40 amp copper run at 100 feet
Learn about the 3% NEC informational note regarding voltage drop for 240V copper branch circuits at 40A over 100 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
6.25 V
2.60% · end 233.7 V
Power loss
250 W
I²R at the operating current
Required ampacity
40.0 A
Each conductor 50 A
Material estimate
$588.00
3 conductors + EMT
Conductor weight
19.2 lb
8.7 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 | 100 ft one way | cuts excluded | 3 conductors | USD | $403.00 | Source Spec Material / Tool |
| EMT conduit | 100 ft | — | 100 ft | USD | $185.00 | 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 | 2.60% | 233.7 V | 250 W |
| aluminum | 6 AWG · 13.3 mm² | 13.3 | 2.69% | 233.5 V | 259 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 100 ft | 8 AWG · 8.37 mm² | 6.25 V | 2.60% | 233.7 V |
| 125 ft | 6 AWG · 13.3 mm² | 4.92 V | 2.05% | 235.1 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.
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Project: 40 amp copper run at 100 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 100 ft, the 3% voltage drop guideline suggests that the maximum allowable voltage drop should not exceed 7.2 volts (3% of 240V). This is crucial for maintaining the performance of electrical equipment and preventing overheating.
It's important to note that derating factors for conductors, such as temperature or conduit fill, do not apply to the calculation of voltage drop in this context. The focus remains solely on the distance and load to ensure compliance with the NEC's recommendations.
Code reference
Refer to NEC Article 310 for conductor ampacity and voltage drop considerations.
Field questions
- What is the significance of the 3% voltage drop guideline?
- The 3% voltage drop guideline helps ensure that electrical devices operate efficiently and safely, preventing issues like overheating and reduced performance.
- Why is derating excluded from voltage drop calculations?
- Derating factors are excluded from voltage drop calculations to provide a clear and straightforward assessment of voltage drop based solely on distance and load, ensuring compliance with NEC guidelines.
- How do I calculate the voltage drop for my circuit?
- To calculate voltage drop, use the formula: Voltage Drop (V) = (2 × Length of Circuit (ft) × Current (A) × Resistance of Conductor (Ω/ft)). Ensure the result does not exceed the allowable drop based on the 3% guideline.