The rules we work to
- No invented formulas. Every number traces to a published NEC table or a first-principles equation. Where the code is banded rather than continuous — ambient temperature correction, for instance — we use the band, because interpolating would give a more precise-looking answer that no inspector would accept.
- Mandatory and advisory are labelled differently. Ampacity is a code requirement. Voltage drop is a design recommendation. Conflating the two is how tools end up telling people that a perfectly compliant installation is illegal.
- The working is always visible. If a result cannot be explained step by step with the code section attached, it does not ship.
- We say what we do not know. The limitations section below is not boilerplate. It is the list of cases where you need something other than this tool.
How the wire size calculator works
A conductor must satisfy two independent requirements, and the recommendation is whichever demands more copper.
Ampacity — two tests, both mandatory
NEC 690.8(B) for PV circuits and 210.19(A)(1) for branch circuits state the requirement the same way: the conductor must satisfy both of the following.
Test A — ampacity at the termination rating, no derating
must be ≥ 125 % × maximum circuit current
Test B — ampacity at the insulation rating, × ambient factor × fill factor
must be ≥ maximum circuit current
The distinction matters. Test A is why a cool, uncrowded run still needs 25 % headroom. Test B is why a hot rooftop conduit needs a bigger conductor even after test A passes. Applying 125 % and the derating factors to the same number in a single step — a shortcut several online calculators take — overstates the requirement.
For a PV source circuit there is an additional multiplier upstream of both tests. NEC 690.8(A)(1) defines the maximum circuit current as 125 % of the sum of parallel module short-circuit currents, because irradiance can exceed the 1000 W/m² standard test condition under snow reflection, cloud-edge effects and at altitude. Combined with test A's 125 %, the total factor on Isc is 1.5625.
Voltage drop
Vdrop = 2 × Lone-way, ft × Iamps × RΩ/1000 ft ÷ 1000
R is the DC resistance from NEC Chapter 9, Table 8, for stranded conductors at 75 °C. We use the tabulated resistance rather than the older K-constant form (2 × K × I × L ÷ circular mils, with K ≈ 12.9 for copper) because the tabulated values account for real stranding instead of assuming an ideal solid conductor. The two agree to within about 1 % on common sizes.
Because Chapter 9 Table 8 publishes DC resistance directly, DC voltage drop is exactly computable — there is no power factor, reactance or skin-effect term to estimate, which is not true of the AC case.
How the voltage drop calculator works
It solves the same problem in reverse. You supply the conductor and it reports the drop, using the identical Chapter 9 Table 8 resistance and the identical formula above. Because the size is an input rather than an output, it can also answer three questions the forward tool cannot: the longest run that conductor can hold at your target, the smallest conductor that would meet it, and what the same circuit would drop one system voltage up.
It also runs the full NEC ampacity check on the size you entered — the same two tests, the same correction and adjustment factors. This is deliberate. Voltage drop being acceptable says nothing about whether a conductor is safe, and a tool that reports only the drop can tell somebody their dangerously undersized wire is fine. Where the two verdicts disagree, the ampacity result is shown first, because it is the mandatory one.
Both calculators call the same ampacity implementation, in conditions.ts. That is a correctness decision rather than a tidiness one: two pages on this site must never disagree about whether the same conductor is legal for the same circuit, and an automated test asserts that they do not.
What these calculators do not do
Source list
Every section of NFPA 70, National Electrical Code, 2023 edition, that the calculators depend on:
| Section | What it is used for |
|---|---|
| Table 310.16 | Allowable ampacity for conductors in a raceway, cable or earth. Copper and aluminum, 60/75/90 °C. |
| Table 310.17 | Allowable ampacity for single insulated conductors in free air. Used for PV wire run openly under an array. |
| Table 310.15(B)(1) | Ambient temperature correction factors, applied to the table ampacity. |
| 310.15(B)(2) | The 33 °C adder for raceways and cables in direct sunlight within 3/4 in. of a roof. |
| Table 310.15(C)(1) | Adjustment factors for more than three current-carrying conductors in a raceway or cable. |
| 310.15(E) | Which conductors count toward the fill adjustment. Equipment grounding conductors do not. |
| 110.14(C) | Termination temperature limits — the reason a 90 °C conductor is often held to the 75 °C column. |
| Chapter 9, Table 8 | Conductor circular-mil area and DC resistance at 75 °C, used for all voltage drop calculations. |
| 690.8(A) and 690.8(B) | Maximum PV circuit current, and the two-part conductor sizing requirement for PV circuits. |
| 210.19(A)(1) | The equivalent two-part conductor sizing requirement for branch circuits. |
| 210.19(A) Informational Note 4 | The 3 % voltage drop recommendation. Advisory, not enforceable — the calculator says so. |
| 240.6(A) | Standard ampere ratings for fuses and inverse time circuit breakers. |
| 240.4(D) and 240.4(G) | Small-conductor overcurrent limits, and the exemption that puts PV conductors under 690.8/690.9 instead. |
| 690.9(B) | Overcurrent device ratings for PV circuits. |
| 310.10(G) | Where paralleling conductors is permitted, referenced when no single conductor is large enough. |
The NEC is published by the National Fire Protection Association and is available to read free of charge through NFPA's online access. We reproduce the numeric tables below because they are what the tools compute from and you should be able to audit them; the code text itself is NFPA's.