NEC Voltage Drop Guidelines: Understanding the 3% and 5% Rules
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% total for feeders plus branch circuits combined. These guidelines, found in NEC sections 210.19(A) and 215.2(A), help ensure efficient equipment operation and energy use. While these are recommendations rather than enforceable requirements, following them is considered best practice in electrical design.
What Does the NEC Say About Voltage Drop?
The National Electrical Code addresses voltage drop in two key sections: NEC 210.19(A) for branch circuits and NEC 215.2(A) for feeders. Both sections contain informational notes that provide guidance on acceptable voltage drop levels.
The NEC uses specific language in these sections. The term "should be sized" indicates a recommendation, not a requirement. This is an important distinction because it means that voltage drop guidelines are not enforceable code requirements, but rather best practices that the NEC advises electricians and engineers to follow.
The specific language from NEC 210.19(A) Informational Note No. 4 states:
This informational note establishes the two key thresholds that have become industry standard: 3% for branch circuits and 5% total. Understanding when and how to apply these guidelines is essential for proper electrical design.
The 3% Rule for Branch Circuits
The 3% voltage drop guideline applies to branch circuits—the circuits that run from the final overcurrent protection device (circuit breaker or fuse) to the outlets, lights, and equipment being served. This is the portion of the electrical system closest to the loads.
Why 3%?
The 3% threshold was established to ensure that equipment receives adequate voltage for proper operation. Most electrical equipment is designed to operate within ±10% of its rated voltage. By limiting branch circuit voltage drop to 3%, even when combined with feeder voltage drop and utility voltage variations, equipment should remain within its operating range.
What Does 3% Mean in Practice?
For common system voltages, 3% voltage drop equals:
| System Voltage | 3% Voltage Drop | Voltage at Load |
|---|---|---|
| 120V | 3.6V | 116.4V |
| 208V | 6.24V | 201.76V |
| 240V | 7.2V | 232.8V |
| 277V | 8.31V | 268.69V |
| 480V | 14.4V | 465.6V |
Calculating Wire Size for 3% Drop
To size a branch circuit conductor for 3% maximum voltage drop, you need to know the circuit length, current, and system voltage. The voltage drop calculator can help you determine the appropriate wire size.
As wire runs get longer, larger conductors are needed to maintain the 3% limit. For example, a 120V, 20A circuit using 12 AWG copper wire will exceed 3% voltage drop at approximately 50 feet one-way distance. For longer runs, 10 AWG or larger may be required.
The 5% Rule for Total Voltage Drop
The 5% rule applies to the combined voltage drop of the feeder circuit (from the service entrance to the subpanel or final breaker) plus the branch circuit (from the breaker to the load). This total system voltage drop should not exceed 5%.
Feeder vs. Branch Circuit
Understanding the distinction between feeders and branch circuits is important:
- Feeder: The conductors between the service equipment (main panel) and the final branch-circuit overcurrent device (subpanel breaker). This includes conductors to subpanels in garages, workshops, or other buildings.
- Branch Circuit: The conductors between the final overcurrent device and the outlets or equipment being served.
Allocating the 5% Budget
The 5% total can be split between feeders and branch circuits in several ways:
| Scenario | Feeder Drop | Branch Drop | Total |
|---|---|---|---|
| Conservative | 2% | 3% | 5% |
| Balanced | 2.5% | 2.5% | 5% |
| Long Feeder | 3% | 2% | 5% |
| Short Feeder | 1% | 4% | 5% |
The most common approach is to design feeders for 2% voltage drop and branch circuits for 3%, which provides a margin of safety while staying within the 5% total limit.
Example: Detached Garage
Consider a detached garage 100 feet from the main panel, with a 60A subpanel. A circuit in the garage runs 40 feet to a workbench outlet.
- Feeder (100 ft, 60A): Using 4 AWG copper, voltage drop is approximately 2.3%
- Branch (40 ft, 20A): Using 12 AWG copper, voltage drop is approximately 1.6%
- Total: 2.3% + 1.6% = 3.9% ✓ Within 5% limit
NEC Code Sections Explained
NEC 210.19(A) – Branch Circuits
Section 210.19 covers branch circuit conductor sizing. The main text requires conductors to have an ampacity not less than the maximum load to be served, with specific rules for continuous loads, multioutlet branch circuits, and household ranges and cooking appliances.
Informational Note No. 4 to this section provides the voltage drop guidance. It's important to understand that informational notes are explanatory material and are not enforceable as code requirements.
NEC 215.2(A) – Feeders
Section 215.2 addresses feeder conductor sizing. Similar to branch circuits, it requires conductors sized for the calculated load with additional requirements for continuous loads.
Informational Note No. 2 to section 215.2(A)(4) provides parallel guidance for feeder voltage drop:
NEC Chapter 9, Table 8
Chapter 9, Table 8 provides the wire resistance values used for voltage drop calculations. This table lists DC resistance values for various conductor sizes in both copper and aluminum. These values are essential for accurate voltage drop calculations using the standard formulas.
NEC Chapter 9, Table 9
Table 9 provides AC impedance values for conductors in various conduit types. For AC circuits, especially those with larger conductors or longer runs, using impedance values from Table 9 produces more accurate results than using the DC resistance values from Table 8.
Requirements vs. Recommendations
One of the most important aspects of NEC voltage drop guidance is understanding that it consists of recommendations, not requirements. This distinction has significant implications for electrical design and inspection.
Key Differences
| Aspect | Requirements | Recommendations |
|---|---|---|
| Language | "Shall," "must" | "Should," "is recommended" |
| Location | Main code text | Informational notes |
| Enforcement | Required for approval | Not enforceable by inspectors |
| Violation | Code violation | Not a code violation |
Can an Inspector Fail You for Voltage Drop?
