50 amp copper run at 40 feet
Learn about the 3% voltage drop rule in the NEC for 240V copper branch circuits at 50A over 40 ft. Understand why derating is excluded and how it impacts your electrical design.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
6 AWG · 13.3 mm²
Single conductor set
Voltage drop
1.97 V
0.82% · end 238.0 V
Power loss
98 W
I²R at the operating current
Required ampacity
62.5 A
Each conductor 65 A
Material estimate
$318.40
3 conductors + EMT
Conductor weight
11.8 lb
5.3 kg · approximate insulated weight
Circle diameters track circular mils. The drawing is a schematic, not a conduit-fill layout.
| Item | Net | Waste | Order | Unit price | Line total | Source Spec Material / Tool |
|---|---|---|---|---|---|---|
| copper 6 AWG · 13.3 mm² | 40 ft one way | cuts excluded | 3 conductors | USD | $244.40 | Source Spec Material / Tool |
| EMT conduit | 40 ft | — | 40 ft | USD | $74.00 | Source Spec Material / Tool |
| 50 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.
| Material | Minimum size | mm² | Drop | End voltage | Loss |
|---|---|---|---|---|---|
| copper | 6 AWG · 13.3 mm² | 13.3 | 0.82% | 238.0 V | 98 W |
| aluminum | 4 AWG · 21.2 mm² | 21.2 | 0.85% | 238.0 V | 102 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 40 ft | 6 AWG · 13.3 mm² | 1.97 V | 0.82% | 238.0 V |
| 65 ft | 6 AWG · 13.3 mm² | 3.20 V | 1.33% | 236.8 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.
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Project: 50 amp copper run at 40 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 acceptable voltage drop in electrical systems to ensure efficient operation. For branch circuits, a 3% voltage drop is recommended to maintain performance and safety. This means that for a 240V circuit, the maximum allowable voltage drop should not exceed 7.2 volts (3% of 240V).
In this scenario, with a 50A load over a distance of 40 ft, it is important to note that derating factors for conductors are excluded when calculating voltage drop. This means that the conductor's ampacity is not reduced due to temperature or other environmental conditions, allowing for a more straightforward calculation of voltage drop.
Code reference
Refer to NEC Article 310 for conductor ampacity and NEC Article 215 for branch circuits.
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 issues such as overheating and reduced performance.
- Why is derating excluded in this voltage drop calculation?
- Derating is excluded to simplify the calculation of voltage drop, allowing for a direct assessment of the conductor's performance under standard conditions.
- How do I calculate the voltage drop for my circuit?
- To calculate voltage drop, you can use the formula: Voltage Drop (V) = (2 * Length of Circuit (ft) * Current (A) * Resistivity of Conductor (Ω/ft)) / 1000.
- What happens if my voltage drop exceeds 3%?
- If the voltage drop exceeds 3%, it can lead to inefficient operation of electrical devices, potential overheating, and may violate code requirements.