Short answer: NEC 210.19(A) Informational Note 4recommends sizing branch-circuit conductors so the voltage drop at the farthest outlet doesn't exceed 3%, with a combined feeder + branch-circuit drop of 5%. The important word is recommends — Informational Notes are not enforceable code under NEC 90.5(C). But many municipalities, engineers, and inspectors enforce 3%/5% by adoption, contract spec, or AHJ practice. Below is what the rule actually says, why it exists, the math to check it, and where it stops being optional and becomes mandatory.

What 210.19(A)(1) actually says

NEC 210.19(A)(1) is the enforceable rule on branch-circuit conductor sizing. Paraphrased:

• Branch-circuit conductors must have an ampacity not less than the maximum load to be served.
• Where a branch circuit supplies continuous loads, the minimum branch-circuit conductor size, before any adjustment factors, must have an allowable ampacity not less than the non-continuous load plus 125% of the continuous load.
• Conductors must be sized to prevent a temperature rise that could damage the insulation.

Then comes Informational Note 4 (paraphrased):

"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, will
provide reasonable efficiency of operation."

That's the full extent of the "rule." Three key observations:

• It says "will provide reasonable efficiency." Not "shall." Recommendation, not requirement.
• It applies to the farthest outlet, not every outlet.
• 3% is for the branch alone; 5% is the combined feeder-plus-branch budget.

The voltage drop formula

Single-phase (2-wire):
  VD  = 2 × K × I × L / cmil

3-phase (3-wire balanced):
  VD  = √3 × K × I × L / cmil
      = 1.732 × K × I × L / cmil

DC:
  VD  = 2 × K × I × L / cmil  (same as single-phase)

K  = conductor resistivity (Ω-cmil/ft)
     Copper:    12.9 at 75°C
     Aluminum:  21.2 at 75°C
I  = circuit current in amps
L  = ONE-WAY length in feet
cmil = circular mils (NEC Ch 9 Table 8)

Worked example: 20 A circuit, 100 ft, #12 AWG

Setup:
  Load:        20 A continuous (lighting / receptacle)
  Length:      100 ft one-way
  Wire:        #12 AWG copper THHN (cmil = 6,530)
  Voltage:     120 V single-phase

VD = 2 × 12.9 × 20 × 100 / 6,530
   = 51,600 / 6,530
   = 7.90 V

VD percentage = 7.90 / 120 = 6.6%

Verdict: 6.6% > 3% — FAILS the 210.19(A) IN 4 recommendation
                     by a large margin.

Solution: upsize.

#10 AWG (cmil = 10,380):
  VD = 51,600 / 10,380 = 4.97 V = 4.1%   STILL FAILS 3%

#8 AWG (cmil = 16,510):
  VD = 51,600 / 16,510 = 3.13 V = 2.6%   PASSES

For a 20 A continuous lighting circuit at 100 ft, you need #8 Cu
to comply with the 3% recommendation — or accept the 4-6% drop
on #10 if the AHJ doesn't enforce.

Where 3% becomes mandatory (not just recommended)

Several places in the NEC do impose voltage drop as an enforceable requirement, even though the general 210.19 IN is advisory:

• NEC 647.4(D) — Sensitive Electronic Equipment: 1.5% max VD on branch circuits supplying audio/video/computing equipment.
• NEC 695.7 — Fire Pumps: voltage at the controller, with motor running at 115% of FLA, must not drop more than 15% below normal voltage. Mandatory.
• NEC 215.2(A)(1)(b) — Feeders for Mobile Home / Park: 1.0% max VD on the feeder, 5% combined.
• NEC 690.45 / 690.7 — PV Systems: 2% max VD recommended on DC strings (Note), but UL inverter listings may force compliance.
• State and local codes: California Title 24, Massachusetts MEC, NYC EC all enforce 3%/5% by adoption.
• Project specs: most consulting engineers and architects specify 3%/5% on drawings as a contract requirement.

Why VD matters even when not enforced

Effect of voltage drop on common loads:

Resistive loads (heaters, baseboards):
  Power ∝ V²
  5% VD = (0.95)² = 90% of rated power output
  → 5% VD on a 1500W heater = 1350W actual delivered (10% loss)

Motors (fan, compressor, pump):
  Torque ∝ V², current rises to compensate, runs hotter
  5% VD = ~10% torque loss, ~10% temperature rise
  → Bearings + windings degrade faster

Lighting (LED):
  Most LED drivers tolerate ±10% V — flicker / dimming at edge
  Older incandescent: 5% VD = ~17% lumen reduction (yes, that high)

EV charging:
  Time to charge ∝ delivered W
  5% VD = ~10% slower charge (Power ∝ V × I)
  → 8 hr charge becomes 8.8 hr

Sensitive electronics:
  Below 5% drop, most equipment is fine.
  Above 5%, you start seeing reset / brownout at peak loads.