Technically, an electrical inspector cannot fail an installation solely based on voltage drop exceeding the NEC informational note recommendations. However, there are some important caveats:
- Local jurisdictions may have adopted amendments that make voltage drop a requirement
- Some states (like California) have specific voltage drop requirements in their energy codes
- Inspectors can require calculations showing the design is adequate for the intended loads
- Equipment manufacturers may void warranties if voltage is outside specifications
Why Follow Recommendations Anyway?
Despite not being enforceable requirements, there are compelling reasons to design within the 3%/5% guidelines:
- Equipment longevity: Motors and electronics last longer with proper voltage
- Energy efficiency: Lower voltage drop means less wasted energy as heat
- Performance: Lights are brighter, motors have more torque
- Professional standards: Following guidelines demonstrates competent design
- Liability protection: Meeting industry standards provides legal protection
When to Exceed NEC Recommendations
There are situations where exceeding the 3%/5% guidelines may be acceptable or even unavoidable. Understanding these scenarios helps make informed design decisions.
Motor Starting Current
Electric motors draw significantly higher current during startup than during running (typically 6-8 times running current). This causes temporary voltage drop that may exceed the 3% guideline. Since this is a transient condition lasting only seconds, it's generally acceptable as long as:
- The voltage drop doesn't cause other equipment to malfunction
- The motor can successfully start under worst-case conditions
- Running voltage drop is within acceptable limits
Infrequently Used Circuits
Circuits that are used rarely or for short durations may be designed with higher voltage drop to save on wire costs. Examples include:
- Emergency generator feeds that only run during outages
- Seasonal equipment like pool pumps or snow melt systems
- Backup or redundant circuits
Cost-Benefit Analysis
For very long runs, the cost of wire sized for 3% voltage drop may be prohibitive. In these cases, a cost-benefit analysis comparing wire cost against energy losses over the equipment lifetime can help determine the optimal wire size.
Voltage-Insensitive Loads
Some loads are less sensitive to voltage variations:
- Resistive heating elements (though they produce less heat at lower voltage)
- Modern switch-mode power supplies with wide input ranges
- Some LED drivers designed for varying input voltages
Impact of Voltage Drop on Equipment
Understanding how voltage drop affects various types of equipment helps explain why the NEC provides these guidelines.
Motors
Motors are particularly sensitive to voltage drop. When voltage decreases:
- Torque decreases proportionally to voltage squared (10% voltage drop = 19% torque loss)
- Motors draw more current to maintain speed, increasing heat
- Motor life is reduced due to overheating
- Starting may fail under load
Lighting
The effect on lighting depends on the type:
- Incandescent: Light output drops dramatically (10% voltage drop = ~30% light reduction)
- LED: Most drivers compensate up to a point, then dim or shut off
- Fluorescent: May flicker, fail to start, or have reduced life
Electronics
Modern electronics typically have switch-mode power supplies that operate over a range of input voltages (often 100-240V). However, at the lower end of their range, efficiency decreases and heat increases. Sensitive equipment may malfunction or produce errors at voltages outside specifications.
Heating Elements
Resistive heating elements produce heat proportional to voltage squared. A 10% voltage drop results in approximately 19% less heat output. While this doesn't damage the equipment, it may cause thermostats to run longer, increasing energy consumption.
State and Local Requirements
While the NEC itself makes voltage drop a recommendation, some jurisdictions have adopted stricter requirements.
California Title 24
California's Energy Code (Title 24, Part 6) includes specific voltage drop requirements for certain circuits. These are mandatory requirements, not recommendations, and are enforced through the building permit process.
Hawaii
Hawaii has adopted amendments to the NEC that make voltage drop requirements more stringent, particularly for photovoltaic systems.
Local Amendments
Many local jurisdictions adopt the NEC with amendments. Some of these amendments may include voltage drop requirements. Always check with your local authority having jurisdiction (AHJ) for specific requirements in your area.
Utility Requirements
Utility companies may have their own requirements for voltage drop on services they provide. These may affect the design of service entrance equipment and main feeders.
Calculate Your Voltage Drop
Use our free calculator to verify your designs meet NEC guidelines.
Open CalculatorFrequently Asked Questions
No, the NEC voltage drop guidelines are recommendations, not enforceable requirements. They appear in informational notes to sections 210.19 and 215.2, which are explanatory material only. However, some states and local jurisdictions have adopted amendments that make voltage drop a requirement, so always check local codes.
Exceeding these limits won't necessarily cause immediate problems, but can lead to reduced equipment performance, shorter equipment life, increased energy consumption, and potential equipment malfunctions. Motors may overheat, lights may dim, and sensitive electronics may behave erratically. The severity depends on how much the limits are exceeded and the type of equipment.
The NEC voltage drop guidelines apply to power circuits of any voltage. For low-voltage systems like 12V landscape lighting or 24V control circuits, voltage drop is actually more critical because the same voltage drop represents a much larger percentage of the system voltage. A 1V drop on a 12V system is 8.3%, which is significant.
For sensitive equipment like computer systems, medical equipment, and precision machinery, targeting 2% or less is often recommended. IEEE 141 (the "Red Book") suggests 2% for sensitive equipment. This provides a larger safety margin and ensures optimal equipment performance. However, for general lighting and standard outlets, 3% is typically adequate.
Use the standard voltage drop formula: VD = (2 × L × I × R) / 1000, where L is one-way length in feet, I is current in amps, and R is wire resistance per 1000 feet from NEC Table 8. For convenience, use our voltage drop calculator which performs this calculation automatically and recommends wire sizes to meet the 3% guideline.