Voltage drop, and the wire size that fixes it
Enter the voltage, load and one-way length. See the drop for every size from 14 AWG to 1000 kcmil, and the smallest copper or aluminum conductor that stays within your target.
Three-phase: enter the line-to-line voltage (208, 480). Length is one way, from the panel to the load. The NEC's informational notes suggest about 3% for a branch circuit or feeder and 5% for the two together; they are recommendations, not requirements.
| Size | Ω/1,000 ft | Drop | % |
|---|
Highlighted: the smallest size within your target. The size you picked is marked on the left. Uses DC resistance at 75°C from NEC Chapter 9, Table 8 and ignores reactance, which is fine for small conductors and an estimate for large ones (Table 9 is more exact). It does not check ampacity: size for the load under Table 310.16 first, then upsize for voltage drop. Local code and the inspector govern.
Single-phase voltage drop = 2 × one-way length × resistance per 1,000 ft × amps ÷ 1,000; for three-phase use 1.732 instead of 2. With NEC Chapter 9 Table 8 values, 12 AWG copper (1.98 Ω) carrying 16 A for 100 ft on 120 V drops 6.34 V, or 5.28%. 8 AWG brings it to 2.07%, under the 3% the NEC's informational notes suggest.
How to use the voltage drop calculator
Describe the circuit
Copper or aluminum, single- or three-phase, voltage, load in amps and one-way length.
Pick a size and target
Choose the wire size you plan to use and your target drop, 3% by default.
Read the result
Drop in volts and percent, voltage at the load, and the smallest size that meets the target.
What the voltage drop calculator does
NEC Table 8 resistances
DC resistance at 75°C for uncoated copper and aluminum, 14 AWG to 1000 kcmil, solid or stranded.
Single- and three-phase
2 × L × R × I ÷ 1,000 for single-phase, √3 × L × R × I ÷ 1,000 for three-phase.
Size finder
Every size in one table, with the smallest that meets your target highlighted.
Voltage at the load
Shows what's left at the far end, not just the percentage.
Honest limits
Says when the estimate ignores reactance and reminds you ampacity is a separate check.
- Resistance data
- NFPA 70 (NEC 2023) Chapter 9, Table 8, DC resistance at 75°C, ohms per 1,000 ft
- Single-phase
- VD = 2 × L × R × I ÷ 1,000 (L = one-way length in ft)
- Three-phase
- VD = 1.732 × L × R × I ÷ 1,000, % of line-to-line voltage
- Default target
- 3% (NEC informational notes in 210.19 and 215.2: 3% branch or feeder, 5% total)
- Not included
- Reactance and power factor (Table 9), ampacity (Table 310.16), temperature correction
Voltage Drop Calculator FAQ
How do you calculate voltage drop?
For a single-phase circuit, multiply 2 by the one-way length in feet, the conductor's resistance in ohms per 1,000 feet and the current in amps, then divide by 1,000. For three-phase, use 1.732 instead of 2. Divide the result by the circuit voltage for the percentage.
What is the maximum voltage drop allowed by the NEC?
For most circuits the NEC doesn't set a limit; informational notes in 210.19 and 215.2 recommend no more than 3% on a branch circuit or feeder and 5% for both together. Some articles, and some local codes and energy codes, do make limits mandatory, so check what applies.
Why use the one-way length?
Because the formula's factor of 2 already counts the current going out and coming back. Measure from the panel or source to the load along the route the wire will take.
Does this calculator check ampacity?
No. First size the conductor for the load and the overcurrent device under the NEC ampacity rules (Table 310.16 with its corrections), then use this to see whether voltage drop calls for a larger size.
Why do larger wires give a slightly different answer than other calculators?
This uses DC resistance from Table 8 and ignores reactance. For large conductors on AC, Table 9's effective impedance with the load's power factor is more exact, so treat big-wire results as estimates.
Which edition of the NEC does this use?
It uses the Table 8 resistances found in recent editions, including the 2023 NEC; they haven't changed for these sizes in years. Your local code and inspector govern, so check the edition your area has adopted.
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