How do I handle voltage drop for a 120/240V circuit?

For a 240V load (connected line-to-line), use 240V as the base voltage and the factor of 2 in the formula. For a 120V load (connected line-to-neutral), use 120V as the base voltage. On a shared neutral multi-wire branch circuit, the neutral carries only the unbalanced current, but voltage drop should be checked for the worst-case 120V load scenario since that produces the highest percentage drop. The same #10 AWG conductor that passes at 240V may fail at 120V.

Why does voltage drop often require larger conductors than ampacity?

Ampacity tables (NEC 310.16) are based on thermal limits — the maximum current before the insulation overheats. Voltage drop is based on Ohm's law (V = I × Z × L). For short runs, the ampacity-selected conductor easily meets VD limits. But for long runs (especially at 120V), the cumulative resistance over distance causes voltage drop to exceed 3% long before the conductor's ampacity is reached. A common rule of thumb: for 120V circuits longer than 50 feet, and 208/240V circuits longer than 100 feet, always check voltage drop.

Can I use aluminium conductors to reduce voltage drop cost on long feeders?

Aluminium has about 1.6× the resistance of copper for the same cross-sectional area, so it does not reduce voltage drop per se. However, aluminium is significantly cheaper per ampere of capacity, so you can upsize the aluminium conductor to meet voltage drop limits at a lower cost than the copper equivalent. For example, 500 kcmil aluminium (R = 0.0519 Ω/1000ft) has similar resistance to 350 kcmil copper (R = 0.0468 Ω/1000ft) but at roughly 40% lower material cost. Aluminium feeders are very common in US commercial and industrial installations for this reason.

Voltage Drop Calculator — NEC/NFPA 70 🇺🇸

United StatesEdition 2023 (National Electrical Code)Free Online Tool

The National Electrical Code (NEC/NFPA 70:2023) addresses voltage drop through informational notes rather than mandatory requirements — a distinctive approach compared to other international standards. NEC 210.19(A) Informational Note No. 4 recommends that branch circuit conductors be sized to limit voltage drop to no more than 3%, while NEC 215.2(A) Informational Note No. 2 recommends that the combined voltage drop of feeder and branch circuit not exceed 5%.

Although these limits are advisory, they represent widely accepted good engineering practice and are routinely enforced by designers, engineers, and many Authorities Having Jurisdiction (AHJs). The calculation methodology uses impedance data from Chapter 9, Table 9, which provides AC resistance and reactance values in ohms per 1000 feet for standard conductor sizes in various conduit types (PVC, aluminium, and steel).

NEC uses the American Wire Gauge (AWG) and kcmil system for conductor sizing, and imperial units (feet) for cable length. Supply voltages in the US include 120V, 208V, 240V (residential and small commercial), 277V (commercial lighting), and 480V (industrial). These multiple voltage levels, combined with long building runs common in US construction, make voltage drop analysis an essential part of NEC-compliant electrical design.

Open Voltage Drop Calculator

How Voltage Drop Works Under NEC/NFPA 70

Voltage Drop Methodology per NEC/NFPA 70:2023

The NEC voltage drop calculation uses conductor impedance values from Chapter 9 tables. Unlike the IEC or BS approach that provides mV/A/m directly, the NEC tables give resistance and reactance in ohms per 1000 feet, requiring the engineer to build up the voltage drop formula from these components.

Step 1: Determine Circuit Parameters

Identify the key design parameters:

Design current I (amperes)
One-way conductor length L (feet)
Supply voltage (120V, 208V, 240V, 277V, or 480V)
Circuit type: single-phase or three-phase
Power factor cosθ (and sinθ)
Conductor size (AWG or kcmil)
Conduit type (steel, aluminium, or PVC)

Step 2: Look Up Impedance Values from Chapter 9

Table 9 (AC Resistance and Reactance for 600-Volt Cables, 3-Phase, 60 Hz, 75°C) provides:

R — AC resistance (Ω/1000ft) at 75°C conductor temperature
X_L — reactance (Ω/1000ft) for the conduit type

The resistance values vary by conductor material (copper or aluminium) and wire coating (coated or uncoated). The reactance varies by conduit material because the magnetic properties of the conduit (especially steel) affect the cable's inductance. PVC conduit has the lowest reactance, steel the highest.

