The 3% and 5% voltage drop rules are general NEC design recommendations: keep branch-circuit voltage drop within 3% and the feeder-plus-branch-circuit total within 5%. The feeder recommendation also uses 3%, but that does not give you permission to add 3% on the feeder to another 3% on the branch circuit. These recommendations appear in informational notes rather than general mandatory requirements; local amendments, special applications, and equipment instructions can establish enforceable limits.
Voltage Drop Targets for Branch Circuits and Feeders
| Circuit or path | General design recommendation | Equivalent drop at 120 V | Equivalent drop at 240 V | How to apply it |
|---|---|---|---|---|
| Branch circuit | No more than 3% | 3.6 V | 7.2 V | Calculate from the branch-circuit supply point to the load |
| Feeder | No more than 3%, while preserving the total budget | 3.6 V | 7.2 V | Evaluate feeder drop under the relevant design load |
| Feeder plus branch circuit | No more than 5% combined | 6.0 V | 12.0 V | Include both portions of the path supplying that load |
These are general design targets, not universal equipment operating limits or automatic pass/fail criteria for every installation. The percentages reflect the NEC’s longstanding efficiency guidance; the voltage values above are calculated by multiplying the applicable circuit voltage by 0.03 or 0.05.
For a 120 V load supplied through a 120/240 V system, evaluate the voltage available to that 120 V load. Do not add a feeder percentage calculated on a 240 V basis to a branch-circuit percentage calculated on a 120 V basis. Combine compatible voltage drops first, then express the total against the same voltage reference.
Why the General Rules Are Recommendations, Not Universal Requirements
NEC 210.19 contains branch-circuit conductor sizing provisions and an informational note addressing voltage drop. NEC 215.2 contains corresponding feeder guidance. Under NEC 90.5(C), informational notes explain the Code but are not themselves enforceable requirements. That distinction is why exceeding the general 3% recommendation does not, by itself, establish a violation of that informational note.
The recommendations still matter. A conductor can have sufficient ampacity yet produce an undesirable voltage drop over a long run. Ampacity addresses current-carrying capability under specified conditions; voltage-drop calculations address the voltage lost between the supply and the load. Passing one check does not establish that the other is satisfactory.
Separate requirements can change the decision. Manufacturer instructions associated with listed equipment may require a minimum supply voltage, and particular NEC applications have mandatory voltage-drop provisions. Local amendments can also make a voltage-drop limit enforceable: New York City’s published amendment to 210.19(A), for example, specifies a maximum 5% combined drop from the service point to the farthest outlet and directs applicable projects to its energy-code requirements.
Use the Adopted Edition, Not Just the Latest Edition
As of October 11, 2026, NFPA identifies the 2026 NEC as its current edition. That does not mean your jurisdiction has adopted it; states and municipalities may enforce earlier editions with local amendments.
Use NFPA’s official NEC information and access instructions to locate the relevant edition. Then confirm the adopted edition, amendments, and applicable energy-code provisions with the local authority having jurisdiction, or AHJ. Check the voltage-drop informational notes associated with 210.19 and 215.2 in that edition rather than assuming subsection and note numbering remain unchanged.
Allocate the 5% Budget Across the Entire Path
A practical starting allocation is 2% for the feeder and 3% for the branch circuit. It is not a mandatory split. A longer feeder may need more of the budget, while a long branch circuit may justify keeping feeder drop lower. Both portions must still be evaluated together.
The following are hypothetical allocations using a common voltage reference:
| Feeder drop | Branch-circuit drop | Combined drop | Relationship to the general recommendations |
|---|---|---|---|
| 2% | 3% | 5% | Meets both individual targets and the combined target |
| 3% | 2% | 5% | Meets both individual targets and the combined target |
| 1% | 3% | 4% | Leaves 1 percentage point of combined margin |
| 3% | 3% | 6% | Meets each individual 3% target but exceeds the combined 5% target |
For a hypothetical 120 V path, a 2% feeder drop equals 2.4 V and a 3% branch drop equals 3.6 V:
[
\Delta V_{\text{total}}=2.4+3.6=6.0\text{ V}
]
[
VD_{\text{total}}=\frac{6.0}{120}\times100=5.0%
]
Calculate feeder drop using the load carried by the feeder, not automatically the current of one downstream branch circuit. Where loads are distributed along a run, calculate the drop segment by segment using the current each segment carries. AC systems with significant phase-angle differences require a more detailed treatment than simply adding scalar drops.
Choose a Formula That Matches the Circuit
For a two-wire single-phase circuit, a common resistance-based estimate is:
[
\Delta V=\frac{2KIL}{CM}
]
[
VD%=\frac{\Delta V}{V_{\text{reference}}}\times100
]
Where:
- (\Delta V) = calculated voltage drop, in volts.
- (K) = conductor resistivity constant, in (\Omega\cdot\text{cmil}/\text{ft}).
- (I) = load current, in amperes.
- (L) = one-way conductor length, in feet.
- (CM) = conductor area, in circular mils.
- (V_{\text{reference}}) = applicable circuit voltage.
The factor of 2 accounts for the outgoing and return conductors. With this formula, enter the one-way length; entering the round-trip length would count the return path twice. Common approximate constants are 12.9 for copper and 21.2 for aluminum, based on resistance at a conductor temperature of approximately 75°C. These are calculation assumptions, not measurements of your installed conductors.
