Feeder Conductor Sizing and Voltage Drop Calculator
Compare feeder conductor ampacity and voltage drop using load current, installation conditions, and a user-set design target.
Load and circuit details
Enter load current after applicable demand factors, at 100%. Include motor running load here once.
Enter the continuous portion of the calculated load. The ampacity check applies 125% to this input.
Enter the applicable motor full-load current at the feeder voltage when the motor-group allowance applies. This is added separately to conductor ampacity.
Enter the one-way distance from the source to the load.
Load current and voltage drop are divided evenly among the entered sets. Parallel conductor rules and physical layout must be verified.
The default 3% target is informational design guidance, not a mandatory code limit.
Ampacity conditions
Use the equipment markings and applicable terminal rules.
Use the temperature around the conductors. Supported range: -40°F to 185°F.
The phase default is 2 for single-phase or 3 for three-phase. Adjust for neutral conductors, non-linear loads, or the actual raceway grouping.
Conductor candidate
| Conductor | Base ampacity | Adjusted ampacity | Small-conductor OCP limit | Voltage drop | Check |
|---|---|---|---|---|---|
| 14 AWG | 20.0 A | 20.0 A | 15 A | 35.34% | Base ampacity too low |
| 12 AWG | 25.0 A | 25.0 A | 20 A | 22.33% | Base ampacity too low |
| 10 AWG | 35.0 A | 35.0 A | 30 A | 13.98% | Base ampacity too low |
| 8 AWG | 50.0 A | 50.0 A | — | 8.77% | Base ampacity too low |
| 6 AWG | 65.0 A | 65.0 A | — | 5.52% | Base ampacity too low |
| 4 AWG | 85.0 A | 85.0 A | — | 3.46% | Base ampacity too low |
| 3 AWG | 100.0 A | 100.0 A | — | 2.75% | Base ampacity too low |
| 2 AWG | 115.0 A | 115.0 A | — | 2.18% | Base ampacity too low |
| 1 AWG | 130.0 A | 130.0 A | — | 1.72% | Meets both |
| 1/0 AWG | 150.0 A | 150.0 A | — | 1.37% | Meets both |
| 2/0 AWG | 175.0 A | 175.0 A | — | 1.10% | Meets both |
| 3/0 AWG | 200.0 A | 200.0 A | — | 0.86% | Meets both |
| 4/0 AWG | 230.0 A | 230.0 A | — | 0.68% | Meets both |
| 250 kcmil | 255.0 A | 255.0 A | — | 0.57% | Meets both |
| 300 kcmil | 285.0 A | 285.0 A | — | 0.49% | Meets both |
| 350 kcmil | 310.0 A | 310.0 A | — | 0.42% | Meets both |
| 400 kcmil | 335.0 A | 335.0 A | — | 0.38% | Meets both |
| 500 kcmil | 380.0 A | 380.0 A | — | 0.29% | Meets both |
| 600 kcmil | 420.0 A | 420.0 A | — | 0.26% | Meets both |
| 750 kcmil | 475.0 A | 475.0 A | — | 0.21% | Meets both |
| 1000 kcmil | 545.0 A | 545.0 A | — | 0.17% | Meets both |
The continuous-load requirement is checked against base ampacity. Corrected ampacity is checked against the load to be served, including the largest-motor allowance. Voltage drop uses the entered operating load current at 75°C resistance.
About this calculator
Compare listed copper or aluminum conductors for a feeder. The worksheet checks adjusted ampacity and a user-selected voltage-drop target, then shows the smallest listed size that meets both.
Enter load current after applicable demand factors. Separate the continuous portion, and include a largest motor full-load current only when the feeder motor rules apply. Keep the motor's running load in the calculated load once; the motor input adds only the feeder sizing allowance.
Method and assumptions
Base feeder ampacity = noncontinuous load + 125% of continuous load + 25% of the largest motor full-load current when the motor allowance applies. The calculator checks this requirement against the uncorrected table ampacity, limited by the selected terminal rating.
It also checks the load to be served, including the largest-motor allowance, against ampacity after the selected ambient-temperature correction, current-carrying conductor adjustment, and terminal limit. The two checks keep continuous-load sizing separate from installation corrections. With parallel sets, each ampacity requirement and load is divided evenly among the entered sets.
