40 amp copper run at 150 feet
Learn about the 3% NEC informational note regarding voltage drop for 240V copper branch circuits at 40A over 150 ft. Understand derating exclusions and ensure compliance.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
6 AWG · 13.3 mm²
Single conductor set
Voltage drop
5.90 V
2.46% · end 234.1 V
Power loss
236 W
I²R at the operating current
Required ampacity
40.0 A
Each conductor 65 A
Material estimate
$1,194.00
3 conductors + EMT
Conductor weight
44.1 lb
20.0 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 6 AWG · 13.3 mm² | 150 ft one way | cuts excluded | 3 conductors | USD | $916.50 | Source Spec Material / Tool |
| EMT conduit | 150 ft | — | 150 ft | USD | $277.50 | 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 | 6 AWG · 13.3 mm² | 13.3 | 2.46% | 234.1 V | 236 W |
| aluminum | 4 AWG · 21.2 mm² | 21.2 | 2.54% | 233.9 V | 244 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 150 ft | 6 AWG · 13.3 mm² | 5.90 V | 2.46% | 234.1 V |
| 175 ft | 6 AWG · 13.3 mm² | 6.88 V | 2.87% | 233.1 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 150 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 branch circuits to ensure efficient operation and safety. For a 240V copper branch circuit carrying 40A over a distance of 150 ft, the 3% voltage drop rule suggests that the maximum allowable voltage drop should not exceed 7.2 volts (3% of 240V).
It's important to note that derating factors for conductors, such as those due to ambient temperature or multiple conductors in a raceway, are not included in this calculation for voltage drop. This means that the voltage drop should be calculated based solely on the length of the run and the load current without additional derating considerations.
Code reference
Refer to NEC Article 210.19(A)(1) for branch circuits and NEC Article 310 for conductor ampacity.
Field questions
- What is the significance of the 3% voltage drop rule?
- The 3% voltage drop rule is significant as it helps ensure that electrical devices operate efficiently and safely. Exceeding this limit can lead to performance issues and potential damage to equipment.
- Why is derating excluded when calculating voltage drop?
- Derating is excluded in voltage drop calculations to provide a straightforward assessment of the circuit's performance based solely on the load and distance, without complicating factors that may vary based on installation conditions.
- How do I calculate the voltage drop for my circuit?
- To calculate voltage drop, use the formula: Voltage Drop (V) = (2 × Length of the circuit in feet × Current in Amperes × Resistivity of the conductor) / 1000. For copper, the resistivity is approximately 10.4 ohms per mil foot.