Voltage Drop Calculator

Calculation steps

1. Conductor circular mils
   6530
   Ref: AWG conversion (physical quantity)
2. Resistance constant K
   12.9
   Ref: DC resistance at 75°C from NEC Chapter 9, Table 8. Reactance is not included; large feeders need Table 9.
3. Circuit factor
   2 (single-phase)
   2
4. Voltage drop
   VD = 2.000 x K x I x L / CM
   7.90 V
5. Voltage drop percentage
   VD / V x 100
   6.58 %
   Ref: NEC 210.19(A)(1) and 215.2(A)(1) Informational Notes (3% on the feeder or branch circuit, 5% combined)

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The PDF contains your inputs, the result, the verdict, the formula with the values substituted, and the code reference for every step. No signup, no watermark, nothing held back.

What voltage drop measures

Voltage drop is the voltage lost along a conductor because of its resistance. The load at the far end sees less than the source voltage, and the loss grows with current, with the length of the run, and with the resistance of the conductor material.

It is usually expressed as a percentage of the nominal voltage, because that is what tells you whether the equipment at the end still works as intended. Five volts lost on a 480 V feeder is about 1%, while the same five volts on a 120 V branch circuit is over 4%. The absolute loss is identical and the effect is not.

Voltage drop for one circuit in copper and aluminum

120 V, 15 A, 50 ft one-way, single phase, each computed with its own resistance constant K. Aluminum loses noticeably more voltage at the same size, which is why it is normally installed one or two sizes larger. Building-wire aluminum is not tabulated below 12 AWG.

Worked examples

Each example is run by the same engine as the calculator above, so the substitution, the result and the code reference come from the same tables.

Where the 3% and 5% numbers come from

The informational notes to NEC 210.19(A)(1) and 215.2(A)(1) recommend that a branch circuit not exceed 3% and that a feeder not exceed 3%, and that the drop from the service point to the farthest outlet not exceed 5% for power, heating and lighting loads combined. Informational notes are recommendations rather than mandatory rules, but they are written into many specifications and are the benchmark most designers work to.

The 3% figure applies to the feeder and to the branch circuit separately, not only to the branch. Each can sit inside 3% and the two can still total more than 5%, which is why a long feeder is checked on its own and again as part of the complete path.

The formula, and what K is

For a single-phase circuit the tool uses VD = 2 x K x I x L / CM. For a three-phase circuit it uses VD = 1.732 x K x I x L / CM. Here I is the load current, L is the one-way length in feet, and CM is the conductor circular-mil area. For three-phase, enter the line-to-line voltage (208, 480 or 600 V), not the line-to-neutral voltage.

K is the DC resistance constant at 75°C: about 12.9 ohm-cmil/ft for copper and 21.2 ohm-cmil/ft for aluminum, derived from NEC Chapter 9, Table 8. It is not a code value, it shifts with temperature, and it ignores reactance. On 14 to 8 AWG the error is small. On large feeders, especially below unity power factor, Chapter 9 Table 9 effective impedance can show a noticeably larger drop. A close call on a kcmil feeder should be recalculated with Table 9 rather than treated as a pass.

Why the factor of 2 is there

In a single-phase circuit the current leaves the source on one conductor and returns on another, so the resistance of both conductors sits in the loop. That is the factor 2. Three-phase circuits return through the other phases, and the 1.732 factor accounts for that geometry.

This is the most common input error: entering the round-trip length and then letting the formula double it again. The calculator wants the one-way length of the run, measured from the source to the load.

When voltage drop, not ampacity, decides the size

Ampacity sets a floor on conductor size. On long runs voltage drop frequently overrides it. A 120 V circuit at 15 A running 150 ft can miss a 3% target on sizes that carry the current comfortably, and the fix is to go up one or two sizes.

Lower nominal voltages are more sensitive because the same number of lost volts is a larger share of a smaller base. A 120 V circuit reaches the 3% mark at half the voltage loss a 240 V circuit absorbs, 3.6 V against 7.2 V, so the run lengths that trigger upsizing are much shorter.

Common mistakes

• Entering the total circuit length instead of the one-way length.
• Using the copper value of K for an aluminum conductor. Aluminum has roughly 64% more resistance at the same size, and the two constants are not interchangeable.
• Treating 3% as a hard legal limit. It is a recommendation in an informational note, while the enforced limit on the same circuit is the conductor ampacity.
• Checking voltage drop and skipping the ampacity correction, or the other way round. Both apply to the same conductor on the same run.
• Using the K result as the final word on a large feeder. The formula omits power factor and reactance, so it can differ materially from an AC impedance calculation for large feeders.

Frequently asked

What is an acceptable voltage drop?

The widely used benchmark is 3% on a feeder and 3% on a branch circuit, with 5% from the service point to the farthest outlet, from the informational notes to NEC 210.19(A)(1) and 215.2(A)(1). Those are recommendations, not mandatory limits, but specifications and inspectors often expect them to be met. Sensitive equipment may need a tighter target than 3%.

How do I calculate voltage drop by hand?

For single phase, multiply 2 by the resistance constant K (about 12.9 for copper, 21.2 for aluminum) by the load current in amps by the one-way length in feet, then divide by the conductor circular-mil area. For three phase, replace the 2 with 1.732. The result is volts lost; divide by the nominal voltage and multiply by 100 for a percentage.

Does the breaker size affect voltage drop?

Only through the current that actually flows. Voltage drop is driven by load current, not by the rating of the overcurrent device, so a 20 A breaker carrying 8 A produces the voltage drop of 8 A. When the load is not known, sizing with the full circuit rating is the conservative choice.

Why does the calculator ask for a one-way length?

Because the formula already accounts for the return path. In a single-phase circuit the factor 2 covers the round trip, and in a three-phase circuit the 1.732 factor covers the line-to-line geometry. Entering the round-trip distance would double-count the conductor resistance and overstate the drop.

Does voltage drop apply to feeders as well as branch circuits?

Yes, and feeders are usually where it matters most because they are long and carry the whole downstream load. The 5% recommendation covers the feeder and branch circuit together, so a feeder that uses 2.5% of the budget leaves only 2.5% for everything past it.

Related guides and references

• The voltage drop 3% rule
• Wire size for a 100 A sub panel
• Copper vs aluminum wire
• Quick reference (printable PDF)