Voltage drop is one of those topics that shows up constantly on Journeyman and Master exams and, unlike some of the more obscure code sections, comes up just as often in the real world — long garage feeders, well pumps out at the edge of a property, EV charger circuits run to a detached carport, subpanel extensions in a renovated basement. Get the concept and the formula down and you have a tool you will genuinely use for the rest of your career, not just a fact you memorized for a test. This article builds the whole idea from the ground up: why voltage drop happens and why it matters, the formula itself broken into pieces you can actually picture instead of just plug numbers into, and then six complete worked examples covering a range of realistic situations, including a three-phase case, so you can see the pattern applied in different contexts.
Why Voltage Drop Happens
Every conductor has some resistance, even copper and aluminum, the two metals almost all building wiring is made from. Resistance is not a flaw in the wire — it is a basic physical property of any real conductor, and it means that as current flows down the length of a wire, a small amount of voltage is "used up" pushing that current through the wire's own resistance before the current ever reaches the load at the other end. The longer the wire, the more resistance it presents, and the more current flowing through it, the more voltage gets consumed along the way. That lost voltage is voltage drop: the difference between the voltage available at the source (the panel) and the voltage actually available at the load (the receptacle, the motor, the fixture) after the wire has taken its cut.
A small amount of voltage drop is unavoidable and harmless — it exists on every circuit ever installed. The concern is when the drop becomes large enough to actually matter to the equipment on the receiving end. Motors run hotter and can experience shortened life or nuisance tripping of protection when they are underpowered. Electronic equipment, especially sensitive control or computer equipment, can misbehave or fail to function correctly when supply voltage sags too far below nominal. Incandescent and even some LED lighting can visibly dim. Heating elements deliver less actual heat output than rated, since power delivered to a resistive load falls off faster than voltage does (power is related to the square of voltage, so a modest voltage sag translates into a proportionally bigger loss of heating or lighting output). None of this is usually a safety hazard in the sense of an immediate shock or fire risk — it is a performance and equipment-longevity issue — which is exactly why the industry treats voltage drop as a design recommendation rather than a strict, universally-enforced numeric code rule.
Is Voltage Drop a Hard Code Requirement?
This is worth being precise about, because it trips people up. The NEC addresses voltage drop primarily through an informational note — guidance intended to inform good design practice — rather than as a strict, universally enforceable requirement with a hard percentage baked into the main body of the code for every installation. The widely used industry guideline that most electricians design around is keeping voltage drop to roughly 3% or less on a branch circuit and roughly 5% or less for the combined total of a feeder and branch circuit together. Treat that 3%/5% guidance as exactly that: a sound, broadly accepted design target worth hitting on essentially every job as good practice, not a number that automatically fails an inspection on its own in every jurisdiction the way an undersized overcurrent device would. That said, specific projects can and do turn this recommendation into a hard requirement — an engineered spec, a commercial contract document, or a local amendment can explicitly require the 3%/5% figures (or some other specific number) to be met, in which case it stops being a "nice to have" and becomes a contractual or code obligation you must meet for that job. The safest approach, and the one worth carrying into the field and onto the exam, is to always design toward the 3%/5% guideline by default, and to check the specific project documents and local amendments for anything that makes it mandatory rather than advisory.
The Voltage Drop Formula, Piece by Piece
The standard formula used throughout the trade is:
VD = (2 × K × L × I) ÷ CM
That looks dense until you break it into its five pieces:
- VD — voltage drop, in volts. This is the answer you are solving for: how many volts are lost between the source and the load.
- 2 — this factor accounts for the fact that current has to travel the length of the circuit twice: once out to the load through the "hot" conductor, and once back through the neutral or the other current-carrying conductor to complete the circuit. If you only counted the one-way length, you would understate the drop by half.
- K — a constant representing the resistivity of the conductor material. The commonly used values are K = 12.9 for copper and K = 21.2 for aluminum. Notice aluminum's K value is considerably higher than copper's, meaning aluminum is a less efficient conductor for a given size — this is exactly why aluminum conductors are typically upsized relative to an equivalent copper installation to keep both ampacity and voltage drop in check.
