instead of 120V, you cut the current in half, which reduces voltage drop proportionally.
Shorten the run — Relocating the panel closer to the load, if feasible.
Use copper instead of aluminum — Copper has lower resistance than aluminum for the same size.

Does voltage drop apply to DC circuits?

Yes, and DC voltage drop is actually simpler to calculate because there is no reactance component. The same basic formula applies: Vd = (2 × L × I × R) / 1000.

For solar installations, battery systems, and low-voltage DC wiring, voltage drop is often more critical because you are working with lower voltages where even a small drop represents a larger percentage.

What's the difference between branch circuits and feeders?

Branch circuits are the final circuits that connect to outlets, lights, or equipment. They originate at the final overcurrent device (breaker or fuse).

Feeders are the conductors that supply power to branch circuit panels. They run between the service equipment and the final panelboard.

The NEC 5% recommendation is for the combined voltage drop of both feeder and branch circuit together—so if your feeder drops 2%, you only have 3% left for the branch circuit.

How does conduit type affect voltage drop?

For small conductors (10 AWG and smaller), conduit type has negligible effect on voltage drop. The resistance values are essentially the same whether you are in EMT, PVC, or rigid steel.

For larger conductors carrying AC current, steel (magnetic) conduit can increase resistance slightly due to eddy current effects. This calculator uses the steel conduit values as a conservative approach. The difference is typically small—a few percent at most.