In-depth guide · NEC 2023 · 6 steps

Voltage drop vs ampacity: the complete circuit-sizing workflow

Every circuit answers two independent questions — will the wire overheat? andwill the load see enough volts? — plus a third the field forgets:do the wires physically fit? This guide chains all three checks in the order electricians actually work, with the numbers shown at every step.

Published October 2026 · Updated October 2026

1 · The two checks (and the forgotten third)

Ampacity is heat: Table 310.16 caps current so insulation stays below its 60/75/90°C rating, after temperature correction, bundling derating, terminal caps per 110.14(C), and breaker caps per 240.4(D). It is mandatory Code — inspectors enforce it, and it protects against fire.Voltage drop is performance: NEC informational notes recommend ≤3% per branch circuit and ≤5% feeder-plus-branch combined. It is not hard code in most jurisdictions, but motors stall, lights dim, contactors chatter, and electronics fault when drop runs high. The formula is 2·K·I·L÷CM single-phase (K = 12.9 copper, 21.2 aluminum, L = one-way feet). The forgotten third check is conduit fill: wires must physically fit under Chapter 9 Table 1 (53% one wire, 31% two, 40% three or more, 60% nipples) — and a packed raceway is exactly what triggers the bundling derating in step 3. For the short version, see ourampacity vs voltage drop reference.

2 · The 6-step workflow

Step 1 — True load with the 125% continuous factor. Loads running 3+ hours (EV chargers, heaters, lighting) size conductors at 125% of load current per 210.19(A)(1). A 48A EV charger is a 60A conductor-sizing problem before any table is opened. Get this wrong and every later step inherits the error.

Step 2 — Conduit fill. List every conductor — hots, neutral, ground — and runconduit fill. The conductor count sets the Table 1 percentage, the ground counts toward area (Note 3), and the count also tells you whether bundling derating applies in step 3. Check jam ratio 2.8–3.2 while you are here.

Step 3 — Ampacity with derating. Start at the insulation column (THHN = 90°C), apply ambient correction per 310.15(B)(1) and bundling per 310.15(C)(1), cap at the terminal column per 110.14(C) (usually 75°C), then apply 240.4(D) small-conductor breaker caps. Run it in thewire size calculator — the classic attic case (#10 THHN, 104°F, 6 conductors: 40 × 0.91 × 0.80 = 29.1A) is where hand math dies and the tool earns its keep.

Step 4 — Voltage drop for the actual distance. Measure one-way feet (panel to load — entering round-trip length is the most common drop mistake) and check 3% branch / 5% total in thevoltage drop calculator. If drop fails while ampacity passes, upsize for performance: the wire is thermally fine but the load disagrees.

Step 5 — Breaker and ground. Breaker = next standard size at or below final ampacity (240.6), subject to 240.4(D) caps. Ground = Table 250.122 by breaker amps. The ground was already counted in fill (step 2) and excluded from derating (step 3) — confirm both.

Step 6 — Print, share, verify. Copy the shareable link for the crew, save the one-page PDF for the permit folder, and verify against your AHJ's adopted edition plus local amendments. FieldCalc follows NEC 2023; your inspector follows whatever your jurisdiction adopted.

3 · When ampacity governs

Short branch circuits in cool, uncrowded raceways are ampacity jobs. The signatures: runs under ~60 ft at 120V (farther at 240V), ambient near 86°F, three or fewer current-carrying conductors, 75°C terminals. Here Table 310.16 plus terminal and breaker caps decide everything — #12 copper on a 20A breaker, #10 on 30A, done. Ampacity also governs whenever heat piles up regardless of length: hot attics (122°F pushes correction to ×0.71–0.82), packed conduits (10–20 conductors derate to ×0.50 before temperature even applies), and continuous loads (the 125% factor in step 1). If your run matches any of those, size for heat first and treat drop as a formality check.