Resistance Estimate Versus AC Impedance Calculation
| Method | Appropriate use | Important limitation |
|---|---|---|
| Single-phase (2KIL/CM) | Transparent resistance-based estimate for a two-wire circuit | Does not explicitly model reactance or power factor |
| Balanced three-phase (\sqrt{3}KIL/CM) | Resistance-based estimate for a balanced three-phase load | Use line-to-line voltage for percentage drop; do not apply it indiscriminately to unbalanced loads |
| AC impedance method | Loads or installations where reactance and power factor materially affect the result | Requires suitable resistance, reactance, and load data |
For AC calculations requiring greater detail, the usual approximation includes (R\cos\phi+X\sin\phi), where (R) is conductor resistance, (X) is reactance, and (\cos\phi) is load power factor. Manufacturer technical data or the applicable NEC Chapter 9 tables help establish the conductor values and installation assumptions.
Worked Example: 100 Feet at 16 Amps
This hypothetical example illustrates the calculation; it is not a conductor selection for a specific installation.
Assume a 120 V, two-wire single-phase branch circuit supplying a steady 16 A load. The outgoing and return conductors have the same material and size, and the resistance-only approximation is used.
Inputs and Assumptions
| Input | Value | Basis |
|---|---|---|
| Circuit voltage | 120 V | Assumed voltage at the branch-circuit supply point |
| Load current | 16 A | Assumed operating load, not a breaker rating |
| One-way conductor length | 100 ft | Assumed routed length from supply point to load |
| Initial conductor | 12 AWG copper | Hypothetical starting size |
| Conductor area | 6,530 cmil | Nominal 12 AWG area |
| Copper constant (K) | 12.9 (\Omega\cdot\text{cmil}/\text{ft}) | Approximate 75°C resistance basis |
| Power factor and reactance | Unity power factor; reactance neglected | Simplified calculation assumptions |
The nominal conductor area is supported by manufacturer wire-property data, and the copper constant follows IAEI’s published calculation method. Actual resistance depends on conductor construction and temperature.
Calculate and Interpret the Result
1. Calculate voltage drop:
[
\Delta V=\frac{2\times12.9\times16\times100}{6{,}530}
=6.32\text{ V}
]
2. Convert volts to a percentage:
[
VD%=\frac{6.32}{120}\times100=5.27%
]
3. Estimate the voltage at the load:
[
V_{\text{load}}\approx120-6.32=113.68\text{ V}
]
4. Compare with the branch-circuit target:
[
\Delta V_{\text{target}}=120\times0.03=3.60\text{ V}
]
Under these assumptions, 12 AWG produces approximately 5.27% branch-circuit drop, exceeding the general 3% recommendation before any upstream feeder drop is included. The estimated 113.68 V load voltage assumes the branch-circuit supply point remains at 120 V; it is not a prediction of actual utility voltage or equipment performance.
Compare Larger Conductors
Keeping the voltage, current, length, and copper constant unchanged gives:
| Copper conductor | Nominal area | Calculated drop | Drop at 120 V | Comparison with 3% branch target |
|---|---|---|---|---|
| 12 AWG | 6,530 cmil | 6.32 V | 5.27% | Above target |
| 10 AWG | 10,380 cmil | 3.98 V | 3.31% | Above target |
| 8 AWG | 16,510 cmil | 2.50 V | 2.08% | Below target |
The conductor areas come from manufacturer reference data; the voltage-drop results are calculated using the stated hypothetical assumptions.
You can also calculate the minimum area required by this model:
[
CM_{\text{required}}
=\frac{2KIL}{\Delta V_{\text{target}}}
=\frac{2\times12.9\times16\times100}{3.60}
\approx11{,}467\text{ cmil}
]
That exceeds the nominal area of 10 AWG. Of the sizes compared, 8 AWG is the first to meet the 3% branch target under this model. It is not automatically the final approved conductor: ampacity, continuous-load provisions, correction and adjustment factors, terminations, overcurrent protection, grounding, and installation requirements still need separate review.
Use the voltage drop calculator to evaluate the same scenario, checking its length convention, temperature basis, and resistance or impedance assumptions before comparing results. A calculator using different conductor data may legitimately produce a slightly different answer.
Common Mistakes That Change the Decision
A branch circuit below 3% can still exceed the combined 5% target once feeder drop is included. Conversely, satisfying the general voltage-drop targets does not establish that conductor ampacity, equipment voltage requirements, or local code provisions are satisfied.
Several input errors can overwhelm otherwise correct arithmetic:
- Doubling the length twice. Use one-way feet in a formula that already includes the factor of 2.
- Mixing feet and meters. The units of (K), length, and conductor area must match.
- Entering kcmil as cmil. One kcmil equals 1,000 circular mils.
- Using breaker amperage as operating current. Calculate for the relevant load condition; a protective-device rating is not a measurement of load current.
- Treating the temperature assumption as a terminal rating. Resistance temperature and permitted ampacity are related but distinct checks.
- Ignoring startup conditions. A steady-state calculation does not establish voltage performance during motor starting or other high-current events.
- Treating larger wire as approval to increase breaker size. Voltage-drop improvement does not replace overcurrent-protection requirements.
Voltage Drop Design Checklist
Use this checklist when documenting the calculation and reviewing the proposed design:
- Confirm the locally adopted NEC edition and amendments with the AHJ.
- Check equipment instructions for minimum voltage or application-specific limits.
- Identify the system configuration and the correct voltage reference.
- Record the relevant load current and one-way routed conductor length.
- Identify conductor material, size, construction, and temperature basis.
- Use a formula appropriate to single-phase, balanced three-phase, or a more detailed AC calculation.
- Include feeder and branch-circuit drop on a consistent voltage basis.
- Compare conductor options without assuming voltage-drop compliance establishes ampacity.
- Review protection, terminations, grounding, and applicable installation requirements.
- Retain the inputs, formula, assumptions, and results so another person can reproduce the calculation.
The general 3% and 5% targets provide a useful design framework, but the final decision belongs to the applicable code requirements, equipment instructions, and installation-specific professional review—not to a percentage result alone.