Conductor voltage drop

Wire has resistance. Current through that resistance creates a voltage loss according to Ohm's law. The load receives source voltage minus drop.

Symptoms at the load end:

• motors hard starting, overheating, shortened life,
• dim lighting, flicker when loads kick in,
• controls dropping out when a compressor starts,
• inconsistent heater output.

A circuit can be code-legal on ampacity and still fail in operation because drop is too high. Always check drop on long feeders and critical motor circuits.

For single-phase, two-wire (hot and neutral return), copper conductors, the common field formula is:

`` VD = (2 × K × I × L) / CM ``

Where:

Percent drop:

`` %VD = (VD / source voltage) × 100 ``

Example: 240 V circuit, VD = 7.2 V → 3% exactly.

Use line-to-neutral voltage for 120 V branch calculations and line-to-line for 240 V single-phase loads—stay consistent with how you report percent.

Three-phase feeders use a different multiplier (often √3 and 1.732 forms) because geometry differs. Delmar and NEC examples cover three-phase separately. On the job, use a listed calculator or NEC information annex for three-phase VD when you are not doing single-phase homeruns.

This lesson focuses on the single-phase 2×K×I×L/CM pattern most helpers use first for branch and small feeder checks.

K bundles resistivity and unit conversions for quick field math.

• Copper: K ≈ 12.9
• Aluminum: K ≈ 21.2 (higher drop per CM—expect larger Al wire for same VD)

CM comes from NEC Chapter 9 or wire tables (#12 ≈ 6530 CM, #10 ≈ 10380 CM, #6 ≈ 26240 CM—verify exact values in current tables).

NEC Informational Notes (not always enforceable as code unless adopted locally) suggest:

• 3% maximum voltage drop on branch circuits (feeder + branch combined often targeted 5% total from service to final outlet).

Many jurisdictions treat these as design standards. Even where not law, they are excellent engineering targets:

Motor circuits especially suffer when drop exceeds ~3% at running voltage—starting current magnifies the problem.

Use actual or design load current, not breaker rating:

• Continuous load → full calculated amps,
• Motor → use FLA or 125% where design rules require for sizing context,
• Receptacle general loads → calculated load per NEC demand rules, not 20 A because breaker is 20 A.

Calculating VD at breaker maximum while load is small gives false comfort. Calculate at expected operating amps.

If calculated %VD exceeds your target:

1. Increase AWG (lower gauge number → larger CM),
2. Reduce length (relocate panel—often not optional),
3. Raise source voltage (240 V instead of 120 V for same power where equipment allows),
4. Parallel conductors per NEC rules (next lesson) to increase effective CM.

Upsizing one step (e.g., 12 → 10 AWG) often fixes marginal residential runs. Long commercial feeders may jump several sizes or use parallel sets.

With equipment running:

1. Measure voltage at source (panel or disconnect).
2. Measure voltage at load terminals under same load.
3. Difference ≈ actual drop (includes connections—corrosion shows up here).

Compare to calculated prediction. Large gap suggests bad connection, not just wire length.

Static no-load voltage checks miss drop under current—always test under load when possible and safe.

Situation. A barn 180 feet from the house has a 120 V, 20 A circuit on 12 AWG. Lights OK idle; air compressor hums and trips on start. No-load at barn: 118 V. Under start: 102 V.

What happened. Drop under inrush is severe. Calculated running drop might pass; motor start pulls multiples of FLA. Undersized feeder length for 12 AWG at that distance.

Applied lesson. Run VD at starting and running current; upsize to 10 AWG or 8 AWG, or install 240 V circuit with step-down at barn if load allows. Measure under load to verify fix.