Voltage drop calculator
Voltage drop is current times the resistance of the conductors on the way out and back. For single phase, VD = 2 × L × R × I; for three phase, VD = 1.732 × L × R × I, with R from NEC Chapter 9 Table 812. A 16 A load at 120 V, 100 ft away on 12 AWG copper drops 6.34 V, or 5.3%. Getting under the 3% the NEC recommends takes 8 AWG3.
Over 3%. About 113.7 V at the load.
| Resistance, copper 12 AWG | 1.98 Ω per 1,000 ft |
| Formula | 2 × L × R × I |
Smallest copper size within 3%: 8 AWG. Check ampacity separately.
What the NEC says about voltage drop
“Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation.”
The same recommendation for feeders is in 215.2(A)(2), Informational Note No. 24. Both are informational notes, so they explain good practice rather than set an enforceable limit. Some articles do set mandatory limits, such as 647.4(D) for sensitive electronic equipment: 1.5% on branch circuits and 2.5% combined. Project specifications often tighten the recommendation too; check yours.5
The same circuit, three wire sizes
| Size | Resistance, Ω per 1,000 ft | Drop | Percent |
|---|---|---|---|
| 12 AWG | 1.98 | 6.34 V | 5.3% |
| 10 AWG | 1.24 | 3.97 V | 3.3% |
| 8 AWG | 0.778 | 2.49 V | 2.1% |
Resistance values used
| Size | Copper | Aluminum |
|---|---|---|
| 14 AWG | 3.14 | - |
| 12 AWG | 1.98 | 3.25 |
| 10 AWG | 1.24 | 2.04 |
| 8 AWG | 0.778 | 1.28 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 3 AWG | 0.245 | 0.403 |
| 2 AWG | 0.194 | 0.319 |
| 1 AWG | 0.154 | 0.253 |
| 1/0 AWG | 0.122 | 0.201 |
| 2/0 AWG | 0.0967 | 0.159 |
| 3/0 AWG | 0.0766 | 0.126 |
| 4/0 AWG | 0.0608 | 0.1 |
| 250 kcmil | 0.0515 | 0.0847 |
| 300 kcmil | 0.0429 | 0.0707 |
| 350 kcmil | 0.0367 | 0.0605 |
| 400 kcmil | 0.0321 | 0.0529 |
| 500 kcmil | 0.0258 | 0.0424 |
Where this method is approximate
- Table 8 is DC resistance at 75°C. For larger conductors on AC, especially in steel conduit, reactance adds to the drop; Chapter 9 Table 9 gives AC impedance for those cases.
- Conductors running cooler than 75°C have lower resistance, so the result leans conservative for lightly loaded circuits.
- Power factor isn't modeled. Motor and other inductive loads behave differently from resistive ones.
- Voltage drop doesn't size the conductor for ampacity. Check 310.16 and the terminal ratings first, then upsize for drop.
Chapter 9 Table 9 and Table 310.16 are in the full code text at NFPA5.
Voltage drop in a bid
Long branch runs in warehouses, parking structures and site lighting are where voltage drop changes the estimate: a size up in wire across a few hundred feet of home runs is real material and labor. Those runs aren't drawn with lengths on the plans, so they're measured or allowed for after the count. The electrical takeoff guide explains how the counted and measured parts of a takeoff fit, and the conduit fill calculator checks whether the larger wire still fits the raceway.
The current to enter for a feeder comes from the Article 220 load calculation, not from the breaker size, which is often larger than the calculated load. Once the sizes are settled, price the upsized wire per 1,000 ft in the electrical estimate spreadsheet, or as a single line in the estimate calculator.
When the specs set a tighter limit than the 3% note, or the engineer left wire sizing to the contractor, write the basis you used into the clarifications of your proposal. The guide on how to bid electrical jobs covers where clarifications go. If you size feeders on every bid, the guide to commercial electrical estimating software compares the suites that price wire and conduit as assemblies.
More calculators and guides
Sources
- NFPA 70 revision cycle, Code-Making Panel 6 public input responses (quotes 2023 Chapter 9 Table 8)
- NEC Chapter 9 Table 8, conductor properties (NECA-IBEW reprint)
- NFPA 70 revision cycle, Code-Making Panel 2 public input responses (quotes the 2023 text of 210.19)
- NFPA 70 revision cycle, Code-Making Panel 10 public input responses (quotes the 2023 text of 215.2)
- NFPA 70, National Electrical Code, 2023 edition (read online at nfpa.org)
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