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

The 3% voltage drop rule helps ensure that electrical devices operate efficiently and safely, preventing performance issues due to insufficient voltage at the load.

Why is derating excluded when calculating voltage drop?

Derating is excluded in this context to simplify calculations and focus on the conductor's ability to handle the specified current over the given distance without additional factors affecting performance.

How do I calculate the 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)). Ensure the result does not exceed 3% of the supply voltage.

What happens if my voltage drop exceeds 3%?

If the voltage drop exceeds 3%, it may lead to inefficient operation of electrical devices, potential overheating, and increased energy costs. It is advisable to adjust the conductor size or reduce the circuit length.

20 amp copper run at 100 feet

Learn about the 3% voltage drop rule from the NEC for 240V copper branch circuits at 20A over 100 ft. Understand how derating does not apply in this context.

Formula SSC-2026.1

K method · SSC-2026.1

Minimum size

10 AWG · 5.26 mm²

Single conductor set

Voltage drop

4.97 V

2.07% · end 235.0 V

Power loss

99 W

I²R at the operating current

Required ampacity

20.0 A

Each conductor 30 A

Material estimate

$432.00

3 conductors + EMT

Conductor weight

11.1 lb

5.0 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	100 ft one way	cuts excluded	3 conductors	USD	$247.00	Source Spec Material / Tool
EMT conduit	100 ft	—	100 ft	USD	$185.00	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	10 AWG · 5.26 mm²	5.3	2.07%	235.0 V	99 W
aluminum	8 AWG · 8.37 mm²	8.4	2.14%	234.9 V	103 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
100 ft	10 AWG · 5.26 mm²	4.97 V	2.07%	235.0 V
125 ft	10 AWG · 5.26 mm²	6.21 V	2.59%	233.8 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 100 feet

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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 for voltage drop in branch circuits to ensure efficient operation and safety. For a 240V copper branch circuit carrying 20A over a distance of 100 feet, the NEC recommends that the voltage drop should not exceed 3%. This means that the maximum allowable voltage drop would be 7.2 volts, ensuring that the voltage at the load remains within acceptable limits for optimal performance.

It is important to note that when calculating voltage drop for this scenario, derating factors for temperature, conduit fill, or other conditions are excluded. This means that the calculations are based solely on the conductor size and length, allowing for a straightforward assessment of whether the circuit meets the 3% guideline.

Code reference

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

Field questions

What is the significance of the 3% voltage drop rule?: The 3% voltage drop rule helps ensure that electrical devices operate efficiently and safely, preventing performance issues due to insufficient voltage at the load.
Why is derating excluded when calculating voltage drop?: Derating is excluded in this context to simplify calculations and focus on the conductor's ability to handle the specified current over the given distance without additional factors affecting performance.
How do I calculate the 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)). Ensure the result does not exceed 3% of the supply voltage.
What happens if my voltage drop exceeds 3%?: If the voltage drop exceeds 3%, it may lead to inefficient operation of electrical devices, potential overheating, and increased energy costs. It is advisable to adjust the conductor size or reduce the circuit length.