For each conductor size, the ampacity screen starts with the supported Table 310.16 ampacity. General small-conductor overcurrent limits are shown separately and included as an additional screen; the calculator does not select a protective device.
Voltage drop uses the entered load current at 100%, divided by the number of parallel sets. The calculation uses one-way length and 75°C AC conductor resistance. It uses the two-conductor relationship for single-phase circuits and the balanced three-phase relationship for three-phase circuits. It does not calculate reactance, power factor, conductor operating temperature, or voltage unbalance.
The 3% default is an informational design target, not a mandatory voltage-drop limit. The selected target does not replace conductor ampacity, overcurrent protection, terminal, parallel-conductor, equipment, or local-code review.
Example
For a 480 V balanced three-phase feeder with 40 A noncontinuous load, 60 A continuous load, and a 20 A largest-motor full-load current, the base feeder ampacity screen is 40 + (60 × 1.25) + (20 × 0.25) = 120 A. The separate corrected-load check is 40 + 60 + (20 × 0.25) = 105 A. At 300 ft one-way, the default copper 1 AWG candidate has a 75°C terminal-limited base ampacity of 130 A, adjusted ampacity of 130 A under the entered conditions, and estimated voltage drop of about 1.72%.
This example illustrates the calculator math only. Verify the motor FLC basis, conductor arrangement, installation conditions, protective device, equipment, and locally adopted code before selecting a feeder.
Interpret the result
A conductor appears as the recommended candidate only when its corrected ampacity and calculated voltage drop both meet the entered criteria. If no listed conductor meets both, review the load, installation inputs, number of parallel sets, and design target against the project requirements.
The smallest candidate shown is not a complete conductor selection. It does not verify overcurrent-device coordination, neutral loading, fault-current performance, raceway fill, pull conditions, equipment listing, or all code exceptions.
FAQs
How does the calculator determine required ampacity?
It adds the noncontinuous load at 100%, the continuous load at 125%, and 25% of the largest motor full-load current when the feeder motor-group allowance applies. Enter the motor running load only once in the noncontinuous or continuous load field.
Which current does the voltage-drop calculation use?
It uses the entered noncontinuous and continuous calculated load currents at 100%. It does not use the additional 125% continuous-load or 25% motor-sizing allowances as operating current.
Is the default 3% voltage-drop target a code limit?
No. It is an informational design target. The target is adjustable and does not establish code compliance or guarantee suitable equipment voltage.
How does the calculator handle parallel sets?
It divides load current and required ampacity evenly among the entered sets. It screens parallel conductor sizes at 1/0 AWG and larger, but it does not verify conductor grouping, equal lengths, raceway arrangement, or all other parallel-conductor requirements.
Does this select an overcurrent device?
No. It checks candidate conductor ampacity and voltage drop only. The overcurrent device, conductor protection, service rating, neutral, grounding, raceway, and final installation design require separate review.
Why can the ampacity result differ from the voltage-drop result?
Ampacity uses the selected conductor temperature rating, terminal rating, ambient temperature, and current-carrying conductor count. Voltage drop uses the entered running load current and published 75°C AC resistance. The calculator evaluates both and identifies a candidate only when it meets both criteria.
Can I transfer the commercial load result into this tool?
Use the commercial worksheet’s calculated-load result as the basis for the running current, then split its noncontinuous and continuous portions for these inputs. Do not use its preliminary conductor-sizing current, because this calculator applies continuous-load and motor allowances again.
Sources
- NFPA 70, National Electrical Code, 2026 edition — feeder ampacity, conductor ampacity, motor, and voltage-drop guidance. Verify the adopted text and local amendments.
- NFPA 70 Second Draft Report, preliminary report — public code-development material used to cross-check feeder load sizing and voltage-drop guidance.
- NFPA 70 Chapter 3 Second Draft Report, preliminary report — public code-development material used to cross-check Table 310.16 and conductor rules.
- NFPA 70 Chapter 2 Second Draft Report, preliminary report — Annex D feeder examples used to cross-check separate terminal and installation-condition ampacity checks.
- Southwire copper THHN/THWN-2 specifications and aluminum XHHW-2 specifications — AC resistance values used for voltage drop.