Table 8 provides DC resistance at 75°C, which can be used for DC circuits or as a starting point when AC resistance from Table 9 is not available for a specific configuration.

Step 3: Calculate Single-Phase Voltage Drop

For single-phase circuits, the formula accounts for the go-and-return path:

VD = 2 × L × I × (R × cosθ + X_L × sinθ) / 1000

The factor of 2 accounts for both the line and neutral (or line and ground for 120V circuits) conductors. The division by 1000 converts the per-1000ft impedance to per-foot.

Step 4: Calculate Three-Phase Voltage Drop

For three-phase balanced circuits:

VD = √3 × L × I × (R × cosθ + X_L × sinθ) / 1000

The √3 factor (1.732) replaces the factor of 2 used in single-phase calculations. Note that Table 9 values are specifically for 3-phase 60Hz systems; the reactance values are appropriate for 60Hz operation.

Step 5: Calculate Percentage Voltage Drop

VD% = (VD / V_source) × 100

Use the appropriate source voltage: 120V or 240V for single-phase residential, 208V or 480V for three-phase commercial/industrial, or 277V for single-phase lighting on a 480V system.

Step 6: Check Against NEC Recommendations

Per the informational notes in NEC 210.19(A) and 215.2(A):

Branch circuit: ≤ 3% voltage drop recommended
Feeder + branch circuit combined: ≤ 5% total voltage drop recommended

While these are not Code requirements, they represent the standard of care expected by most AHJs and engineers. Some jurisdictions and specifications may impose stricter limits (e.g., 2% feeder, 3% branch, 5% total is a common engineering specification).

Key Reference Tables

Chapter 9, Table 9 — AC Resistance and Reactance (600V, 3-Phase, 60Hz, 75°C)

The primary table for NEC voltage drop calculations. Provides AC resistance (Ω/1000ft) and reactance (Ω/1000ft) for copper and aluminium conductors from #14 AWG to 2000 kcmil. Separate reactance columns for PVC conduit, aluminium conduit, and steel conduit. Resistance values assume 75°C conductor temperature.

Use for all AC voltage drop calculations. Select the conductor size row and the appropriate conduit column. For non-metallic (NM) cable, use the PVC conduit reactance column. The combined effective impedance Z_eff = R×cosθ + X_L×sinθ.

Chapter 9, Table 8 — DC Resistance of Conductors

Provides DC resistance (Ω/1000ft) at 75°C for both coated (tinned) and uncoated copper conductors and aluminium conductors. Covers sizes from #18 AWG to 2000 kcmil. Does not include AC effects (skin effect, proximity effect).

Use for DC circuit voltage drop (solar PV, battery systems, DC feeders). Also useful as a baseline to understand the AC resistance increase shown in Table 9 for larger conductors where skin effect is significant.

NEC 210.19(A) Informational Note No. 4 — Branch Circuit Voltage Drop

Recommends that branch circuit conductors be sized so that the maximum voltage drop does not exceed 3% at the farthest outlet of utilisation under full load. Notes that the total voltage drop for feeders and branch circuits should not exceed 5%.

The primary reference for branch circuit VD design targets. Although labelled as an informational note (non-mandatory), it establishes the standard of care and is widely enforced by AHJs and specifying engineers.

NEC 215.2(A) Informational Note No. 2 — Feeder Voltage Drop

Recommends that feeder conductors be sized so that the maximum voltage drop does not exceed 3% at the farthest outlet of utilisation when combined with the branch circuit drop (or 5% total). Effectively limits the feeder VD to 2% when the branch circuit uses its full 3% allocation.