- L — the one-way length of the circuit, in feet, from the source to the load. This is measured as the actual route the conductor takes, not the straight-line distance — if the conduit run has to go up, over, and down to get around an obstacle, use that real routed length, not a shortcut distance.
- I — the current the circuit will actually carry, in amps. Use the real expected load current for the calculation, not the wire's or breaker's rating, unless the load is expected to draw at that maximum continuously.
- CM — circular mils, a measure of the conductor's cross-sectional area (explained in detail below). Larger CM values mean a thicker conductor with less resistance, which is why CM sits in the denominator: bigger wire, less voltage drop, for the same length and current.
Read as a whole, the formula is really just Ohm's Law (V = I × R) applied specifically to the conductor itself, with the conductor's resistance expressed through the K, L, and CM terms instead of a flat resistance value, and the factor of 2 built in to account for the round-trip path of current in a typical two-conductor circuit.
What Circular Mils Actually Are
A circular mil is a unit of cross-sectional area used specifically for round wire. One mil is one-thousandth of an inch, and one circular mil is defined as the area of a circle exactly one mil in diameter. Instead of expressing wire cross-sectional area in square inches or square millimeters, the electrical trade in the U.S. traditionally expresses it in circular mils, and every standard AWG (American Wire Gauge) conductor size has a published circular mil value — you look this number up from a standard wire size table rather than calculating it from the wire's diameter by hand for everyday work. The key intuition to hold onto: circular mils go up as wire gauge number goes down. A 12 AWG conductor has fewer circular mils (a smaller cross-section) than a 10 AWG conductor, which has fewer than an 8 AWG conductor, and so on — smaller gauge number always means a physically bigger, lower-resistance wire with a larger circular mil value.
How the Three-Phase Version of the Formula Differs
The formula shown above is built for a single-phase circuit, where current genuinely does make a round trip through two conductors, which is exactly what the factor of 2 represents. A three-phase circuit's conductors relate to each other differently because of the phase relationships between the three legs, and the standard adjustment is to replace the factor of 2 with 1.732 (the square root of 3):
VD (three-phase) = (1.732 × K × L × I) ÷ CM
Everything else about the formula — K, L, I, and CM — works exactly the same way; only the multiplier out front changes to reflect the three-phase relationship instead of the simple round-trip of a single-phase circuit. Forgetting to swap this multiplier on a three-phase problem, or accidentally using 1.732 on a single-phase problem, is one of the single most common mistakes made under exam time pressure.
Solving for Minimum Wire Size When Drop Is Too High
Sometimes the question is not "what is the voltage drop on this circuit" but the reverse: "what is the smallest wire I can use and still stay within an acceptable voltage drop." The same formula can be rearranged to solve for CM directly:
CM = (2 × K × L × I) ÷ VD
(or with 1.732 in place of 2 for a three-phase circuit). Here, VD is not the unknown anymore — it is the target maximum voltage drop you have decided you are willing to accept, in volts, based on the 3%/5% guideline applied to the circuit's voltage. Once you calculate the required CM value, you compare it against a standard wire size table and select the next standard conductor size at or above that calculated CM figure — never round down to the next smaller size, since that would allow more voltage drop than your target permits. This "solve for CM" version of the formula is exactly what you reach for whenever an initial voltage drop calculation comes back too high and you need to figure out how much bigger the conductor needs to be to bring it back under the target.
Calculating Percentage Voltage Drop
Once you know the voltage drop in volts, converting it to a percentage — which is what the 3%/5% guideline is actually expressed in — is a simple final step:
Percentage VD = (VD ÷ source voltage) × 100
This percentage is what you actually compare against the 3% branch circuit / 5% combined guideline, not the raw volts figure by itself, because a 4-volt drop means something very different on a 120-volt circuit than it does on a 480-volt circuit.