4 · When voltage drop governs

Drop scales with distance; ampacity does not — so every long run eventually becomes a drop job. The crossover for 20A at 120V on #12 copper lands around 60 ft one-way; at 240V the same load reaches ~120 ft before 3% bites (double the volts, double the distance at equal percent). Three-phase stretches further still per amp. Low-voltage work is the extreme: 1V lost is 8.3% of a 12V solar homerun but 0.8% of 120V, so a 10A 12V run on #10 fails 3% past ~14 ft. Rule of thumb: feeders to detached garages, EV chargers at the far end of the driveway, landscape lighting, and anything 12/24V — check drop first, because it will decide the wire. See precomputed limits in themax distance chart.

5 · Three worked circuits

Example A — 20A kitchen receptacles, 120V, 80 ft, 3/4″ EMT.Fill: four #12 THHN + ground fit easily. Ampacity: #12 THHN at base conditions = 25A at 75°C terminals, 20A breaker per 240.4(D) — passes. Drop: 2 × 12.9 × 20 × 80 ÷ 6530 = 6.3V = 5.3% — fails 3%. Verdict: drop governs; pull #10 (3.3%) or #8 for headroom.

Example B — 48A EV charger, 240V, 150 ft, continuous.Step 1: 48 × 1.25 = 60A minimum conductor. Ampacity: #6 THHN copper = 65A at 75°C — passes barely. Drop: 2 × 12.9 × 48 × 150 ÷ 26240 = 7.1V = 2.9% — passes barely. Verdict: both constraints bite; #6 works on paper but #4 buys margin against a hot garage and future 60A charging. This is the exact case the integrated check in thewire size calculator was built for.

Example C — 100A subpanel feeder, 240V, 200 ft.#3 THHN copper: 100A at 75°C terminals, 100A breaker — passes exactly. Drop at full load: 2 × 12.9 × 100 × 200 ÷ 52620 = 9.8V = 4.1% on the feeder alone — over a 3% feeder budget and eating the whole 5% total. Verdict: drop governs hard; #2 copper (≈3.2%) minimum, #1/0 aluminum as the value alternative. Confirm with thevoltage drop calculator before trenching.

6 · Breaker, ground, and sign-off

With the wire settled, the breaker protects it: standard sizes per 240.6, never above final ampacity, and never above the 240.4(D) ceiling for #14/#12/#10 copper. The ground follows Table 250.122 by breaker size — a 100A feeder takes #8 copper ground, confirmed in the worked sizing examples. Then sign off properly: shareable link to the crew, one-page PDF to the permit folder, and a final check against the edition your AHJ enforces — including the local amendments that no national calculator can know.

Field checklist

Continuous load? ×1.25 before opening any table.
Every conductor listed — hots, neutral, ground — for fill.
Derating started at the insulation column, capped at terminals.
One-way feet (not round-trip) entered for drop.
Breaker ≤ ampacity and ≤ 240.4(D); ground per 250.122.
Link shared, PDF saved, AHJ edition verified.

FAQ

Sizing questions, answered

Do I check ampacity or voltage drop first?

Ampacity first — it is mandatory Code (NEC 310.16), while drop is a recommended performance target (3% branch / 5% total). Size for heat, then verify volts delivered. The larger wire wins.

When does voltage drop govern over ampacity?

On long runs. A 20A 120V load on #12 copper passes ampacity easily yet exceeds 3% drop past ~60 ft one-way. Higher voltage (240V vs 120V) and three-phase both push the crossover farther out.

Is the 3% voltage drop a code requirement?

No — NEC 210.19 presents 3% per branch circuit and 5% feeder-plus-branch as informational recommendations, not hard code (fire pumps excepted). Most engineers and inspectors still treat them as pass/fail.

Do grounds count for derating like they do for fill?

No — and that split causes real mistakes. Equipment grounds count toward conduit fill (Chapter 9 Note 3) but never count as current-carrying for 310.15(C)(1) derating. Fill and ampacity are two independent tests.

What breaker do I use once the wire is sized?

The next standard size at or below the final ampacity per 240.6 — then apply 240.4(D) small-conductor caps (#14→15A, #12→20A, #10→30A copper). Size the ground from Table 250.122 by that breaker.

Size a circuit →Check voltage drop →Check conduit fill →