40 amp copper run at 80 feet
Learn about the 3% NEC informational note regarding voltage drop for 240V copper branch circuits at 40A over 80 ft. Understand derating exclusions and ensure compliance.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
8 AWG · 8.37 mm²
Single conductor set
Voltage drop
5.00 V
2.08% · end 235.0 V
Power loss
200 W
I²R at the operating current
Required ampacity
40.0 A
Each conductor 50 A
Material estimate
$470.40
3 conductors + EMT
Conductor weight
15.4 lb
7.0 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 8 AWG · 8.37 mm² | 80 ft one way | cuts excluded | 3 conductors | USD | $322.40 | Source Spec Material / Tool |
| EMT conduit | 80 ft | — | 80 ft | USD | $148.00 | Source Spec Material / Tool |
| 40 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 | 8 AWG · 8.37 mm² | 8.4 | 2.08% | 235.0 V | 200 W |
| aluminum | 6 AWG · 13.3 mm² | 13.3 | 2.15% | 234.8 V | 207 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 80 ft | 8 AWG · 8.37 mm² | 5.00 V | 2.08% | 235.0 V |
| 105 ft | 8 AWG · 8.37 mm² | 6.56 V | 2.73% | 233.4 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: 40 amp copper run at 80 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 electrical systems to ensure efficiency and safety. For a 240V copper branch circuit carrying 40A over a distance of 80 feet, the NEC suggests that the voltage drop should not exceed 3% of the system voltage.
This means that for a 240V circuit, the maximum allowable voltage drop would be 7.2V (3% of 240V). It is important to note that derating factors are not included in this calculation, as the 3% guideline is based on the nominal voltage drop without considering additional derating due to temperature, conduit fill, or other factors.
Code reference
Refer to NEC Article 210 for branch circuits and NEC Article 310 for conductors.
Field questions
- What is the significance of the 3% voltage drop rule?
- The 3% voltage drop rule helps ensure that electrical systems operate efficiently and safely, reducing the risk of overheating and equipment malfunction.
- Why is derating excluded from the 3% calculation?
- Derating is excluded from the 3% calculation to provide a baseline for voltage drop that is independent of other factors, ensuring that the circuit can handle the load without additional complications.
- 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) * Resistance of Conductor (Ω/ft)). Ensure to use the correct resistance value for copper conductors.