Voltage Drop vs. Ampacity — Two Separate Checks on the Same Wire
One of the most persistent points of confusion for people newer to the trade is thinking that sizing a conductor for ampacity and sizing it for voltage drop are the same calculation, or that getting one right automatically takes care of the other. They are not the same thing, and they do not automatically track together. Ampacity sizing is a heat and insulation question: how much current can this conductor carry, in this environment, with this insulation type, without exceeding a temperature the insulation can safely handle over the long term. It depends on things like the conductor's insulation rating, the ambient temperature it is installed in, and how many other current-carrying conductors are bundled alongside it. Voltage drop sizing is an entirely different question about the electrical "cost" of pushing current down a long path: how many volts get consumed by the conductor's own resistance before the current reaches the load, which depends heavily on distance in a way that ampacity does not.
Here is the practical consequence, illustrated already in Example 5 above: a conductor can be perfectly adequate for ampacity — rated to carry the circuit's current all day long without overheating — and still be undersized from a voltage drop standpoint purely because the run is long. Distance is the variable that ampacity calculations mostly ignore (aside from ambient temperature effects) and that voltage drop calculations are built entirely around. This is exactly why an experienced electrician runs both checks separately on any circuit with a long run, a sensitive load, or a project spec that calls for it, rather than assuming that picking a wire size off an ampacity table alone is the end of the sizing decision. On a short run, ampacity is almost always the tighter constraint and voltage drop is nearly irrelevant. On a long run, that relationship frequently flips, and voltage drop becomes the number that actually decides how big the conductor needs to be, exactly as it did with the garage circuit worked through in Examples 1 and 5.
Where Voltage Drop Fits Into the Overall Design Process
In practice, most electricians treat voltage drop as a check performed after an initial wire size has already been chosen for ampacity and code-minimum reasons, rather than as the very first step. The typical sequence looks like this: first, determine the load current and select a conductor that satisfies ampacity requirements (including any needed adjustment or correction factors) and any applicable minimum size rules. Second, take that same conductor size and run it through the voltage drop formula for the circuit's actual length. If the resulting percentage comfortably clears the 3%/5% guideline (or whatever stricter figure a specific project spec calls for), the ampacity-driven wire size stands as the final answer. If it does not, the conductor gets upsized specifically to satisfy voltage drop, as shown in Example 5, and that larger size — not the original ampacity-only size — becomes what actually gets pulled. Understanding this two-step sequence is useful both for the exam, where questions sometimes describe a wire size that is "code minimum" for ampacity but still fails a voltage drop check, and for real design work, where catching a voltage drop problem on paper before conductors are purchased and pulled is far cheaper than discovering it after the fact when equipment underperforms.
Worked Examples
Example 1 — Long detached garage run, single-phase, copper.
A 120-volt, single-phase circuit feeds a detached garage 100 feet from the house panel. The circuit carries 16 amps, wired with 12 AWG copper (CM = 6,530).
VD = (2 × K × L × I) ÷ CM
VD = (2 × 12.9 × 100 × 16) ÷ 6,530
VD = 41,280 ÷ 6,530
VD = 6.32 volts
Percentage VD = (6.32 ÷ 120) × 100 = 5.27%
This exceeds the 3% branch circuit guideline by a wide margin, so the electrician needs to solve for a larger conductor before finalizing this run — see Example 5 below for exactly that follow-up calculation on a very similar scenario.
Example 2 — Well pump circuit, single-phase, copper.
A 240-volt well pump draws 12 amps and sits 250 feet from the panel, wired with 8 AWG copper (CM = 16,510).
VD = (2 × 12.9 × 250 × 12) ÷ 16,510
VD = 77,400 ÷ 16,510
VD = 4.69 volts
Percentage VD = (4.69 ÷ 240) × 100 = 1.95%
This comfortably meets the 3% guideline. Notice how the much higher 240-volt supply voltage keeps the percentage drop low even over a long 250-foot run — voltage drop in raw volts still adds up over distance, but it represents a smaller percentage of a higher supply voltage.
