What is the 3% voltage drop rule?

The 3% voltage drop rule suggests that the voltage drop in a branch circuit should not exceed 3% of the supply voltage to ensure efficient operation and minimize energy loss.

Why is derating excluded in this calculation?

Derating is excluded in the voltage drop calculation to provide a clear guideline for maintaining voltage levels without complicating the assessment with additional factors that may not directly affect the voltage drop.

How do I calculate voltage drop for my circuit?

To calculate voltage drop, use the formula: Voltage Drop (V) = (2 x Length of Circuit (ft) x Current (A) x Resistance of Conductor (ohms/ft)). For copper conductors, you can find resistance values in NEC tables.

What happens if the voltage drop exceeds 3%?

If the voltage drop exceeds 3%, it can lead to inefficient operation of electrical devices, potential overheating, and reduced lifespan of equipment.

30 amp copper run at 80 feet

Learn about the 3% voltage drop rule for 240V copper branch circuits at 30A over 80 feet. Understand why derating is excluded and how it affects your electrical installations.

Formula SSC-2026.1

K method · SSC-2026.1

Minimum size

10 AWG · 5.26 mm²

Single conductor set

Voltage drop

5.97 V

2.49% · end 234.0 V

Power loss

179 W

I²R at the operating current

Required ampacity

30.0 A

Each conductor 30 A

Material estimate

$345.60

3 conductors + EMT

Conductor weight

8.9 lb

4.0 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 10 AWG · 5.26 mm²	80 ft one way	cuts excluded	3 conductors	USD	$197.60	Source Spec Material / Tool
EMT conduit	80 ft	—	80 ft	USD	$148.00	Source Spec Material / Tool
30 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	10 AWG · 5.26 mm²	5.3	2.49%	234.0 V	179 W
aluminum	8 AWG · 8.37 mm²	8.4	2.57%	233.8 V	185 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
80 ft	10 AWG · 5.26 mm²	5.97 V	2.49%	234.0 V
105 ft	8 AWG · 8.37 mm²	4.92 V	2.05%	235.1 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: 30 amp copper run at 80 feet

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Date

Verified by (PE / electrician / inspector)

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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 electrical systems operate safely and efficiently. One such guideline is the 3% voltage drop rule for branch circuits, which helps maintain optimal performance and safety in electrical installations.

For a 240V copper branch circuit carrying 30A over a distance of 80 feet, the 3% voltage drop would be 7.2 volts. This means that the voltage at the load should not drop below 232.8 volts to ensure proper operation. It's important to note that derating factors, such as temperature or conduit fill, are excluded when calculating the voltage drop for this specific scenario.

Code reference

Refer to NEC Article 210.19(A)(1) for branch circuits and the informational note regarding voltage drop.

Field questions

What is the 3% voltage drop rule?: The 3% voltage drop rule suggests that the voltage drop in a branch circuit should not exceed 3% of the supply voltage to ensure efficient operation and minimize energy loss.
Why is derating excluded in this calculation?: Derating is excluded in the voltage drop calculation to provide a clear guideline for maintaining voltage levels without complicating the assessment with additional factors that may not directly affect the voltage drop.
How do I calculate voltage drop for my circuit?: To calculate voltage drop, use the formula: Voltage Drop (V) = (2 x Length of Circuit (ft) x Current (A) x Resistance of Conductor (ohms/ft)). For copper conductors, you can find resistance values in NEC tables.
What happens if the voltage drop exceeds 3%?: If the voltage drop exceeds 3%, it can lead to inefficient operation of electrical devices, potential overheating, and reduced lifespan of equipment.