Use to determine feeder conductor sizing. In practice, allocate 2% to the feeder and 3% to branch circuits for a 5% total budget, though the specific allocation depends on the installation layout.

NEC 310.16 — Ampacity Table (75°C Column)

The standard ampacity table for conductors at 75°C in raceway or cable. While primarily for ampacity (current-carrying capacity), the conductor size selected from this table is the starting point for the voltage drop check. If the ampacity-selected conductor fails the VD check, it must be upsized.

Use to select the initial conductor size based on ampacity, then verify voltage drop compliance. Voltage drop frequently governs conductor sizing for long runs, requiring a larger conductor than the ampacity table alone would indicate.

Chapter 9, Table 5 — Dimensions of Insulated Conductors

Provides the outside diameter and cross-sectional area of insulated conductors for various insulation types (THHN, THWN-2, XHHW, etc.). While not directly used in voltage drop calculations, it is needed for conduit fill calculations that accompany conductor sizing.

Reference when upsizing conductors for voltage drop to verify the larger conductor still fits within the specified conduit without exceeding fill limits per Table 1 (Chapter 9).

Worked Example — NEC/NFPA 70 Voltage Drop

Scenario

A 120V single-phase branch circuit supplies a 30A load via #10 AWG THWN-2 copper conductors in a steel (EMT) conduit. The one-way conductor length is 150 feet (46m). Calculate the voltage drop and check against NEC recommendations.

Identify circuit parameters

Design current I = 30A, one-way length L = 150 ft, supply voltage = 120V single-phase, conductor = #10 AWG copper, insulation = THWN-2 (75°C rated), raceway = steel EMT conduit, power factor cosθ = 1.0 (resistive load).

Look up R and X_L from Chapter 9 Table 9

For #10 AWG copper conductor at 75°C: AC resistance R = 1.29 Ω/1000ft (uncoated copper column). Reactance for steel conduit X_L = 0.063 Ω/1000ft.

R = 1.29 Ω/1000ft, X_L = 0.063 Ω/1000ft (Table 9, #10 AWG row)

R = 1.29, X_L = 0.063 Ω/1000ft

Calculate effective impedance per 1000ft

For unity power factor (cosθ = 1.0, sinθ = 0), the reactive component is zero.

Z_eff = R × cosθ + X_L × sinθ = 1.29 × 1.0 + 0.063 × 0 = 1.29 Ω/1000ft

Z_eff = 1.29 Ω/1000ft

Calculate single-phase voltage drop

Apply the single-phase formula with the factor of 2 for go-and-return path.

VD = 2 × L × I × Z_eff / 1000 = 2 × 150 × 30 × 1.29 / 1000

VD = 11.61V

Calculate percentage voltage drop

Express as a percentage of the 120V source voltage.

VD% = (11.61 / 120) × 100

VD% = 9.68%

Check against NEC recommendations

The 9.68% voltage drop far exceeds both the 3% branch circuit recommendation (3.6V at 120V) and the 5% total recommendation (6.0V at 120V). This conductor must be upsized. Trying #6 AWG (R = 0.510 Ω/1000ft): VD = 2 × 150 × 30 × 0.510 / 1000 = 4.59V (3.83%). Trying #4 AWG (R = 0.321 Ω/1000ft): VD = 2 × 150 × 30 × 0.321 / 1000 = 2.89V (2.41%). The #4 AWG conductor at 2.41% satisfies the 3% branch recommendation.

9.68% > 3% — FAIL. Upsize to #4 AWG: VD = 2.41% — PASS

The #10 AWG conductor is grossly inadequate for voltage drop on this 150ft, 30A, 120V circuit — despite being rated for 30A ampacity per NEC 310.16 (75°C column). This demonstrates why voltage drop frequently governs conductor sizing for long runs at lower voltages. The conductor must be upsized to #4 AWG (three sizes larger) to achieve 2.41% voltage drop, meeting the 3% branch circuit recommendation. This example underscores that 120V circuits are particularly sensitive to voltage drop: the same 150ft run at 240V would only have 4.84% drop with #10 AWG.

