40 amp copper run at 40 feet
Learn about the 3% NEC informational note for 240V copper branch circuits at 40A over 40 ft. Understand derating exclusions and ensure compliance with electrical codes.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
8 AWG · 8.37 mm²
Single conductor set
Voltage drop
2.50 V
1.04% · end 237.5 V
Power loss
100 W
I²R at the operating current
Required ampacity
40.0 A
Each conductor 50 A
Material estimate
$235.20
3 conductors + EMT
Conductor weight
7.7 lb
3.5 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 | 40 ft one way | cuts excluded | 3 conductors | USD | $161.20 | Source Spec Material / Tool |
| EMT conduit | 40 ft | — | 40 ft | USD | $74.00 | 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 | 1.04% | 237.5 V | 100 W |
| aluminum | 8 AWG · 8.37 mm² | 8.4 | 1.71% | 235.9 V | 164 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 40 ft | 8 AWG · 8.37 mm² | 2.50 V | 1.04% | 237.5 V |
| 65 ft | 8 AWG · 8.37 mm² | 4.06 V | 1.69% | 235.9 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 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 the safe installation of electrical systems. One important aspect is the 3% voltage drop recommendation for branch circuits. For a 240V copper branch circuit carrying 40A over a distance of 40 feet, the NEC suggests that the voltage drop should not exceed 3% to ensure efficient operation and minimize energy loss.
It's important to note that derating factors, which may apply to certain conditions such as temperature or conduit fill, are excluded from this voltage drop calculation. This means that the 3% guideline is based on the circuit's nominal capacity without adjustments for these factors.
Code reference
Refer to NEC Article 210 for branch circuits and Article 310 for conductors.
Field questions
- What is the significance of the 3% voltage drop recommendation?
- The 3% voltage drop recommendation helps ensure that electrical devices receive adequate voltage for optimal performance, reducing the risk of overheating and energy loss.
- Why is derating excluded from the 3% voltage drop calculation?
- Derating is excluded to provide a clear guideline for voltage drop based solely on the circuit's nominal capacity, ensuring that installations remain efficient 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 x Length of Circuit (ft) x Current (A) x Resistance of Conductor (Ω/ft)). Ensure that the result does not exceed 3% of the system voltage.