Formulas and sources
Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.
Voltage drop, single phase
On a single-phase run
- V
- voltage drop, in volts
- ρ
- resistivity, in ohm-circular mils per foot: copper 12.9 and aluminum 21.2 at 75 °C, moved to the rating you pick
- I
- current, in amperes
- L
- one-way length, in feet
A is the conductor area in circular mils, from the AWG table. The 2 counts the out-and-back conductors. DC resistance only: on AC, the cable's reactance adds a little more drop.
Source: Standard formula; no single publisher. The resistivities are CalcTrail's working values: copper 12.9 is 2.3% above the NBS copper standard at 75 °C [1], and aluminum 21.2 is 2.1% above the NBS 61.0% IACS aluminum at 75 °C [2].
Voltage drop, three phase
On a three-phase run
- V
- line-to-line voltage drop, in volts
- ρ
- resistivity, as for single phase
- I
- line current, in amperes
- L
- one-way length, in feet
A balanced load. √3 gives the line-to-line drop.
Source: Standard formula; no single publisher. Resistivities from NBS Handbooks 100 [1] and 109 [2], as for single phase.
Ampacity and the code checks
In conduit
The table ampacity comes from Table 310.16, by metal and the 60, 75 or 90 °C column, and is multiplied by the ambient factor of Table 310.15(B)(1)(1). A size passes when the table ampacity covers the load, or 125% of a continuous load (210.19(A)), and the derated ampacity covers the load. It must also pass the 240.4(D) limit on the breaker for 14, 12 and 10 AWG. The recommendation is the smallest size that passes these checks and your drop target.
Source: Uses NFPA 70, the National Electrical Code, 2023 edition [3]. As of August 3, 2026, NFPA counts the 2023 edition in force in 20 states, 2020 in 15, and the 2026 edition, issued August 20, 2025, in six [4]. Ampacity is the current a conductor can carry continuously without exceeding its temperature rating, as OSHA defines it [5].
Open air
In open air
Copper only, from a chassis-wiring column that is not an NEC table and does not vary with the rating. The ambient factor of the rating you pick still applies. Aluminum in open air is sized on voltage drop alone.
Source: Widely used; the earliest traceable source is a vendor page that cites none, so no primary source found (NEC Table 310.17 is the free-air table).
Power lost in the wire
- I
- current, in amperes
- R
- one conductor's resistance over the run
- n
- conductors: 2 single phase, 3 three phase
Source: Standard formula; no single publisher.
Sources
- National Bureau of Standards (now NIST). Copper Wire Tables, Handbook 100, Part I, §1.1 (pp. 3-4), and §2.2, the American Wire Gauge (p. 6). February 21, 1966.
- National Bureau of Standards (now NIST). Aluminum Wire Tables, Handbook 109, introduction (p. 1) and Table 1 (p. 5). 1972.
- National Fire Protection Association (NFPA). NFPA 70, National Electrical Code, Table 310.16, Table 310.15(B)(1)(1), 210.19(A) and its informational note, 210.20(A), 240.4(D). 2023 edition.
- National Fire Protection Association (NFPA). NEC Adoption Maps. As of August 3, 2026.
- Occupational Safety and Health Administration, Code of Federal Regulations. 29 CFR 1910.399, Definitions applicable to this subpart, Ampacity. eCFR, current as of October 1, 2026.
What this calculator does not cover
- This sizes one circuit of up to three current-carrying conductors from Table 310.16 and its ambient factors. It does not adjust for more conductors in one raceway, size the breaker, or size motor or EV charging circuits.
- The data follows NEC 2023. NFPA's second-revision drafts for the 2026 edition add a 16 AWG copper row and a 14 AWG copper-clad aluminum row, which this tool does not hold; they also renumber some sections. Which edition applies depends on where you build.
- It does not use the table's 16 and 18 AWG rows, rated at 90 °C only for Type MI and MTW cable.
- Below 26 °C the NEC allows an ambient factor above 1.00; the tool keeps 1.00 there. The bands are whole degrees, so a fractional entry above 30 °C rounds up to the next whole degree.
- The voltage drop is DC resistance only, so it leaves out the cable's AC reactance.
Sources checked on .