Example 3 — EV charger circuit, single-phase, copper.
A Level 2 EV charger circuit is rated for a continuous 40 amps at 240 volts, run 60 feet from the panel to the charger with 8 AWG copper (CM = 16,510).
VD = (2 × 12.9 × 60 × 40) ÷ 16,510
VD = 61,920 ÷ 16,510
VD = 3.75 volts
Percentage VD = (3.75 ÷ 240) × 100 = 1.56%
Well within the 3% guideline. EV charger circuits carry substantial continuous current, so even though this run is relatively short, it is worth always double-checking voltage drop on these circuits rather than assuming a short run is automatically fine.
Example 4 — Subpanel extension, single-phase, aluminum feeder.
A 120/240-volt subpanel feeder carries 80 amps over 150 feet of 2 AWG aluminum conductor (CM = 66,360, K = 21.2 for aluminum).
VD = (2 × 21.2 × 150 × 80) ÷ 66,360
VD = 508,800 ÷ 66,360
VD = 7.67 volts
Percentage VD = (7.67 ÷ 240) × 100 = 3.19%
This is a feeder, so it is judged against the 5% combined guideline (feeder plus whatever branch circuit drop follows downstream of the subpanel) rather than the tighter 3% branch-circuit-only figure, and on its own this feeder's 3.19% still leaves reasonable room within that combined 5% budget for the branch circuits fed from this subpanel.
Example 5 — Solving for minimum wire size, the garage run from Example 1.
Using the same 120-volt, 16-amp, 100-foot garage circuit from Example 1, find the minimum copper conductor size needed to stay at or under a 3% (3.6-volt) target.
CM = (2 × K × L × I) ÷ VD
CM = (2 × 12.9 × 100 × 16) ÷ 3.6
CM = 41,280 ÷ 3.6
CM = 11,466.7 circular mils required
Checking a standard wire size table, 12 AWG copper (6,530 CM) is not enough, and 10 AWG copper (10,380 CM) is still short of 11,466.7 CM. The next standard size up, 8 AWG copper (16,510 CM), clears the requirement comfortably. Conclusion: upsize this run to 8 AWG copper to meet the 3% voltage drop target, even though 12 AWG would otherwise be an adequate ampacity choice for a 16-amp circuit — this is a case where voltage drop, not ampacity, is what actually drives the wire size decision.
Example 6 — Three-phase commercial feeder.
A 208-volt, three-phase feeder supplies a commercial mechanical room panel 180 feet away, carrying 60 amps through 3 AWG copper (CM = 52,620).
VD = (1.732 × K × L × I) ÷ CM
VD = (1.732 × 12.9 × 180 × 60) ÷ 52,620
VD = 241,302.24 ÷ 52,620
VD = 4.59 volts
Percentage VD = (4.59 ÷ 208) × 100 = 2.21%
Well within the 3% guideline for this feeder. Notice the 1.732 multiplier in place of the single-phase circuit's factor of 2 — using 2 here by mistake would have overstated the drop and could have led to an unnecessarily oversized (and more expensive) conductor.
Example 7 — Combined feeder and branch circuit against the 5% guideline.
A 240-volt residential subpanel feeder runs 120 feet from the main panel carrying 50 amps through 6 AWG copper (CM = 26,240), and a branch circuit from that subpanel runs 40 feet carrying 16 amps through 12 AWG copper (CM = 6,530).
Feeder VD = (2 × 12.9 × 120 × 50) ÷ 26,240 = 154,800 ÷ 26,240 = 5.90 volts. Percentage = (5.90 ÷ 240) × 100 = 2.46%.
Branch VD = (2 × 12.9 × 40 × 16) ÷ 6,530 = 16,512 ÷ 6,530 = 2.53 volts. Percentage = (2.53 ÷ 240) × 100 = 1.05%.
