40 amp copper run at 60 feet
Learn about the 3% NEC informational note for 240V copper branch circuits at 40A over 60 ft, and understand why derating is excluded in this context.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
8 AWG · 8.37 mm²
Single conductor set
Voltage drop
3.75 V
1.56% · end 236.2 V
Power loss
150 W
I²R at the operating current
Required ampacity
40.0 A
Each conductor 50 A
Material estimate
$352.80
3 conductors + EMT
Conductor weight
11.5 lb
5.2 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 | 60 ft one way | cuts excluded | 3 conductors | USD | $241.80 | Source Spec Material / Tool |
| EMT conduit | 60 ft | — | 60 ft | USD | $111.00 | 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 | 1.56% | 236.2 V | 150 W |
| aluminum | 8 AWG · 8.37 mm² | 8.4 | 2.57% | 233.8 V | 247 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 60 ft | 8 AWG · 8.37 mm² | 3.75 V | 1.56% | 236.2 V |
| 85 ft | 8 AWG · 8.37 mm² | 5.31 V | 2.21% | 234.7 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 60 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 to ensure safe and efficient electrical installations. One important aspect is the 3% voltage drop rule, which suggests that for branch circuits, the voltage drop should not exceed 3% of the supply voltage to maintain performance and safety.
In the case of a 240V copper branch circuit carrying 40A over a distance of 60 feet, the NEC informational note indicates that while voltage drop is a consideration, derating of the conductors is not applied in this specific context. This means that you can use the full ampacity of the conductors without needing to adjust for the distance in this scenario.
Code reference
Refer to NEC Article 310 for conductor ampacity and Article 215 for branch circuit requirements.
Field questions
- What is the 3% NEC informational note?
- The 3% NEC informational note suggests that the voltage drop in branch circuits should not exceed 3% of the supply voltage to ensure efficient operation and safety.
- Why is derating excluded for this circuit?
- Derating is excluded in this case because the NEC allows for the full ampacity of the conductors to be used without adjustment for the distance involved, as long as the voltage drop remains within acceptable limits.
- How do I calculate voltage drop for my circuit?
- Voltage drop can be calculated using the formula: Voltage Drop (V) = (2 * Length of Circuit (ft) * Current (A) * Resistance of Conductor (Ω/ft)).
- What type of conductor should I use for a 240V, 40A circuit?
- For a 240V, 40A circuit, copper conductors with appropriate gauge should be used, typically a minimum of 8 AWG, but always refer to NEC guidelines for specific applications.