Voltage Drop Calculations for a 100 Amp Feeder: NEC Method

Why Voltage Drop on a Feeder Matters

Voltage drop is the reduction in electrical potential along the path of a current-carrying wire. While some drop is unavoidable, excessive voltage drop can lead to inefficient operation, poor equipment performance, and energy waste converted into heat. For a professional electrician, managing voltage drop is not just about following rules; it’s about delivering a quality, long-lasting installation.

The National Electrical Code addresses this in its informational notes, which are considered best practices. Specifically for feeders, NEC 215.2(A)(4) Informational Note No. 2 recommends sizing conductors to prevent a voltage drop of more than 3% at the farthest outlet for power, heating, and lighting loads. Furthermore, it suggests a maximum combined drop of 5% for both the feeder and the final branch circuit. While these are not enforceable rules in most jurisdictions, they are the industry standard for a reliable electrical system and a key part of professional electrician training.

Adhering to the allowable voltage drop percentage is especially critical in installations with sensitive electronic loads or long conductor runs, where the effects of voltage drop are more pronounced.

Understanding the Key Components of the Voltage Drop Formula

To correctly perform a wire size computation, you must understand the variables in the standard voltage drop formulas. These formulas are used to determine the minimum conductor size needed to stay within a desired voltage drop limit.

• Single-Phase Voltage Drop Formula: CM = (2 x K x I x D) / VD_allowed
• Three-Phase Voltage Drop Formula: CM = (√3 x K x I x D) / VD_allowed

Here’s a breakdown of each component:

• CM (Circular Mil area): This is the cross-sectional area of the conductor in circular mils. The formula calculates the minimum CM required. You then consult NEC Chapter 9, Table 8 to find a conductor with at least that CM area.
• K (Conductor Material Constant): This “K-factor” represents the DC resistance of a 1,000-foot-long conductor with a cross-sectional area of one circular mil, at 75°C. For calculations, use:
  • 12.9 for uncoated copper
  • 21.2 for aluminum
• I (Current in Amps): This is the actual operating current (load) that the feeder will carry. It is crucial to use the calculated load, not simply the overcurrent protection device (OCPD) rating. For more on this, read about the NEC method for calculating voltage drop.
• D (Distance): This is the one-way length of the conductor run from the source to the load, measured in feet.
• VD_allowed (Allowable Voltage Drop): This is the maximum acceptable voltage drop in volts, not a percentage. You calculate this by multiplying the system voltage by the desired percentage (e.g., 240V x 3% = 7.2V).

Step-by-Step Voltage Drop Calculation for a 100 Amp Feeder

Let’s walk through the process for a hypothetical 100A feeder. This step-by-step guide mirrors the process a master electrician would follow to ensure a compliant and efficient installation.

1. Determine the Actual Load (I): First, perform an electrical load calculation per NEC Article 220. A feeder rarely operates at its full OCPD rating. Distinguish between continuous and non-continuous loads. A continuous load calculation involves multiplying any load running for three or more hours by 125%. For example, if a 100A feeder supplies 60A of continuous load and 20A of non-continuous load, the load for sizing conductors is (60A x 1.25) + 20A = 95A. However, for voltage drop, the value for ‘I’ can vary. While industry best practice is to use the actual expected operating current, the NEC informational notes reference the “load at the farthest outlet”. To ensure a conservative and safe design, some Authorities Having Jurisdiction (AHJs) may require using the full calculated load (including the 125% factor). Always confirm local requirements. For this example, let’s assume the typical operating load is 80A.
2. Establish System Parameters: Identify the system voltage (e.g., 240V), phase (single-phase or three-phase), and the one-way distance (D) of the run in feet. For this example, we’ll use a 240V, single-phase feeder running 150 feet.
3. Calculate Allowable Voltage Drop in Volts (VD_allowed): Using the NEC recommendation of 3% for feeders, calculate the maximum voltage drop in volts.VD_allowed = System Voltage x Allowable Percentage

   VD_allowed = 240V x 0.03 = 7.2V

4. Apply the Voltage Drop Formula: Insert your values into the single-phase formula to find the minimum required circular mil (CM) area. We will use copper conductors (K=12.9).CM = (2 x K x I x D) / VD_allowed

   CM = (2 x 12.9 x 80A x 150 ft) / 7.2V

   CM = 309,600 / 7.2 = 43,000 CM

5. Select the Conductor from NEC Chapter 9, Table 8: Look up your calculated CM value (43,000) in the “Circular Mils” column of Table 8. You must select the next size up.
   • 3 AWG Copper = 52,620 CM
   • 4 AWG Copper = 41,740 CM

   Since 41,740 CM is less than our required 43,000 CM, we must choose the next larger size: 3 AWG Copper.