Common Mistakes When Using NEC/NFPA 70

1
Treating NEC voltage drop limits as mandatory Code requirements — NEC 210.19(A) and 215.2(A) voltage drop limits are contained in Informational Notes, which per NEC 90.5(C) are explanatory material and are not enforceable as Code. However, they represent accepted good practice and many AHJs, specifications, and engineering standards of care treat them as effectively mandatory. Not designing to these limits may expose the engineer to liability.
2
Using DC resistance from Table 8 instead of AC resistance from Table 9 — Table 8 gives DC resistance only, which does not account for skin effect and proximity effect at 60Hz. For large conductors (above 250 kcmil), the AC resistance can be 10-25% higher than DC. Using Table 8 values for AC circuits underestimates voltage drop and may result in undersized conductors.
3
Forgetting that conduit type affects reactance — Chapter 9 Table 9 provides different reactance values for PVC, aluminium, and steel conduits. Steel conduit has the highest reactance because the magnetic steel influences the cable's inductance. For large conductors where reactance is significant, using PVC reactance for a steel conduit installation underestimates the voltage drop.
4
Not accounting for conductor temperature rise on resistance — Table 9 values are at 75°C. If the conductor operates at a higher temperature (e.g., 90°C for THWN-2 used at its full 90°C ampacity rating), the resistance increases approximately 6%. Some engineers use the 75°C ampacity column with Table 9, which is consistent. But if using 90°C ampacity and Table 9's 75°C resistance, the voltage drop is slightly underestimated.
5
Neglecting the impact of low supply voltage on VD percentage — a 120V circuit has double the percentage voltage drop of a 240V circuit for the same current and cable length, because the same absolute voltage drop represents a larger fraction of the lower source voltage. Designers accustomed to 208V or 480V systems often underestimate the severity of voltage drop on 120V runs.

How Does NEC/NFPA 70 Compare?

The NEC is unique among major wiring standards in that its voltage drop limits are informational notes (advisory) rather than mandatory requirements. BS 7671 and AS/NZS 3000 impose binding voltage drop limits. The NEC uses AWG/kcmil conductor sizes and imperial units (Ω/1000ft, feet), while IEC-based standards use metric (mm², Ω/m, metres). NEC Chapter 9 Table 9 provides separate columns for conduit type (steel, aluminium, PVC), reflecting that conduit material affects inductance — a level of detail not typically provided in BS 7671 or AS/NZS 3008 tables. The fundamental formula, however, is the same impedance-based approach used by IEC 60364.

Frequently Asked Questions

Advisory. The 3% branch circuit and 5% total limits appear in Informational Notes (NEC 210.19(A) IN No. 4 and 215.2(A) IN No. 2), which per NEC 90.5(C) are not enforceable Code requirements. However, they are universally recognized as good engineering practice, are frequently cited in project specifications, and many AHJs treat non-compliance as a deficiency. Designing to exceed these limits may also create professional liability exposure.

Table 8 provides DC resistance only, at 75°C, for both coated and uncoated conductors. Table 9 provides AC resistance and reactance at 75°C for 3-phase 60Hz systems, with separate reactance columns for PVC, aluminium, and steel conduits. For AC circuit voltage drop calculations, always use Table 9. Table 8 is appropriate for DC circuits (solar PV, battery feeders, DC drives) or when Table 9 does not cover your specific conductor configuration.

The conduit material affects the cable's reactance (inductive component). Steel conduit has the highest reactance because ferrous metal concentrates the magnetic field. PVC conduit has the lowest reactance. For small conductors (#14 to #6 AWG), the reactance is negligible and conduit type has minimal impact. For large conductors (250 kcmil and above), using steel conduit instead of PVC can increase the total impedance by 5-15%, depending on conductor size and power factor.

Voltage Drop Calculator for Other Standards

🇦🇺AS/NZS 3008.1.1

🇬🇧BS 7671

🌍IEC 60364-5-52