30 amp copper run at 40 feet
Learn about the 3% NEC informational note regarding voltage drop for 240V copper branch or feeder circuits at 30A over 40 ft, and understand why derating does not apply.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
10 AWG · 5.26 mm²
Single conductor set
Voltage drop
2.98 V
1.24% · end 237.0 V
Power loss
89 W
I²R at the operating current
Required ampacity
30.0 A
Each conductor 30 A
Material estimate
$172.80
3 conductors + EMT
Conductor weight
4.4 lb
2.0 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 10 AWG · 5.26 mm² | 40 ft one way | cuts excluded | 3 conductors | USD | $98.80 | Source Spec Material / Tool |
| EMT conduit | 40 ft | — | 40 ft | USD | $74.00 | Source Spec Material / Tool |
| 30 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 | 10 AWG · 5.26 mm² | 5.3 | 1.24% | 237.0 V | 89 W |
| aluminum | 8 AWG · 8.37 mm² | 8.4 | 1.28% | 236.9 V | 92 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 40 ft | 10 AWG · 5.26 mm² | 2.98 V | 1.24% | 237.0 V |
| 65 ft | 10 AWG · 5.26 mm² | 4.85 V | 2.02% | 235.2 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: 30 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 maintaining voltage drop within acceptable limits to ensure efficient operation of electrical systems. For a 240V copper branch or feeder circuit operating at 30A over a distance of 40 ft, the NEC suggests that the voltage drop should not exceed 3% of the nominal voltage.
This means that for a 240V circuit, the maximum allowable voltage drop is 7.2 volts (3% of 240V). However, it is important to note that derating factors, which may apply to certain conditions such as ambient temperature or conduit fill, do not apply in this context when calculating voltage drop for a standard installation.
Code reference
Refer to NEC Article 310 for conductor sizing and NEC Article 215 for branch circuits.
Field questions
- What is the 3% rule in the NEC?
- The 3% rule in the NEC refers to the recommendation that voltage drop in branch circuits should not exceed 3% of the nominal voltage to ensure efficient operation.
- Why is derating excluded when calculating voltage drop?
- Derating is excluded in voltage drop calculations because the 3% rule focuses on maintaining voltage levels for performance rather than adjusting for factors like temperature or conduit fill.
- 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)). For copper conductors, you can find resistance values in NEC tables.