What is the significance of the 3% voltage drop in electrical installations?

The 3% voltage drop guideline helps ensure that electrical devices receive adequate voltage for optimal performance, reducing the risk of overheating and equipment failure.

Why is derating excluded when calculating voltage drop for this circuit?

Derating is excluded in this context because the 3% voltage drop guideline is a general recommendation for maintaining efficiency and does not account for additional factors that may apply in other scenarios.

How do I calculate the voltage drop for my circuit?

To calculate the voltage drop, you can use the formula: Voltage Drop (V) = (2 * Length of Circuit (ft) * Current (A) * Resistivity of Copper (Ω/ft)) / 1000. For a 240V circuit at 20A over 40 ft, ensure the drop does not exceed 7.2V.

20 amp copper run at 40 feet

Learn about the 3% NEC informational note regarding voltage drop for 240V copper branch circuits at 20A over 40 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

3.16 V

1.32% · end 236.8 V

Power loss

63 W

I²R at the operating current

Required ampacity

20.0 A

Each conductor 20 A

Material estimate

$138.48

3 conductors + EMT

Conductor weight

2.8 lb

1.3 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	40 ft one way	cuts excluded	3 conductors	USD	$64.48	Source Spec Material / Tool
EMT conduit	40 ft	—	40 ft	USD	$74.00	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	1.32%	236.8 V	63 W
aluminum	10 AWG · 5.26 mm²	5.3	1.36%	236.7 V	65 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
40 ft	12 AWG · 3.31 mm²	3.16 V	1.32%	236.8 V
65 ft	12 AWG · 3.31 mm²	5.14 V	2.14%	234.9 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 40 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 such guideline is the 3% voltage drop recommendation for branch circuits, which helps maintain electrical efficiency and equipment performance.

For a 240V copper branch circuit operating at 20A over a distance of 40 feet, the 3% voltage drop equates to a maximum allowable drop of 7.2V. This means that the voltage at the load should not fall below 232.8V to ensure optimal performance. It's important to note that derating factors, which may apply to other scenarios, are excluded when calculating the voltage drop for this specific case.

Code reference

Refer to NEC Article 210 for branch circuits and related voltage drop considerations.

Field questions

What is the significance of the 3% voltage drop in electrical installations?: The 3% voltage drop guideline helps ensure that electrical devices receive adequate voltage for optimal performance, reducing the risk of overheating and equipment failure.
Why is derating excluded when calculating voltage drop for this circuit?: Derating is excluded in this context because the 3% voltage drop guideline is a general recommendation for maintaining efficiency and does not account for additional factors that may apply in other scenarios.
How do I calculate the voltage drop for my circuit?: To calculate the voltage drop, you can use the formula: Voltage Drop (V) = (2 * Length of Circuit (ft) * Current (A) * Resistivity of Copper (Ω/ft)) / 1000. For a 240V circuit at 20A over 40 ft, ensure the drop does not exceed 7.2V.