What is the significance of the 3% voltage drop rule?

The 3% voltage drop rule ensures that electrical equipment operates efficiently and reliably by maintaining adequate voltage levels throughout the circuit.

Why is derating excluded when calculating voltage drop?

Derating is excluded to simplify the voltage drop calculations and focus on maintaining voltage levels, rather than adjusting for temperature or other factors that could complicate the assessment.

How do I calculate voltage drop for my circuit?

To calculate voltage drop, you can use the formula: Voltage Drop (V) = (2 * Length of Circuit (ft) * Current (A) * Resistance of Conductor (Ω/ft)). Ensure to compare the result against the 3% guideline for your specific voltage.

20 amp copper run at 80 feet

Learn about the 3% NEC informational note regarding voltage drop for 240V copper branch circuits at 20A over 80 ft. Understand why derating is excluded in this context.

Formula SSC-2026.1

K method · SSC-2026.1

Minimum size

12 AWG · 3.31 mm²

Single conductor set

Voltage drop

6.32 V

2.63% · end 233.7 V

Power loss

126 W

I²R at the operating current

Required ampacity

20.0 A

Each conductor 20 A

Material estimate

$276.96

3 conductors + EMT

Conductor weight

5.5 lb

2.5 kg · approximate insulated weight

Single-line and conductor gaugeLive scaled blueprint

Circle diameters track circular mils. The drawing is a schematic, not a conduit-fill layout.

Bill of materials and shopping list
Item	Net	Waste	Order	Unit price	Line total	Source Spec Material / Tool
copper 12 AWG · 3.31 mm²	80 ft one way	cuts excluded	3 conductors	USD	$128.96	Source Spec Material / Tool
EMT conduit	80 ft	—	80 ft	USD	$148.00	Source Spec Material / Tool
20 A breaker	2-pole	—	1	Match the calculated load	—	Source Spec Material / Tool

Source links open a product search on the Amazon storefront for the selected currency. SiteSpecCalc may earn a commission. The link does not change the quantities above.

Copper versus aluminum at this run
Material	Minimum size	mm²	Drop	End voltage	Loss
copper	12 AWG · 3.31 mm²	3.3	2.63%	233.7 V	126 W
aluminum	10 AWG · 5.26 mm²	5.3	2.72%	233.5 V	131 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
80 ft	12 AWG · 3.31 mm²	6.32 V	2.63%	233.7 V
105 ft	10 AWG · 5.26 mm²	5.22 V	2.17%	234.8 V

Show the deterministic formula

Single phase: Vdrop = 2 × K × I × D / cmil. Three phase: Vdrop = 1.732 × K × I × D / cmil. K is 12.9 for copper and 21.2 for aluminum. D is one-way feet.

Percent drop = Vdrop ÷ source voltage × 100. Green is 3% or less, amber is 3 to 5%, and red is over 5%. Power loss is I²R on the current-carrying conductors.

The smallest conductor that meets both the ampacity (125% when continuous) and the drop limit is selected. Ampacity does not include bundling or temperature correction.

Contractor sign-off

Project: 20 amp copper run at 80 feet

Prepared by

Date

Verified by (PE / electrician / inspector)

License number

Signature

This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.

Engineering notes

The National Electrical Code (NEC) provides guidelines to ensure safe and efficient electrical installations. One important aspect is the allowable voltage drop in branch circuits. For a 240V copper branch circuit carrying 20A over a distance of 80 feet, the NEC recommends a maximum voltage drop of 3% to maintain equipment performance and efficiency.

This 3% guideline is an informational note, meaning it serves as a recommendation rather than a strict requirement. It's important to note that derating of conductors due to temperature or other factors is excluded when calculating this voltage drop, as the focus is on maintaining voltage levels for optimal operation.

Code reference

National Electrical Code (NEC) - Article 210

Field questions

What is the significance of the 3% voltage drop rule?: The 3% voltage drop rule ensures that electrical equipment operates efficiently and reliably by maintaining adequate voltage levels throughout the circuit.
Why is derating excluded when calculating voltage drop?: Derating is excluded to simplify the voltage drop calculations and focus on maintaining voltage levels, rather than adjusting for temperature or other factors that could complicate the assessment.
How do I calculate voltage drop for my circuit?: To calculate voltage drop, you can use the formula: Voltage Drop (V) = (2 * Length of Circuit (ft) * Current (A) * Resistance of Conductor (Ω/ft)). Ensure to compare the result against the 3% guideline for your specific voltage.