NEC Voltage Drop Calculator

Calculator

Source Voltage

Load Current

One Way Length

Length Unit

Circuit Type

Conductor Material

Conductor Size

Custom Circular Mils

Calculation Method

Custom K Value

Resistance Per 1000 ft

Reactance Per 1000 ft

Power Factor

Design Target

Custom Target Percent

Existing Upstream Drop Percent

Formula Used

Single phase and DC: Voltage drop = 2 × I × L × R ÷ 1000.

Three phase: Voltage drop = 1.732 × I × L × R ÷ 1000.

K-factor resistance: R per 1000 ft = K × 1000 ÷ circular mils.

AC effective impedance: R effective = R × power factor + X × sinθ.

Percent drop: Percent drop = voltage drop ÷ source voltage × 100.

The calculator uses one way length. The return path factor is added by the formula.

How to Use This Calculator

Enter the source voltage and expected load current.
Enter the one way conductor length.
Select DC, single phase, or three phase service.
Choose conductor material and conductor size.
Use K-factor mode for size comparison.
Use direct resistance mode when you know exact cable data.
Select a branch, feeder, combined, or custom target.
Press calculate and review the result above the form.
Download the CSV or PDF report when needed.

Example Data Table

Voltage	Current	Length	Type	Conductor	Approx Drop	Percent
120 V	20 A	100 ft	Single phase	10 AWG copper	4.97 V	4.14%
240 V	40 A	150 ft	Single phase	6 AWG copper	5.90 V	2.46%
480 V	80 A	250 ft	Three phase	2/0 AWG copper	3.36 V	0.70%

Electrical Planning With Voltage Drop

Voltage drop is the reduction in voltage between the source and the load. It happens because every conductor has resistance. Long runs, high current, and small conductors increase the drop. A small drop is normal. A large drop can reduce motor torque, dim lights, heat conductors, and lower equipment performance.

Why NEC Style Targets Matter

The National Electrical Code treats many voltage drop values as design guidance. Common planning targets are three percent for a branch circuit or feeder. Five percent is often used for the combined feeder and branch path. These targets help designers keep utilization voltage close to the equipment rating. Local rules, plans, and product instructions can still be stricter.

How This Calculator Helps

This calculator estimates conductor voltage drop for DC, single phase, and three phase circuits. You can use standard circular mil sizes or enter a custom conductor area. You can use copper, aluminum, custom k values, or direct resistance per thousand feet. A reactance input is also included for AC runs. Power factor is used when reactance matters.

Inputs That Affect Results

Start with system voltage and load current. Enter the one way distance from source to load. Select the circuit type. Choose the material and conductor size. Then select a target percent. Add any upstream voltage drop if you want a combined review. The calculator reports dropped volts, load voltage, percent drop, heat loss, and pass status.

Using Results Safely

The result is an engineering estimate. It is not a substitute for conductor ampacity checks, termination ratings, temperature correction, conduit fill, short circuit study, or local inspection rules. Voltage drop can suggest a larger conductor, but ampacity and overcurrent protection must also be correct. Motors and continuous loads may need extra review.

Practical Design Notes

Keep runs short when possible. Balance three phase loads. Avoid unnecessary splices. Confirm actual load current instead of guessing. Review sensitive equipment before accepting high drop. For larger installations, coordinate the feeder and branch calculations together. Export the report for records. Compare several conductor sizes before final design. For buried or hot locations, resistance may change. Always verify conductor data from the selected cable table before final purchase and installation in the field.

FAQs

What is voltage drop?

Voltage drop is the voltage lost as current moves through a conductor. It increases with longer distance, higher current, and smaller conductor area.

Is NEC voltage drop a hard rule?

Many NEC voltage drop values are informational guidance. Some special systems may have enforceable requirements. Always check the adopted code and local authority.

Why does the calculator use one way length?

Electrical plans usually measure from source to load. The formula adds the return path for DC and single phase circuits using the factor of two.

What does K value mean?

K value represents conductor resistance behavior by material. Copper commonly uses a lower value than aluminum. Lower K means less voltage drop for the same size.

When should I use direct resistance mode?

Use direct resistance mode when a cable datasheet gives ohms per 1000 feet. This can improve results for special conductors or exact manufacturer data.

Does power factor affect voltage drop?

Yes. For AC circuits, power factor affects how resistance and reactance combine. Low power factor can increase the effective voltage drop.

Can this choose the final conductor size?

It suggests a listed size by voltage drop only. Final selection must also satisfy ampacity, temperature, termination, protection, and installation rules.

Why include upstream voltage drop?

Upstream drop helps review the combined feeder and branch path. This is useful when checking a total design target across the full circuit route.