The 5% combined feeder + branch budget

The 5% combined ceiling is what catches most engineers: you can have a 3% branch drop AND a 3% feeder drop and still violate the recommendation, because combined is what matters at the load. Worked:

Scenario:
  Service:                  240 V at the meter
  Feeder to subpanel:       125 ft, 50 A continuous → some drop
  Branch from subpanel:     100 ft, 20 A continuous → some drop
  Total path length:        225 ft

If feeder drops 3.0% and branch drops 3.0% (each within IN 4
"recommendation"), combined drop is 6.0% — over the 5% budget.

The fix: pick wire sizes so feeder drops ~2% and branch drops ~3%,
or feeder ~2.5% and branch ~2.5%. Combined = 5.0%, at the edge.

In practice: solve for VD across the whole path at once, not per
section. Long-run feeder = upsize the feeder.

Quick-reference: max single-phase 120 V run lengths at 3% VD

Wire size, copper THHN, single-phase 120 V, 3% VD = 3.6 V max:

Ampere     #14    #12    #10    #8     #6     #4     #2
─────────  ─────  ─────  ─────  ─────  ─────  ─────  ─────
10 A       91 ft  144    230    365    580    922    1466
15 A       60 ft   96    154    243    387    615     978
20 A       46 ft   72    115    183    290    461     733
30 A         —     48     77    122    194    307     489
40 A         —      —     58     91    145    230     367
50 A         —      —     46     73    116    184     294
60 A         —      —      —     61     96    154     244

Cells "—" mean the wire fails ampacity OR is too short for the rated
current to make a reasonable run before VD limits.

For 240V single-phase, double the listed lengths (V drop budget doubles).
For 3-phase 208V, multiply by 0.866. For 3-phase 480V, multiply by 4.

Common voltage drop misconceptions

• "My breaker doesn't trip, so the wire is fine." Voltage drop doesn't trip breakers — it dims lights and cooks motors.
• "Code says 3%, period." No, the IN says 3% is recommended for 'reasonable efficiency.' Local code adoption makes it enforceable in some states.
• "Use #12 for 20 A always." Only true for runs under ~46 ft on a 20 A continuous circuit at 120 V.
• "Aluminum and copper drop the same." No. Aluminum's K is 21.2 vs copper's 12.9 — aluminum drops ~64% more voltage at the same length and amperage.
• "Voltage drop only matters in commercial." Wrong. EV chargers, well pumps, and detached garages in residential are the most common VD failures.

How to design for VD up front

• Estimate the longest run before pulling. The garage feeder is usually the worst case in a residential project.
• Pick the wire size that satisfies BOTH ampacity (310.16 + derating) and 3% VD, whichever is bigger.
• If the run is 200+ ft, default to upsize one or two AWG over ampacity-required size.
• For motors and EV chargers, use 2% VD as the target, not 3%. Saves equipment and saves callbacks.
• When in doubt, run the math. Forum rules of thumb stop being correct past 100 ft.

The 100/200/400 rule of thumb

Quick mental check before pulling wire:

Run length              Default to
────────────            ─────────────────────
< 75 ft                  Ampacity table only
75–150 ft                Ampacity + check 3% VD
150–250 ft               One AWG up from ampacity-required
250–400 ft               Two AWG up; consider 240V/3-phase
> 400 ft                 Re-think — buck-boost transformer, sub-feed,
                         or run at higher voltage

Run it on your phone

The ElectricianCalc app sizes conductors for ampacity AND voltage drop simultaneously, so the recommended wire is the bigger of the two checks — without you having to run two calculations. Type the load, distance, voltage, and material; get the size that passes both. 100% offline. Free on the App Store and Google Play.

Related

• Wire size for 40A continuous, 180 ft, 240V single-phase
• Conduit fill: 4× #10 + #6 THHN in 3/4" EMT
• NEC wire sizing for branch circuits
• ElectricianCalc — NEC calculator on iOS + Android

Note: NEC 210.19 Informational Note 4 is a recommendation, not a code requirement, in the 2023 NEC. Always verify enforceability against the code edition adopted by your AHJ and any project-specific spec.