Combined percentage = 2.46% + 1.05% = 3.51%
This stays comfortably under the 5% combined guideline, even though the branch circuit alone (1.05%) is well under 3% and the feeder alone (2.46%) is also under 3% — the combined check is what actually matters when a load is fed through both a feeder and a branch circuit in series, since each stage's drop stacks on top of the other by the time current reaches the final load.
Voltage Drop Reference Table (Selected Copper Conductor CM Values)
| AWG Size | Circular Mils (CM) | Typical Use Context |
|---|---|---|
| 14 AWG | 4,110 | Light-duty branch circuits, short runs only |
| 12 AWG | 6,530 | Standard 20-amp residential branch circuits |
| 10 AWG | 10,380 | 30-amp circuits, moderate-length runs |
| 8 AWG | 16,510 | 40–50 amp circuits, well pumps, EV chargers |
| 6 AWG | 26,240 | Larger appliance circuits, subpanel feeders |
| 4 AWG | 41,740 | Feeders, longer high-current runs |
| 3 AWG | 52,620 | Commercial feeders |
| 2 AWG | 66,360 | Subpanel feeders, larger service runs |
Common Mistakes
How This Changed: NEC 2020 → 2023 → 2026
Voltage drop guidance has remained conceptually stable across recent code cycles as an informational note rather than a blanket enforceable numeric rule, and this article does not have a verified, specific list of subsection-level changes to voltage drop language across the 2020, 2023, and 2026 editions to report. Rather than invent specific renumbering or threshold changes that are not confirmed, the accurate approach is to say that the underlying 3%/5% industry guideline and its status as a recommendation (rather than a blanket hard limit) has been the long-standing, broadly consistent position across recent editions, but you should always verify the exact current wording and placement of any informational note on this topic against the specific edition of the NEC adopted in your jurisdiction, since edition-to-edition renumbering can move where guidance is located even when the underlying substance stays consistent.
Frequently Asked Questions
Is a 3% voltage drop always required by code?
Generally no — the 3% branch circuit / 5% combined figures are a widely used industry design guideline referenced through informational note-style guidance rather than a blanket, universally enforceable numeric requirement written into the main body of the code for every installation. It becomes a hard requirement when a specific project's engineered specification, contract documents, or a local amendment explicitly calls for it.
Why does the formula use "2" for single-phase but "1.732" for three-phase?
The factor of 2 in the single-phase formula represents current making a genuine round trip: out through the hot conductor and back through the neutral or other current-carrying conductor. In a three-phase circuit, the relationship between the three phase conductors is different because of their phase angles relative to each other, and 1.732 (the square root of 3) is the standard multiplier that correctly accounts for that three-phase relationship instead of a simple round trip.
What is the difference between K for copper and K for aluminum, and why does it matter?
K represents the resistivity of the conductor material — copper's is 12.9, aluminum's is 21.2. Aluminum's higher K value means it is a less efficient conductor for a given cross-sectional area, so aluminum conductors generally need to be sized larger than an equivalent copper conductor to deliver comparable ampacity and voltage drop performance over the same run.
Do I calculate voltage drop using the breaker size or the actual expected load current?
Use the actual expected current the load will draw during normal operation, not the breaker or conductor's rated capacity, unless the load genuinely draws at that maximum level continuously (as some continuous loads do). Using the breaker size instead of real load current will overstate the drop and can lead to unnecessarily oversizing the conductor.
What are circular mils and why does the trade use them instead of square inches?
A circular mil is the cross-sectional area of a circle exactly one mil (one-thousandth of an inch) in diameter. It is a traditional unit specific to round wire, and every standard AWG wire size has a published circular mil value in reference tables, which makes circular mils the conventional and convenient unit for this kind of calculation in the U.S. electrical trade rather than converting to square inches or metric units.
If my calculated voltage drop is too high, is upsizing the wire the only fix?
Upsizing the conductor is the most common fix and the one the rearranged CM formula is built to solve for directly, but shortening the run, reducing the load current where practical, or in some feeder situations raising the system voltage are other ways the underlying drop can be reduced. In everyday branch circuit and feeder work, upsizing the wire is usually the simplest and most practical option.