6. Final Verification: Always cross-reference your result with NEC Table 310.16 to ensure the selected conductor meets the ampacity requirements for a 100A feeder (based on the 75°C column for most terminations). A 3 AWG copper conductor is rated for 100 amps at 75°C, so it is suitable for both ampacity and voltage drop in this scenario. If the voltage drop calculation had resulted in a smaller wire (e.g., 6 AWG), you would still be required to use the 3 AWG wire to meet the minimum ampacity for the 100A OCPD. To learn more about conductor sizing for panels, check out this guide on how to size wire for a 100 amp subpanel.

Worked Example (Three-Phase)

The process is similar for a three-phase system, but we use the three-phase formula and the corresponding line-to-line voltage. Let’s calculate for a 208V, three-phase feeder carrying an 80A load over 150 feet.

1. System Parameters: 208V, three-phase, 80A load, 150 ft distance, copper wire (K=12.9).
2. Allowable Voltage Drop (VD_allowed): A 3% drop on a 208V system is 208V x 0.03 = 6.24V.
3. Apply the Three-Phase Formula:CM = (√3 x K x I x D) / VD_allowed

   CM = (1.732 x 12.9 x 80A x 150 ft) / 6.24V

   CM = 268,113.6 / 6.24 = 42,967 CM

4. Select the Conductor: The required 42,967 CM is slightly more than what a 4 AWG copper wire provides (41,740 CM). Therefore, we must again size up to a 3 AWG Copper conductor (52,620 CM) to meet the voltage drop requirement.

Important Considerations for Feeder Sizing

Accurate feeder conductor sizing involves more than just plugging numbers into a formula. Here are some critical points to remember:

• Ampacity vs. Voltage Drop: You must perform two checks. The conductor must be large enough for ampacity based on NEC Article 310 and large enough to mitigate voltage drop. You must use the larger of the two resulting wire sizes.
• Conductor Operating Temperature: The K-factor of 12.9 (copper) and 21.2 (aluminum) is based on a conductor temperature of 75°C (167°F). If operating conditions differ significantly, a more precise calculation may be needed.
• Use a Professional Voltage Drop Calculator: While manual calculation is a vital skill, especially for a journeyman electrician exam, using a verified size electrical wire calculator in the field can save time and reduce errors. Always ensure it uses the correct formulas and NEC tables.
• Understanding Different Calculation Methods: There are multiple ways to approach the problem. For more examples and methods, see these resources on how to calculate voltage drop and alternative NEC calculation methods.

Related Resources

• Acceptable Voltage Drop: NEC Code

Frequently Asked Questions (FAQ)

What is the correct voltage drop formula for a 100 amp feeder?

The correct formula depends on the phase. For single-phase, use CM = (2 x K x I x D) / VD. For three-phase, use CM = (√3 x K x I x D) / VD. ‘I’ should be the calculated load in amps, not necessarily 100A.

How does conductor material affect the voltage drop calculation 100 amp?

The conductor material is represented by the K-factor. Aluminum (K=21.2) has a higher resistance than copper (K=12.9), meaning you will need a larger aluminum conductor to achieve the same low voltage drop as a copper one under the same conditions.

What is the maximum allowable voltage drop percentage for a feeder?

The NEC, in section 215.2(A)(4) Informational Note No. 2, recommends a maximum voltage drop of 3% for a feeder. It also recommends a total of 5% for the combined feeder and branch circuit. This is a best practice, not a mandatory rule, except in specific cases like for sensitive electronic equipment (NEC 647.4(D)).

Can I use a voltage drop calculator instead of the manual formula?

Yes, online calculators are excellent tools for field use. However, every journeyman and master electrician must understand the manual wire size computation to verify results, troubleshoot issues, and pass licensing exams, which frequently test this knowledge.

What is the minimum AWG wire size for a 100A feeder before voltage drop?

Based on ampacity alone from NEC Table 310.16 (at 75°C), you would typically need a 3 AWG copper or 1 AWG aluminum conductor for a 100A feeder. However, this size is often insufficient for longer runs, and a voltage drop calculation 100 amp is required to determine if a larger wire is necessary.