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

The 3% voltage drop rule is significant because it helps ensure that electrical devices receive adequate voltage for optimal performance and safety. Exceeding this drop can lead to inefficient operation and potential damage.

Why is derating excluded from the 3% voltage drop calculation?

Derating is excluded from the 3% voltage drop calculation to provide a clear guideline for standard circuit lengths and loads, ensuring that the basic performance of the circuit is maintained without additional complexity from environmental factors.

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) * Resistance of Wire (ohms/ft)). For copper wire, you can find the resistance values in NEC tables.

What happens if my voltage drop exceeds 3%?

If your voltage drop exceeds 3%, it may lead to insufficient voltage at the load, potentially causing equipment malfunction or failure. It is advisable to adjust the wire size or reduce the length of the circuit to comply with the NEC recommendations.

20 amp copper run at 60 feet

Learn about the 3% NEC informational note regarding voltage drop for 240V copper branch circuits at 20 A over 60 ft. Understand derating exclusions and ensure compliance with electrical codes.

Formula SSC-2026.1

K method · SSC-2026.1

Minimum size

12 AWG · 3.31 mm²

Single conductor set

Voltage drop

4.74 V

1.98% · end 235.3 V

Power loss

95 W

I²R at the operating current

Required ampacity

20.0 A

Each conductor 20 A

Material estimate

$207.72

3 conductors + EMT

Conductor weight

4.1 lb

1.9 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	60 ft one way	cuts excluded	3 conductors	USD	$96.72	Source Spec Material / Tool
EMT conduit	60 ft	—	60 ft	USD	$111.00	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	1.98%	235.3 V	95 W
aluminum	10 AWG · 5.26 mm²	5.3	2.04%	235.1 V	98 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
60 ft	12 AWG · 3.31 mm²	4.74 V	1.98%	235.3 V
85 ft	12 AWG · 3.31 mm²	6.72 V	2.80%	233.3 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 60 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 for voltage drop in branch circuits, recommending that the voltage drop should not exceed 3% for feeders and branch circuits. For a 240V copper branch circuit at 20 A over a distance of 60 ft, this means that the maximum allowable voltage drop is 7.2 volts (3% of 240V).

It's important to note that derating factors, which may apply to certain installations due to temperature or other conditions, are excluded from this specific calculation. This means that the 3% guideline is based solely on the length and load of the circuit without adjustments for derating.

Code reference

Refer to NEC Article 210 for branch circuits and NEC Article 215 for feeders regarding voltage drop considerations.

Field questions

What is the significance of the 3% voltage drop rule?: The 3% voltage drop rule is significant because it helps ensure that electrical devices receive adequate voltage for optimal performance and safety. Exceeding this drop can lead to inefficient operation and potential damage.
Why is derating excluded from the 3% voltage drop calculation?: Derating is excluded from the 3% voltage drop calculation to provide a clear guideline for standard circuit lengths and loads, ensuring that the basic performance of the circuit is maintained without additional complexity from environmental factors.
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) * Resistance of Wire (ohms/ft)). For copper wire, you can find the resistance values in NEC tables.
What happens if my voltage drop exceeds 3%?: If your voltage drop exceeds 3%, it may lead to insufficient voltage at the load, potentially causing equipment malfunction or failure. It is advisable to adjust the wire size or reduce the length of the circuit to comply with the NEC recommendations.