Does voltage drop ever create an actual safety hazard, not just a performance issue?
In most ordinary residential and commercial scenarios, voltage drop is primarily a performance and equipment-longevity concern rather than an immediate shock or fire hazard. That said, severely undersized conductors carrying excessive current for their size can overheat for reasons tied to ampacity and insulation rating rather than voltage drop specifically — which is exactly why ampacity sizing and voltage drop sizing are treated as two separate, complementary calculations, and a conductor sized correctly for one is not automatically sized correctly for the other.
Why does the same conductor produce a lower percentage voltage drop on a 240-volt circuit than on a 120-volt circuit carrying the same current and length?
The raw voltage drop in volts is identical in both cases, since it depends only on K, L, I, and CM, none of which changed. But percentage voltage drop divides that same raw figure by the source voltage, and 240 volts is double 120 volts, so the identical volt-for-volt drop represents half the percentage on the higher-voltage circuit.
Should I include the neutral conductor's length twice or just the hot conductor's length in L?
L represents the one-way length of the circuit route from source to load — the factor of 2 (or 1.732) already built into the front of the formula is what accounts for the return path, so L itself should only be the one-way distance, not doubled again on top of that multiplier.
Do voltage drop calculations change if the circuit uses stranded conductor instead of solid conductor?
For everyday voltage drop work, stranded and solid conductors of the same AWG size are treated as having essentially the same circular mil area and the same standard reference values in the wire size tables, so the calculation itself does not change based on strand construction. The practical difference between stranded and solid conductor mostly comes down to flexibility and ease of pulling through conduit, not the voltage drop math.
How do I check voltage drop when a load is fed through both a feeder and a branch circuit?
Calculate the percentage voltage drop for each segment separately — the feeder from the main panel to the subpanel, and the branch circuit from the subpanel to the final load — then add the two percentages together and compare that combined figure against the 5% guideline, rather than checking each segment only against the tighter 3% figure on its own.
Does a longer wire always mean I need a thicker conductor?
Not automatically — it depends on the combination of length, current, and voltage together. A long run carrying very little current at a high voltage might still produce an acceptable percentage voltage drop on a smaller conductor, while a shorter run carrying heavy current at a low voltage can produce a surprisingly large percentage drop. Always run the actual numbers for the specific combination rather than assuming length alone determines the outcome.
Key Terms
- Voltage drop (VD) — The amount of voltage lost between the source and the load due to the resistance of the conductor carrying the current.
- Circular mil (CM) — A unit of cross-sectional area for round wire, defined as the area of a circle one mil in diameter; the standard way conductor size is expressed in voltage drop calculations.
- K constant — A value representing the resistivity of a conductor material; 12.9 for copper and 21.2 for aluminum in the standard voltage drop formula.
- Percentage voltage drop — Voltage drop expressed as a percentage of the source voltage, which is the figure actually compared against the 3%/5% design guideline.
- Informational note — Guidance in the NEC intended to inform good design practice, as opposed to a strict, universally enforceable requirement in the main body of the code.
- One-way length — The actual routed distance from the source to the load along the real path the conductor takes, used as L in the voltage drop formula.
Keep Practicing
Voltage drop math becomes second nature with repetition. Work through the voltage drop calculations practice test, and check your worked answers against the voltage drop calculator. If Ohm's Law itself still feels shaky as a foundation, start with Ohm's Law and basic circuit theory explained and the related Ohm's Law practice test. Voltage drop also interacts closely with conductor ampacity sizing, so it is worth reviewing branch circuits explained and the ampacity calculator as companion material. For more math-focused practice, browse the electrician math category.
This article is a study aid for exam preparation and general understanding. It is not a substitute for the official NEC or for the specific code edition, local amendments, and project specifications enforced in your jurisdiction. Always verify current guidance and any project-specific voltage drop requirements against your official code book, engineered specifications, and local authority having jurisdiction before performing actual installation work.